Pay attention to dissimilar-metal welds

Guidelines for dissimilar metals

Reprinted with permission from Chemical Engineering Progress May 1991 ©1991 American Institute of Chemical Engineers All rights reserved

NiDI

NICKEL DEVELOPMENT INSTITUTE NiDI Reprint Series NO 14 018

Richard E. Avery The material presented in this publication has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice. The Nickel Development Institute, its members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein. Pay Attention to Dissimilar-Metal Welds

issimilar-metal welding refers to (FCAW), and gas tungsten arc (GTAW). Recent the joining of two different alloy With these processes there is a well-defined systems. Actually all fusion weld that preferably contains a substantial experience with D welds are dissimilar-metal welds filler-metal addition. With the GTAW boiler tubing (DMWs) because the metals being joined process, however, the amount of filler have a wrought structure and the welds added is controlled by the . The reveals how have a cast structure. Frequently the match- welder should be trained to make the prop- welding ing-composition filler metal is deliberately er filler-metal addition used for the partic- altered from that of the base alloys. For this ular welding procedure. practices affect discussion a dissimilar-metal weld will be 2. Low-dilution welds. Low-dilution weld joint that between metals of two different alloy welds include electron beam, laser, and systems. pulsed arc; the amount of base metal melt- performance On this matter, the chemical process ed is relatively small, and filler metals are in service. industries can learn something from the not normally added. power industry. A very common DMW ap- 3. Nonfusion joining: Typical nonfusion plication is joining ferritic [e.g., 2 1/4% joining processes are , and Cr-1% Mo (UNS K21590)] tubes to explosion welding, austenitic boiler tubes such as 304H along with and . (S30409) or a similar austenitic stainless Dissimilar-metal joints can usually be steel. Because these welds are so impor- made by any of these methods, but low-di- tant, they are treated separately in this lution and nonfusion joining processes are article. more often used for high-production, spe- cial-application joining. DMWs encoun- Metallugical factors tered in power and process industries are Richard E. Avery, In dissimilar-metal welding, the prop- most often fusion welds made by the more Avery Consulting erties of three metals must be considered: common welding processes. Associates, Inc. the two metals being joined and the filler In fusion welding, the weld metal is a metal used to join them. For example, if mixture of the two metals being joined and one of the metals being joined is welded the filler metal. In arc welds made with using preheat when welding to itself, pre- consumable processes such as heat should be used in making a DMW. SMAW, GMAW, SAW, and FCAW, the Another variable might be heat input con- weld metal is well mixed or stirred by the trol. On occasion there may be a conflict in arc action and the composition is quite uni- that the optimum control for one metal is form from one area to another. By sampling undesirable for the other. In this case, a any place in the weld bead, the weld com- compromise is needed. This is one reason position is determined and weld properties the development of a DMW procedure reasonably predicted. While the bulk of the often requires more study than for a con- weld is well mixed, there is an unmixed ventional, similar-metal welding proce- zone (UMZ) at the weld interface, which is dure. a very narrow boundary layer of melted Fusion welds and other joining meth- base metal that froze before mixing with ods. The processes available for joining the weld metal. Fortunately, the UMZ is dissimilar metals are: seldom important in normal service envi- l. Fusion welds. The processes for fu- ronments but, on rare occasions, has ex- sion welds include shielded metal arc hibited selected corrosion attack. There is (SMAW), gas metal arc (GMAW), sub- also a zone of unmelted base metal that will merged arc (SAW), flux cored arc have been altered by the heat of welding.

