Guidelines for the Welded Fabrication of Nickel-Containing Stainless Steels for Corrosion Resistant Services

NiDl Nickel Development Institute

Guidelines for the welded fabrication of nickel-containing stainless steels for corrosion resistant services

A Nickel Development Institute Reference Book, Series No 11 007

Table of Contents Introduction ...... i PART I – For the welder ...... 1 Physical properties of austenitic steels ...... 2 Factors affecting corrosion resistance of stainless steel welds ...... 2 Full penetration welds ...... 2 Seal welding crevices ...... 2 Embedded iron ...... 2 Avoid surface oxides from welding ...... 3 Other welding related defects ...... 3 Welding qualifications ...... 3 Welder training ...... 4 Preparation for welding ...... 4 Cutting and joint preparation ...... 5 Weld joint designs ...... 5 Cleaning in preparation for welding ...... 7 Oxides and other surface layers ...... 7 Contamination elements ...... 8 Chlorinated solvents ...... 9 Health hazards ...... 9 Fixturing, fitting and tack welding ...... 9 Fixtures and positioners ...... 10 Backing materials ...... 10 Tack welding ...... 10 Fitting pipe joints for GTAW root welds ...... 11 Purging during pipe root welding ...... 11 Welding Processes ...... 12 Shielded metal arc welding ...... 12 Electrode types ...... 13 Other guides in SMAW ...... 13 Electrode handling and storage ...... 13 Welding current ...... 15 Arc starting and stopping ...... 15 Gas tungsten arc welding ...... 15 GTAW equipment ...... 16 Consumables ...... 16 Operator technique guides ...... 17 Gas metal arc welding ...... 18 Arc transfer modes ...... 18 GMAW equipment ...... 18 Consumables ...... 19 Other welding processes ...... 19 Post-fabrication cleaning ...... 19 Surface contaminants ...... 20 Detection ...... 20 Removal ...... 20 Embedded iron ...... 20 Detecting embedded iron ...... 21 Removing embedded iron ...... 21 Mechanical damage ...... 22 Safety and welding fumes ...... 22

PART II – For the materials engineer ...... 23 Stainless steel alloys ...... 23 Austenitic stainless steels ...... 23 Effect of welding on corrosion resistance ...... 23 Role of weld metal ferrite ...... 27 Measuring weld metal ferrite ...... 27 Duplex stainless steels ...... 28 Characteristics of duplex stainless steels ...... 30 High temperature exposure ...... 30 Effect of welding on duplex stainless steels ...... 30 Nickel-enriched filler metal ...... 31 Heat input control ...... 31 Interpass temperature control ...... 31 Preheat ...... 32 Other stainless steels ...... 32 Martensitic stainless steels ...... 33 Ferritic stainless steels ...... 33 Precipitation hardening stainless steels ...... 34 Corrosion resistant stainless steel castings ...... 34 Heat treatment of stainless steel ...... 35 Alternate 1 ...... 35 Alternate 2 ...... 35 Material procurement and storage guides ...... 36 Surface finishes ...... 36 Purchasing guidelines ...... 38 PART III – For the design engineer ...... 39 Design for corrosion services ...... 39 Tank bottoms ...... 39 Tank bottom outlets...... 40 Bottom corner welds ...... 40 Attachments and structurals ...... 40 Heaters and inlets ...... 42 Pipe welds ...... 42 Appendix A: Specifications for stainless steel for welded fabrication ...... 45 Additional requirements ...... 46 Bibliography ...... 46 Acknowledgement ...... 46 Tables Table 1 – Influence of physical properties on welding austenitic stainless steels compared to carbon steel ...... 1 Table 2 – Stainless steel cutting methods ...... 5 Table 3 – Melting temperatures of metals and metal oxides ...... 7 Table 4 – Suggested filler metals for welding stainless steels ...... 14 Table 5 – Comparison of GMAW arc modes for stainless steels ...... 18 Table 6 – Wrought austenitic stainless steels chemical analysis ...... 24 Table 7 – Corrosion resistant stainless steel castings chemical analysis ...... 25 Table 8 – Duplex stainless steels chemical analysis ...... 29 Table 9 – Duplex stainless steel filler metals typical composition ...... 32 Table 10 – Suggested filler metals for welding some of the martensitic, ferritic and precipitation hardening stainless steels ...... 33 Table 11 – Stainless steel products forms ...... 36 Table 12 – Standard mechanical sheet finishes ...... 37 Figures Figures 1-1 to 1-7 –Typical weld joint designs ...... 6 to 7 Figure 2 – Backing bar groove designs ...... 10 Figure 3 – The correct tack weld sequence ...... 11 Figure 4 – Typical pipe purging fixtures ...... 12 Figure 5 – Shielded metal arc welding ...... 12 Figure 6 – Gas tungsten arc welding ...... 15 Figure 7 – Gas metal arc welding ...... 17 Figure 8 – Typical fabrication defects ...... 20 Figure 9 – Crevice corrosion ...... 20 Figure 10 – A scratch serves as an initiation site for corrosion ...... 20 Figure 11 – Effect of carbon control on carbide precipitation in Type 304 ...... 26 Figure 12 – Revised constitution diagram for stainless steel weld metal ...... 28 Figure 13 – Typical microstructure of cold rolled, quench annealed Alloy 2205 seamless tube ...... 28 Figures 14-1 to 14-36 – Designs for improved corrosion service ...... 39 to 44 Introduction

Nickel-containing stainless steels are In the section FOR THE WELDER, it is indispensable in the construction of assumed that the welders or others equipment for the process industries. involved in welded fabrication are These steels are used in place of familiar with the basic techniques used conventional steels for properties such in carbon steel fabrication, but have had as excellent corrosion resistance, limited experience with nickel-containing toughness at low temperatures and stainless steels. The Welder section good elevated-temperature properties. employs a “how to do it” approach for The stainless steels are an excellent the non-engineer but may also serve as choice for chemical, dairy, food, archi- a reference for the welding and metal- tectural, biotechnology equipment and lurgical engineer. similar services. The wrought nickel The section FOR THE MATERIALS stainless steels widely used for corro- ENGINEER describes the various types sion services range from Type 304 of stainless steels, how their metallurgi- (Unified Numbering System, UNS, cal and corrosion characteristics are S30400) through the newer 6% molyb- affected by welding and some of the denum alloys, along with the compara- more specialized aspects of fabrication ble cast alloys and the duplex stainless such as heat treating. Guidelines for steels. material procurement and handling are This publication is presented in three also covered. sections identified as, FOR THE The section FOR THE DESIGN WELDER (page 1), FOR THE MATERI- ENGINEER provides a number of ALS ENGINEER (page 23) and FOR design examples of how the corrosion THE DESIGN ENGINEER (page 39). performance of stainless steels can be enhanced through good design.

i Part I For the welder

Part I focuses on the fabrication and molybdenum stainless steels. Duplex welding of austenitic stainless steels, stainless steels which are half austenite Types 304, 316, 321 and 347 (UNS and half ferrite are discussed in the S30400, S31600, S32100 and S34700) section entitled “For the materials and the more highly alloyed 4 and 6% engineer.”

Table I Influence of physical properties on welding austenitic stainless steels compared to carbon steel

Austenitic stainless Carbon steel steel Remarks Melting point 2550-2650°F 2800°F Type 304 requires less heat to produce (Type 304) (1400-1450°C) (1540°C) fusion, which means faster welding for the same heat or less heat for the same speed. Magnetic Non-magnetic Magnetic to over Nickel stainless steels are not subject to arc response all temperatures (1) 1300°F blow. (705°C)

Rate of heat Type 304 conducts heat much more slowly conductivity (Type 304) than carbon steel thus promoting sharper (% at 212°F) (100°C) 28% 100% heat gradients. This accelerates warping, (% at 1200°F) (650°C) 66% 100% especially in combination with higher expansion rates. Slower diffusion of heat expansion through the base metal means that weld zones remain hot longer, one result of which may be longer dwell in the carbide precipitation range unless excess heat is artificially removed by chill bars, etc.

Electrical resistance This is of importance in electrical fusion (Annealed) methods. The higher electrical resistance of (Microhm-cm, approx.) Type 304 results in the generation of more At 68°F (20°C) 72.0 12.5 heat for the same current or the same heat At 1625°F (885°C) 126.0 125 with lower current, as compared with carbon steel. This, together with its low rate of heat conductivity, accounts for the effectiveness of resistance welding methods on Type 304.

Thermal Expansion 9.8 6.5 Type 304 expands and contracts at a faster over range indicated (68-932°F) (68-1162°F) rate than carbon steel, which means that in./in./F x 10-6 increased expansion and contraction must be allowed for in order to control warping and the development of thermal stresses upon cooling. For example, more tack welds in./in./C x 10-6 17.6 11.7 are used for stainless steel than for carbon (20-500°C) (20-628°C) steel.

(1) Duplex stainless steels are magnetic.

1 For the welder Physical properties of Full penetration welds austenitic stainless steels It is well recognized that for optimum strength, butt welds should be full- The physical properties of ordinary penetration welds. In corrosion service, carbon steels and austenitic stainless any crevice resulting from lack of steels are quite different and these call penetration is also a potential site for for some revision of welding proce- crevice corrosion. A typical example of dures. Physical properties, Table l, an undesirable crevice is incomplete include such items as melting point, fusion of a pipe root pass weld such as thermal expansion, thermal conductivity shown in Figure 14-31 (see page 43). and others that are not significantly In some environments, corrosion takes changed by thermal or mechanical place in the crevice which, in turn, can processing. As illustrated in Table l, the lead to early failure of the weld joint. melting point of the austenitic grades is lower, so less heat is required to pro- Seal welding crevices duce fusion. Their electrical resistance Crevices between two stainless steel is higher than that of mild steel so less surfaces such as tray supports tacked electrical current (lower heat settings) is to a tank, as shown in Figure 14-16 required for welding. These stainless (see page 40), also invite crevice steels have a lower coefficient of ther- corrosion. Avoiding such crevices is a mal conductivity, which causes heat to design responsibility and discussed concentrate in a small zone adjacent to further in the section For the Design the weld. The austenitic stainless steels Engineer, as well as calling for also have coefficients of thermal expan- corrective action. However, it is helpful sion approximately 50% greater than for those actually making the mild steel, which calls for more attention equipment to assist in eliminating to the control of warpage and distortion. crevices whenever possible. Embedded iron Factors affecting When new stainless steel equipment corrosion resistance of develops rust spots, it is nearly always stainless steel welds the result of embedded free iron. In some environments, if the iron is not Before discussing welding guidelines, it removed, deep attack in the form of is useful to describe the types of welds pitting corrosion may take place. In less and stainless steel surfaces which will extreme environments, the iron rust give the best performance in corrosive may act as a contaminant affecting environments. These are factors that product purity, or present an unsightly the welders or others on the shop floor rusty appearance to a surface that control, rather than alloy selection, should be clean and bright. which is usually made by the end user Free iron is most often embedded in or Materials Engineer. The manufacture stainless steels during welding or of corrosion resistant equipment that will forming operations. Some cardinal give superior service should be viewed fabrication rules to follow in avoiding as a joint effort of selecting the correct free iron are: alloy and then employing the proper welding and fabrication practices. Both
DO NOT bring iron or steel surfaces elements are essential. into intimate contact with stainless steel. The contact could come from lifting tools, steel tables or storage racks, to name a few.

2 For the welder
DO NOT use tools such as abrasive When, after normal precautions are disks or wheels that have been previ- taken and there are still surface ously used on ordinary iron or steel and oxides, they can be removed by acid could have iron embedded. pickling, glass-bead blasting or one of the other methods discussed in Post-
Use only stainless steel wire brushes Fabrication Cleaning. that have never been used on carbon steel. Never use brushes with carbon Other welding related defects steel wire. Three other welding related defects and their removal procedure are listed
DO NOT leave stainless steel sheets below. or plates on the floor exposed to traffic. Sheet and plate are best stored in the
Arc strikes on the parent material vertical position. damage stainless steel's protective film and create crevice-like imperfections.
Locate stainless steel fabrication Weld stop points may create pinpoint away from carbon steel fabrication, if at defects in the weld metal. Both imper- all possible, to avoid iron contamination fections should be removed by light from steel grinding, cutting and blasting grinding with clean fine grit abrasive operations. tools.

The detection of free iron and removal
Weld spatter creates a tiny weld methods are discussed under Post- where the molten slug of metal touches Fabrication Cleaning in this section. and adheres to the surface. The protec- tive film is penetrated and tiny cervices Avoid surface oxides from are created where the film is weakened the most. Weld spatter can easily be welding eliminated by applying a commercial For best corrosion resistance, the spatter-prevention paste to either side stainless steel surface should be free of of the joint to be welded. The paste and surface oxides. The oxides may be in the spatter are washed off during cleanup. form of heat tint resulting from welding on the reverse side or heat tint on the weld
Slag on some coated electrode welds or in the heat affected zone, HAZ. Oxides is difficult to remove completely. Small can also develop on the root inside slag particles resist cleaning and diameter, ID, surface of pipe welds made particularly remain where there is a with an inadequate inert gas purge. slight undercut or other irregularity. The oxides may vary from thin, straw Slag particles create crevices and must coloured, a purple colour to a black heavy be removed by wire brushing, light oxide. The darker the colour and heavier grinding or abrasive blasting with iron the oxide, the more likely pitting corrosion free materials. will develop, causing serious attack to the underlying metal. It should be understood that the oxides are harmful in corrosive Welding qualifications environments. Oxides normally need not It is standard practice for fabricators of be removed when the stainless steel will process equipment to develop and operate at high temperatures where maintain welding procedure specifica- oxides would normally form. Heat tint tions, WPS, for the various types of seldom leads to corrosion in atmospheric welding performed. The individual or other mild environments but is welders and welding operators are frequently removed for cosmetic tested and certified by satisfactorily purposes.

