2000 Stainless Steels: an Introduction to Their Metallurgy and Corrosion

Stainless Steels: An Introduction to Their Metallurgy and Corrosion Resistance

Roger A. Covert and Arthur H. Tuthill*

and why they sometimes do not. In most cases, selection of the proper stainless steel leads to satisfactory performance.

COMPOSITION, NOMEN- CLATURE AND GENERAL PROPERTIES

Most metals are mixtures of a primary metallic element and one or more intentionally added other ele- This article has been peer-reviewed by two professionals. ments. These mixtures of elements are called alloys. Stainless steels are alloys, as are brasses (copper + zinc), bronzes (copper + tin), the many alu- INTRODUCTION better understanding of stainless minum alloys, and many other me- Worldwide, in industry, in busi- steels, especially to the non-metal- tallic materials. In general, solid ness and in the home, metals called lurgist. metals and alloys consist of randomly stainless steels are used daily. It is Industries are concerned with oriented grains that have a well-de- important to understand what these integrity of equipment and product fined crystalline structure, or lattice, materials are and why they behave purity. To achieve these, stainless within the grains. In stainless steels, the way they do. This is especially steels are often the economical and the crystalline structures within the true because the word “stainless” is practical materials of choice for pro- grains have been given names such as itself somewhat of a misnomer; these cess equipment. However, before ferrite, austenite, martensite, or a materials can stain and can corrode intelligent decisions can be made mixture of two or more of these. under certain conditions. People need regarding the proper selection from Many of the properties of stainless to know why these metals are the various types of stainless steel, it steels depend upon which crystalline usually bright and shiny and is necessary to have an understanding lattice occurs. Examples of these why they sometimes depart from of what stainless steels are. It is crystal structures are given in Fig. 1, this expected appearance. In this important to know what different where the black dots represent atoms paper, we hope to explain some grades of stainless steel are available, and the lines are present to help the of these phenomena and provide a why they perform satisfactorily structure to be seen.

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Figure 1. Crystal structures of stainless steels (Fig. 1b) becomes stable at room temperature if nickel, manganese, nitrogen, or carbon is added, singly or in combination, to iron or iron/ chromium alloys. The resulting materials are called austenitic stainless steels. In general, they are easier to shape and bend, more weldable, and less brittle than ferritic alloys. Martensite is a stable structure at ambient temperature and more similar to ferrite than to austenite. It also has a body-centred structure (Fig. 1c), but one axis of the cube has been elongated to form a tetragonal structure, that is, a crystal having all three axes at right angles Figure 2. The influence of chromium on the atmospheric corrosion of low carbon steel and with two equal sides and one unequal. It is produced by heat treat- ing or cold working cubic crystals of ferrite or austenite. Martensite is the hardest, and strongest of the three crystalline forms, but it is also the least workable. In fact, these alloys are seldom intentionally deformed. As mentioned previously, alloys are combinations of two or more elements, at least one of which is always a metal. All of the many and varied stainless steels are alloys. They are always iron-chromium alloys, but they often contain other elements, such as molybdenum or nickel. The better known varieties of stainless steel are wrought (hot- rolled or hot-forged after casting into an ingot). There are also cast counterparts that have properties similar to those of most of the wrought grades but that are altered slightly in composition in order to improve casting properties. To define the different materials, the publication Metals and Alloys in the Ferrite is the basic crystal struc- grains of other substances. They can Unified Numbering System (1) lists ture of iron or low-alloy steel at am- be quite complicated and often play over 250 types within the broad bient temperatures. To understand it, an important role in the mechanical definition of stainless steels. These envision a cube with an atom at each properties and corrosion behaviour are iron base alloys containing more of the eight corners and in the geo- of metals. than 11% chromium. Various grades metric centre of the cube. This body- Austenite is the crystal form of also contain nickel, molybdenum, centred cubic structure (Fig. 1a) is unalloyed iron in the grains at higher manganese, nitrogen and other repeated regularly in three dimensions temperature (>800°C). It is different alloying elements. As can be seen in throughout the grain until it meets a from ferrite. As in ferrite, there is an Fig. 2 (6), chromium's primary grain of different orientation. At these atom at each corner of a cube, but effect is to impart corrosion contacts are areas termed grain instead of one in the geometric resistance. The diagram shows the boundaries. Grain boundaries consist centre, there is one in the centre of influence of chromium on corrosion of many things, including the each of the six faces of the cube. when it is added to iron or steel. As interface, defects, impurities and This face-centred cubic array can be seen, when it reaches