1

This heat-affected zone (HAZ) can Table 1. Determining DMW composition influence service life. Determining weld composition. Method Advantages Limitations It is necessary to know the approxi- mate weld metal composition before the service performance can be pre- 1. Chemical analysis of Most accurate Time consuming dicted. Table 1 lists three methods of weld determination Expensive determining the weld metal compo- sition along with advantages and lim- itations. The technique for method 1 2. Approximation of Less expensive and Estimating the is obvious: metal is removed from base metal dilution usually shorter than percentage the weld and an analysis performed. by weldcross section chemical analysis often difficult in Method 2 approximates weld dilu- and composition welds such as tion by area measurement as shown calculated multipass welds in Figure 1. Method 3 uses the fol- lowing base metal dilution percent- ages for some of the common weld- 3. Approximate dilution Very fast way of Welding ing processes: figures for common estimating “rough” technique can • SMAW (covered electrode): 20 welding processes composition have a strong to 25% dilution and composition No laboratory work influence of • GMAW (spray arc): 20 to 40% calculated involved dilution in some dilution processes, e.g., • GTAW: 20 to 50% dilution GMAW, GTAW • SAW (submerged arc): 20 to 50% dilution The figures are approximate be- metal welding. In other words, it is sion and oxidation resistance equal cause the welding technique has a assumed both metals in a DMW are to the least resistant base metal being strong influence on the dilution, par- basically weldable. joined. When a DMW is in an envi- ticularly with GTAW. Dilution in the Service condition effects. A ronment where the liquid can be an SMAW process is most predictable, properly engineered DMW matches electrolyte, the weld metal should be which is an advantage in making weld properties to the service condi- cathodic to (more corrosion resis- DMWs. tions. Some of the more important tant than) both base metals. If the When the amount of dilution from factors to be considered are me- weld is anodic (less corrosion re- the base metal is determined by ei- chanical and physical sistant), it can suffer ther method 2 or 3 of Table 1, the properties and weld accelerated galvan- average percentage of a specific corrosion/oxidation re- Ductility ic corrosion. element, X, is determined by the sistance. Dissimilar-metal formula below. In this example, the Mechanical prop- comparable to combinations dilution is 15% from each base metal erties. The weld metal the metals being A and B, while the filler metal con- should be equal to or Nickel-containing tributes 70% of the weld volume. stronger than the joined is desirable, and nickel alloys are easily welded to XX = (XA)(0.15) + (XB)(0.15) + weaker material being but not always (X )(0.70) joined, although the most commercially F possible. where XX is the average percentage American Society of used metals. of element X in the weld metal, XA is Mechanical Engineers Exceptions are fu- the percentage of element X in (ASME) code allows a sion welding to alu- base metal A, XB is the percentage weld strength of 95% in some cases. minum, titanium, and most refracto- of element X in base metal B, and XF Ductility comparable to the metals ry metals and alloys. Some of the is the percentage of element X in the being joined is desirable, but not al- most commonly encountered com- filler metal F. ways possible. binations will now be discussed. Calculations are normally made Physical properties. Weld metal Steel-to- welds for only major alloy constituents, physical properties similar to the below 800°F. These are probably the e.g., iron, chromium, nickel, copper, base metals are desirable. In joints most frequently encountered DMWs and molybdenum, while elements that are heat cycled, a gross mis- in industry, with the possible excep- such as carbon or manganese are sel- match in the coefficient of thermal tion of boiler tube welds. In devel- dom figured. Carbon is an important expansion can lead to an early ther- oping a DMW procedure, it is im- factor in the of iron base mal fatigue failure. portant to note the welding alloys, but it is of no more signifi- Weld corrosion/oxidation resis- parameters normally used for each cance in a DMW than in similar tance. The weld should have corro- of the metals being joined so that

