ASM Metals Handbook - Desk Edition (ASM_ 1998) WW part 6 pdf

Martensitic stainless steels are specified when the application requires good tensile strength, creep, and fatigue strength properties, in combination with moderate corrosion resistance

indicates that yield and tensile strengths are virtually identical from room temperature to 600 °C (1100 °F), but that ductility is moderately lower in cast material Hot hardness of cast H13 is higher than that of wrought H13 at temperatures above about 300 °C (about 600 °F); this hardness advantage increases with temperature and measures about eight points

on the HRC scale at 650 °C (1200 °F)

Because cast dies exhibit uniform properties in all directions, no problem of directionality (anisotropy) exists Dimensional control of castings is very consistent after an initial die is made and any necessary corrections are incorporated in the pattern Reasonable finishing allowances are 0.25 to 0.38 mm (0.010 to 0.015 in.) on the impression faces, 0.8 to 1.6 mm ( to in.) at the parting line of the mold, and 1.6 to 3.2 mm ( to in.) on the back and outside surfaces The hot-work tool steels most commonly cast include H12, H13, H21, and H25

Specialty Tool Steels

Through-Hardening Stainless Steels. Type 420 martensitic stainless steel (and modifications of this alloy) is

commonly used for injection molds for all thermoplastic materials It is particularly adaptable for molding vinyls or other corrosive plastics, or when the atmospheric or storage conditions are unusually severe, because it does not require chromium plating to resist these types of corrosive attack Other stainless steels used for plastic molds include type 414, free-machining grade 420F, and Elmax, a high-hardness (58 to 60 HRC) wear-resistant P/M grade Chemical compositions of these stainless steels are given in Table 8

Table 8 Chemical compositions of martensitic stainless steel plastic mold materials

(a) P/M stainless steel produced by hot isostatic pressing of gas-atomized

stainless steel powder

Type 440C martensitic stainless steel, both in wrought and P/M versions, is also used for some cold-work applications CPM 440V is a high-vanadium, high-chromium tool steel for applications requiring both high wear resistance and good corrosion resistance The composition of this material (Table 7) is essentially that of wrought type 440C to which about 5.75% V and increased carbon have been added to improve wear resistance

Maraging Steels. Certain high-nickel maraging steels are being used for special noncutting tool applications; 18Ni(250) is the type most frequently used However, for the most demanding applications, the higher-strength 18Ni(300)

is often preferred For applications requiring maximum abrasion resistance, any of the maraging steels can be nitrided

Maraging steels achieve full hardness nominally 500 HRC for 18Ni(250), 54 HRC for 18Ni(300), and 58 HRC for 18Ni(350) by a simple aging treatment, usually 3 h at about 480 °C (900 °F) Because hardening does not depend on cooling rate, full hardness can be developed uniformly in massive sections, with almost no distortion Decarburization is

of no concern in these alloys because they do not contain carbon as an alloying element If the long-time service temperature exceeds the aging temperature, maraging steels overage with a significant drop in hardness

The 18Ni(250) grade is used for aluminum die-casting dies and cores, aluminum hot forging dies, dies for molding plastics, and various support tooling used in extrusion of aluminum In die casting of aluminum, maraging steel dies can

be used at higher hardness than is possible for dies made of H13 tool steel because maraging steel is not as prone to heat checking Because the aging process results in very little size change, it is possible to machine the intricate impressions for plastic molding dies to final size prior to final hardening

For molding extremely abrasive types of plastics, the higher surface hardness provided by 18Ni(300) maraging steel is desirable

Wrought Stainless Steels: Selection and Application

Introduction

STAINLESS STEELS are iron-base alloys that contain a minimum of approximately 11% Cr, the amount needed to prevent the formation of rust in unpolluted atmospheres (hence the designation stainless) Few stainless steels contain more than 30% Cr or less than 50% Fe They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film This oxide forms and heals itself in the presence of oxygen Other elements added to improve particular characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades Figure 1 provides a useful summary of some of the compositional and property linkages in the stainless steel family

Fig 1 Compositional and property linkages in the stainless steel family of alloys

Production of stainless steels is a two-stage process involving the melting of scrap and ferro-alloys in an electric-arc furnace followed by refining by oxygen-inert gas injection (argon oxygen decarburization) or oxygen injection under vacuum (vacuum oxygen decarburization) to adjust carbon content and remove impurities (Both of these processes are described in the Section "Iron and Steelmaking Practices" in this Handbook.) The refined molten metal is then poured into molds to form ingots, followed later by blooming or slabbing, or is poured directly into a continuous casting machine to form slabs, blooms, or billets Cast ingots can be rolled or forged; and flat products (sheet, strip, and plate) can be produced from continuously cast slabs The rolled product can be drawn, bent, extruded, or spun Stainless steels can be further shaped by machining, and they can be joined by welding, brazing, soldering, and adhesive bonding Stainless steels can also be used as an integral cladding on plain carbon or low-alloy steels, as well as some nonferrous metals and alloys

Stainless steels are used in a wide variety of applications Most of the structural applications occur in the chemical and power engineering industries, which account for more than a third of the market for stainless steel products (see the following table) These applications include an extremely diversified range of uses, including nuclear reactor vessels, heat exchangers, oil industry tubulars, components for chemical processing and pulp and paper industries, furnace parts, and boilers used in fossil fuel electric power plants The relative importance of the major fields of application for stainless steel products are as follows:

Industrial equipment

Chemical and power engineering 34

Food and beverage industry 18

Consumer goods

Domestic appliances, household utensils 28

Small electrical and electronic appliances 6

Some of these applications involve exposure to either elevated or cryogenic temperatures; austenitic stainless steels (see the following discussion) are well suited to either type of service

Designations for Stainless Steels

In the United States, wrought grades of stainless steels are generally designated by the American Iron and Steel Institute (AISI) numbering system, the Unified Numbering System (UNS), or the proprietary name of the alloy In addition, designation systems have been established by most of the major industrial nations Of the two institutional numbering systems used in the U.S., AISI is the older and more widely used Most of the grades have a three-digit designation; the