3 For the welder making acceptable performance qualifi- P-Number Base metal cation weldments. There are a number of Society or Industry codes that govern 8 Austenitic stainless steels in welding qualifications, but the two most Table VI from Type 304 widely used in the US for corrosion through 347 and Alloy 254 resistance equipment are: SMO plus the similar CF cast - American Society of Mechanical alloys of Table VII Engineers, ASME, Boiler and Pres- 10H Duplex stainless steels include- sure Vessel Code – Section IX, ing Alloys 255 and 2205 and Welding and Brazing Qualification; cast CD 4MCu - American Welding Society, AWS, 45 Alloys 904L and 20Cb-3 and Standard for Welding Procedure and 6% molybdenum alloys of Ta- Performance Qualification – AWS ble VI except Alloy 254 SMO.

B2.1. Not all alloys have been assigned a P- Number. Alloys without a number Internationally, each country typically require individual qualification even has its own individual codes or stand- though similar in composition to an alloy ards. Fortunately, there is a trend already qualified. If an alloy is not listed toward the acceptance and interchange in the P-Number tables, the alloy manu- of specifications in the interest of elimi- facturer should be contacted to deter- nating unmerited requalification. mine if a number has been recently Common to these codes is the identifi- assigned by the code. cation of essential variables that estab- lish when a new procedure qualification test weldment is required. Essential Welder training variables differ for each welding process In complying with welding qualification but common examples might be: specifications such as ASME and AWS, - change in base metal being welded welders must pass a performance test. (P-Number); A welder training program is not only - change in filler metal (F-Number); essential prior to taking the performance - significant change in thickness being test but also insures quality production welded; welding. Stainless steels are sufficiently - change in shielding gas used; different in welding characteristics from - change in welding process used. ordinary steels that the welders should be provided training and practice time. ASME Section IX classification of P- Once they are familiar with the stainless Numbers often first determines if a steels, many welders develop a prefer- separate WPS is needed. A change in a ence over regular steels. In addition to base metal from one P-Number to the particular base metal and welding another P-Number requires process, training should also cover the requalification. Also joints made be- shapes to be welded such as pipe and tween two base metals of different P- thin sheets or unusual welding positions. Numbers require a separate WPS, even though qualification tests have been made for each of the two base metals Preparation for welding welded to themselves. P-Numbers are shown below: Stainless steels should be handled with somewhat greater care than carbon steels in cutting and fitting. The care taken in preparation for welding is time

4 For the welder well spent in improved weld quality and costs. Butt welds should be full penetra- a finished product that will give optimum tion welds for corrosive services. Fillet serviceability. welds need not be full penetration as long as both sides and ends are welded Cutting and joint preparation to seal off voids that could collect liquid With the exception of oxyacetylene and allow crevice corrosion. cutting, stainless steels can be cut by the Fillet welding branch connections on same methods used for carbon steel. pipe headers leaves a large and severe Oxyacetylene cutting stainless steel crevice on the ID. This practice invites (without iron rich powder additions) crevice and microbiologically influenced results in the formation of refractory corrosion and should be prohibited for chromium oxides, preventing accurate, stainless steel pipe fabrications in all smooth cuts. The thickness and shape of services. the parts being cut or prepared for weld- The molten stainless steel weld metal ing largely dictates which of the methods is somewhat less fluid than carbon steel shown in Table II is most appropriate. and depth of weld penetration is not as great. To compensate, stainless steel Weld joint designs weld joints may have a wider bevel, thinner land and a wider root gap. The The weld joint designs used for welding process also influences opti- stainless steels are similar to those mum joint design. For example, spray used for ordinary steels. The weld joint arc, gas metal arc welding, GMAW, design selected must produce welds of gives much deeper penetration than suitable strength and service perform- short circuiting GMAW, so thicker lands ance while still allowing low welding are used with the former process.

Table II Stainless steel cutting methods Method Thickness cut Comments Shearing Sheet/strip, thin plate Prepare edge exposed to environment to remove tear crevices.

Sawing & abrasive cutting Wide range of thicknesses Remove lubricant or cutting liquid before welding or heat treating.

Machining Wide range shapes Remove lubricant or cutting liquid before welding or heat treating.

Plasma arc cutting (PAC) Wide range of thicknesses Grind cut surfaces to clean metal.

Powder metal cutting with Wide range of thicknesses Cut less accurate than iron-rich powder PAC, must remove all dross,

Carbon arc cutting Used for gouging backside Grind cut surfaces to clean of welds and cutting metal. irregular shapes.

5 For the welder

Typical joint designs for sheet and plate welding are shown in Figures 1-1 through 1-5. Typical pipe joint designs for gas tungsten arc welding, GTAW, root welds with and without consum- able inserts, are shown in Figures 1-6 and 1-7. Consumable insert rings are widely used and are recommended for consistent root penetration.

Figure 1-3 Typical double “V” joint for plate.

Figure 1-1 Typical square butt joint for sheet.

Figure 1-4 Typical single “U”joint for plate.

Figure 1-2 Typical single “V” joint for sheet and plate.

Figure 1-5 Typical double “U” joint for plate.

6 For the welder

Figure 1-6 Typical joint design for pipe with consumable insert. Figure 1-7 Typical joint design for pipe welded without consumable insert.

Cleaning in preparation for thermal cutting. Stainless steel oxides are comprised mainly of those of chro- welding mium and nickel. Because these oxides The weld area to be cleaned includes melt at a much higher temperature than the joint edges and two or three inches the weld metal, they are not fused of adjacent surfaces. Improper cleaning during welding. Often an oxide film can cause weld defects such as cracks, becomes trapped in the solidifying weld porosity or lack of fusion. The corrosion resulting in a defect that is difficult to resistance of the weld and HAZ can be detect by radiography. This is a basic substantially reduced if foreign material difference from welding steel. With is left on the surface before welding or a steel, iron oxides melt at about the same heating operation. After cleaning, joints temperature as the weld metal. While it should be covered unless welding will be is considered poor practice to weld over immediately performed. a heavy steel mill scale, it does not present the problem caused by a stain- Oxide and other surface layers – less steel oxide film. The differences The joints to be welded should be free of between metal and metal oxide melting the surface oxides frequently left after temperatures is shown in Table lll.

Table III Melting temperatures of metals and metal oxides Metal Melting temperatures Metal Melting temperatures °F (°C) oxide °F (°C)

Iron 2798 (1537) Fe 2O3 2850 (1565)

Fe 3O4 2900 (1593)

Nickel 2650 (1454) NiO 3600 (1982)

304 S/S 2550-2650 (1400-1454) Cr 203 4110 (2266)

7 For the welder Stainless steel wrought products sulphur, carbon - hydrocarbons delivered by the mills are normally free such as cutting of objectionable oxides and do not fluids, grease, oil, need special treatment prior to welding. waxes and Any oxide layer would be thin and not primers likely the cause of welding problems. Very thin metals, such as strip under sulphur, - marking crayons, 0.010 in. (0.25mm) may need special phosphorous, paints and tem- cleaning such as vapour honing since carbon perature indicating even light oxide layers may be trapped markers in small, fast solidifying welds. lead, zinc, - tools such as Stainless steels that have been in copper hammers (lead), service often require special pre-weld hold down or cleanup. If the alloy has been exposed backing bars to high temperatures, the surface is (copper), zinc rich often heavily oxidized or may have a paint carburized or sulphurized layer. Such layers must be removed by grinding or shop dirt - any or all of the machining. Wire brushing polishes and above does not remove the tightly adhering oxides. Stainless steel equipment that The presence of sulphur, phosphorous has been in chemical service may be and low-melting metals may cause contaminated by the product media. A cracks in the weld or HAZ. Carbon or good example is caustic. If caustic is carbonaceous materials left on the left on the surface during welding, the surface during welding may be taken in weld and HAZ often develops cracks. solution, resulting in a high carbon layer Neutralizing caustic residue with an which in turn lowers the corrosion acid solution is part of an effective resistance in certain environments. cleanup prior to welding. It is good Cleaning to remove the above con- practice to give a neutralizing treatment taminants should be accomplished by prior to repair welding chemical equip- following a few guidelines, along with ment. That is, neutralize acid contami- common sense. Metallic contaminants nated surfaces with a mild basic solu- and materials not having an oil or grease tion and an alkaline contaminated base are often best removed by surface with a mild acidic solution. A mechanical means such as abrasive hot water rinse should always follow the blasting or grinding. It is essential that neutralizing treatment. the blasting material or abrasive disk be free of contaminants such as free iron. A Contamination elements – There are nitric acid treatment, followed by neu- a number of elements and compounds tralization can also effectively remove that must be removed from the surface some low melting metals without dam- prior to welding. If not removed, the heat age to the stainless steel. from welding can cause cracking, weld Oil or grease (hydrocarbon) base defects or reduced corrosion resistance contaminants must be removed by of the weld or HAZ. The elements to be solvent cleaning because they are not avoided and common sources of the removed by water or acid rinses. Large elements are: weldments are usually hand cleaned by wiping with solvent saturated cloths. Other acceptable methods include immersion in, swabbing with or spraying with alkaline, emulsion, solvent or

8 For the welder detergent cleaners or a combination of misapplication, some organizations these; by vapour degreasing; by steam, prohibit the use of any chlorinated with or without a cleaner; or by high- solvent across the board. Non-chlorin- pressure water jetting. American Society ated solvents are preferred for cleaning for Testing and Materials, ASTM, A380, stainless steels and should always be Standard Recommended Practice for used for equipment and crevices. Cleaning and Descaling Stainless Steel Parts, Equipment and Systems, is an Health hazards – The term health excellent guide for fabricators and users. hazard has been defined as including A typical procedure to remove oil or carcinogens, toxic agents, irritants, grease includes: corrosives, sensitizers and any agent - remove excess contaminant by that damages the lungs, skin, eyes or wiping with clean cloth; mucous membranes. Each organization - swab the weld area (at least 2 should assure that the solvents used are in.(5cm) each side of the weld) with not harmful to personnel or equipment. an organic solvent such as aliphatic In addition to the toxic effect, considera- petroleums, chlorinated hydro- tion must be given to venting of explo- carbons or blends of the two. (See sive fumes, safe disposal of spent cautionary remarks below.) Use only solutions and other related handling clean solvents (uncontaminated with practices. Compliance with state and acid, alkali, oil or other foreign local regulations is obviously a require- material) and clean cloths; ment. - remove all solvent by wiping with Solvents used for pre-weld cleaning clean, dry cloth; include, but are not limited to, the - check to assure complete cleaning. following: A residue on the drying cloth can - non-chlorinated: toluene, methyl - indicate incomplete cleaning. Where ethyl ketone and acetone size allows, either the water-break or - chlorinated solvent: 1.1.1. atomized test are effective checks. Trichloroethane All must be handled in compliance with Selecting the solvent cleaner involves regulator requirements and manu- considerations more than just the facturers' instructions. ability to remove oil and grease. Two Fixturing, fitting and tack precautions are as follows. welding Chlorinated solvents – Many Good alignment of the assembly prior commercial solvents contain chlorides to welding can reduce welding time. It is and are effective in cleaning machined essential that the mating pieces to be parts and crevice free components. The joined should be carefully aligned for potential problem with chlorinated good quality welding. When one mem- solvents is that they may remain and ber is considerably thicker than the concentrate in crevices and later initiate other, for example a tank head thicker crevice corrosion and stress corrosion than the shell, the head side should be cracking, SCC. There have been unnec- machined to a taper of 3:1 or more to essary and costly SCC failures of reduce stress concentrations. Joints stainless steel heat exchangers after with varying root gap require special cleaning with chlorinated solvents. adjustment by the welding operator and Cleaning of open, bold areas with may result in burn through or lack of chlorinated solvents does not present a penetration. When the volume of identi- problem, but rather than risk a cal parts is large, use of fixtures is often economically justified.

9 For the welder Fixtures and positioners – Fixtures most often used for backing bars. are usually designed for each particular Typical backing bar designs for use with assembly and hold the parts together and without a backing gas are shown in throughout the welding operation. When Figure 2. In the normal course of weld- fixtures are attached to positioners, ing, the copper bar chills the weld to there is a further advantage in that solid metal without melting the copper. welding can be done in the most con- The arc should not be misdirected to the venient position. Some advantages of extent that copper is melted and incor- using fixtures are: porated into the stainless steel weld or - better joint match-up; weld cracking can result. It is good - less tacking and welding time; practice to pickle after welding to remove - distortion from welding is minimized; traces of copper from the surface and - finish assembly is made to closer essential to pickle if solution annealing is tolerance. to follow welding. It is important that fixture surfaces Argon backing gas provides excellent holding the stainless steel parts do not protection to the underneath side of introduce iron contamination. This can be GTAW welds. It helps control penetration avoided by surfacing the fixture contact- and maintain a bright, clean under ing surfaces with stainless steel and surface. Nitrogen is also used as a using the fixtures only for stainless steel. backing gas and has a price advantage over argon. However, nitrogen should Backing materials – A backing not be introduced into the arc atmos- material should be used in welding sheet phere which, in turn, could alter the weld or plate, unless both sides of the joint metal composition balance. can be welded. Without a backing, the When a copper backing bar or an inert underneath side may have erratic gas backing purge is impractical, there penetration with crevices, voids and are commercially available tapes, pastes excessive oxidation. Such defects and ceramic backing products. These reduce weld strength and can initiate offer some protection from burn-through accelerated corrosion. Copper, with its but give little protection from oxidation, high thermal conductivity, is the material so final cleaning by abrasive means or acid pickling is needed after welding when these backing materials are used. Tack welding – Joints not held in fixtures must be tack welded to maintain a uniform gap and alignment along the entire length. The tacks should be placed in a sequence to minimize the effect of shrinkage. In fitting two sheets, tack welds should be placed at each end and then the middle section as shown in Figure 3 (A). Figure 3 (B) shows how the sheets close up when tack welding progresses from one end. Tack welds in stainless steel must be

spaced considerably closer than would Figure 2 Backing bar groove designs. be needed for ordinary carbon steel (A) Standard groove for use without a since the higher thermal expansion of backing gas. (B) Square-corner stainless steel causes greater distortion. groove employed with backing gas. A rough guide is to use about half the