2

2. Austenitic-covered corrosion resistance and adversely R.E. AVERY, of Avery or flux-cored wires should have low affect the mechanical properties of Consulting Associates, Inc., moisture content to prevent hydro- many standard grades of stainless Londonderry, NH (603/434- gen-associated defects in the low- steel. 2625; Fax: 603/425-2542), alloy HAZ. Coating moisture levels 2. Heating unstabilized stainless and consultant to the Nickel acceptable for welding austenitic al- steels that have a carbon content of Development Institute, has loys may cause hydrogen-related 0.03% or higher can significantly re- had more than 35 years’ ex- problems such as underbead crack- duce the intergranular corrosion re- perience in the fabrication ing in the HAZ of a low-alloy steel. sistance. If heat treatment is a ne- and joining of stainless Electrodes can be rebaked in accor- cessity and full corrosion resistance steels and high-nickel alloys. dance with manufac- of the austenitic stain He has authored more than turers’ recommenda- less steel is needed, 12 articles on welding as tions to reduce columbium- or titani- well as sections of the moisture. High-restraint um-stabilized types or “American Welding Society 3. High-restraint joints are the low-carbon grades joints are susceptible to (less than 0.03% C) Handbook.” A registered susceptible to professional engineer, cracking unless preheat should be used. is used. The degree of Filler-metal con- he graduated from the cracking unless restraint varies with siderations. One of Rensselaer Polytechnic joint design and metal preheat is used. the most common Institute with a B.S. in thickness. Material DMW combinations is metallurgical engineering. over about 1 1/4 in. (32 type 304 (UNS mm) can be highly restrained and S30400) stainless to a low-carbon or those that are appropriate are in- usually requires preheat. mild steel. Type 308 (S30800), the cluded in the welding procedure. 4. When a preheat is needed, a standard filler metal for welding Carbon and low-alloy side con- temperature of 300°F is usually ad- type 304 to itself, should not be used siderations. A simple guide in equate with 400°F used in severe to make this weld. Some type 308 making DMWs is to use the same conditions. Upon completion, the welds may be satisfactory, but even- parameters such as preheat, inter- weld should be slow cooled to allow tually there will be quality problems pass temperature, heat input, post- hydrogen to diffuse from the HAZ. because of iron dilution. weld heat treatment, etc. that are Stainless steel side considera- A higher alloy filler metal such as used in welding the alloys to them- tions. As with welding stainless steel type 309 (S30900) with a ferrite selves. Some of these controls are to itself, good practice includes such number (FN) over 10 or type 312 as follows. items as proper cleaning before (S31200) with an FN over 25 should 1. Carbon steels with less than welding, good fitup, and proper be used. The effect of dilution on an 0.20% carbon can normally be weld- shielding gases. Other considerations austenitic stainless steel weld can be ed with austenitic fillers without pre- include the following: illustrated using the WRC 1988 dia- heat, but when the carbon is greater 1. Postweld heat treatments such gram in Figure 2. The structure of a than 0.30% temperature control is nec- as a 1,100-1,300°F stress relief are stainless steel weld may be fully essary. As alloy content increases, i.e., often beneficial in improving HAZ austenitic, such as type 310 in the case of low-alloy steels, preheat properties in ferritic steels. This heat (S31000), or contain varying control is usually essential. treatment can, however, reduce the amounts of delta ferrite, as with types

■ Figure 1. Weld bead with 30% dilution, 15% from Metal A and 15% from Metal B.

3

■ Figure 2. Effect of 25% mild steel dilution types 308, 309, and 312 weld metals. Structure of the diluted 308 is austenite and martensite while 309 and 312 austenite and ferrite.

308, 309 or 312. The amount of fer- While types 309 and 312 are now A5.11 Class ENiCrFe-2 or Class rite is determined by the composition widely used for DMWs, type 310 has ENiCrFe-3 electrodes. Nickel alloy and weld cooling rates; the faster the a long history of use in dissimilar- welds have a coefficient of thermal cooling, the higher the ferrite con- metal welding and for welding diffi- expansion (COE) between ordinary tent. Fully austenitic welds are more cult metals including high-hardening steel and austenitic stainless. With susceptible to hot cracking or fis- alloys such as tool steels. Type 310 the higher COE type 309 and 312 sures than welds containing about welds often have given excellent welds, there is a high stress concen- 5% or more ferrite. service in spite of minor fissures de- tration at the steel-side fusion line Figure 2 also shows that marten- tectable by liquid penetrant testing. that, during thermal cycling, invites site (M) may be formed as the nick- One caution in using 310 for “weath- thermal fatigue failures. el and chromium equivalents are re- ering” steels containing 0.07–0.15 % Another caution in using stainless duced. Martensite is a hard, low- phosphorus is the probable weld steel filler metals occurs when the ductile phase that is prone to hy- metal cracking. Type 309 or 312 weldment is heat treated between drogen-related defects. In DMWs, it filler metals can better tolerate this 1100 and 1300°F. Welds containing is best to avoid martensite. If type level of phosphorus and should be higher amounts of delta ferrite, e.g., 308 filler metal is diluted by 25% used. type 312 (FN more than 25) or type with mild steel, the weld metal is in Steel-to-stainless steel welds 309 (FN more than 10), can lose the austenite–martensite over 800°F. When room temperature ductility and suf- (A + M) phase area of service temperatures fer reduced corrosion resistance as a Figure 2. Types 309 and Martensite is a are above 800°F, the result of sigma formation in this tem- 312 electrodes both ideal filler is a nick- perature range. If postweld heat treat- have more nickel and hard, low-ductile el–chromium or nick- ment in this range is required, a low- chromium and when di- phase that is el–chromium–iron ferrite composition weld metal luted by carbon steel are metal such as reduces the chance of sigma forma- still in the austenite–fer- prone to American Welding tion. Another method is to first “but- rite (A + F) phase area hydrogen-related Society (AWS) ter” (surface by weld overlay) the and maintain excellent A5.14 Class ERNiCr- ferritic side with type 309 followed crack resistance. defects. 3 bare wire or AWS by the heat treatment for the ferritic