JULY 2000 - Dairy, food and Environmental Sanitation 507 steels), valve steels and iron-base Figure 3. Effect of adding nickel to Fe-Cr alloys “superalloys”, the J to cast steels (except tool steels), and N to nickel and nickel alloys. The UNS system also provides for classification of many of the newer, more complex alloys that would not fit into the old system, and it covers many types of alloys in addition to the stainless steels. It should be noted that in other countries different nomenclatures and systems may be used. For example, in Europe the EN system of numbering alloys is in common usage. With this method, S30400 (304) and S31600 (316) are replaced by the numbers 1.4301 and 1.4401, respectively. Most stainless steels have similar designations, some of which are given in Tables 2 and 3. Figure 4 (6) shows how composition variations have led to many related stainless steels that have evolved from the basic S30400 (304) composition. By altering the composition, as indicated by the arrows 11-14%, corrosion is practically Institute, applied to the cast grades. and text in the figure, various com- negligible in the atmosphere. These early systems divided positions are produced to meet par- Nickel in stainless steel promotes stainless steel alloys into groups ticular needs. In many cases, this is austenite stability and reduces the according to crystal structure. done by adding or omiting small temperature at which austenite can However, many of the newer alloys amounts of other constituents without exist. Figure 3 illustrates this did not fit into the earlier categories, making major changes in the primary effect. Above the diagonal line in and it became necessary to have a alloy content. the diagram, austenite is stable at more complete system. Therefore, the indicated temperature; below these older nomenclatures are now the line, either ferrite or martensite being replaced by the, Unified COMMON STAINLESS STEEL is the stable crystal structure. Numbering System (UNS)(1) ALLOY SYSTEMS Manganese is similar to nickel developed by the Society for when it is added to or substituted Automotive Engineers (SAE) and Austenitic alloys - iron-chrom- for nickel and also increases the American Society for Testing ium-nickel and iron-chromium- strength. Molybdenum increases and Materials (ASTM). These groups manganese-nickel alloys the resistance to localized have developed a six-character corrosion phenomena, such as notation that assigns a unique Some of these alloys also con- pitting and crevice corrosion. designator to metals and alloys in a tain nitrogen, copper, silicon, and Nitrogen also improves resistance way that consistently defines a other elements for special purposes. to crevice corrosion, as well as material. For example, the UNS They have an austenitic, i.e., face- increasing strength and acting as number S30403 replaces AISI 3041; centred cubic crystal structure an austenite stabilizer. Elements the final two digits, 03, indicate the within the grains. To obtain this such as copper and silicon improve maximum permitted carbon content. structure, the austenite/ferrite corrosion resistance in special In other alloys, the various digits transition temperature is suppressed environments, and silicon also may refer to other parameters; so it by the addition of alloying agents, improves casting properties. cannot always be assumed that the primarily nickel, but also To reduce confusion and sim- latter numbers mean carbon content. manganese and nitrogen, so that plify nomenclature, standard The equivalent cast alloy is J92500, the resulting austenite is stable numbering systems have been which formerly was ACI CF-3. It at ambient temperature (see Fig. 3). developed for the various stainless should also be noted that the letter These alloys are grouped in the steel alloys. For many years in the before the numbers in the Unified 300 and 200 series, respectively, in United States, the three-digit Numbering System pertains to the old AISI system. They are method of the American Iron and different alloy classes. All letters non-magnetic, unless heavily cold- Steel Institute (AISI) was common used will not be defined here, but worked, and hardenable only for wrought stainless steels. those of importance will be by cold work. The primary Another letter and number system, mentioned. The S refers to heat and alloy of this type is S30400 that of the Alloy Casting corrosion steels (including stainless (304), 18-20% Cr, 8-10.5%

508 Dairy, Food and Environmental Sanitation - JULY 2000 Figure 4. Some compositional modifications of 18/8 austenitic stainless steel to produce special strength and corrosion resistance is properties. Dashed lines show compositional links to other alloy systems. needed. Many of them can be shaped and formed in the soft or annealed conditioned and subsequently hardened or “aged”. One of the best known uses of age-hardened stainless steels is for golf club heads.

Duplex alloys These are usually iron-chromium-nickel alloys with a nickel content lower than that of the austenitic grades. Some may also contain molybdenum or other elements. The duplex structure has grains of both austenite and ferrite. Duplex alloys are typically stronger than alloys that are solely austenitic, and their corrosion resistance is often at least as good as that of the alloys they replace. Duplex alloys are used in chemical, process, and petroleum industries, especially where better resistance to chloride stress corrosion cracking is required.