10 For the welder to pull the joint closed. To maintain the desired gap, it may be necessary to use spacers and to increase the size and number of tack welds. Spacers are usually short lengths of suitable diameter clean stainless steel wire. Any cracked or defective tack welds should be ground out. Both ends of the tacks on open root welds should be tapered to aid in fusing into the root weld. The need to maintain a proper gap during root pass welding is two-fold. First, a consistent and uniform gap aids the welder in producing the optimum ID root contour. When the joint closes up, there is a tendency for concave roots rather than the desired slightly convex contour. The other reason for a uniform Figure 3 The correct tack weld se- root gap is the need to maintain the quence is shown in A above. When tack optimum root pass chemical composition. welding from one end only, as shown in For many corrosion services, the filler B, the edges close up. metal addition is essential to provide a weld with corrosion resistance spacing between stainless tacks as used comparable to the base metal. As the for carbon steel when distortion is a joint closes, it is usually impossible to factor. melt a proper amount of filler metal into The length of tack welds may be as the weld root. For example, the 6% short as 0.125 in. (3 mm), or a small spot molybdenum stainless steels require of weld metal for thin material to over 1 proper root gap and adequate filler metal in. (254 mm) long for heavy plate addition for high integrity root welds. sections. More important, the shape of the tack should not cause a defect in the Purging during pipe root welding – The final weld. Heavy or high tacks or abrupt pipe interior must be purged with an inert starts and stops should be contour gas prior to the GTAW root pass. Failure to ground. Bead shape is easier controlled use a purge can result in a heavily oxidized with the GTAW process, making it a ID root surface with substantially lower good choice for tack welding. Tack welds corrosion resistance. Purging is usually with to be incorporated into the final weld pure argon, but nitrogen is sometimes used must be wire brushed or ground to clean because of lower cost. With duplex stainless metal. They should be inspected for steels, nitrogen backing gas compensates crater cracks and any cracks ground out. for nitrogen lost in the weld metal and re- stores weld pitting resistance. In Europe, a Fitting pipe joints for GTAW root nitrogen-10% hydrogen mixture is widely welds – Tack welding is important used for purging austenitic steel pipe, but because the tack normally becomes a would not generally be acceptable for duplex part of the root weld. Inert gas purging steels. prior to tacking is needed for protection Purging is a two-step operation, the first against oxidation. In tacking joints being done prior to welding to displace air without consumable inserts, or open root inside the pipe. To save time and purging welds as they often are called, there is a gas, baffles on either side of the weld joint strong tendency for the shrinkage forces are often used to reduce the purge area.

11 For the welder GMAW processes. The areas covered in earlier sections of this publication such as base metal properties, joint designs and preparation for welding are common to all welding procedures and are not repeated. Shielded metal arc welding SMAW is a versatile process, widely used for welding stainless steel when the shapes or quantity do not justify automatic welding. The electrode is a solid wire covered by an extruded flux coating, although some manufacturers use a cored wire in lieu of the solid core wire. SMAW is frequently referred to as covered electrode or stick welding. The arc zone in the SMAW process is shown Figure 4 Typical pipe purging fixtures. in Figure 5. Open root weld joints should be taped and dead air spaces vented prior to purging. The internal oxygen content should be reduced to below 1 % prior to welding. Typical purging fixtures are shown in Figure 4. Before the start of welding, the purge flow rate should be reduced to the point where there is only a slight positive pressure. Tape covering weld joints should be removed just in advance of the area to be welded. After the root pass, the internal purge should be maintained during the next two filler passes in order to minimize heat tint Figure 5 Shielded metal arc welding. (oxidation) on the inside weld surface. The welding is performed manually This is especially important when it is with the welder maintaining control over impractical to pickle after welding. the arc length and directing the arc into For those needing more information the weld joint. The electrode coating on GTAW root pass pipe welding, there has these functions: are a number of technical articles and - the outer flux does not burn off as specifications available. Two excellent fast as the electrode core which, in sources are the American Welding turn, helps control the arc action and Society publications listed in the Gen- ability to weld out-of-position; eral References. - the flux is used to provide alloy addition to the weld metal. The core wire is not always the same Welding processes composition as the deposited weld This section provides information to metal and therefore it is poor assist in formulating stainless steel practice to remove the flux and use welding procedures for the shielded the core wire for filler with another metal arc welding, SMAW, GTAW and process such as GTAW;

12 For the welder

- the gaseous envelope from flux ing current. They are more popular than decomposition excludes oxygen and the lime type because of better operat- nitrogen from the molten weld metal; ing characteristics. The arc is stable - the molten slag formed on top of the and smooth with a fine metal transfer. weld protects the weld metal from The weld bead is uniform with a flat to contamination by the atmosphere slightly concave contour. Slag is easily and helps to shape the bead. removed without a secondary film remaining on the weld bead. Electrode types – The electrodes are selected first on the basis of weld metal Other guides in SMAW – Factors composition and then according to the which contribute to high quality stainless type of coating. Normally, they are of steel welds include proper handling and matching or higher alloy composition to storage of electrodes, correct welding the base metal. In some cases, it is an current along with good arc starting and engineering decision to use a special stopping techniques. composition electrode. The electrode coating type is usually left to the indi- Electrode handling and storage – vidual fabricators. Electrodes for stain- Stainless steel electrodes are normally less steel base metals are shown in furnished in packages suitable for long Table IV. storage. After the package is opened, The flux formula is usually each the electrodes should be stored in manufacturer's jealously guarded heated cabinets at the temperature proprietary information. The flux coating recommended by the manufacturer. If influences how the electrode operates in the electrodes have been overexposed various positions, shape and uniformity to moisture, they should be recondi- of weld bead and that hard-to-define tioned by a higher temperature bake operator appeal. There are two basic using the manufacturer's suggested classifications, namely -15 (lime) and - time and temperature. It is preferable to 16 (basic-titania). For example, an obtain the manufacturer's specific electrode may be either type 308-15 or recommendations, since the tempera- 308-16. Electrode manufacturers often ture often varies with the particular establish their own suffix to designate coating, but lacking this information, special electrodes but AWS A 5.4 - 81 commonly used temperatures are: recognizes only -15 and -16. - storage of opened electrodes Lime coated electrodes (-15) are also 225°F (110°C); known as lime-fluorspar or basic type. - recondition bake 500°F (260°C). They are used on direct current, elec- trode positive, DCEP, (reverse polarity) current, but some brands operate on Moisture in the coating is a concern alternating current, AC. Lime coated because the hydrogen gas generated electrodes give the cleanest weld metal, can cause weld porosity. The pores may lowest in nitrogen, oxygen and inclu- be in the weld metal or may reach the sions. The weld metal tends to be surface just as the metal solidifies, tougher, more ductile, more crack resist- forming visible surface pores. The ant and have the best corrosion resist- porosity can occur in butt welds when ance. The electrodes have good pen- the moisture content of the coating is etration and all-position weldability, high, but more often occurs in fillet which is desirable for field work. welds. Excessive moisture in duplex AC-DC coated electrodes (-16) covered electrodes has the added risk of generally have a mixture of lime and causing hydrogen embrittlement in titania and are often used with alternat-

13 Table IV Suggested filler metals for welding stainless steels

Bare welding Bare welding Covered welding electrodes and Covered welding electrodes and electrode rods – AWS electrode rods – AWS Base AWS or common or common Base AWS or common or common metal name name metal name name AISI AWS A 5.4 AWS A 5.9 AISI AWS A 5.4 AWS A 5.9 (UNS) (UNS) (UNS) (UNS) (UNS) (UNS)

304 E 308(1) ER 308(1) 20 MO-6(2) (3) (3) (S30400) (W30810) (S30880) (N08026)

304L E 308L ER 308L 20Cb-3(2) E 320LR ER 320LR (S30403) (W30813) (S30883) (N08020) (W88022) (N08022)

309 E 309(1) ER 309(1) (S30900) (W30910) (S30980) Castings

310 E310 ER 310 ACI type AWSA5.4 AWSA5.9 (S31000) (W31010) (S31080) (UNS) (UNS) (UNS)

316 E 316(1) ER 316(1) CF-8 E 308(1) ER 308(1) (S31600) (W31610) (S31680) (J92600) (W30810) (S30880)

316L E 316L ER 316L CF-3 E 308L ER 308L (S31603) (W31613) (S31683) (J92500) (W30813) (S30883)

317 E 317(1) ER 317(1) CF-8M E 316(1) ER 316(1) (S31700) (W31710) (S31780) (J92900) (W31610) (S31680)

317L E 317L ER 317L CF-3M E 316L ER 316L (S31703) (W31713) (S31783) (J92800) (W31613) (S31683)

317 LM (3) (3) CN-7M E 320 LR ER 320 LR (S31725) (J95150) (W88022) (N08022)

321 E 347 ER 321 CK-3MCu (3) (3) (S32100) (W34710) (S52180) (S32154)

347 E 347 ER 347 CA-6NM E 410 NiMo ER 410 NiMo (S34700) (W34710) (S34780) (J91540) (W41016) (S41086)

Alloy 904L (3) (3) Notes: (N08904) (1) The “L” or low carbon grade or a stabilized grade is always used for welded fabrication except in a few instances where the slightly (2) higher strength of the regular grades is more important than best Alloy 254 SMO (3) (3) corrosion resistance. (S31254) (2) Trade name. (3) A filler metal with 9% or more molybdenum such as the two listed below is normally used to weld these stainless steels AL-6XN(2) (3) (3) (N08367) Bare welding 1925 hMo(2) (3) (3) Covered electrode electrodes and rods (N08926) AWS A5.11 AWS 5.14 (UNS) (UNS) 25-6 Mo(2) (3) (3) E NiCrMo-3 ER NiCrMo-3 (N08926) (W86112) (N06625) E NiCrMo-4 Er NiCrMo-4 (W80276) (N10276)

14 For the welder the ferrite phase which is not a concern - Avoid excessive weaving of the with the 300-series austenitic stainless electrode. Acceptable weave limits steels. Wet electrodes should not be vary with the particular electrode baked but discarded. and some weave or oscillation is Moisture in the coating is not the only often necessary to obtain accept- cause of weld metal porosity. Welding able bead contour in a lime-type on painted, greasy or oily surfaces may electrode. However, an excessive lead to pores of the worm-hole type. weave results in a high heat input that can cause hot cracking and Welding current – Electrode manufac- increased deformation to the turers usually print on each package the weldment. Weaving is usually recommended current ranges for each limited to 2 to 2.5 times the core diameter. Since stainless steels have a wire diameter. higher electrical resistance than ordinary steels, the current ranges may be 25 to Gas tungsten arc welding 50% of that used for steel electrodes. The GTAW process or TIG, tungsten Excessive current overheats the elec- inert gas, as it is frequently called, is trode coating which in turn causes a widely used and is well suited for weld- loss of arc force and difficulty in directing ing stainless steels. An inert gas (usually the arc near the end of the electrode. argon) is used to protect the molten weld metal and the tungsten electrode from Arc starting and stopping – The same the air. Filler metal in the form of bare good operator techniques for arc start- wire is added as needed, either by ing and stopping used for low hydrogen manual or automatic feeding into the arc. carbon steel electrodes such as type The process is illustrated in Figure 6. E7018, are applicable to stainless steel GTAW can weld material as thin as a welding. few mils to heavy gauges, but usually faster welding processes are used over Some guides are: 0.25 in. (6.4 mm). - Strike the arc at some point in the joint so that the metal is remelted. An arc strike away from the weld may have cracks and unless re- moved, will result in lower corrosion resistance in that area;

- Do not abruptly extinguish the arc leaving a large weld crater. A depression will form as the metal solidifies, often with a slag-filled pipe or cracks in the center of the crater depression. One acceptable tech- nique is to hold the arc over the weld pool for a few moments and then move quickly back, lifting the arc from the completed weld. Another technique is to extinguish the arc against one of the joint side walls after filling the crater;

Figure 6 Gas tungsten arc welding.

15 For the welder Some of the advantages of this specific current ranges while others use process for welding stainless steels a size such as .09 in. (2.4 mm) for a include: much wider current range. Also the - no slag to remove which minimizes electrode end preparation preferences post weld cleanup; vary but one commonly used is a 20 to - an all position welding process which 25° taper with the tip blunted to a 0.010 is particularly useful in pipe welding; in. (0.25 mm) diameter. - no weld spatter to clean; Nozzle or gas cups come in a wide - essentially no alloy loss during variety of shapes and sizes and it is welding. often best to match the nozzle to the weld joint or application. Larger cup GTAW equipment – Direct current, diameters provide better shielding gas electrode negative, DCEN, (straight protection to the weld while smaller polarity) current is standard. One option nozzles help maintain a more stable arc is the pulsed-current where there is a and allow better visibility. An alternate is pulsating high rate of current rise and the gas lens which creates a laminar decay. This current mode is well suited to flow by special screens inside the welding thin material and for joints which nozzle. The flow of inert gas is projected have poor fit-up. Pulsed-current is also a considerable distance beyond the end useful in making the root pass of pipe of the nozzle, giving both better gas joints. A high-frequency starting feature is protection and good visibility. commonly a part of the power source. With any welding process using inert This allows an arc to be initiated without gas, it is important that all gas lines and a scratch start that may result in contami- connections be checked to ensure nation of the tungsten electrode. Some freedom from leaks in the system. If a power sources are provided with a leak is present, for example in a gas feature that allows the electrode to be line, air will aspirate into the inert gas positioned on the work but power does stream rather than the internal gas not flow until the torch is lifted. An advan- exiting as is sometimes believed. tage over high frequency starting is that it eliminates possible interference to nearby Consumables – Pure argon, helium components such as computers. or mixtures of the two are used for In addition to current controls at the shielding gas in welding stainless steels. power source, it is often useful to have a The oxygen bearing argon mixtures used foot pedal current control. This control in GMAW should not be used in GTAW allows the welder to increase or decrease because of rapid deterioration of the current during welding to adjust to tungsten electrode. Nitrogen additions conditions such as poor fit-up. A further are not recommended for the same advantage is at arc stops where slowly reason. In manual welding and joining reducing the current and in turn the weld thicknesses below.06 in. (1.6 mm), pool, effectively eliminates crater cracks. argon is the preferred shielding gas. It Torches are either air or water cooled. provides good penetration at lower flow The air-cooled variety is limited to lower rates than helium and less chance of currents than the water-cooled. The 2% melt-through. Helium produces a higher thoriated tungsten electrodes are most heat input and deeper penetrating arc commonly used because of their excel- which may be an advantage in some lent emissive qualities, although other automatic welding applications. Argon- tungsten electrode types are acceptable. helium mixtures may improve the bead Opinions differ regarding electrode size contour and wetability. for various amperages. Some favour The correct filler metals for GTAW using a different diameter for a number of stainless steels are shown in Table IV.