200 and 300 series are generally austenitic stainless steels, whereas the 400 series are either ferritic or martensitic Some

of the grades have a one- or two-letter suffix that indicates a particular modification of the composition

The UNS system includes a considerably greater number of stainless steels than AISI because it incorporates all of the more recently developed stainless steels The UNS designation for a stainless steel consists of the letter S, followed by a five-digit number For those alloys that have an AISI designation, the first three digits of the UNS designation usually correspond to an AISI number When the last two digits are 00, the number designates a basic AISI grade Modifications

of the basic grades use two digits other than zeroes For stainless steels that contain high nickel contents ( 25 to 35% Ni), the UNS designation consists of the letter N followed by a five-digit number Examples include N08020 (20Cb-3), N08024 (20Mo-4), N08026 (20Mo-6), N08366 (AL-6X), and N08367 (AL-6XN) Although classified as nickel-base alloys by the UNS system, the previously mentioned materials constitute the "superaustenitic" category of stainless steel shown in Fig 1 and described in the following section "Classification of Stainless Steels."

Classification of Stainless Steels

Stainless steels can be divided into five families Four are based on the characteristic crystallographic structure/microstructure of the alloys in the family: martensitic, ferritic, austenitic, or duplex (austenitic plus ferritic) The fifth family, the precipitation-hardenable alloys, is based on the type of heat treatment used, rather than microstructure

Martensitic Stainless Steels

Characteristics and Compositions. Martensitic stainless steels are essentially Fe-Cr-C alloys that possess centered tetragonal (bct) crystal structure (martensitic) in the hardened condition They are ferromagnetic, hardenable by heat treatments, and generally resistant to corrosion only in relatively mild environments Chromium content is generally

body-in the range of 10.5 to 18%, and carbon content can exceed 1.2% The chromium and carbon contents are balanced to ensure a martensitic structure Elements such as niobium, silicon, tungsten, and vanadium can be added to modify the tempering response after hardening Small amounts of nickel can be added to improve corrosion resistance in some media and to improve toughness Sulfur or selenium is added to some grades to improve machinability Table 1 provides chemical compositions for standard (AISI) and nonstandard grades

Table 1 Chemical compositions of martensitic stainless steels

S43100 431 0.20 1.00 1.00

15.0-17.0

2.50

0.30- 0.50

0.10- 12.5

11.0- 0.80

(b) Optional

(c) German (DIN) specification

Properties and Applications. The most commonly used alloy within the martensitic stainless steel family is type

410, which contains approximately 12 wt% Cr and 0.1 wt% C to provide strength The carbon level and, consequently, strength increase in the 420, 440A, 440B, and 440C alloy series The latter three alloys, in particular, have an increased chromium level in order to maintain corrosion resistance Molybdenum can be added to improve mechanical properties or corrosion resistance, as it is in type 422 stainless steel Nickel can be added for the same reasons in types 414 and 431 When higher chromium levels are used to improve corrosion resistance, nickel also serves to maintain the desired microstructure and to prevent excessive free ferrite The limitations on the alloy content required to maintain the desired fully martensitic structure restrict the obtainable corrosion resistance to moderate levels

In the annealed condition, martensitic stainless steels have a tensile yield strength of approximately 275 MPa (40 ksi) and can be moderately hardened by cold working However, martensitic alloys are typically heat treated by both hardening and tempering to yield strength levels up to 1900 MPa (275 ksi), depending primarily on carbon level These alloys have good ductility and toughness properties, which decrease as strength increases Depending on the heat treatment, hardness values range from approximately 150 HB (80 HRB) for materials in the annealed condition to levels greater than 600 HB (58 HRC) for fully hardened materials

Martensitic stainless steels are specified when the application requires good tensile strength, creep, and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance up to approximately 650 °C (1200 °F)

In the United States, low- and medium-carbon martensitic steels (for example, type 410 and modified versions of this alloy) have been used primarily in steam turbines, jet engines, and gas turbines In Europe, alloy HT9 (12Cr-1Mo-0.3V) has been widely used in elevated-temperature, pressure-containment applications, including steam piping and steam generator reheater and superheater tubing used in fossil fuel power plants Type 420 and similar alloys are used in cutlery, valve parts, gears, shafts, and rollers Other applications for higher carbon-level grades (type 440 grades) include cutlery, surgical and dental instruments, scissors, springs, valves, gears, shafts, cams, and ball bearings

Ferritic Stainless Steels

Characteristics and Compositions. Ferritic stainless steels are essentially iron-chromium alloys with body-centered cubic (bcc) crystal structures Chromium content is usually in the range of 11 to 30% Some grades may contain molybdenum, silicon, aluminum, titanium, and niobium to confer particular characteristics Sulfur or selenium can be added to improve machinability Table 2 lists compositions of ferritic stainless steels

Table 2 Chemical compositions of ferritic stainless steels

S44400 Type 444 0.02 18 bal 2 0.4 0.02 0.5Ti

S44660 SEA-CURE 0.02 27.5 bal 3.4 1.7 0.025 0.5Ti

S44635 Nu Monit 0.025 25 bal 4 4 0.025 0.4Ti

Properties and Applications. Typical annealed yield and tensile strengths for ferritic stainless steels are 35 to 55 ksi (240 to 380 MPa) and 60 to 85 ksi (415 to 585 MPa), respectively Ductilities tend to range between 20 and 35% Higher strengths, up to 75 ksi (515 MPa) for yield strength and 95 ksi (655 MPa) for tensile strength, are obtained in the more highly alloyed "superferritic" steels shown in Fig 1

Whereas the martensitic stainless steels offer only moderate corrosion resistance, that of the ferritic stainless steels can range from moderate for the low-to-medium, chromium-content alloys to outstanding for the superferritics such as type