PRODUCTION OF STAINLESS STEELS For many years, stainless steels Ni, and the balance iron. It is com- Martensitic alloys were both melted and refined in an monly referred to as 18-8 stainless These are iron-chromium alloys electric arc furnace. These steps are steel because of the approximate but higher in carbon and other hard- now frequently separated, with the chromium and nickel contents. Com- ening agents than the ferritic alloys. molten charge in the electric furnace mon applications are for an almost They are magnetic, hardenable by transferred to a separate unit for endless variety of equipment, includ- heat treatment, and somewhat diffi- adjusting of composition and removal ing vessels, piping, and tubing used cult to weld and fabricate. S41000 of impurities. Such operations in producing and processing indus- (410), 11.5-13.5% Cr, 0.15max% C, normally use oxygen-inert gas injec- trial products. Many consumer prod- and the balance iron is typical of tion (Argon Oxygen Decarburization, ucts such as sinks and wash basins, these grades. Common uses are in AOD) or oxygen injection under cooking utensils, pots and pans, and making corrosion resistant bearings, vacuum (Vacuum Oxygen De- flatware are made from this alloy. knife and shear blades, and valve and carburization, VOD). These tech- When welded fabrication is em- compressor parts. niques permit the production of purer, ployed, the low-carbon grade S30403 cleaner steels with much more (304L) is frequently used. Precipitation or age-hardening carefully controlled compositions. alloys Stainless steels require care dur- Ferritic alloys - iron-chromium These are primarily iron-chro- ing fabrication (5, 6). Their properties alloys mium-nickel alloys to which other can vary depending on prior thermal These have a body-centred elements have been added to form and mechanical operation. Austenitic cubic crystal structure. They have compounds of small grains which stainless steels are almost always the corrosion behaviour of stainless precipitate when heated to inter- placed in service in the annealed steels but are sometimes difficult mediate or high temperature (500°C condition. This means they are to weld and fabricate. They are to 900°C) for a period of time. When quenched in water or other fast magnetic and hardenable only by present, these small grains strain the cooling media from the annealing cold work. An example is 543000 crystal and “harden” or strengthen the temperature (1040°C -1130°C). Thus, (430), 16-18%Cr, in the iron base. alloy. S17400 (17-4PH), 15-17.5% annealing has a different meaning Less expensive consumer products, Cr, 3-5% Cu, 0.15-0.45% Cb, 3-5% than it has for carbon or low- including automotive and Ni, and the balance iron is a common alloy steels, which are generally slow- appliance trim and flatware, are composition. These alloys are used cooled from the annealing tempera- often made from ferritic alloys. where a combination of high ture. For austenitic stainless steels,

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Figure 5. Pitting corrosion

lack of, or improper, annealing may tive passive film that covers the sur- cause only some form of lo- result in intergranular corrosion face. This film can conveniently be calized corrosion. To explain this problems because of precipitated thought of as chromium oxide, but it further, the various types of localized carbides at grain boundaries of the also contains small amounts of the corrosionpitting, crevice corrosion, microstructure. Producers are well other elements in the alloy. Some intergranular corrosion, stress aware of this and ship only annealed investigators of the subject consider corrosion cracking, and galvanic material unless they are asked to do the film to be something other than corrosionwill be considered otherwise, although it is always best an exact oxide, and they may be cor- separately. to specify the heat treatment. How- rect, but it is easier to think of the ever, using low carbon or titanium or film as an oxide. Many people think columbium stabilized grades is stainless steel must be given a “pas- Pitting and crevice corrosion additional protection from this prob- sivating” treatment for this film to Because pitting and crevice cor- lem. form properly. This is not true; if the rosion are very similar and the factors surface is clean and free of contami- that affect their occurrence are nation, the film forms instantaneously essentially the same, these two phe- INTRODUCTION TO THE on exposure to air, aerated water, nomena will be considered together. CORROSION BEHAVIOUR nitric acid, or other oxidizing media. Pitting (Fig. 5) is highly localized corrosion at individual sites on the OF STAINLESS STEELS It is extremely durable and reforms spontaneously. surface of the metal. The figure also The aqueous corrosion of metals Because of this protective film, shows that pits vary in size, shape, is generally considered an elec- stainless steels do not corrode as and morphology. Some pits are broad trochemical action. That is, there are carbon or low alloy steels or cast iron and not very deep while some alternating sites of differing electro- do. These materials “rust” or corrode penetrate quite deeply and others chemical activity on a metal surface. uniformly through constantly may undercut the passive film and These sites act like the anodes and changing anodes and cathodes on the spread out beneath it. Crevice cor- cathodes in a battery. At the anode, surface. However, except in solutions rosion (Fig. 6) is the attack that oc- the metal oxidizes (corrodes), react- such as hydrochloric acid, this curs at the interface between the ing with the environment to form rust general corrosion or uniform attack corroding metal and another sub- or some other corrosion product. At practically never occurs on stainless stance, usually one that is not elec- the cathode, a reduction reaction steels. The terms “corrosion rate” trically conductive. The corrosion such as the reduction of oxygen and “corrosion allowance” are usually spreads into the crevice be- takes place. This completes the usually meaningless when applied yond the point of contact. Both types electrochemical cell and corrosion to stainless steels. While factors of corrosion happen on stainless proceeds. In order to prevent corro- such as chemical environment, pH, steel in certain media, especially sion, these cells must be interrupted temperature, equipment design, those containing chlorides. Pitting in some manner. fabrication methods, surface finish, can occur because of minor The unique corrosion resistance contamination, and maintenance discontinuities in the passive film, of stainless steels is attributed to the procedures can affect the corrosion inclusions or defects in the stainless existence of a thin, adherent, inac- of stainless steels, they usually steel, or dirt and contamination on