16 For the welder

Straight lengths are commonly used for concave bead that has a tendency for manual welding and spool or coil wire centerline cracking. Adequate filler metal for automatic welding. Conventional addition produces a slightly convex weld quality control practices to assure clean bead and in some alloys enhances the wire and absence of material mix-up are ferrite level, both of which improve essential. Bare wire for GTAW should cracking resistance. be wiped clean before using and stored In welds subject to severe corrosive in a covered area. environments, it is often necessary for the welds to be of higher alloy content Operator technique guides – Arc than the base materials being joined to initiation is made easier by devices such give comparable corrosion resistance. as a high frequency start or a pilot arc. In Alloy-enriched welds are possible only the absence of these devices, a scratch when ample filler metal additions are start is used which risks contaminating made. It is difficult to define just how the electrode and the metal being much is ample and to measure it. A welded. Where practical, starting tabs rough guide is that at least 50% of the adjacent to the weld joint are useful in weld metal should be from filler metal eliminating damage to the base metal. addition. However, it is also important The welder must also be careful when that adequate filler metal mixing takes extinguishing the arc. The size of the place before the weld solidifies, other- weld pool must be decreased, otherwise wise segregated spots of high and low crater cracking is likely as the weld alloy may exist. One cause of this type solidifies. In the absence of a foot pedal of segregation is from uneven melting of current control described earlier or a the filler metal along with fast solidifica- power source current decay system, the tion rates. An example of where this type arc pool should be decreased in size by of weld segregation could adversely increasing the travel speed before lifting affect service performance is a root weld the electrode from the joint. Good arc of pipe used in a severe environment. stopping practice is particularly important in the root pass of welds that are welded from only one side, otherwise the cracks may extend completely through the root and are difficult to repair. After the arc is broken, the welder should hold the torch over the crater for several seconds to allow the weld to cool under protection of the argon atmosphere. Stainless steels are easy to weld with the GTAW process. The alloys are relatively insensitive to marginal shielding compared to reactive metals such as titanium or zirconium. However, it is good practice to provide ample shielding protection to both the weld puddle and backside as well as keeping the filler metal within the inert gas envelope during welding. If the process has a potential short- coming, it is that the weld may look good but have inadequate filler metal. In some weld joints, this practice can result in a Figure 7 Gas metal arc welding

17 For the welder process are shown in Figure 7. Gas metal arc welding Arc transfer modes – The type of In the GMAW process (often referred metal transfer in GMAW has a profound to as MIG when an inert shielded gas is influence on the process characteristics used or MAG when an active gas is to the extent that it is often misleading used), an arc is established between a to make general statements about consumable, bare wire electrode and GMAW without indicating the arc the work piece. The arc and deposited transfer mode. The three modes most weld metal are protected from the used in welding stainless steels are atmosphere by a gas shield, comprised spray, short circuiting and pulsed arc. mainly of the inert gases, argon and/or Table V compares some parameter and helium. Small amounts of active gases usability differences in the three. such as carbon dioxide, oxygen and hydrogen are optional for better wetting GMAW equipment – The same and arc action. Some advantages of power sources, wire feed mechanisms GMAW over GTAW and SMAW include: and torches used for welding ordinary - faster welding speeds; steels are used for stainless steels. - no slag to remove which minimizes Plastic liners in the wire feed conduit post weld clean-up; have been found helpful in reducing - ease of automation; and, drag with stainless wire. The GMAW - good transfer of elements across the process has more welding parameters arc. to control than GTAW and SMAW such The basic components of the GMAW as amperage, voltage, current slope,

Table V Comparison of GMAW arc modes for stainless steels Spray Short circuiting Pulsed arc

arc welding type transfer welding Typical thickness 0.125 in. (3 mm) min. welded 0.25 in. (6 mm) 0.06 in. (1.6 mm) 0.06 in. (1.6 mm) and thicker normal and up and up

Welding positions Flat & horizontal all all

Relative deposition rate highest lowest intermediate

Typical wire 0.06 in. 0.030 or 0.035 in. 0.035 or 0.045 in. diameter (1.16 mm) (0.8 or 0.9 mm) (0.9 or 1.2 mm)

Typical welding 250-300 amps 50-225 amps up to 250 amps peak current

Shielding gas(1) Argon - 1 % O2 90 % Helium 90 % Helium

Argon - 2 % O2 7.5 % Argon 7.5 % Argon

2.5% CO2 2.5 % CO2 or or 90 % Argon 90 % Argon 7.5 % Helium 7.5 % Helium

2.5%CO2 2.5%CO2 or

Argon -1 % O2 (1) Other gas mixtures are used, however, the shielding gas should contain at least 97.5 % inert gas, i.e., argon, helium or a mixture of the two.

18 For the welder wire feed, pulse rate and the arc advances in flux cored arc products transfer mode. Consequently the which produce quality welds at higher GMAW power sources are often more efficiency than SMAW. Cored wires are complex and expensive. Some of the often easier to produce to special newer power sources such as the compositions or ferrite ranges than it is synergic pulsed arc have made opera- to melt large heats for solid wire. tion simpler by providing only one Submerged arc welding, SAW, has control dial for the operator, with other been used extensively for welding parameters adjusted automatically. The thickness about 0.25 in. (6.4 mm) and welding current used more than 95% of thicker and for overlay welding. Com- the time is DCEP (reversed polarity). mercial fluxes are available for use with This current gives deeper penetration standard filler metals used for GMAW. than DCEN (straight polarity) and a Plasma arc, electroslag, electron stable arc. DCEN is limited to applica- beam, laser and friction welding are tions requiring shallow penetration used more and more and the such as overlay welding. resistance welding processes; spot, seam, projection and flash welding are Consumables – Some of the more readily adaptable to stainless steels. common shielding gases used in Stainless steel may be joined to itself GMAW are shown in Table V. Spray arc or a number of other metals by brazing. shielding gas is usually argon with It is not usually used when the joint will either 1 % or 2% oxygen. Short be exposed to severe corrosive envi- circuiting and pulsed arc welding use a ronments but there are instances in greater variety of shielding gases. A food and other process industries popular mixture in North America is where brazing provides adequate

90% helium, 7.5% argon and 2.5% CO2 properties. but in Europe, helium is quite expensive Oxyfuel welding, OFW, is not recom- and 90% argon, 7.5% helium and 2.5% mended for stainless steels. The

CO2 is widely used. Whatever the chromium oxide formed on the surface combination, the shielding gas should makes oxyacetylene welding difficult. contain at least 97.5% inert gases However, more important is the ex- (argon, helium or a mixture of the two). treme care needed in welding to avoid Carbon dioxide should not exceed 2.5% reducing the corrosion resistance of the or the weld quality and corrosion weld and weld area. resistance may be reduced. The preferred filler metals to be used Post-fabrication cleaning in GMAW stainless steels are shown in Table IV. The most widely used diam- All too often, it is assumed the fabrica- eters are 0.035 in. 0.045 in. and 0.062 tion, be it a tank, pressure vessel, pipe in. (0.9 mm, 1.2 mm and 1.6 mm) but assembly etc., is ready for service after others are available. the final weld is made and inspected. Post-fabrication cleaning may be as Other welding processes important as any of the fabrication steps discussed above. The surface condition Stainless steels can be welded by of stainless steels is critical, both where most of the commercial welding proc- the product must not be contaminated, esses. These processes may offer e.g., pharmaceutical, food and nuclear advantages not obtainable in SMAW, plants; and where the stainless must GTAW and GMAW processes and resist an aggressive environment such should not be overlooked for high as in a chemical or other process production or special fabrications. As an industry plant. Surface conditions that example, there have been recent

19 For the welder can reduce corrosion resistance may be must be free of organic contaminants grouped into four categories; surface for the acid to be effective in removing contamination, embedded iron, free iron, surface oxides or similar mechanical damage or welding related conditions. Because little can be done defects. Figure 8 illustrates some of the during fabrication to reduce organic common conditions. contamination, the fabricator must do this during the final cleanup. Detection – Visual inspection is usually used for organic contamination, while cloth or paper can be used for oil or grease detection. Removal – Degreasing, using a nonchlorinated solvent, is effective. The water-break test is a simple way to judge the effectiveness of degreasing. A thin sheet of water (applied by a hose) directed on a vessel wall will break Figure 8 Typical fabrication defects or around oil, grease or similar surface surface conditions commonly encountered. contamination. Degreasing should be redone until the water stops breaking. A chlorinated solvent is not recom- Surface contaminants mended because residual chlorides may remain in crevices and cause crevice In aggressive environments, organic corrosion or chloride stress corrosion contaminants on stainless steel sur- cracking later when the unit is placed in faces can foster crevice corrosion. service. Such contaminants include grease, oil, crayon marks, paint, adhesive tape and other sticky deposits. Figure 9 shows Embedded iron crevice corrosion pits (in the area Sometimes, new stainless-steel tanks marked 33) on a stainless steel vessel. or vessels rust shortly after delivery The pits formed where crayon markings from a fabricator. This may be due to were not removed from the surface iron embedded in the surface during before the vessel was put in service. fabrication. The iron particles corrode in Surfaces to be pickled or acid treated moist air or when wetted, leaving telltale

Figure 9 Crevice corrosion occurred Figure 10 A deep scratch made during where crayon marks were made and not fabrication served as an initiation site for removed on a stainless steel vessel. corrosion in this vessel.

20 For the welder rust streaks. In addition to being un- trained to administer it in only a few sightly as they corrode, the larger hours. This test is generally required particles of embedded iron may initiate for stainless steel equipment used in, crevice corrosion in the underlying for example, pharmaceutical, food and stainless steel. Figure 10 shows corro- nuclear plants, as well as for equip- sion at several points along a scratch ment used to process chemicals. An where iron had been embedded. In excellent basic guide to these tests is corrosive service, crevice corrosion ASTM A380, “Standard Recom- initiated by large embedded iron parti- mended Practice for Cleaning and cles may lead to corrosion failure that Descaling Stainless Steel Parts.” would not otherwise occur. In the pharmaceutical, food, and other Removing embedded iron – processing industries in which stainless Pickling, which is carried out after is used primarily to prevent con- degreasing, is the most effective tamination of the product, embedded method for removing embedded iron. iron cannot be tolerated. In pickling, the surface layer, less than 0.001 in. (0.025 mm), is removed by Detecting embedded iron – The corrosion, normally in a nitric/ simplest test for embedded free iron is hydrofluoric acid bath at 120°F, to spray the surface with clean water (50°C). Pickling not only removes and drain the excess. After 24 hours, embedded iron and other metallic the surface is inspected for rust streaks. contamination, it leaves the surface This is a minimum test that any fabricat- bright and clean, and in its most ing shop can conduct. To ensure resistant condition. Since pickling is against rust-streaked units, the water controlled corrosion, low-carbon or test should be specified in procurement stabilized grades of stainless are documents. preferred. The process may initiate A more sensitive indication of embed- intergranular corrosion in the HAZ of ded iron is obtained by use of the regular unstabilized grades. Because ferroxyl test for free iron. The test pickling is aggressive, it will destroy a solution is prepared by mixing the polished or high-luster surface. following ingredients: Using nitric acid alone will remove superficial iron contamination but few, Ingredient Amount if any, of the larger, deeply embedded % volume or weight particles. Nitric acid treatment is Distilled water 94 1,000 cm3 referred to as passivation. This can be 3 Nitric acid, 60-67% 3 30 cm misleading, since the pickled surface Potassium ferrocyanide 3 30 g is also passivated when it contacts air. The solution is best applied using a Small objects are best pickled by one-quart spray applicator, the type that immersion. Piping, field-erected tanks applies bleach to laundry. Iron contami- and vessels too large to immerse can nation is indicated by the appearance of be treated by circulating the pickling a blue colour after a few minutes. The solution through them. Typically, depth of colour roughly indicates the chemical-cleaning contractors are degree of contamination. The solution hired to do this. should be removed after a few minutes When ferroxyl testing shows only with a water spray or a damp cloth. spotty patches of iron, these can be The ferroxyl test is not only sensitive removed by local application of nitric/ but it can be performed in the field as hydrofluoric acid paste. For large easily as in the shop. Personnel can be tanks, filling to about 6 in. (150 mm)

21 For the welder to pickle the bottom, and locally remov- GTAW is usually used because of greater ing embedded iron on side-walls is often ease in making small repair welds. Filler a practical alternative to circulating metal should always be added and wash pickling solution throughout them. passes or cosmetic welds never allowed When pickling is not practical, blasting because of the risk of weld cracking and can be used, but not all abrasives yield reduced corrosion resistance. good results. Glass-bead blasting produces good results but, before blasting, a test should be made to Safety and welding fumes determine that it will remove the surface Safety rules for welding stainless steels contamination. Also, periodic tests are essentially the same as for all metals should be made to see how much reuse as they pertain to areas such as electri- of beads can be tolerated before they cal equipment, gas equipment, eye and begin to recontaminate the surface. face protection, fire protection, labeling Walnut shells have also performed well hazardous materials and similar items. A as an abrasive. good reference guide on welding safety Abrasive blasting with steel shot or grit is American National Standard Institute / is generally unsatisfactory because of Accredited Standards Committee, ANSI/ the possibility of embedding iron ASC, Z49.1-88, “Safety in Welding and particles. Also, grit blasting leaves a Cutting,” published by the American rough profile that makes the stainless Welding Society. steel susceptible to crevice corrosion, Proper ventilation to minimize the whether or not the surface is free of iron. welders’ exposure to fumes is important Sand blasting should also be avoided if in welding and cutting all metals, includ- possible, because even new sand is ing stainless steels. In addition to good seldom free of iron particles or other ventilation, the welders and cutters contaminants. should try to avoid breathing the fume plume directly, by positioning the work so Mechanical damage that their head is away from the plume. The composition of welding fumes varies When the surface has been damaged with the welding filler metal and welding and reconditioning is needed, the repair process. Arc processes also produce is usually made by grinding or by gaseous products such as ozone and welding and grinding. Shallow defects oxides of nitrogen. Concern has been are first removed by grinding, preferably expressed in welding with stainless steel with a clean fine grit abrasive disk, a and high alloy steel consumables be- flapper wheel or a pencil type grinder. cause of the chromium and, to a lesser The maximum grinding depth to remove extent, the nickel usually present in the defects is often specified by the fabrica- welding fume. Good ventilation will tion specification and may vary from minimize the potential health risk. The 10% to 25% of the total thickness. International Institute of Welding has When weld repair is needed, it can be developed a series of “Fume information made by SMAW, GMAW or GTAW. sheets for welders” which offer interna- tionally accepted suggested guidelines for fume control.