444 and UNS No S44627, S44635, S44660, S44700, and S44800 The low-chromium (11%) alloys, such as types 405 and 409, have fair corrosion and oxidation resistance and good fabricability at low cost Type 409, the most widely used ferritic stainless steel, has gained wide acceptance for use in automotive exhaust systems The intermediate-chromium (16

to 18%) alloys include type 430, which resists mild oxidizing acids and organic acids and is used in food-handling equipment, and type 434, which includes a molybdenum addition for improved corrosion resistance and is used for automotive trim The high-chromium (19 to 30%) alloys, which include types 442 and 446 as well as the superferritics, are used for applications that require a high level of corrosion and oxidation resistance By controlling interstitial element content via argon oxygen decarburization (AOD) processing, it is possible to produce grades with unusually high chromium and molybdenum (up to 4.5%) contents and very low carbon contents (as low as 0.01%) Such highly alloyed superferritics offer exceptional resistance to localized corrosion induced by exposure to aqueous chlorides Localized corrosion, such as pitting, crevice corrosion, and stress-corrosion cracking (SCC) are problems that plague many austenitic stainless steels Therefore, the superferritics are often used in heat exchangers and piping systems for chloride-bearing aqueous solutions and seawater

Austenitic Stainless Steels

Characteristics and Compositions. Austenitic stainless steels constitute the largest stainless family in terms of number of alloys and usage Like the ferritic alloys, they cannot be hardened by heat treatment However, their similarity ends there The austenitic stainless steels are essentially nonmagnetic in the annealed condition and can be hardened only

by cold working They usually possess excellent cryogenic properties and good high-temperature strength and oxidation resistance Chromium content generally varies from 16 to 26%; nickel content is less than or equal to approximately 35%; and manganese content is less than or equal to 15% The 200 series steels contain nitrogen, 4 to 15% Mn, and lower nickel contents (up to 7% Ni) The 300 series steels contain larger amounts of nickel and up to 2% Mn Molybdenum, copper, silicon, aluminum, titanium, and niobium can be added to confer certain characteristics, such as halide pitting resistance or oxidation resistance Table 3 provides chemical compositions for standard (AISI) and nonstandard grades

Table 3 Chemical compositions of austenitic stainless steels

10.5-0.045 0.03

19.0-21.0

12.0

10.0-0.045 0.03

22.0-24.0

15.0

12.0-0.045 0.03

S30908 309S 0.08 2.0 1.00

22.0-24.0

15.0

12.0-0.045 0.03

24.0-26.0

22.0

19.0-0.045 0.03

S31008 310S 0.08 2.0 1.50

24.0-26.0

22.0

19.0-0.045 0.03

1.5-3.0

26.0

23.0- 22.0

19.0-0.045 0.03

16.0-18.0

14.0

10.0-0.045 0.03 2.0-3.0 Mo

S31620 316F 0.08 2.0 1.00

16.0-18.0

14.0

10.0-0.20 0.10 min

10.0-0.045 0.03 2.0-3.0 Mo

S31603 316L 0.03 2.0 1.00

16.0-18.0

14.0

10.0-0.045 0.03 2.0-3.0 Mo

S31653 316LN 0.03 2.0 1.00

16.0-18.0

14.0

10.0-0.045 0.03 2.0-3.0 Mo; 0.10-0.16 N

S31651 316N 0.08 2.0 1.00

16.0-18.0

14.0

10.0-0.045 0.03 2.0-3.0 Mo; 0.10-0.16 N

18.0-20.0

15.0

11.0-0.045 0.03 3.0-4.0 Mo

S31703 317L 0.03 2.0 1.00

18.0-20.0

15.0

17.0- 37.0

3.00- 18.0

15.0- 6.00

0.3- 18.5

17.0-0.75 0.045 0.030 0.35 N

17.0- 15.5

14.0-0.020 14.0-0.020 0.2 Mo

S30615 RA 85 H(c) 0.20 0.80 3.50 18.5 14.50 1.0 Al

20.0- 12.0

12.0-0.045 0.030 10 × %C min to 1.10 max Nb

S31040 Type 310 Cb 0.08 2.00 1.50

24.0-26.0

22.0

10.00-0.030 0.015 3.00-4.00 Cu; 2.00-3.00 Mo

S31725 Type 317 LM 0.03 2.00 1.00

18.0-20.0

17.5

13.5-0.045 0.030 4.0-5.0 Mo; 0.10 N

S31726 17-14-4 LN 0.03 2.00 0.75

17.0-20.0

17.5

13.5-0.045 0.030 4.0-5.0 Mo; 0.10-0.20 N

S31753 Type 317 LN 0.03 2.00 1.00

18.0-21.0

15.0

11.0-0.030 11.0-0.030 0.10-0.22 N

S37000 Type 370

0.03-0.05

2.35

1.65- 1.0

0.5- 14.5

12.5- 16.5

17.0- 18.5

17.5-0.030 17.5-0.030

0.48-0.58

10.00

8.00-0.25

20.0-22.0

4.50

3.25-0.030 0.09

1.50-0.050 0.09

0.04-0.20-0.40 N

S63017 21-12N

0.15-0.25

1.50

1.00- 1.25

0.70- 22.0

20.0- 12.50

10.50-0.03 0.03 0.15-0.25 N

0.28-0.38

3.50

1.50- 0.90

0.60- 24.0

22.0-7.0-9.0 0.04 0.015 0.28-0.35 N; 0.50 Co

S63198 19-9 DL

0.28-0.35

1.50

0.75- 0.8

0.03- 21.0

33.0-0.03 0.03 5.00-6.70 Mo; 2.00-4.00 Cu

N08028 Sanicro 28 0.02 2.00 1.00

26.0-28.0

32.5

29.5-0.020 0.015 3.0-4.0 Mo; 0.6-1.4 Cu

N08366 AL-6X 0.035 2.00 1.00

20.0-22.0

25.5

23.5-0.030 23.5-0.030 6.0-7.0 Mo

N08367 AL-6XN 0.030 2.00 1.00

20.0-22.0

25.50

23.50-0.040 0.030 6.0-7.0 Mo; 0.18-0.25 N

N08700 JS-700 0.04 2.00 1.00

19.0-23.0

26.0

24.0-0.040 0.030 4.3-5.0 Mo; 8 × %C min to

0.5 max Nb; 0.5 Cu; 0.005 Pb; 0.035 S

N08800 Type 332 0.01 1.50 1.00

19.0-23.0

35.0

30.0-0.045 0.015 0.15-0.60 Ti; 0.15-0.60 Al

N08904 904L 0.02 2.00 1.00

19.0-23.0

28.0

23.0-0.045 0.035 4.0-5.0 Mo; 1.0-2.0 Cu

N08925 Cronifer 1925 hMo 0.02 1.00 0.50

24.0-26.0

21.0

Properties and Applications. The yield strengths of chromium-nickel austenitic stainless steels are rather modest and are comparable to those of mild steels Typical minimum mechanical properties of annealed 300 series steels are yield strengths of 205 to 275 MPa (30 to 40 ksi), ultimate tensile strengths of 520 to 760 MPa (75 to 110 ksi), and elongations