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Figure 6. Crevice corrosion

the surface. Examples of common tent, nitrogen) to the alloys. The Other than heat treatment, there are crevices are joints with gaskets, at pitting resistance of a common ma- usually two solutions to sensitization: points where scale or hard biofouling terial such as S30400 with no added use of a low-carbon alloy such as attaches and in places where molybdenum can be markedly im- S30403 (304L) or use of an alloy materials overlap. Because the area of proved in this way. Alloys such as containing, or “stabilized” with, the attack is very small in comparison S31600 (2-3% Mo), S31700 (3-4% titanium, S32100 (321), or colum- to the overall area of the metal Mo), N08904 (4-5% Mo), and the bium, S34700 (347). In the first case, surface, corrosion can be very intense 6-7% Mo alloys have increasing pit- there is insufficient carbon in the and rapid at the site of attack. The ting and crevice corrosion resistance alloy to form large amounts of chro- most important single fact in the with increasing molybdenum content. mium carbides and thus reduce initiation of crevice corrosion is the Good design and fabrication chromium in the grain boundaries. In presence of chloride ions, although techniques that produce smooth, the latter case, the carbon is pre- higher environmental temperatures, clean surfaces, rounded corners, and combined with titanium or colum- oxygen or easily reducible ions such “drain away” designs also help resist bium and is therefore not available to as ferric ions, and acid pH values can pitting and crevice corrosion. the chromium. The titanium and also have deleterious effects. Pitting columbium carbides are dispersed in is less apt to occur in aqueous Intergranular corrosion the matrix of the grains and not solutions moving at moderate to high If an austenitic stainless steel of localized at grain boundaries to velocities than in stagnant ones. normal carbon content (0.03-0.08% promote intergranular corrosion. In Although we have previously C) is heated in the temperature range recent years, as the AOD and VOD said that pitting and crevice corrosion from 425°C (800°F) to 815°C processes have become more suc- are essentially the same, some (1500°F), chromium carbides are cessful and low carbon alloys easier differences should be mentioned. precipitated at grain boundaries and to produce, the low carbon grades of Crevice corrosion can occur in en- the structure is said to be “sensitized.” stainless steel have largely supplanted vironments that normally do not The chromium-depleted zone around the stabilized alloys for welded cause pitting in boldly exposed sheet each grain is more susceptible to fabrication. Because of their ease of or plate, particularly in tight attack in some media, particularly production, they are also replacing stationary crevices in slow-moving acids. Exposure to this critical the standard carbon grades for many solutions. temperature range can result from applications. If the environment cannot be improper annealing, stress relieving, controlled, by reducing acidity, or or heating during forming and Stress corrosion cracking chloride content or by increasing welding. Figure 7 is a representation The phenomenon of stress cor- solution velocity, more highly al- of what can happen. As is seen, rosion cracking (Fig. 8) of austenitic loyed grades may be used to control corrosion has proceeded from the stainless steels in chloride-containing pitting and crevice corrosion. This is surface down the grain boundaries environments is not unique to usually done by adjusing chromium to the extent that the grains can stainless steels. Many types of alloys and nickel content and adding more become detached and the surface is are susceptible to similar effects in molybdenum (and, to a lesser ex- sometimes said to have “sugared”. different media, such as brass alloys