22 Part II For the materials engineer

This section is for the engineer who developed for an austenitic stainless needs further information about the steel to weld a martensitic stainless wrought and cast stainless steel steel could result in low quality welds. alloys, how their corrosion resistance is affected by welding and typical heat treating practices. Also included are Austenitic stainless steels guides for material procurement and Austenitic stainless steels are non- good storage practices. magnetic or only slightly magnetic in the annealed state and can be hardened only by cold working. They possess Stainless steel alloys excellent cryogenic (low temperature) Steel is made corrosion resistant by the properties and good strength at high addition of 11 % or more chromium. The temperatures. Corrosion resistance is term stainless describes the non-rusting, outstanding in a wide range of environ- bright appearance of these alloys. The ments. They exhibit good weldability and earliest types of stainless steel were the are easy to fabricate provided suitable straight chromium grades with procedures are maintained. chromium ranging from over 10% to The composition of common grades of about 18%, but through the years a wrought stainless steels and corrosion- number of different types of stainless resistant stainless steel castings is steel alloys have been developed and shown in Tables Vl and Vll. It includes categorized into five groups, namely: alloys commercially available worldwide - martensitic (AISI* 400-series) and those most frequently used for - ferritic (AISI* 400-series) corrosion resistant applications. The - austenitic (AISI* 300-series) UNS numbers in Table Vl have either an - precipitation hardening S or N prefix. Stainless steels are - duplex defined by ASTM as having at least 50% *American Iron and Steel Institute iron which UNS identifies with a S number. Alloys with a N number are The austenitic stainless steels are the classified as nickel alloys, but the most widely used but the use of duplex distinction is purely artificial. The alloys is increasing, although they still fabricability of the high alloy S grades represent a small part of the stainless and the nickel alloys in Table VI is steels used. This publication describes essentially the same. these two alloy families and their use. The other three groups, martensitic, Effect of welding on corrosion ferritic and precipitation hardening are also identified as stainless steels but the resistance fabrication and welding is often quite Austenitic stainless steels are usually different from the austenitic and duplex specified for their excellent corrosion grades. When discussing welding and resistance. Welding can reduce base fabrication techniques, the particular metal corrosion resistance in aggressive stainless steel group must be identified, environments. In welding, heat is otherwise serious mistakes could be generated that produces a temperature made. For example, using a procedure gradient in the base metal, i.e. the HAZ.

23 For the materials engineer

Table VI Wrought austenitic stainless steels chemical analysis, %, of major elements (Max. except as noted)

AISI Type or common name (UNS) C Cr Ni Mo Other

304 0.08 18.0-20.0 8.0-10.5 - 0.10N (S30400)

304 L 0.03 18.0-20.0 8.0-12.0 - 0.10N (S30403)

309 0.20 22.0-24.0 12.0-15.0 - - (S30900)

310 0.25 24.0-26.0 19.0-22.0 - - (S31000)

316 0.08 16.0-18.0 10.0-14.0 2.0-3.0 0.10N (S31600)

316L 0.03 16.0-18.0 10.0-14.0 2.0-3.0 0.10N (S31603)

317 0.08 18.0-20.0 11.0-15.0 3.0-4.0 0.10N (S31700)

317L 0.03 18.0-20.0 11.0-15.0 3.0-4.0 0.10N (S31703)

317 LM 0.03 18.0-20.0 13.0-17.0 4.0-5.0 0.10N (S31725)

321 0.08 17.0-19.0 9.0-12.0 - 5 x %C min, (S32100) 0.70 max. Ti

347 0.08 17-0-19.0 9.0-13.0 - 10 x %C min, (S34700) 1.10 max. (Nb + Ta) Alloy 904L 0.02 19.0-23.0 23.0-28.0 4.0-5.0 1.0-2.0 Cu (N08904)

Alloy 254 SM0* 0.02 19.5-20.5 17.5-18.5 6.0-6.5 0.18-0.22N (S31254) 0.50-1.00 Cu

AL-6XN* 0.03 20.0-22.0 23.5-25.5 6.0-7.0 0.18-0.25N (N08367) 0.75 Cu

1925 h Mo* 0.02 20.0-21.0 24.5-25.5 6.0-6.8 0.18-0.20N (N08926) 0.8-1.0 Cu

20 Mo-6* 0.03 22.0-26.0 33.0-37.0 5.0-6.7 2.0-4.0 Cu (N08026)

20Cb-3* 0.07 19.0-21.0 32.0-38.0 2.0-3.0 3.0-4.0 Cu (N08020) 8 x C min, 1.00 %max.

25-6 MO* 0.02 19.0-21.0 24.0-26.0 6.0-7.0 0.15-025N (N08926) 0.5-1.5 Cu

* 254 SMO is a trademark of Avesta AB AL-6XN is a trademark of Allegheny Lundlum Steel Corporation 1925 hMo is a trademark of VDM Nickel Technologie A.G. 20 Mo-6 and 20 Cb-3 are trademarks of Carpenter Technology Corp. 25-6 MO is a trademark of Inco Alloys International, Inc.

24 For the materials engineer

Table VII Corrosion resistant stainless steel castings chemical analysis, %, of major elements (Max. except as noted) ACI Similar Most Type wrought common Anneal at (UNS) type C Cr Ni Mo Others structure °F (°C) CF-8 304 0.08 18.0-21.0 8.0-11.0- - Ferrite in 1900-2050 (J92600) austenite (1035-1120)

CF-3 304L 0.03 17.0-21.0 8.0-11.0- - 1900-2050 (J92500) (1035-1120)

CF-8M 316 0.08 18.0-21.0 9.0-12.02.0-3.0 - 1900-2050 (J92900) (1035-1120)

CF-3M 316L 0.03 17.0-21.0 9.0-13.02.0-3.0 - 1950-2050 (J92800) (1065-1120)

CN-7M 20Cb-3 (1) 0.07 19.0-22.0 27.5-30.5 2.0-3.0 3.0-4.0Cu Austenite 2050 (1120) (N08007) Min.

CK-3M Cu Alloy254SMO(2) 0.02 19.5-20.5 17.5-18.5 6.0-6.5 0.18-0.22N Austenite 2100-2200 (J93254) 0.50-1.00 Cu (1150-1205)

CA-6NM 0.06 11.5-14.0 3.5-4.5 0.4-1.0 - Martensite 1900-1950 (J91540) (1035-1065) followed by double

temper CD-7MCuN FERRALIUM (3) 0.04 24.0-27.0 4.5-6.5 2.0-4.0 0.10-0.25N Duplex- 1925 (1050) 255 1.5-2.5 Cu austenite & Min. ferrite CD-3MN ASTM-A- 2205 0.03 21.0-23.5 4.5-6.5 2.5-3.5 0.10-0.30N Duplex- 2050 (1120) 890, Gr4A austenite & Min. (J92205) ferrite

Zeron 100 Zeron 100 (4) 0.03 24.0-26.0 6.0-8.5 3.0-4.0 0.2-0.3N (4) Duplex- 2050 (1120) (J93380) austenite & Min. ferrite

(1) 20Cb-3 is a tradename of Carpenter Technology Corporation (3)FERRALIUM is a trademark of Langley Alloys, Ltd (2) 254SMO is a trademark of Avesta AB (4) Zeron 100 is a trademark of Weir Material Services, Ltd.

Welding may also induce residual attack, IGA, in the weld HAZ. In the stresses in the weld area which in temperature range of about 800°F to certain environments can lead to 1650°F (425°C to 900°C), carbon SCC. Heat treatments to reduce combines with chromium to form chro- residual stresses are discussed in the mium carbides at the grain boundaries. section Heat Treatment of Austenitic The area adjacent to the carbides is Stainless Steels. depleted in chromium. When the car- One of the early corrosion problems bide network is continuous, the low related to welding was intergranular chromium envelope around grains may

25 For the materials engineer

Figure 11 Effect of carbon control on carbide precipitation in Type 304.

be selectively attacked, resulting in chromium carbides have formed at the intergranular corrosion. In the worst grain boundaries, when the time at case, the depleted chromium layer is temperature for any particular carbon corroded away and complete grains are content Type 304 is to the right of the % separated from the base metal and can carbon curve. It can be seen that the even fall out. Alloys are said to be temperature where sensitization occurs sensitized when welding or heat treat- most rapidly varies from 1300°F ment results in chromium depleted (700°C), with an alloy of 0.062% car- areas that will be attacked in these bon, to 1100°F (600°C), for a 0.03% corrosive environments. Sensitized carbon alloy. From Figure 11, an alloy alloys may still provide good service in with 0.062% carbon could become many of the milder environments in sensitized in as little time as 2 to 3 which stainless steels are used. Today, minutes at 1300°F (700°C ). On the with the trend of mills to furnish lower other hand, 304L with 0.030% carbon carbon products, IGA of austenitic could be held at 1100°F (595°C) for 8 stainless steels occurs less often. hours before being sensitized. For this The degree of sensitization, i.e., the reason the low carbon “L” grades are amount of grain boundary chromium most commonly used for corrosion carbides formed, is influenced by the resistant equipment where IGA is a amount of carbon, exposure tempera- possibility. With the “L” grade, the weld ture and time at this temperature. Figure HAZ is not at temperature long enough 11 illustrates the time-temperature- to become sensitized. sensitization curves for Type 304 Grain boundary chromium carbides stainless steel. Curves of other can be prevented from forming when austenitic stainlesses would be similar titanium or niobium-tantalum are but the actual values would be some- present in the alloy. (Niobium,Nb, is what different. To explain Figure 11, the also known by the name columbium, alloy is sensitized, that is a network of Cb, in some references). These ele-

26 For the materials engineer ments have a greater affinity for carbon cooling, the higher the ferrite content. than chromium and form evenly distrib- Unfortunately, ferrite is not obtainable uted carbides away from the grain in all nickel stainless steel alloys. For boundaries, where there is no adverse example, it is not possible to adjust the affect on the corrosion resistance. Type composition to obtain ferrite in Type 310 321 (UNS S32100) contains titanium (UNS S31000). In spite of being fully and 347 (UNS S34700) contains nio- austenitic and prone to fissures, the bium-tantalum. Both are stabilized alloy has been used over 50 years with versions of type 304. The stabilized excellent service. In the absence of grades are usually preferred where weld metal ferrite, it is more important there will be long time service in the for the filler metal manufacturer to sensitizing temperature range of 800°F control minor elements such as silicon, to 1650°F (425°C to 900°C). phosphorus and sulphur to as low a A third method of preventing IGA in level as possible to prevent cracking. the weld HAZ in alloys containing over When a filler metal is required with a 0.03% carbon is to redissolve the specific ferrite level, the purchaser or chromium carbides by a solution anneal user should specify the level to the at 1900°F to 2150°F (1040°C to supplier. Stainless steel filler metal 1175°C), followed by rapid cooling. specifications, ANSI/AWS A5.4 for The solution anneal is a good method to electrodes and ANSI/AWS A5.9 for bare restore full corrosion resistance when wire do not specify ferrite levels for any the shape, size and geometry of the of the alloy classes. weldment allows the heat treatment. Solution annealing must be closely Measuring weld metal ferrite – controlled in both heating and cooling to While there is wide agreement on the minimize distortion within acceptable beneficial affect of ferrite in the weld, limits. it is not always easy to measure the amount accurately in a given weld Role of weld metal ferrite deposit. One of the three following Microfissures or cracks have been methods can be used. known to occur in austenitic stainless 1. Magnetic instruments can measure steel welds. They can appear in the weld ferrite on a relative scale and this is the metal during or immediately after method most used by filler metal pro- welding, or they may occur in the HAZ of ducers. Calibration of the instruments is previously deposited weld metal. The very critical and AWS has developed a microstructure of the weld metal strongly special calibration procedure. AWS also influences susceptibility to details how the weld pad is to be made microfissuring. A fully austenitic weld is and prepared for testing, since this can more prone to microfissuring than a weld influence the measurement. Ferrite with some ferrite. determination using sophisticated Ferrite levels of 5% to 10% or more in laboratory magnetic instruments is often welds or castings can be quite beneficial not practical for the average user. in reducing hot cracking and Portable magnetic instruments are microfissuring. For example, a Type 308 commercially available that, even (UNS W30840) weld with zero to 2% though they may be less accurate, are ferrite might be quite crack sensitive easier for the fabricator to use. while another 308 weld with 5% to 8% ferrite would have good crack resistance. 2. Using the weld chemical composi- The amount of ferrite in a 300-series tion, ferrite content can be estimated weld is controlled by the composition from a constitution diagram for stainless and the weld cooling rate, the faster the

27 For the materials engineer

Figure 12 Revised constitution diagram for stainless steel weld metal. (from Metals Handbook, Volume 6, Ninth Edition)

steel weld metal, Figure 12. Earlier, temperatures 900°F to 1700°F (480°C ferrite diagrams represented ferrite in to 925°C) to avoid a loss of room units of volume-%. The most recent temperature ductility as a result of a Welding Research Council, WRC, high temperature sigma phase. Sigma diagram determines ferrite number, FN, forms more readily from ferrite than by the magnetic response. The FN and from austenite and is discussed in the volume-% are the same up to 6% but duplex stainless steel section. differ at higher levels. Ferrite determina- tion using the diagram is easy and quite accurate, provided a reliable chemical Duplex stainless steels analysis has been made. Duplex stainless steels are an alloy family that have two phases – ferrite 3. The ferrite content can be estimated and austenite – with ferrite typically by metallographic examination. It is most accurate when ferrite is in the range of 4% to 10% and should be performed by an experienced techni- cian. One advantage to this method is that it can be used on small specimens removed from weldments or where the two other methods are not practical. There are services where ferrite in the weld structure is not a benefit. At cryrogenic temperatures,viz., -320°F (-195°C), toughness and impact strength are reduced by ferrite and it is Figure 13 Typical microstructure of cold common practice to specify welds with rolled, quench annealed Alloy 2205 no more than 2 FN and preferably 0 FN. seamless tube. Dark phase – ferrite, It is also desirable to have low ferrite light phase – austenite. when the welds are exposed to service (from Sandvik AB)