of 40 to 60% Annealed 200 series alloys have higher yield strengths ranging from 345 to 480 MPa (50 to 70 ksi) Higher strengths are possible in cold-worked forms, especially in drawn wire, in which a tensile strength of 1200 MPa (175 ksi)

or higher is possible Figure 2 compares the work-hardening characteristics of 300 series and type 430 (ferritic) grades The 200 series have work-hardening characteristics similar to types 301 and 302 in Fig 2

Fig 2 Typical effect of cold rolling on the tensile strength of selected stainless steels

Even the leanest austenitic stainless steels (e.g., types 302 and 304) offer general corrosion resistance in the atmosphere,

in many aqueous media, in the presence of foods, and in oxidizing acids such as nitric acid Types 321 and 347 are essentially type 304 with additions of either titanium or niobium, respectively, which stabilize carbides against sensitization (see the subsection "Thermally Induced Embrittlement" ) The addition of molybdenum in types 316/316L (Fig 1) provides pitting resistance in phosphoric and acetic acids and dilute chloride solutions, as well as corrosion resistance in sulfurous acid An even higher molybdenum content, as in type 316L (3%), and even richer alloys further enhance pitting resistance Nitrogen is added to enhance strength at room temperature and, especially, at cryogenic temperatures (e.g., type 304N) Nitrogen is also added to reduce the rate of chromium carbide precipitation and, therefore, the susceptibility to sensitization It is also added to molybdenum-containing alloys to increase resistance to chloride-induced pitting and crevice corrosion Higher amounts of chromium and/or nickel are used to enhance high-temperature oxidation resistance (e.g., types 309, 310, and 330) Copper and nickel can be added to improve resistance to reducing acids, such as sulfuric acid (type 320) Some of the more corrosion-resistant alloys, such as N08020 (20Cb-3) have nickel contents high enough (32 to 37%) to rate UNS classification as nickel-base alloys Alloys containing nickel, molybdenum ( 6%), and nitrogen (0.15 to 0.25%) are sometimes referred to as "superaustenitics," as shown in Fig 1 These alloys were developed for improved resistance to chloride corrosion

Duplex Stainless Steels

Characteristics and Compositions. Duplex stainless steels are two-phase alloys based on the Fe-Cr-Ni system These materials typically comprise approximately equal proportions of ferrite and austenite phases in their microstructure and are characterized by their low carbon contents (<0.03%) and additions of molybdenum, nitrogen, tungsten, and copper Typical chromium and nickel contents are 20 to 30% and 5 to 8%, respectively Table 4 gives compositions of duplex stainless steels The specific advantages offered by duplex stainless steels over conventional 300 series stainless steels are strength (approximately twice that of austenitic stainless steels), improved toughness and ductility (compared to ferritic grades), and superior chloride SCC resistance and pitting resistance

Table 4 Chemical compositions of duplex stainless steels

18.0- 5.25

As with all stainless steels, composition also plays a major role in the corrosion resistance of duplex stainless steels Pitting corrosion resistance is most easily affected To determine the extent of pitting corrosion resistance offered by the material, the pitting resistance equivalent (PRE) is commonly used The PRE is calculated by adding the weight percentages of elements that affect pitting corrosion resistance namely, chromium, molybdenum, and nitrogen and then normalizing them with respect to the effect of 1% Cr The most commonly used formula for pitting resistance equivalent is:

PRE = %Cr + 3.3(%Mo) + 16(%N) (Eq 1)

As indicated in the following table, the PRE values for duplex stainless steels range from approximately 24 for grades containing no molybdenum to greater than 40 for the more highly alloyed (Fe-25Cr-7Ni-3.5Mo-0.25N-W-Cu) grades:

UNS number PRE range

Precipitation-Hardening Stainless Steels

Characteristics and Compositions. Precipitation-hardenable (PH) stainless steels are chromium-nickel grades that can be hardened by an aging treatment These grades are classified as austenitic (e.g., A-286), semiaustenitic (e.g., 17-7PH), or martensitic (e.g., 17-4PH) The classification is determined by their solution-annealed microstructure The semiaustenitic alloys are subsequently heat treated so that the austenite transforms to martensite Cold work is sometimes used to facilitate the aging reaction Various alloying elements, such as aluminum, titanium, niobium, or copper, are used

to achieve aging Table 5 lists compositions of PH stainless steels

Table 5 Chemical compositions of precipitation-hardening stainless steels

(b) Typical values

Properties and Applications. Like the hardenable 400 series martensitic stainless steels, PH alloys can attain high tensile yield strengths, up to 1700 MPa (250 ksi) Cold working prior to aging can result in even higher strengths The PH grades generally have good ductility and toughness with moderate-to-good corrosion resistance A better combination of strength and corrosion resistance is achieved than with the 400 series martensitic alloys These improved properties are related to their higher chromium, nickel, and molybdenum contents, as well as their restricted carbon (0.040 max) levels The low carbon content of the martensitic PH stainless steels is especially critical for toughness and good ductility Because of their high strengths, most of the applications for PH stainless steels are in the aerospace and other high-technology industries