JULY 2000 - Dairy, Food and Environmental Sanitation 511 Figure 7. Intergranular attack in a sensitized austenitic alloy produced by exposure to a boiling sulfuric acid-ferric sulfate solution. Prolonged exposure causes grains to detach from surface. (100x)

Figure 8. Transgranular chloride stress corrosion cracking of an austenitic stainless steel cracking is characteristically transgranular (across the grains). In poorly heat-treated and in weld heat-affected zones where carbides have precipitated at grain boundaries, the cracking is intergranular (at the grain boundaries). Minimum levels of chloride content, temperature, and stress are not known, because these variables are interrelated. The phenomenon is usually controlled by proper alloy selection, although altering the environment and reducing residual stresses can sometimes be effective. In general, ferritic and duplex stainless steels have more resistance to chloride stress corrosion cracking and are often substituted. Austenitic iron-nickel-chromium alloys also have increased resistance at nickel contents above 20%. In fact, some of the 6-7% Mo alloys with 17-23% Cr and 17-26% Ni have good resistance to chloride stress corrosion cracking. However, virtual immunity is probably found in ammoniacal environments and ture. If these are moderate to low, in austenitic alloys only when carbon and alloy steels, including oxygen is also required for stress nickel levels are above 35% (4). stainless steels, in strongly corrosion cracking to occur. The alkaline solutions. Chloride stress necessary tensile stresses are corrosion cracking, the most almost always residual rather than Galvanic corrosion common form of environmentally applied. It is not the load Galvanic corrosion, or dissimi- induced cracking in austenitic put on a stainless steel vessel lar metal corrosion, is usually not a stainless steels, requires the that leads to cracking, but how problem for stainless steels but can presence of chloride ions, tensile it is formed and welded. In affect other metals in contact with stresses, and elevated tempera- properly annealed material, the them. For galvanic corrosion to take

512 Dairy, Food and Environmental Sanitation -JULY 2000 stainless steel sheet or plate. Some- TABLE 1. Galvanic series of some metals and alloys in sea water. times, when the film is disrupted, Metal or Alloy Potential vs. SHE1 stainless steel can become the active metal (as shown in Table 1) and Active (anodic) Magnesium -1.49 corrode in an active manner. Also, once pitting and crevice corrosion Zinc -0.81 begin, these forms can be considered galvanic corrosion. In both cases, the result is a small active area (the pits Cadmium -0.64 or the crevices) surrounded by a large area of film-protected, inactive Aluminum -0.61 stainless steel. Most galvanic corrosion problems can be avoided Steel -0.3 8 by proper design or electrical insula- tion. S30400 Stainless Steel (active) -0.36

DESIGN AND SELECTION OF Lead -0.32 STAINLESS STEEL EQUIPMENT

Tin -0.27 By factoring the properties of stainless steel into the design of Admiralty Brass -0.12 equipment, a great number of ben- efits can be realized. Unwanted corrosion can be prevented and product Hydrogen 0.00 purity ensured. Because stainless steels are easy to clean and main- Copper +0.02 tain, a number of different products can be produced in the same equip- Nickel +0.10 ment. If properly utilized, equipment made of stainless steel can be N04400 (Monel Ni/Cu Alloy) +0.13 expected to last for many years. In selecting austenitic stainless Titanium +0.14 steels, a number of factors other than corrosion performance should be considered. Among these are S30400 Stainless Steel (passive) +0.15 their usually attractive appearance, good mechanical properties, and Silver +0.16 excellent fabrication characteristics. On a life-cycle basis, the alloys are Graphite +0.49 often the most cost-effective. The common alloys are usually readily Platinum +0.50 available. They are a valuable recycling product and because of Passive (cathodic) Gold +0.50 their lack of reactivity do not contaminate the environment. Recently many of the low carbon 1SHE: Standard Hydrogen Electrode grades have been “dual” certified. That is, they are guaranteed to have not only low carbon contents but also the mechanical properties of the higher carbon grade. place, two or more metals of different active (-) or more passive (+) with Tables 2 and 3 give the electrochemical activity need to be in regard to it. In this table, the more nominal chemical composition and intimate contact in an electrolyte negative or active metals (at the top minimum mechanical properties solution. An abbreviated galvanic of the table) will corrode preferen- of some representative wrought series, or electrochemical activity se- tially to any less active metal to stainless steel alloys. The ries, of materials in seawater is given which they are electrically coupled. If compositions are for wrought alloys in Table 1 (5). The standard hydrogen the surface area of the active compo- and are taken from Metals and electrode is used as a reference nent is small in relation to that of the Alloys in the Unified Numbering against which electrochemical activ- other member of the couple, the System (1). The mechanical ity of a material is measured: The corrosion rate can be very high. Such properties are also for wrought activity of hydrogen is set at zero and would be the case if carbon steel alloys and are from the Steel Prod- other materials are measured as more bolts or rivets were used to connect ucts Manual of the Iron and Steel

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TABLE 2. Chemical composition of some common stainless steels.