28 For the materials engineer between 40% and 60%.The ferrite/ is improved pitting and crevice corrosion austenite ratio is accomplished in resistance. With proper welding proce- wrought alloys by composition adjust- dures, as-welded second-generation ment along with controlled hot working duplex stainless steels can have nearly and annealing practices at the mill. The the same level of corrosion resistance as alloys could properly be called ferritic- mill annealed material. Nitrogen is also austenitic stainless steels but the term beneficial in the manufacture of second- “duplex” is more widely used. A typical generation alloy plates, where the duplex stainless steel microstructure is ductile-brittle transition is depressed well shown in Figure 13. The matrix which below room temperature, making heavy appears as the darker background is section weldments practical. However, ferrite and the elongated, island-like duplex alloys are generally not used lighter phase is austenite. below about -50°F (-45°C) whereas Duplex alloys date to the 1930s and some fully austenitic alloys may be used the early alloys are now identified as to -456°F (-270°C). first-generation. Unfortunately, the early Alloy 2205 (UNS S31803) is the most alloys had a problem of significant loss widely used duplex alloy and is available of corrosion resistance in the as-welded from a number of producers. Comparing condition and it has taken some time for the duplex composition to a fully the new second-generation alloys to austenitic stainless steel such as Type overcome this reputation. All the alloys 316, 2205 is higher in chromium, lower shown in Table Vlll are second-genera- in nickel and contains nitrogen. The tion alloys and typically contain 0.15% to nitrogen addition is very critical in duplex 0.30% nitrogen. One benefit of nitrogen alloys as will be discussed shortly. Table VIII Duplex stainless steels chemical analysis, %, of major elements

(Max. except as noted) Common name

(UNS) C Cr Ni Mo Others 7-Mo PLUS(1) 0.03 26.0-29.0 3.5-5.2 1.0-2.5 0.10-0.35N (S32950)

Alloy 2205 0.03 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20N (S31803)

FERRALIUM 255(2) 0.03 24.0-27.0 4.5-6.5 2.0-4.0 0.10-0.25N (S32550) 1.5-2.5Cu

SAF 2507(3) 0.03 24.0-26.0 6.0-8.0 3.0-5.0 0.24-0.32N (S32750)

(4) 0.03 24.0-26.0 6.0-8.0 3.0-4.0 0.5-1.0 Cu Zeron 100 0.5-1.0 W (S32760) 0.2-0.3 N

(1) 7-Mo PLUS is a trademark of Carpenter Technology Corporation (2) FERRALIUM is a trademark of Langley Alloys, Ltd. (3) SAF 2507 is a trademark of Sandvik AB (4) Zeron 100 is a trademark of Weir Material Services, Ltd.

29 For the materials engineer Characteristics of duplex producing or fabricating the alloys, a high temperature solution anneal at stainless steels 1900°F (1040°C) or higher, depending The duplex alloys offer two important on the alloy, followed by rapid cooling is advantages over austenitic alloys such employed to give optimum mechanical as 304L and 316L, namely greater properties and corrosion resistance. In resistance to chloride stress corrosion exposure to the temperature range of cracking, CSCC, and higher mechanical 600°F to 1750°F (315°C to 950°C) the properties. The yield strength of duplex duplex alloys act differently than the alloys is typically two to three times austenitics but once the differences are higher and the tensile strength 25% recognized, no problems should arise. higher while still maintaining good An intermetallic phase called sigma ductility at normal operating tempera- can form when duplex alloys are held in tures. the 1200°F to 1750°F (650°C to 950°C) The susceptibility of austenitic stain- temperature range. Sigma causes room less steels to CSCC at temperatures temperature embrittlement and, when above about 140°F (60°C) is a well present in appreciable amounts, corro- known concern. The ferritic stainless sion resistance is lowered. However, steels are highly resistant but are more attention to minimum time in the sigma difficult to fabricate and weld. The forming range during annealing and duplex alloys have intermediate resist- welding, improved processing control at ance to CSCC which, in many environ- the steel mill and the beneficial effect of nitrogen can essentially eliminate any ments, represents a substantial im- sigma problem. In normal second- provement over the austenitics. The generation duplex welding procedures, duplex alloys also offer: the weld or HAZ is not at temperature - general and pitting corrosion resist- long enough for sigma to be a factor. ance equal to or better than type Another high temperature occurrence is 316L stainless steel in many envi- a phenomenon called 885°F (475°C) ronments; embrittlement. It can occur when a - resistance to intergranular corrosion duplex alloy (or any iron-chromium alloy due to the low carbon content; containing 13% to 90% Cr) is held within or slowly cooled through the - good resistance to erosion and temperature range of 600°F to 1000°F abrasion; and, (315°C to 540°C). With the second- - a thermal expansion coefficient close generation duplex alloy and using to that of carbon steel which can standard annealing and welding prac- result in lower stresses in weldments tices, the weld or HAZ is not at tempera- involving duplex stainless and ture long enough for this embrittlement carbon steel. to occur. It is mentioned here as a precaution should there be need to There are metallurgical differences deviate from standard procedures. compared to the austenitic alloys that when known and recognized are easily Effect of welding on duplex handled. The differences occur as a result of high temperature exposure. stainless steels The weldability of second-generation High temperature exposure – duplex alloys has been greatly improved Duplex stainless steels are normally through controlled nitrogen additions used in the temperature range of about and the development of nickel-enriched -50°F to 500°F (-45°C to 260°C). In filler metals. Using a few welding procedure controls, sound welds with

30 For the materials engineer corrosion resistance comparable to the current AWS stainless steel filler metal base metal are obtained. The impor- specifications but will be included in tance of controls on heat input, interpass future editions. temperature, preheat and nickel-en- riched filler metal are as follows: Heat input control – There is not complete agreement on the part of Nickel-enriched filler metal – Duplex producers and welding investigators as stainless steel welds made with matching to the proper limits on heat input. The composition filler metal or autogenously argument for high heat input (see welded (no filler metal) may exhibit 80% formula) is that it allows more time for or more ferrite in the fusion zone in the ferrite to transform to austenite, particu- as-welded condition. A weld with such a larly in the heat affected zone. The high ferrite level has poor toughness and concern for high heat input is that it ductility and often will not pass a bend could allow embrittling phases, such as test. The higher ferrite content of such sigma and 885°F (475°C) embrittlement welds also markedly reduces corrosion to develop in the ferrite: With the resistance in many aggressive environ- second-generation duplex stainless ments. An anneal at 1900°F to 2100°F steels, longer time at temperature is (1040°C to 1150°C) restores the desired needed for these phases to develop, so ferrite/austenite balance but the treat- there should be no significant ment is not practical for many embrittlement. A generally accepted fabrications and is expensive. Increasing heat input range in kilo joules (kJ) is 15 the nickel content of the filler metal to 65 kJ/in. (0.6 to 2.6 kJ/mm) although allows more austenite to form so that levels as high as 152 kJ/in. (6.0 kJ/mm) welds in the as-welded condition have are claimed to have been successfully typically 30% to 60% ferrite. Welds made used. When a welding process with with nickel-enriched filler metals have less than 15 kJ/in. (0.6 kJ/mm) heat good as-welded ductility, are able to input must be used, preheating to pass bend tests, and have corrosion 200°F-400°F (95°C - 205°C) is helpful in resistance comparable to the base metal. reducing the cooling rate and increasing It is desirable that all weld passes be austenite in the weld. Where there is a made with substantial filler metal addition question on heat input for a particular to provide a nickel enhanced weld metal duplex alloy, it is a good practice to compostition. A large amount of base contact the material supplier for specific metal dilution can result in welds having recommendations. a high ferrite content with lower ductility and toughness. An example of where Heat input in kJ/in. is calculated: this can occur is the root pass of a pipe Voltage x Amperage x 60 weld with high base metal dilution. Special care should be taken to add Travel speed (inch/minute) x 1000 sufficient nickel-enriched filler metal. Joints with a feather edge and tight fit- Interpass temperature control – An ups favor high dilution and are best early concern was that a high interpass avoided. Joints with an open root temperature could result in 885°F spacing and a land are preferred since (475°C) embrittlement and a limit of they require the addition of filler metal. 300°F (150°C) maximum interpass Nickel-enriched filler metal products temperature was suggested. This limit for the duplex alloys are available as is conservative and in some instances a covered electrodes, bare filler metal and maximum limit of 450°F (230°C) could flux cored wire as shown in Table IX. be acceptable. However, in the interest Duplex filler metals are not covered by of consistency, fabricators often specify

31 For the materials engineer Table IX Duplex stainless steel filler metals typical composition

Filler metal For welding common name base metal (UNS) C Cr Ni Mo Others Covered electrodes 2209-16(1) 2205 0.03 23 9.7 3.0 0.10N (W39209) tentative (S31803)

22.9.3.L-16(2) 3RE60 (S31500) 0.03 22 9.5 3 0.15N 22.9.3.L-15(2) 2205 (S31803) 22.9.3.LR(2) 2304(S32304)

7-Mo PLUS Enriched Ni(3) 7-MO PLUS 0.03 26.5 9.5 1.5 0.20N (S32950)

FERRALIUM 255(4) FERRALIUM 255 0.03 25 7.5 3.1 0.20N (W39553) tentative (S32550) 2.0 Cu

Bare filler wire 22.8.3L(2) 3 RE60 (S31500) 0.01 22.5 8 3 0.10N 2205(S31803) 2304(S32304)

7-Mo PLUS Enriched Ni(3) 7-Mo Plus 0.02 26.5 8.5 1.5 0.20N (S32950)

FERRALIUM 255(4) FERRALIUM 255 0.03 25 5.8 3.0 0.17N (S39553) Tentative (S32550)

Zeron 100 filler wire(5) Zeron 100 0.03 25 10* 3.5 0.25N (S32760) 0.7 Cu 0.7 W Flux Cored Wire

In-Flux 2209-0 (1) 2205 0.02 22.0 8.5 3.3 0.14N (W31831) (S31803)

In-Flux 259-0(1) FERRALIUM 255 0.02 25 10 3.2 0.14N (S32550) 2.0 Cu

(1) 2209-16, In-Flux 2205-0 and In-Flux 259-0 are (4) FERRALIUM is a trademark of Langley Alloys, Ltd. trademarks of Teleldyne McKay (5) Zeron 100 is a trademark of Weir Material Services, Ltd. (2) 22.9.3L-16, 22.9.3L-15, 22.9.3.LR and 22.8.3L are * When the joint is fully heat treated after welding, Ni should trademarks of Sandvik AB be 6.0-8.0% (3) 7-Mo PLUS is a trademark of Carpenter Technology Corporation

the same value used for austenitic low heat input welding process, below stainless steel, 300°F to 350°F (150°C 15 kJ/in. (0.6 kJ/mm), must be used, a to 175°C). preheat of 200°F-400°F (95°C-205°C) reduces rapid cooling and decreases Preheat – There is no need for the amount of ferrite in the weld metal preheat on thicknesses 0.25 in. (6 mm) and HAZ. and less on welds made with nickel- enriched filler metals. In heavier sec- tions and high restraint welds, preheat Other stainless steels may be used to advantage in minimizing Other types of stainless steels are the chance of weld cracking. When a martensitic, ferritic and precipitation

32 For the materials engineer three types follows and illustrates some extensively used for heavy components of the basic differences from the such as pump bowls, valve bodies and austenitic alloys. Some of the more compressor cases. CA-15 (UNS J91150) common alloys and their welding filler was the standard alloy but has been metals are shown in Table X. largely replaced by CA-6NM (UNS J91540). Compared to CA-15, CA-6NM has improved toughness and weldability, Table X along with better resistance to cavitation. Suggested filler metals for welding It is preferable to weld CA-6NM cast- some of the martensitic, ferritic and ings in the heat treated condition rather precipitation hardening stainless than the as-cast. Welding is usually done steels at room temperature although a preheat Covered Bare welding of 250°F to 300°F (120°C to 150°C) may Base welding electrodes and be beneficial for large welds in heavy or metal electrode rods highly stressed sections. After welding, AISI AWS A5.4 AWS A5.9 (UNS) (UNS) (UNS) the casting is heated to not higher than 1100°F to 1150°F (590°C to 620°C) and Type 410 (wrought) E410 ER410 air-cooled. When there is a special (S41000) (W41010) (W41040) hardness requirement, CA-6NM may be CA-15 (casting) E410 ER410 given a normalizing heat treatment above (J91540) (W41010) (W41040) 1750°F (950°C) and air cooled, followed

CA-6NM (casting) E410NiMo ER410NiMo by a double temper of 1100°F to 1150°F (J91540) (W41016) (W41046) (590°C to 620°C). The casting should be cooled to room temperature between Type 430 (wrought) E430 ER430 (S43000) (W43010) (W43040) each tempering treatment.

17-4PH E630 (1) ER630 (1) (S17400) (W37410) (W37440) Ferritic stainless steels The ferritic alloys are not hardenable (1) When weld does not need to match bare metal by heat treatment and only slightly strength, E 308 (UNS W30810) or ER 308 hardenable by cold working. They are (UNS W30840) are often used. magnetic and have good resistance to corrosion in many environments. Most Martensitic stainless steels typical of the ferritic stainless steels is The martensitic alloys can be hard- type 430 (UNS S43000), a straight- ened and strengthened by heat treat- chromium alloy with 16% to 18% chro- ment and only slightly hardened by cold mium, 0.12% max. carbon, some minor working. They are strongly magnetic, elements and the balance iron. resist corrosion in mild environments Weldability of the ferritic stainless and have fairly good fabricating steels is generally better than the qualities. The alloys are often selected martensitic. Exposure to high tempera- because of high mechanical properties tures, such as in the weld heat affected and low cost. zone, causes a reduction in ductility and Weldability of the martensitic toughness along with grain coarsening. stainlesses varies with alloy content, Solution annealing to prevent IGA is particularly the amount of carbon. The done at 1450°F (790°C) for ferritic higher the carbon content, the greater stainless steels instead of 1900°F to the need for preheat and postweld heat 1950°F (1040°C to 1065°C) as for treatment to produce sound welds. austenitic stainless steels. There is While the wrought martensitic stainless greater need for preheating and steels have limited use in process postweld annealing as the thickness and industries, the cast grades have been joint restraint increases.