Physical and Mechanical Properties of Stainless Steels

The physical properties of stainless steels are quite different from those of commonly used nonferrous alloys such as aluminum and copper alloys However, when comparing the various stainless families with carbon steels, many similarities in properties exist, although there are some key differences Like carbon steels, the density of stainless steels

is 8.0 g/cm3, which is approximately three times greater than that of aluminum alloys (2.7 g/cm3) Like carbon steels, stainless steels have a high modulus of elasticity (200 MPa, or 30 ksi) that is nearly twice that of copper alloys (115 MPa,

or 17 ksi) and nearly three times that of aluminum alloys (70 MPa, or 10 ksi)

Differences among these materials are evident in thermal conductivity, thermal expansion, and electrical resistivity, as well Figure 3 shows the large variation in thermal conductivity between various types of materials; 6061 aluminum alloy (Al-1Mg-0.6Si-0.3Cu-0.2Cr) has a very high thermal conductivity, followed by aluminum bronze (Cu-5Al), 1080 carbon steel, and then stainless steels For stainless steels, alloying additions, especially nickel, copper, and chromium, greatly decrease thermal conductivity

Fig 3 Comparison of thermal conductivity for carbon steel, copper alloy, aluminum, and stainless steels

Thermal expansion (Fig 4) is greatest for type 6061 aluminum alloy, followed by aluminum bronze and austenitic stainless alloys, and then ferritic and martensitic alloys For austenitic stainless alloys, additions of nickel and copper can decrease thermal expansion Stainless steels have high electrical resistivity (Fig 5) Alloying additions tend to increase electrical resistivity Therefore, the ferritic and martensitic stainless steels have lower electrical resistivity than the austenitic, duplex, and PH alloys, but higher electrical resistivity than 1080 carbon steel Electrical resistivity of stainless steels is 7.5 times greater than aluminum bronze and nearly 20 times greater than type 6061 aluminum alloy

Fig 4 Comparison of thermal expansion for carbon steel, copper alloy, aluminum, and stainless steels

Fig 5 Comparison of electrical resistivity for carbon steel, copper alloy, aluminum, and stainless steels

Mechanical Properties. Tables 6 and 7 list tensile properties and toughness for selected stainless alloys representing the five families The grades listed under austenitic alloys have relatively low yield strength, compared with the heat-treatable alloys, but have the highest tensile ductility and toughness The ferritic stainless steels (type 405 and 409) listed have tensile yield strengths similar to those of the austenitic grades but lower values for ultimate tensile strength, ductility, and toughness The duplex stainless steels have twice the tensile yield strength of the austenitic and ferritic grades and approximately half the toughness Again, their toughness is far superior to that of alloys that are heat treated and hardened

Table 6 Minimum room-temperature mechanical properties for selected stainless steels

Tensile strength Yield strength(a)

S35000 AM-350(i) 1140 165 1000 145 2-8 36 HRC (min)

S35500 AM-355(i) 1170 170 1030 150 12 37 HRC (min)

S66286 A-286(j) 896-965 125-140 655 95 4-15 24 HRC (min)

(a) At 0.2% offset

Table 7 Tensile and impact properties of selected stainless steels

Average tensile properties

Yield strength, 0.2% offset

Ultimate tensile strength

Charpy notch impact

The PH alloys, such as the martensitic grades S45500 and S17400, have higher annealed strength and lower ductility than the 400 series martensitic alloys and are aged at temperatures ranging from 480 to 620 °C (895 to 1150 °F) Their strength

is dependent on the hardener (titanium, niobium, and copper), the amount of hardener, and the aging temperatures used Toughness is either similar or superior to the 400 series martensitic alloys at a given strength level

Factors in Selection

The selection of stainless steels can be based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges, and product cost However, corrosion resistance and mechanical properties are usually the most important factors in selecting a grade for a given application

Characteristics to be considered in selecting the proper type of stainless steel for a specific application include:

• Corrosion resistance

• Resistance to oxidation and sulfidation

• Strength and ductility at ambient and service temperatures

• Suitability for intended fabrication techniques

• Suitability for intended cleaning procedures

• Stability of properties in service

• Toughness

• Resistance to abrasion, erosion, galling, and seizing

• Surface finish and/or reflectivity

• Physical property characteristics, such as magnetic properties, thermal conductivity, and electrical resistivity

• Sharpness, or retention of cutting edge

• Rigidity

• Dimensional stability

Corrosion resistance is frequently the most important characteristic of a stainless steel but often is also the most difficult to assess for a specific application General corrosion resistance to pure chemical solutions is comparatively easy

to determine, but actual environments are usually much more complex

General corrosion is often much less serious than localized forms such as SCC, crevice corrosion in tight spaces or under deposits, pitting attack, and intergranular attack in sensitized material such as weld heat-affected zones Such localized corrosion can cause unexpected and sometimes catastrophic failure while most of the structure remains unaffected, and therefore it must be considered carefully in the design and selection of the proper grade of stainless steel Corrosive attack can also be increased dramatically by seemingly minor impurities in the medium that may be difficult to anticipate but that can have major effects, even when present in only parts-per-million concentrations: heat transfer through the steel to

or from the corrosive medium, contact with dissimilar metallic materials, stray electrical currents, and many other subtle factors At elevated temperatures, attack can be accelerated significantly by seemingly minor changes in atmosphere that affect scaling, sulfidation, or carburization

Despite these complications, a suitable steel can be selected for most applications on the basis of experience, perhaps with assistance from the steel producer Laboratory corrosion data can be misleading in predicting service performance Even actual service data have limitations, because similar corrosive media may differ substantially because of slight variations

in some of the corrosion conditions listed previously For difficult applications, extensive study of comparative data may

be necessary, sometimes followed by pilot plant or in-service testing Other important factors that must be considered when selecting a stainless steel for a corrosion application include:

• Chemical composition of the corrosive medium, including impurities

• Physical state of the medium: liquid, gaseous, solid, or combinations thereof

• Temperature

• Temperature variations

• Aeration of the medium

• Oxygen content of the medium

• Bacteria content of the medium

• Ionization of the medium

• Repeated formation and collapse of bubbles in the medium

• Relative motion of the medium with respect to the steel

• Chemical composition of the metal

• Nature and distribution of microstructural constituents

• Continuity of exposure of the metal to the medium

• Surface condition of the metal

• Stresses in the metal during exposure to the medium

• Contact of the metal with one or more dissimilar metallic materials

• Stray electrical currents

• Differences in electric potential

• Marine growth such as barnacles

• Sludge deposits on the metal

• Carbon deposits from heated organic compounds

• Dust on exposed surfaces

• Effects of welding, brazing, and soldering

More detailed information on selection of stainless steels for use in various corrosive environments can be found in the article "Corrosion of Wrought Stainless Steels" in this Section

Mechanical properties at service temperature are obviously important, but satisfactory performance at other temperatures must also be considered Thus, a product for arctic service must have suitable properties at subzero temperatures even though steady-state operating temperatures may be much higher; room-temperature properties after extended service at elevated temperature can be important for applications such as boilers and jet engines, which are intermittently shut down

Wear and Galling Resistance. Stainless steels are characterized as having relatively poor wear and galling resistance, but they are often required for a particular application because of their corrosion resistance Therefore, finding the most effective alloy to withstand wear and galling can be a difficult problem for design engineers Lubricants and coatings are often used to reduce wear, although lubricant use is precluded in many applications, such as high-temperature environments, in which they can break down, or food and pharmaceutical processing equipment, which require sanitation Additionally, a critical part, such as a valve in a power plant, must resist galling or seizing, because it can shut down or endanger the entire plant Tables 8 and 9 present a comparison of wear compatibility and galling resistance of selected stainless steels

Table 8 Wear compatibility of dissimilar-mated stainless steels

S17400 (43 HRC)

S24100 (95 HRB)

S20910 (99 HRB)

S21800 (95 HRB)

Type 440C (57 HRC)

Type 304 16.4

Type 316 13.5 16.4

S17400 31.7 23.7 67.7

Table 9 Threshold galling stress for selected stainless steels

Type 410 Type 416 Type 430 Type 440C Type 303 Type 304 Type 316 S17400 S24100 S21800 Alloy Condition and nominal

Rockwell hardness

MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi MPa ksi

Type 410 Tempered (38 HRC) 21 3 28 4 21 3 21 3 28 4 14 2 14 2 21 3 317 46 345(a) 50

Type 416 Tempered (36 HRC) 28 4 90 13 21 3 145 21 62 9 165 24 290 42 14 2 310 45 345(a) 50

Type 430 Annealed (84 HRB) 21 3 21 3 14 2 14 2 14 2 14 2 14 2 21 3 55 8 248 36

Type 440C Tempered (56 HRC) 21 3 145 21 14 2 76 11 34 5 21 3 255 37 21 3 345(a) 50 345(a) 50

Type 303 Annealed (82 HRB) 28 4 62 9 14 2 34 5 14 2 14 2 21 3 21 3 345(a) 50 345(a) 50

Type 304 Annealed (77 HRB) 14 2 165 24 14 2 21 3 14 2 14 2 14 2 14 2 207 30 345(a) 50

Type 316 Annealed (81 HRB) 14 2 290 42 14 2 255 37 21 3 14 2 14 2 14 2 21 3 262 38

S17400 Aged (84 HRB) 21 3 14 2 21 3 21 3 14 2 14 2 14 2 14 2 345(a) 50 345(a) 50

S21800 Annealed (94 HRB) 345(a) 50 345(a) 50 248 36 345(a) 50 345(a) 50 345(a) 50 262 38 345(a) 50 345(a) 50 345(a) 50

Based on button-on-block test, ASTM G 98, unlubricated ground finish; condition and hardness apply to both horizontal and vertical axes

(a) Did not gall

Fabrication and Cleaning. Frequently a particular stainless steel is chosen for a fabrication characteristic such as machinability, formability, or weldability Even a required or preferred cleaning procedure can dictate the selection of a specific type For instance, a weldment that is to be cleaned in a medium such as nitric-hydrofluoric acid, which attacks sensitized stainless steel, should be produced from stabilized or low-carbon stainless steel even though sensitization may not affect performance under service conditions

Surface Finish. Other characteristics in the stainless steel selection checklist are vital for some specialized applications but are of little concern for others Among these characteristics, surface finish is important more often than any other except corrosion resistance Stainless steels are sometimes selected because they are available in a variety of attractive finishes Surface finish selection can be made on the basis of appearance, frictional characteristics, or sanitation The effect of finish on sanitation sometimes is thought to be simpler than it actually is, and tests of several candidate finishes may be advisable The selection of finish can in turn influence the selection of the alloy because of differences in availability or durability of the various finishes for different types

Product Forms

Plate is a flat-rolled or forged product more than 254 mm (10 in.) in width and at least 4.76 mm (0.1875 in.) in thickness Stainless steel plate generally is produced in the annealed condition and is either blast cleaned or pickled Blast cleaning generally is followed by further cleaning in appropriate acids to remove surface contaminants such as particles of steel picked up from the mill rolls

Sheet is a flat-rolled product in coils or cut lengths at least 610 mm (24 in.) wide and less than 4.76 mm (0.1875 in.) thick Stainless steel sheet is produced in nearly all types except the free-machining and certain martensitic grades

Strip is a flat-rolled product, in coils or cut lengths, less than 610 mm (24 in.) wide and 0.13 to 4.76 mm (0.005 to 0.1875 in.) thick Cold finished material 0.13 mm (0.005 in.) thick and less than 610 mm (24 in.) wide fits the definitions of both strip and foil and can be referred to using either term

Cold rolled stainless steel strip is manufactured from hot rolled, annealed, and pickled strip (or from slit sheet) by rolling between polished rolls Depending on desired thickness, various numbers of cold rolling passes through the mill are required for effecting the necessary reduction and for securing the desired surface characteristics and mechanical properties

Hot rolled stainless steel strip is a semifinished product obtained by hot rolling slabs or billets and is produced for conversion to finished strip by cold rolling