(Composition in Weight Per Cent - Balance Iron)

3 UNS EN AISI ACI C Cr Mo Ni Structure Number Number Type Type

5174001 1.4542 17-4PH2 CB-7CU-1 .07max 15.0-17.5 - 3.0-5.0 PH

541000 1.4006 410 CA-15 .15max 11.5-13.5 - - Mart

S43000 1.4016 430 - .12max 16.0-18.0 - - Ferr

S30400 1.4301 304 CF-8 .08max 18.0-20.0 - 8.0-10.5 Aus

S30403 1.4306 304L CF-3 .03max 18.0-20.0 - 8.0-12.0 Aus

S31600 1.4401 316 CF-8M .08max 16.0-18.0 2.0-3.0 10.0-14.0 Aus

S31603 1.4404 316L CF-3M .03max 16.0-18.0 2.0-3.0 10.0-14.0 Aus

S31703 1.4438 317L CG-3M .03max 18.0-20.0 3.0-4.0 11.0-15.0 Aus

N08904 1.4539 904L2 CN-3M .02max 19.0-23.0 4.0-5.0 23.0-28.0 Aus

S318031 1.4462 22052 CD3MN .03max 21.0-23.0 2.5-3.5 4.5-6.5 Dup

S32205 1.4462 2205N2 CD3MN .03max 22.0-23.0 3.0-3.5 4.5-6.5 Dup

1S17400 also contains 3.0-5.0% Copper and .15-.45% Niobium (Columbium).

S31803 also contains .08-.20% Nitrogen.

S32205 also contains .14-.20% Nitrogen.

2These are not AISI Types, but the common names used in North America.

3Structure names are abbreviated. PH is a Precipitation Hardening Martensite, Mart is Martensite, Ferr is Ferrite, Aus is Austenite and Dup is Duplex (Ferrite + Austenite).

Society (2). In general, mechanical Si. Except for slightly higher amounts purity levels in the various systems properties are not the critical factor in of Cr, Mn and Ni, only silicon is can also be different, but not to any selecting stainless steels, but they are noticeably higher, at 2%. This increased significant degree. more than adequate for most uses. silicon is permitted for higher fluidity Almost all of these wrought alloys and better casting properties in the COMMON STAINLESS STEEL have cast counterparts, which differ liquid phase. Similarly, S31600 (316) ALLOYS only slightly in chemical composition has a cast version, J92900 (CF-8M), and in mechanical properties. These which has similar variations permitted. The following list of some of the are indicated by the ACI numbers. Castings also are heat-treated to more common stainless steel alloys For example, S30400 (304) has a cast produce a small amount of ferrite in the currently in use is not complete, but version, J92600 (CF-8). The wrought microstructure, which reduces cracking it gives examples of the various alloy has a composition of 0.08% during welding. The EN-numbered grades of alloys. max C, 18-20% Cr, 2% max Mn, alloys may also differ slightly S43000 (430). This common fer- 8-10.5% Ni, 1% max Si. The cast in chemical composition and me- ritic, iron-chromium stainless steel is alloy has 0.08% max C, 18-21% Cr, chanical properties but are very used for applications such as 1.5% max Mn, 8-11 % Ni, 2% max similar. Minor alloying elements and im- tableware and appliance trim where

514 Dairy, Food and Environmental Sanitation -JULY 2000 TABLE 3. Minimum mechanical properties of some common wrought stainless steels alloys are in the annealed condition except where noted. UNS EN AISI Yield Tensile Elongation Number Number Type Strength3 Strength Mpa(ksi) Mpa (ksi) %

S174001 1.4542 17-4PH2 1172 (170) 1310 (190) 10

S41000 1.4006 410 207 (30) 448 (65) 22

S43000 1.4016 430 207 (30) 448 (65) 22

S30400 1.4301 304 207 (30) 517 (75) 40

S30403 1.4306 304L 172 (25) 483 (70) 40

S31600 1.4401 316 207 (30) 517 (75) 40

S31603 1.4404 316L 172 (25) 483 (70) 40

S31703 1.4438 317L 207 (30) 517 (75) 40

N08904 1.4539 904L2 220 (31) 490 (71) 35

S31803 1.4462 22057 450 (65) 620 (90) 25

S32205 1.4462 2205N2 450 (65) 620 (90) 25

1Solution annealed at 927°C (1700°F), cooled and hardened at 482°C (900°F) for 1 hour, and air cooled.