4

■ Figure 3. Typical dissimilar-metal weld defects in boiler tubes after a long time in service. side. The butt weld is then made of the joint are nonmagnetic. Even bon steel to various grades of using using a conventional filler such when all of materials are magnetic, chromium-molybdenum steels to as type 308. An alternative is a the degree of ferromagnetism can austenitic stainless steels such as nickel alloy filler metal that is not vary because of composition dif- type 304H (UNS S30409). This in- subject to sigma formation. ferences, and the magnetic differ- volves a number of DMWs. The fer- Other dissimilar-metal combi- ences can give false indications at ritic-to-austenitic welds have expe- nations. Nickel- and copper-base al- the fusion line. Because of this, liq- rienced early service life failures. loys are often welded to carbon and uid penetrant inspection is most fre- These welds have traditionally been low-alloy steels as well as to each quently used for surface inspection. made with either an austenitic stain- other. After determining the approx- Nondestructive radiographic less steel or a nickel–chromium alloy imate composition of the DMW, the inspection. DMWs can be inspected filler metal. Failures that occur after approximate maximum tolerance using the same procedures and in- about five years have not been relat- limits for major alloying elements spection standards employed in sim- ed to ordinary weld defects such as can be determined; see Table 2. ilar-metal joints. The exposure , lack of fusion, or porosity but should be selected for the material are related to metallurical changes Inspection and testing and thickness of greatest interest. due to service conditions. The num- In qualifying a welding proce- Because of differences in the radio- ber of DMW failures increased sig- dure specification, DMWs are usu- graphic density, interpretation of nificantly in the mid to late 1970s, ally evaluated by tensile radiographs can be and investigations were initiated in and bend tests like sim- somewhat different North America under the direction of ilar-metal welds. When DMWs can be than with similar the Electrical Power Research either of the base metals metal welds. Institute (EPRI). A brief summary of or the weld metal is sig- inspected using Nondestructive ul their findings follows. nificantly weaker, which the same trasonic testing. Nature of failures. Typical is often the case, a inspection When the weld metal is DMW defects in boiler tubes after longitudinal bend test is coarse grained (such as long times in service are shown in preferable because all standards an austenitic stainless Figure 3. Through an examination of elements are forced to employed in steel, nickel–chromi- numerous DMWs with 50,000 to elongate the same um or nickel–copper 200,000 h of service, the EPRI stud- amount and a better similar-metal weld joining a ferritic ies identified three distinct failure evaluation is possible. joints. alloy), there is a major modes, all in 2 1/4 Cr–1 Mo next to With a transverse bend problem with interpret- the fusion line. test, the specimen may tation at the fusion line. 1. Failures that occur along prior move in the bend die, causing all of For this reason, the ultrasonic testing austenite grain boundaries in the the elongation to take place in the of DMWs is seldom practical. low-alloy steel about one or two weaker member and often resulting grains away from the weld fusion in fracturing. Boiler tube DMWs line; this failure is most commonly Nondestructive surface in- To make the most effective use of seen in DMWs made with stainless spection. Magnetic particle testing is the materials in modern boilers, steel filler metal and occasionally in not possible if one or more parts tubes range in composition from car- nickel-base filler-metal welds.