Foil is a flat-rolled product, in coil form, up to 0.13 mm (0.005 in.) thick and less than 610 mm (24 in.) wide Foil is

produced in slit widths with edge conditions corresponding to No 3 (as-slit) and No 5 (square edge produced by rolling

or filing after slitting) edge conditions for strip

Bar is a product supplied in straight lengths; it is either hot or cold finished and is available in various shapes, sizes, and surface finishes This category includes small shapes, whose dimensions do not exceed 127 mm (5 in.), and hot-rolled flat stock at least 3.2 mm (0.125 in.) thick and up to 254 mm (10 in.) wide

Hot finished bar commonly is produced by hot rolling, forging, or pressing ingots to blooms or billets of intermediate size, which are subsequently hot rolled, forged, or extruded to final dimensions Whether rolling, forging, or extrusion is selected as the finishing method depends on several factors, including composition and final size

Cold-finished bar is produced from hot-finished bar or rod by additional operations such as cold rolling or cold drawing, which result in the close control of dimensions, a smooth surface finish, and higher tensile and yield strengths Sizes and shapes of cold-reduced stock classified as bar are essentially the same as for hot-finished bar, except that all cold-reduced flat stock less than 4.76 mm (0.1875 in.) thick and over 9.5 mm (0.375 in.) wide is classified as strip

Wire is a coiled product derived by cold finishing hot rolled and annealed rod Cold finishing imparts excellent dimensional accuracy, good surface smoothness, fine finish, and specific mechanical properties Wire is produced in several tempers and finishes

Wire is customarily referred to as round wire when the contour is completely cylindrical and as shape wire when the contour is other than cylindrical Shape wire is cold finished either by drawing or by a combination of drawing and rolling

Special Wire Commodities. There are many classes of stainless steel wire that have been developed for specific components or for particular applications The unique properties of each of these individual wire commodities are developed by employing a particular combination of composition, steel quality, process heat treatment, and cold-drawing practice

Cold heading wire is produced in any of the various types of stainless steel In all instances, cold heading wire is subjected to special testing and inspection to ensure satisfactory performance in cold heading and cold forging operations

Of the chromium-nickel group, types 305 and 302Cu are used for cold heading wire and generally are necessary for severe upsetting Other grades commonly cold formed include 303Se, 304, 316, 321, 347, 384, 410, 420, 430, and 431

Spring wire is drawn from annealed rod The types of stainless steel in which spring wire is commonly produced include 302, 304, 316, and 20Cb-3 (N08020)

Spring wire in large sizes can be furnished in a variety of finishes such as dry-drawn lead, copper, lime and soap, or oxide and soap Fine sizes usually are wet drawn, although they can be dry drawn

Rope wire is used to make rope, cable, and cord for a variety of uses such as aircraft control cables, marine ropes, elevator cables, slings, and anchor cables

Rope wire is made of type 302 or 304 unless a higher level of corrosion resistance is required, in which case type 316 is generally selected Special nonmagnetic characteristics may be required, which necessitates selection of heats having little

or no ferrite or martensite in the microstructure and use of special drawing practices to limit or avoid deformation-induced transformation to martensite

Weaving wire is used in weaving of screens for many different applications in coal mines, sand-and-gravel pits, paper mills, chemical plants, dairy plants, oil refineries, and food-processing plants Annealing and final drawing must be carefully controlled to maintain uniform temper and finish throughout each coil or spool Because weaving wire must be ductile, it usually is furnished in the annealed temper with a bright annealed finish or in the soft temper with either a lime-soap finish or an oil- or grease-drawn finish Most types of stainless steels are available as weaving wire

Stainless welding wire is available for many grades to provide good weldability with optimized mechanical properties and corrosion resistance of the weldment Stainless steel weld wire is produced in layer-level wound spools, straight lengths (both included in the American Welding Society AWS A5.9), and coated electrodes (AWS A5.4)

Semifinished Products. Blooms, billets, and slabs are hot rolled, hot forged, or hot pressed to approximate sectional dimensions and generally have rounded corners Round billets are also produced, typically for extrusion or closed-die forging These semifinished products, as well as tube rounds, are produced in random lengths or are cut to specified lengths or to specified weights There are no invariable criteria for distinguishing between the terms bloom and billet, and often they are used interchangeably

cross-The nominal cross-sectional dimensions of blooms, billets, and slabs are designated in inches and fractions of an inch The size ranges commonly listed as hot-rolled stainless steel blooms, billets, and slabs include square sections 100 × 100

mm (4 × 4 in.) and larger, and rectangular sections at least 10,300 mm2 (16 in.2) in cross-sectional area

Pipe, tubes, and tubing are made either by piercing rounds or by rolling and welding strip They are used for conveying gases, liquids, and solids and for diverse mechanical and structural purposes (Cylindrical forms intended for use as containers for storage and shipping purposes, and products cast to tubular shape, are not included in this category.)

Pipe is distinguished from tubes chiefly by the fact that it is commonly produced in relatively few standard sizes Tubing

is generally made to more exacting specifications regarding dimensions, finish, chemical composition, and mechanical properties than either pipe or tubes

Stainless steel tubular products are classified as follows according to intended service:

• Stainless steel tubing for general corrosion-resistant service

• Stainless steel pressure pipe

• Seamless steel pressure tubes

• Stainless steel sanitary tubing

• Stainless steel mechanical tubing

• Stainless steel aircraft tubing

• Aircraft structural tubing

• Aircraft hydraulic-line tubing

Notch Toughness and Transition Temperature

Notched-bar impact testing of stainless steels is likely to show a wide scatter in test results, regardless of type or test conditions Because of this wide scatter, only general behavior of the different classes can be described

Austenitic types have good notched-bar impact resistance Charpy impact energies of 150 J (110 ft · lbf) or greater are typical of all austenitic types at room temperature (Table 7) Cryogenic temperatures have little or no effect on notch toughness; ordinarily, austenitic stainless steels maintain values exceeding 130 J (95 ft · lbf), even at very low temperatures (Table 10) Conversely, cold work lowers the resistance to impact at all temperatures