2These are not AISI Types, but common names used in North America.

3Stainless steels do not have a true yield strength as do carbon and low alloy steels. This property has been measured at the 0.2% offset strength on the stress/strain curve for stainless steels.

extensive welding and forming are not is fabricated by welding and cannot be used for special applications in pulp required and low cost is desired. subsequently annealed. and paper, food and beverage, and S30400 (304). The most widely S31600 (316). This most popular chemical process industries. used of all stainless steels, this is an austenitic iron-chromium-nickel- S31803 (2205). This example of a austenitic iron-chromium-nickel alloy. molybdenum stainless steel has cor- duplex, austenitic-ferritic iron- S30400 finds applications in a broad rosion resistance superior to that of chromium – nickel - molybdenum-ni spectrum of industries including S30400 (304), particularly where trogen stainless steel has good resis- beverage, food, pharmaceutical, pitting and crevice corrosion may be a tance to chloride stress corrosion petroleum refining, consumer product, problem. cracking. A more controlled chemistry electric power, chemical process and S31603 (316L). This low-carbon version, S32205, is commonly architecture. It has good corrosion version of S31600 (316) has inter- available. Both have higher strength resistance in a wide range of granular corrosion resistance similar to than either the austenitic or ferritic environments as well as good that of S30403 (304L). It is suggested grades. formability, weldability, and moderate where welding is required and N08904 (904L). This material, a cost. improved corrosion resistance is very low-carbon austenitic iron-chro- S30403 (304L). This low-carbon desired. mium-nickel-molybdenum-copper version of S30400 (304) has superior S31703 (317L). The higher mo- stainless steel, has corrosion resistance resistance to intergranular corrosion lybdenum, low-carbon version of superior to that of S31703 (317L). following welding or stress relieving S31600, with even better resistance to The addition of about 1.5% and is suggested for equipment that pitting and crevice corrosion is copper improves resistance to cor-