5

2. Failures along a line of globu- Table 2. Approximate limit of diluting lar carbides, formed in service, next elements in welds.* to the fusion line; this is more com- mon in DMWs made with nickel- Diluting Elements base filler metal. Weld Metal Iron Nickel Chromium Copper 3. Failures that result because of an oxide notch formed on the outside Nickel 30% — 30% Unlimited of the tube at the weld to low-alloy junction. The notches do not usually propagate to failure, but can in the Nickel-Copper 2.5% SMAW Unlimited 8% Unlimited case of thin wall tubes subject to high 15% GMAW bending stresses; this failure can occur in both stainless steel and Ni-Cr-Fe† 25% Unlimited 30% 15% nickel base welds. There is a difference in the service Copper-Nickel 5% Unlimited 3-5% Unlimited life of stainless steel and nickel base welds. The nickel joints last three to five times longer. Another finding * The limit values should be treated only as guides. Absolute limits are was that a wider bevel on the ferrit- influenced by the welding process, weld restraint and small variations in ic side extended service life. weld filler and base metal compositions, Service conditions and predict- † Silicon should be less than 0.75% in the weld. ed life. The service life of a boiler tube DMW is strongly influenced by the following factors: 1. Shop-welded transition pieces, the weld metal composition of •operating temperature: higher often called “dutchmen,” are used DMWs. Knowing the composition, temperatures shorten life; because the DMW can be made weld properties can be predicted for •number of thermal cycles: the under optimum conditions, e.g., a wide range of DMWs. greater the number of cycles, the down hand, automatic welding, etc.; In establishing a DMW proce- greater the damage; the i.d. root can be ma- dure, the more restrictive require- •type of thermal cy- chined or ground to ments for each base metal (such as cles: the cycle can be Knowing the provide a smooth sur- preheat, temperature control, weld cold, warm, or hot; face; and inspection is heat treatment, etc.) should be cold cycling causes the composition, easier. The field welds used. On occasion, there will be a most stress; weld properties are then between sim- conflict that needs special study •temperature ilar metals, i.e., stain- and testing. excursions: the higher can be predicted less steel to stainless the temperature and. for a wide range steel and low alloy to CEP number of excursions, of DMWs. low alloy. the greater the 2. Making the damage; DMW in the boiler; •total time at temperature: service some companies prefer making one Literature Cited life is shortened by longer times at DMW field weld rather than the temperature. total of three welds described pre- 1. Bailey, N., ed., “Welding Dissimilar By using these factors and other viously. Metals,” The Welding Institute, engineering data, EPRI developed a 3. Nickel-base filler metals are Cambridge, United Kingdom (1986). software program called Prediction used by most utilities intead of stain- 2. “Welding Stainless Steels,” Teledyne of Damage in Service (PODIS) that less steel. The most widely used McKay, York, PA (1984). estimates the remaining life of a filler metals are AWS A5.14 Class 3. “Joining,” technical bulletin, Inco given DMW. PODIS can be helpful ERNiCrFe-2 for the GTAW root and Alloys International, Huntington, WV in establishing a monitoring inspec- covered electrodes conforming to (1985). tion program for DMWs as they ap- AWS A5.11 Class ENiCrFe-2 or 4. Viswanathan, R., “Dissimilar Metal proach the end of their expected life. ENiCrFe-3. Weld and Boiler Creep Damage Evaluations for Plant Life Extension,” Replacement weld joints. J. Pressure Vessel Technol., 107, pp. Various utilities employ different In conclusion 218-225 (1985). practices in making replacement The nickel-containing stainless 5. Roberts, D. I., R. H. Ryder, and R. boiler tube DMWs, which probably steels, nickel- and copper-base alloys Viswanathan, “Performance of indicates that there is no single best are readily fusion welded to carbon Dissimilar Welds in Service,” J. method. Some of the following ap- and low alloy steel and to each other. Pressure Vessel Technol., 107, pp. 247-254 (1985). proaches are used: Methods are described to estimate

6