33 For the materials engineer Precipitation hardening areas worn and corroded in later service stainless steels will sensitize the HAZ to IGA. Users can Iron-chromium-nickel alloys containing and should request a 0.03% C maxi- precipitation-hardening elements such mum as an exception to the specifica- as copper, aluminum and titanium have tion in order to prevent IGA in this alloy. good weldability, comparable to that of Normally, these castings are welded the austenitic alloys, but are often used without problems, but where welding is for components which require little or extensive, special techniques may be no welding. When welding is required, it needed to prevent microfissures next to is best to weld these alloys in the the weld. Techniques available are low annealed condition prior to the final age interpass temperatures, low heat input hardening heat treatment. These stain- and peening of the weld to relieve less steels are hardenable by a mechanical stresses. combination of cold working and a low- The austenitic and duplex castings in temperature heat treatment, 850°F to Table Vll are usually purchased to one 1100°F (455°C to 595°C). of the following specifications: ASTM A 743 – Castings, Iron- Corrosion resistant Chromium, Iron-Chromium-Nickel, stainless steel castings Nickel-Base, Corrosion Resistant, For General Application. Stainless steel castings are classified, based on their end use, as corrosion ASTM A 744 – Castings, Iron- resistant or heat resistant, and are Chromium, Nickel-Base, Corrosion designated accordingly by the first letter Resistant, For Severe Service. C or H. The heat resistant grades are generally higher in alloy content than Both specifications require that the the corrosion types and in nearly all casting be solution annealed which cases have higher carbon. The follow- largely removes alloy segregation and ing remarks apply to corrosion resistant dendritic structures occurring in cast- types and may not be applicable to the ings, particularly in heavy sections. The heat resistant alloys. The chemical high temperature anneal of 1900°F composition of the more common (1040°C) or higher, depending on the austenitic and duplex cast alloys is alloy, promotes a more uniform chemi- shown in Table Vll (see page 25). cal composition and microstructure, as The most widely used, CF-3, CF-3M, well as dissolving carbides. As a result CF-8 and CF-8M grades, normally have of the anneal, the casting is in the most 5% to 20% ferrite in the austenitic corrosion resistant state. For best matrix. The amount will vary with corrosion resistance, the widely used composition, thermal history of the CF-3M and CF-8M grades must be casting and at different locations in the annealed at 2050°F (1120°C), not casting. Ferrite is beneficial in minimiz- 1900°F (1040°C) as allowed by the ing casting cracks and improving specification. weldability. Some cast corrosion resist- Stainless steel castings are often ant stainless steel grades such as CN- welded, either by fabricators making 7M are fully austenitic by nature of their assembly welds, during service life or composition. ASTM Specifications do by the foundries weld repairing defects. not as yet include the 0.03% carbon When the casting will be placed in a grade for CN-7M as they do for the severe corrosive environment, selecting standard grades. Weld repair of cast- a low carbon version such as CF-3 or ings at the foundry or weld buildup of CF-3M can avoid problems resulting

34 For the materials engineer from the formation of chromium car- When weldments are solution annealed bides in the weld HAZ. The same effect and rapidly cooled, new residual of chromium carbides on IGA discussed stresses are often introduced. These in wrought alloys is true for cast alloys. stresses can cause movement after The need for a low carbon version machining, with the result that the part applies not only for the initial fabrication exceeds dimensional tolerance limits. welds but also for later maintenance Stress relieving of mild steel weldments overlay and weld buildup of cast compo- is frequently performed but it is best to nents. When a low carbon grade is not avoid stress relief treatments of stain- included in ASTM A743 or A744, an less steel weldments unless absolutely exception to the specification can necessary. When it is necessary, two usually be reached with the foundry. alternates are available, namely: One difference between A743 and Alternate 1 – Use a low temperature A744 is that A744 requires a full solu- stress equalizing treatment at 600°F to tion anneal after all weld repairs except 800°F (315°C to 425°C) with a hold of 4 for minor repairs as defined in the hours per inch of thickness, followed by specification. Austenitic A743 castings a slow cool. Since the alloys have which are intended for general service excellent creep strength, the low tem- do not require the solution anneal to be perature treatment removes only peak made after all weld repairs. Knowledge stresses. The treatment is safe to use of the intended service conditions is with the standard grades such as 304 helpful in selecting the correct material and 316 as well as the stabilized and specification and casting grade but if low carbon grades since the tempera- this information is not available, a low ture is below that at which harmful carbon grade of W744 is usually a good chromium carbides form. choice. Heat treatment of Alternate 2 – If the 600°F to 800°F (315°C to 425°C) treatment is inad- stainless steel equate in reducing stresses to the level Austenitic stainless steels, both wrought desired, stress relieving in the range of forms and castings, are normally sup- 800°F to 1700°F (425°C to 925°C) may plied in the solution annealed condition. be required. The higher the temperature In solution annealing, the alloy is and the longer the time, the more heated to a high temperature, 1900°F to complete the stress relief. For example, 2150°F (1040°C to 1175°C) depending one hour at 1600°F (870°C) removes on the alloy type, and rapidly cooled, about 85% of the residual stresses. usually by a water quench. At the However, the standard grades such as annealing temperature, chromium Types 304 and 316 cannot be heated in carbides are put back into solution as this range without sacrificing corrosion chromium and carbon, restoring the full resistance as a result of carbide precipi- resistance to IGA of the alloy. The tation. A stabilized grade, e.g., 321, 347 anneal also removes the effect of cold or 348 or a low carbon grade, e.g., working and places the alloy in a soft, 304L, 316L etc. should be used when ductile condition, however, the quench- stress relief in this temperature range is ing operation may leave considerable required. Refer to Figure 11, page 26, residual stresses. in developing stress relief treatments In fabrication, even higher residual that will avoid carbide precipitation and stresses may be developed as a result IGA in Type 304. of forming operations and welding.

35 For the materials engineer Table XI Stainless steel products forms

Diameter Item Description Thickness Width or size

Coils and cut lengths: under .187 in. 24 in. & over – Sheet Mill finishes Nos. 1, 2D & 2B under .187 in. all widths – Pol. finishes Nos. 3, 4, 6, 7 & 8

Strip Cold finished, coils or cut lengths under .187 in. under 24 in. –

Plates Flat rolled or forged .187 in. & over over 10 in. –

Hot finished rounds, squares, – – .25 in. & over Bars octagons and hexagons .25 in. to 10 in. .125 in. & over – Hot finished flats incl.

Cold finished rounds, squares, – – over 1.2 in. octagons and hexagons – .375 in. & over – Cold finished flats

Hot rolled rounds, squares, Rods octagons, and hexagons in coils – – .25 in. to .75 in. for cold rolling or cold drawing.

Cold finished only: 0.010 in. to .062 in. to Wire round, square, octagon, .5 in. & under under.187 in. under.375 in. hexagon, flat wire

Not considered “standard” shapes, but of wide interest. Currently limited in size to approxi- Extrusions mately 6.5 inches diameter circle, or structurals to 5 inches diameter.

Tubing Dimensions by the outside diameter and wall thickness.

Nominal pipe size is the inside diameter from .125 inch to and including 12 inches. Nominal pipe size is the outside diameter for 14 inches and larger diameters. The wall thickness is Piping dimensioned by schedules (5S, 10S, 20, 30, 40, 80, 120, 160, XX and variations thereof) for all nominal pipe sizes.

Material procurement are included in weldments, it is useful to review the extent to which the heat and storage guides affected zone of welds may suffer IGA Table XI shows the principal stainless in the process fluid of interest. steel wrought product forms – plate, sheet, pipe and tubing – most com- Surface finishes monly used in welded fabrication. These Table Xll shows the standard finishes are normally purchased in the low for sheet and strip. The most widely carbon modification for each grade, used finish for sheet is 2B. Polished although the alloys stabilized with finishes are also available but are not titanium or niobium-tantalum are also normally used in welded fabrications for used as discussed in the section on the chemical and other process welding for corrosion resistance. Bars industries except food and medical and structural shapes are sometimes equipment. included in the fabrication. With bars There is no standard surface finish for and shapes, the higher carbon grades plate as there is for sheet and strip. are normally used because of their Plate is normally hot rolled, annealed somewhat higher strength. When they and pickled. Surface defects and

36 For the materials engineer Table XII Standard mechanical sheet finishes

Unpolished or rolled finishes: No. 1 A rough, dull surface which results from hot rolling to the specified thickness followed by annealing and descaling.

No. 2D A dull finish which results from cold rolling followed by annealing and descaling, and may perhaps get a final light roll pass through unpolished rolls. A 2D finish is used where appearance is of no concern.

No. 2B A bright, cold-rolled finish resulting in the same manner as No. 2D finish, except that the annealed and descaled sheet receives a final light roll pass through polished rolls. This is the general-purpose cold-rolled finish that can be used as is, or as a prliminary step to polishing.

No. 2BA or BA Non-standard but widely offered bright annealed finish, highly reflective surface.

Polished finishes: No. 3 An intermediate polished surface obtained by finishing with a 100-grit abrasive. Generally used where a semi-finished polished surface is required. A No. 3 finish usually receives additional polishing during fabrication.

No. 4 A polished surface obtained by finishing with a 120 –150 mesh abrasive, following initial grinding with coarser abrasives. This is a general-purpose bright finish with a visible “grain” which presents mirror reflection.

No. 6 A dull satin finish having lower reflectivity than No. 4 finish. It is produced by Tampico brushing the No. 4 finish in a medium of abrasive and oil. It is used for architectural and ornamental applications where a high luster is undesirable, and to contrast with brighter finishes.

No. 7 A highly reflective finish that is obtained by buffing finely ground surfaces but not to the extent of removing the “grit” lines. It is used cheifly for architecural and ornamental purposes.

No. 8 The most reflective surface, which is obtained by polishing with successively finer abrasives and buffing extensively until all grit lines from peliminary grinding operations are removed. It is used for applications such as mirrors and reflectors.

Standard mechanical strip finishes: No. 1 Approximates a 2D finish for sheet.

No. 2 Approximates a 2B finish for sheet:

BA Bright annealed, highly reflective finish. Used extensively for automotive trim.

Mill-buffed No. 2 or BA strip finish followed by buffing to produce a uniform colour and uniform reflectivity. Used for automotive trim, household hardware and for subsequent chromium plating.

For more information consult: NiDI 9012, “Finishes for stainless steels”.

roughness in plate can initiate crevice about 8 in. (200mm), and from sheet in attack in severe environments. For such larger sizes. The finish on welded pipe services, it is necessary to negotiate normally approaches the 2B or 2D finish surface finish required with the on sheet, except in the area of the weld. producer. The finish on extruded seamless pipe is Pipe is not normally furnished to a not quite as smooth but is normally specific finish. Welded pipe is made satisfactory from the standpoint of from cold finished coils in sizes up to corrosion.

37 For the materials engineer Electropolishing is an electrochemical welded and removed from the rest of process which provides a high luster the surface just prior to final cleaning finish and is finding increased use in and inspection. applications where cleanability is a major concern, such as bioprocessing and 4. Pipe is normally ordered to either paper mill head box equipment. The ASTM A 312, which requires a final process may be described as the re- heat treatment after welding, or to verse of electroplating in that there is an ASTM A 774, which does not. Pipe electrolyte but the current is reversed to A 312 is standard for most ware- and metal is removed rather than plating house stock. Only five of the more a new layer. The electropolishing proc- common grades are covered in A ess selectively reduces the peaks and 778. A 778 tends to be used for the sharp edges that exist on the metal larger diameter sizes where the low surfaces, which in turn lessens the carbon grades have proven to have chance of product build up and eases adequate corrosion resistance in the cleaning. There is evidence that the as-welded condition. ASTM A 403 corrosion resistance is improved over and A 774 are the comparable that of mechanically polished surfaces. specifications for stainless steel Electropolishing may be performed on fittings. Large diameter welded the completed fabrication rather than stainless steel pipe spiral can be sheet, strip or other starting products. obtained to ASTM A 409. The surface roughness, that is the distance between the peaks and valleys, 5. The interior finish in the area of the is reduced about 25% through weld is often of concern. Most electropolishing. The surface may be welded pipe producers achieve a ground to a 180 to 250 grit finish before good finish in the area of the weld electropolishing but mechanical polishing but the finish desired should be on electropolished surfaces is avoided. identified in procurement to avoid misunderstandings. Purchasing guidelines 6. Standard pipe lengths are 20 ft. The following guidelines are offered (6m), but longer lengths up to 60 ft. when purchasing stainless steel to be (18m) are available. For smaller used in the fabrication of corrosion diameter lines, considerable savings resistant equipment. can be made by ordering in longer 1. Select the low carbon grades or the lengths and utilizing bends in lieu of stabilized alternates for welded fittings. fabrications that will not be solution annealed after fabrication. Standard specifications for stainless steel product forms for welded fabrica- 2. Specify a 2B finish for sheet. tion are shown in APPENDIX A. The Specify the finish the application ASTM specifications do not cover requires for plate in the assembly welds such as required to procurement documents. fabricate pipe assemblies, tanks and 3. Specify protective paper for sheet other process equipment. Specfications and plate to be applied at the mill for fabrication weld quality are the when special surface protection responsibility of the user and must be during storage and fabrication war- included in procurement documents. rants. The protective paper can be stripped back in the area to be

38 Part III For the design engineer

Design for corrosion design, but can be significant and have led to failures in flat bottom tanks. The services concave and dished head arrangements Much can be done in the detailed design can withstand much greater fatigue to improve corrosion resistance and loadings than can flat bottoms. obtain better service from less expen- sive grades.