JULY 2000 - Dairy, Food and Environmental Sanitation 515 rosion in some acids. N08904 may be purchased “off-the-shelf” from ware- COMMON SURFACE FINISHES available only on special order from houses and producers in standard ON STAINLESS STEEL selected mills. shapes and sizes, but less common alloys often require special requests The product forms mentioned and long delays. above are commercially available in OTHER STAINLESS STEEL Plate is a flat-rolled product over various surface finishes, most of ALLOYS 254 mm (10 in) in width and over which are described in ASTM Speci- fication A480/A480M - 96a (3). It is As mentioned previously, there 4.76 mm (0.1875 in) in thickness. It is produced from hot-rolled material generally necessary to specify a par- are many stainless steel alloys other ticular surface finish when ordering than the ones discussed in this paper and has a relatively rough surface finish compared to cold-rolled, or stainless steel products and equip- and shown in Tables 2 and 3. One of ment. The different finishes are de- them, S30300 (303), has sulfur added cold-rolled and polished, sheet or strip. scribed by a system of numbers, let- to it to improve machinability. ters and, sometimes, words. How- However, corrosion resistance suffers Sheet is a flat-rolled product 610 mm (24 in) and over in width and ever, a given finish is often produced greatly, especially at sites of sulfide by different sequences and methods or similar inclusions. Other under 4.76 mm (0.1875 in) in thickness. of operation by different producers. It compositions, such as S30900 (309) may be important to know these and S31000 (310) and their Strip is also a flat-rolled product, but it is under 610 mm (24 in) in processing steps, if surface finish or variations, contain increased chro- appearance is critical. A few standard mium and nickel to improve their width and, like sheet, under 4.76 mm (0.1875 in) in thickness. finishes in common use are defined strength and corrosion resistance at below. high temperatures. Cast alloys such Bar or rod are straight lengths that can be round, oval, square, rect- No. 1 Finish or HRAP. Hot- as J92600 and J92620 are basically rolled, annealed and pickled (chemi- S30400 (304) and S30403 (304L) angular, or other in cross-section. They are produced by a number of cally descaled) is the common finish with up to 2% added silicon to in- on stainless steel plate. Other finishes crease fluidity in the liquid phase different methods such as hot-rolling, forging, extruding, and/or cold- must be specially requested for and improve casting properties. product over 3/16 in. in thickness. Another group of stainless steel drawing. Wire is usually round or oval in This finish is rougher and may have alloys to which we have previously more defects than the cold-rolled referred but that are not in Tables 2 cross-section. It is a cold-reduced product that is drawn from small and/or abraded finishes to be de- and 3 are those containing 6-7% mo- scribed. It is generally used in indus- lybdenum. These so-called “super diameter bars or rods. Tubing of various types are hol- trial applications where smoothness austenitic stainlesses” also contain is not particularly important. about 17-23% Cr and 17-26% Ni, low products, round or any other shape in cross-section. They are made No. 2B Finish. This bright, cold- with some variations. There are six or rolled finish is produced when an- eight alloys in this class, some of from sheet or strip and can be either seamless or welded. nealed and descaled flat products which contain nitrogen or other receive a final light cold rolling pass elements. They are mostly prop- Pipe is often a welded, relatively large diameter, hollow, round product on polished rolls. This general pur- rietary to their manufacturers and it is pose finish can be used as is or for difficult to choose between them; it made from strip, sheet or plate, it can also be extruded from billets and be products to be subsequently polished, does not seem fair to emphasize one and is most often seen on sheet and over the others. Their main attributes seamless. Shapes is a catchall term that strip products. are their resistance to pitting and No. 4 Finish. This is a general- crevice corrosion. In most cases they includes a wide variety of angles, U-sections, and similar forms pro- purpose polished finish primarily are superior to lower-molybdenum used on sheet and strip for a wide alloys in saline solutions at ambient duced by rolling or extruding. Fittings, flanges, forgings, etc. variety of industrial and consumer and slightly elevated temperature. As products. To produce it, a 2B finish mentioned before, they also have are specialty products that are widely available in numerous standard and surface is initially ground with useful resistance, but not immunity, coarser abrasives but is polished last to chloride stress corrosion cracking. non-standard sizes. Castings refer to the cast coun- with abrasives of approximately 120 terparts of most of the common to 150 mesh. It is commonly called a grades of wrought stainless steel. The “brushed” finish. It does not show AVAILABLE PRODUCT FORMS fingerprints or water spots as readily There is considerable composition of these may be slightly altered to ensure good castability and as unabraded finishes. variation in the availability of all Electropolished Finish. Surface alloys in all product forms. The properties, but their corrosion resistance is comparable to the material is electrochemically dis- more common materials such as S30400, S30403, S31600, equivalent wrought products. The and S31603 can usually be shapes and applications are almost limitless.

516 Dairy, food and Environmental Sanitation -JULY 2000 solved, leaving a bright, mirror-like ABOUT THE AUTHORS REFERENCES appearance. Some people believe that 1. Anonymous. 1999. ASTM and SAE, electropolished items are easier to *Covert Consulting, Inc., 94 Metals and alloys in the unified numbering system, HS-1086, 8th ed., SAE clean and sanitize and have better Deepdale Dr., Middletown, NJ 07748; Phone: 732.671.0601; Fax: & ASTM. Warrendale, PA. corrosion resistance. This finish is 2. Anonymous. 1999. Steel products widely used for process equipment in 732.671.0602; Tuthill Associates, manual. Stainless and heat resisting the food and beverage industries. Inc., P.O. Box 204, Blacksburg, VA steels. Iron and steel society of AIME. Options other than these four 24060; Phone: 540.953.2626; Fax: Warrendale, PA. 540.953.2636. 3. ASTM. 1997. Standard specification finishes and those in ASTM A480 for general requirements for flat-rolled 480M96a (3) may be requested. stainless and heat resisting steel plate, Some of them are produced by roll- ACKNOWLEDGMENTS sheet and strip. West Conshohocken, ing, some by abrading with different The authors express their ap- PA. size grit, and some by a combination preciation to the Nickel Development 4. Copson, H. R. (ed.). 1959. Physical Institute for its support, en- metallurgy of stress corrosion fracture. of the two processes. Mirror-like fin- Interscience, New York. ishes similar to electropolished ones couragement and guidance in pre- 5. Dillon, C. P. 1986. Corrosion control in can be produced by abrading with paring this paper. We also thank our the chemical process industries, very fine polishing grit or com- numerous colleagues who have pro- McGraw-Hill, New York. pounds. Embossed patterns made vided suggestions, information and 6. Sedricks, A. J. 1979. Corrosion of other assistance in reviewing the stainless steels, John Wiley & Sons, with special rolls are common. New York. manuscript.

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