There are two cardinal rules: 1. DESIGN FOR COMPLETE AND FREE DRAINAGE. 2. ELIMINATE OR SEAL WELD Figure 14-1 Flat Figure 14-2 Flat CREVICES. bottom, square bottom, rounded corners – worst. corners – good Tank bottoms – Figures 14-1 through corners, poor 14-6 show six common tank bottom outside. arrangements. The square corner flat bottom arrangement, Figure 14-1, invites early failure from the inside at the corner weld where sediment will collect, in- creasing the probability of crevice attack. Moisture penetrating the flat bottom to pad support invites rapid crevice corro- sion from the underside. The rounded bottom shown in Figure Figure 14-3 Flat Figure 14-4 Flat 14-2 is much more resistant from the bottom, rounded bottom, rounded inside, but is actually worse from the corners, grouted – corners, drip skirt outside as condensation is funnelled poor inside, poor – good inside, directly into the crevice between the tank outside. good outside. bottom and pad support. The grout used to divert such condensation, Figure 14-3, does help initially but soon shrinks back and becomes a maintenance demand itself. The drip skirt shown in Figure 14-4 is much the best arrangement for flat bottom tanks. The concave bottom and the dished head bottom on supports, Figure 14-5 Figure 14-6 Figures 14-5 and 14-6, are very good Concave bottom, Dished head – best and superior to all flat bottom tanks not rounded corners – inside, best out- only in corrosion resistance but also in good inside, good side, fatigue fatigue. Fatigue stresses from filling and outside, fatigue resistant. emptying are seldom considered in resistant.

39 For the design engineer Tank bottom outlets – Water left Bottom corner welds – When the side standing in the bottom of stainless steel wall forms a right angle with the bottom, tanks has been a source of tank bottom the fillet weld is seldom as smooth as failures in both fresh and saline waters. shown in Figure 14-13. It is usually Side outlets and centre outlets, shown rough and frequently varies in width in Figures 14-7 and 14-8, allow for compensating for variations in fit up. convenient construction but invite early Because of the location, it is very difficult failure of stainless steel tank bottoms. to grind and blend the weld into the Not only is a layer of stagnant water adjacent sides. Debris tends to collect held on the tank bottom but sediment and is difficult to remove, leading to cannot be easily flushed out. A flush under sediment type crevice attack. side outlet and a recessed bottom outlet Unless welded from the outside as in as in Figures-14-9 and 14-10 allow the Figure 14-14, the crevice is vulnerable to bottom to be completely drained and all crevice attack. Rounding the corner and debris and sediment to be flushed out, moving the weld to the side wall over- leaving the bottom clean and dry. The comes both shortcomings as shown in sloped arrangements shown in Figures Figure 14-15. This construction has 14-11 and 14-12 make it easier to flush much improved corrosion resistance and out and clean. also has better fatigue resistance.

Figure 14-7 Side Figure 14-8 Centre Figure 14-13 Figure 14-14 outlet, above outlet, above Corner weld from Corner weld from bottom – poor. bottom – poor. inside – poor both sides – poor inside, worst inside, good outside. outside.

Figure 14-9 Side Figure 14-10 outlet, flush – Centre outlet, good. recessed – good. Figure 14-15 Side wall in lieu of corner weld – best inside, good outside, fatigue resistant.

Attachments and structurals – All attachments create potential crevice sites. Figure 14-16 shows a tray support angle with intermittent welds adequate for strength. There is a severe crevice Figure 14-11 Side Figure 14-12 Centre outlet, between the angle and the inside wall of outlet, flush, the vessel which will become filled with sloped – best. recessed, sloped – best. debris and invite premature failure from

40 For the design engineer crevice corrosion. always be done. It is good practice to Figure 14-17 shows drill a weep hole through the outer wall the same tray and must be done if the vessel is to support with a receive a stress relief or solution anneal, continuous seal weld otherwise the expansion of the trapped at the top preventing air could damage the vessel wall. unwanted material Figure 14-21 shows structural angles from finding its way positioned so they can drain, an impor- down the wall and Figure 14-16 Tray tant factor when shutting down and into the crevice. The support, staggered flushing out. Angles should never be angle to side wall strength weld – positioned as in the top section of Figure crevice is still open adequate support, 14-22. The best position for complete from the bottom but severe crevice. drainage is shown in the lower view. this is a much less severe crevice which When channels are used, drain holes vapors but not material can still enter. Figure 14-18 shows a full seal weld at the top and bottom of the tray support angle. Here the crevice is fully sealed.

Figure 14-21 Figure 14-22 Position of angles. Position of angles.

should be drilled about every 12 in. Figure 14-17 Tray Figure 14-18 Tray (300 mm) in the centre, unless they can support, full seal, support, full seal be positioned as in the right hand view weld top – good weld top & bottom of Figure 14-23. crevice resistance. – best crevice Continuous fillet welds on support resistance. beams will seal the severe beam to horizontal plate crevice shown in the upper section of Figure 14-24. Baffles in tanks and heat exchangers create dead spaces where sediment

Figure 14-19 Figure 14-20 Reinforcing pad, Reinforced pad, staggered welds – seal weld – best Figure 14-23 Position of channels. adequate strength. crevice resistance. Figure 14-19 shows a reinforcing pad to which other attachments are fre- quently welded. The intermittent weld creates a severe pad-to-sidewall crevice inviting premature failure. It takes very little more time to complete the seal weld as shown in Figure 14-20 which should Figure 14-24 Vertical beams.

41 For the design engineer can collect and where full cleaning is difficult. Figure 14-25 shows a cut out at the lower corner of a tank baffle and Figure 14-26 a cut out in the lower portion of a heat exchanger tube sup- port plate. Both arrangements reduce debris collection and facilitate cleaning.

Heaters and inlets – Heaters should be located so they do not cause hot Figure 14-28 Poor and good designs for mixing concentrated and dilute solutions.

stainless steel pipe rather than butt weld, as in Figure 14-29. On larger diameter pipe, over 2 in. (50 mm), a Figure 14-25 Figure 14-26 Heat backing ring as shown in Figure 14-30 is Corner baffle cut exchanger, baffle often used. Both designs may be out – good. cut out – good. satisfactory for those services where the spots on the vessel wall. In Figure 14-27, stainless steel alloy has adequate the poor location of heaters creates hot crevice corrosion resistance. Because of spots which in turn may result in higher the crevice formed, these designs often corrosion in the area between the heater lead to unnecessary corrosion in and vessel wall. The good design avoids aggressive environments and are not hot spots by centrally locating the heater. recommended. The backing ring has the When a concentrated solution is further disadvantage of protruding into added to a vessel, it should not be the flow stream, which in turn can cause introduced at the side as shown in the product build up or unnecessary poor design of Figure 14-28. Side turbulence. introduction causes concentration and Very often stainless steel piping is uneven mixing at the side wall. With the installed as commercial quality, that is good design, mixing takes place away from the side wall. It is also good design practice to introduce feed below the liquid level to avoid splashing and drying above the liquid line.

Pipe welds – It is frequently conven- ient to socket weld small diameter Figure 14-29 Figure 14-30 Pipe Socket weld joint weld made with severe crevice. backing ring – severe crevice. without imposing code standards such as the ASME or the American Petro- leum Institute, API, which require full penetration butt welds. When procure- ment does not require full penetration smooth ID butt welds, it is all too com- Figure 14-27 Poor and good designs mon to have a beautiful looking weld for the location of heaters in a vessel. from the outside but incomplete pen-

42 For the design engineer etration on the The hand fed filler metal method is inside, such as more widely used in the chemical shown in Figure process industry but the experience of 14-31. Many the particular company or welders, corrosion failures strongly influences the selection. It is originate in crev- important that the root bead have ices created by adequate and uniform amounts of filler incomplete pen- metal melted into the weld for best Figure 14-31 Pipe etration at the root corrosion resistance. This addition is weld with incom- of pipe butt welds. readily obtained with consumable plete penetration – Since ASTM does inserts or by skilled welders using the severe crevice. not cover fabrica- hand fed filler metal method. tion, procurement specifications must There are a number of automatic specify full penetration and smooth ID GTAW machines available for root for the root bead of butt welds when the pass and fill welding. The root pass weld quality is not covered by other can be made using an insert, with specifications. automatic wire feed or in thin wall pipe, The preferred pipe butt welding single pass welds can be made without procedure to insure high quality root filler metal addition. The ID root welds is the use of GTAW for the root contour of automatic welds is very pass with an inert gas backing. In consistent and it is an excellent manual root pass welds, the hand fed process to use where the economics filler metal technique or the use of are favourable. Automatic GTAW is a consumable inserts is commonly used. particular advantage for tubing and Figure 14-32 shows some standard pipe 2 in. (50 mm) diameter and less. consumable insert designs. Properly Three good pipe-to-flange welding made welds with either technique can arrangements are shown in Figures provide a crevice free ID surface with 14-33, 14-34 and 14-35. The recessed minimum bead convexity or concavity. arrangement shown in 14-33 avoids the need for machining or grinding smooth the surface of the weld on the flange face in Figure 14-34. Both these arrangements are suitable when the flange is of the same material as the pipe. Neither is suitable when carbon steel or ductile iron flanges are used on stainless steel pipe. In this case a stub end arrangement shown in Figure 14-35 is preferred. In the case of pressure

Figure 14-33 Pipe Figure 14-34 Pipe recessed flange Figure 14-32 Standard consumable flush, pipe and and pipe, same flange same alloy – inserts, (from AWS A5.30). alloy – good. better.

43 For the design engineer piping, the flange design must also be in accordance with the applicable design or fabrica- tion specification. For piping and heat exchanger Figure 14-35 Stub tubing to drain end, flange carbon completely, it is steel or ductile iron necessary to slope – very good. Figure 14-36 the piping or heat (A) Horizontal (standard) – poor. exchanger just enough so that water will (B) Horizontal sloped – very good. drain and not be trapped where the pipe or tubing sags slightly between support points. Figure 14-36 shows how a water film tends to remain in horizontal runs of pipe or tubing and how water drains when sloped.

44 Appendix A Specifications for stainless steel for welded fabrication

ASTM A240 – “Heat resisting chro- ASTM A312 – “Seamless and welded mium and chromium-nickel stainless austenitic stainless steel pipe.” steel plate, sheet and strip for pres- ASTM A403 – “Wrought austenitic sure vessels.” stainless steel piping fittings.” A240 is the basic specification for A312 (pipe) and A403 (fittings) are the procurement of stainless steel for older specifications for welded welded fabrication. A240 requires austenitic stainless steel pipe for solution annealing at the mill. This aggressive environments developed specification includes 40 austenitic, 4 for, and widely used by, the chemical duplex and 16 ferritic grades. Caution: industry. Both products require a Care must be taken to select the low solution anneal after welding. Most of carbon or stabilized grades for the common stainless steels are corrosion resistant services, as the covered. Caution: Care must be taken higher carbon grades, used primarily in to select the low carbon or stabilized heat resistant applications, are included. grades for corrosion resistant services, as the higher carbon grades are also ASTM A262 – “Detecting included in A 312. Sizes from 1/8 inch susceptibility to intergranular attack to 30 inch diameter are included. in austenitic stainless steels.” A262 is a supplemental specification ASTM A778 – “Welded, unannealed that covers five tests that can be in- austenitic stainless steel tubular cluded in procurement specifications products.” when maximum resistance to ASTM A774 – “As-welded austenitic intergranular attack is required. When stainless steel fittings for general A262 is used, the criteria to be met in corrosive services at low and moder- the test (practice) must be included as ated temperatures.” pass/fail criteria are not part of A262. A778 (pipe) and A774 (fittings) are used where the low carbon and stabilized ASTM A264 - “Stainless chromium- grades can be used in the as-welded nickel steel-clad plate, sheet and condition. Solution annealing after strip.” welding is not required. Only low carbon A264 is the specification for the clad and stabilized grades are included in construction using the austenitic grades these specifications. Sizes from 3 covered in A240. inches to 48 inches are covered. ASTM A265 - “Nickel and nickel-base ASTM A409 - “Welded large diameter alloy-clad steel plate.” austenitic steel pipe for corrosive or A265 is the specification for clad con- high temperature service.” struction using the ten highly alloyed A409 covers light wall, spiral welded, as nickel and nickel-base grades covered well as straight seam welded, pipe 14 under separately identified ASTM B inches to 30 inches in diameter. section specifications. Solution annealing is required unless waived. Fourteen grades are covered. Caution: Care must be taken to select the low carbon or stabilized grades for corrosion resistant services, as the higher carbon grades are also included. There is no specification for fittings.

45

Additional requirements:

Few ASTM pipe and fitting specifica- 2. Inert gas backup on the inside of the tions require pickling after production. pipe during welding to minimize oxidation and heat tint. ASTM specifications do not cover shop fabrication of pipe and fittings. The user 3. Matching composition or higher Mo must develop his own specifications for content filler metal for Mo-containing butt welding and fabrication to pipe grades. drawings. Important points to include are the following: 4. Protection of piping with protective 1. Full penetration, smooth ID, TIG end caps to minimize contamination made root beads. during shipment and storage.

Bibliography

ANSI/AWS D10.4 – 86, Recommended American Welding Society, Welding Practices for Welding Austenitic Chro- Handbook, Volume 4, Seventh Edition. mium-Nickel Stainless Steel Piping and Tubing. ASM International, Metals Handbook, Ninth Edition, Volume 6, Welding, ANSI/AWS D10.11 – 87, Recommended Brazing and Soldering. Practices for Root Pass Welding of Pipe Without Backing. AWS B2.1 – 84, Standard for Welding Procedure and Performance Qualifica- ASTM A380, Standard Recommended tion. Practice for Cleaning and Descaling Stainless Steel Parts, Equipment, and ASME Boiler and Pressure Vessel Systems. Code, Section Nine.

American Iron and Steel Institute, Fabrication and Post-Fabrication Welding of Stainless Steels and Other Cleanup of Stainless Steels, by Arthur Joining Methods, distributed by Nickel H. Tuthill, NiDI Technical Series Development Institute, Publication No 10 004. No 9002.

Acknowledgement

The authors are indebted to Richard B. valuable technical comments. Hitchcock and David E. Jordan for their

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