Table 10 Typical cryogenic properties of selected annealed austenitic stainless steels

-73 -100 648 94 1138 165 64 74 285 210

Martensitic and ferritic stainless steels exhibit a decreasing resistance to impact with decreasing temperature, and the fracture appearance changes from a ductile mode at mildly elevated temperatures to a brittle mode at low temperatures This fracture transition is characteristic of martensitic and ferritic materials Both the upper-shelf energy and the lower-shelf energy are not greatly influenced by heat treatment in these stainless steels However, the temperature range over which transition occurs is affected by heat treatment, minor variations in composition, and cold work Heat treatments that result in high hardness move the transition range to higher temperatures, and those that result in low hardness move the transition range to lower temperatures

Fracture toughness data for austenitic stainless steels are limited because of the high ductility and high toughness of these

grades The fracture toughness data that are available were obtained by the J-integral method (Jc initial toughness values) and are given in units of kJ/m2 Extensive fracture toughness testing of types 304 and 316 stainless steels shows that they

are extremely resistant to fracture Both types exhibit a ductile fracture response under a wide variety of conditions, but Jc

values are highly variable, typically ranging from 169 to 1660 kJ/m2 at room temperature and from 130 to 1420 kJ/m2 at approximately 400 °C (750 °F)

Fatigue Properties

Fatigue crack initiation tests are procedures in which a specimen or part is subjected to cyclic loading to failure A large portion of the total number of cycles in these tests is spent initiating the crack Although crack initiation tests conducted on small specimens do not precisely establish the fatigue life of a large part, such tests do provide data on the intrinsic fatigue crack initiation behavior of a stainless steel As a result, such data can be used to develop criteria to prevent fatigue failures in engineering design Examples of the use of small-specimen fatigue test data can be found in the basis of the fatigue design codes for boilers and pressure vessels, complex welded, riveted, or bolted structures, and automotive and aerospace components Factors influencing the fatigue life of stainless steels include temperature, specimen orientation (e.g., longitudinal versus transverse), heat treatment (e.g., annealed versus quench hardened), hardness, and surface condition (Fig 6)

Fig 6 Factors affecting fatigue properties of stainless steels

Three types of fatigue tests are used to develop data on fatigue behavior of stainless steels The most common of these tests is the rotating-beam test, which most closely approximates the kind of loading to which shafts and axles are subjected The flexural fatigue test is used to evaluate the behavior of sheet and most closely simulates the action of leaf springs, which are expected to flex without deforming or breaking The axial-load fatigue test subjects a fatigue specimen

to unidirectional loading that can range from full reversal (tension-compression) to tension-tension loading, and can have virtually any conceivable ratio of maximum stress to minimum stress In general, fatigue conditions involving tension-

compression loading (stress ratio, R, between 0 and -1) lead to shorter fatigue lives than conditions involving tension loading (stress ratio, R, between 0 and +1) at the same value of maximum stress

tension-Fatigue crack growth (FCG) rates of austenitic stainless steels have been extensively studied, given their widespread use in high-temperature structural parts with cyclic stressing over a wide range of frequencies, temperatures, environments, and load ratios Results of FCG tests have been analyzed on the basis of fracture mechanics concepts, leading to a well-documented collection of fatigue crack growth rate data for the austenitic stainless steels particularly types 304 and 316 Data are also available on duplex grades; martensitic grades; and martensitic, semi-austenitic, and austenitic PH stainless steels Figure 7 shows the effects of condition (annealed versus cold worked) and temperature on the FCG rates of type 304 stainless steel This figure shows that the high- K crack growth rates were lower for the cold-

worked specimens than for the annealed specimens Crack growth rates were higher for the specimens tested at 427 °C (800 °F) than for corresponding specimens tested at room temperature

Fig 7 Fatigue crack growth rates for annealed and cold worked type 304 stainless steel at 25 and 427 °C (77

and 800 °F), 0.17 Hz, and an R ratio of 0

Elevated-Temperature Mechanical Properties

Many stainless steels particularly the austenitic types 304, 309, 310, 316, 321, 330, and 347; certain hardening types such as PH 15-7 Mo, 15-5 PH, 17-4 PH, 17-7 PH, AM-350 and AM-355; and certain martensitic types such as the so-called "Super 12 Chrome" steels that contain molybdenum (up to 3%), tungsten (up to 3.5%), and/or vanadium are used extensively for elevated-temperature applications As shown in Fig 8, the austenitic types retain their strength at higher temperatures (up to 815 °C, or 1500 °F) than do the other types of stainless steels For the best creep strength and creep-rupture strength, the H grades of austenitic stainless steels are specified These steels have carbon contents of 0.04 to 0.10% (Table 3) and are solution annealed at temperatures high enough to produce improved creep properties

precipitation-Fig 8 General comparison of the hot-strength characteristics of austenitic, martensitic, and ferritic stainless

steels with those of low-carbon unalloyed steel and semiaustenitic precipitation- and transformation-hardening steels

Valve steels are austenitic nitrogen-strengthened steels that have been used extensively in automotive/internal combustion engine valve applications Examples of such alloys include 21-2N (21Cr, 8Mn, 2Ni + N), 21-4N (21Cr, 9Mn, 4Ni + N), 21-12N (21Cr, 12Ni, 1.25Mn + N), and 23-8N (21Cr, 8Ni, 3.5Mn + N) (see Table 3) The nitrogen contents in these alloys range from 0.20 to 0.50% These engine valve grades are used at temperatures up to 760 °C (1400 °F), but they provide fairly low strength at the upper end of their temperature capability

As described in the section "Thermally Induced Embrittlement," extended service at elevated temperature can result in embrittlement (sensitization) of austenitic stainless steels, which degrades the ability of the material to withstand corrosion and induces embrittlement Most often, such degradation is caused by the precipitation of secondary phases such as carbides and sigma phase Precipitation depends on both time and temperature: longer times at temperature and higher temperatures both promote more extensive precipitation

Thermally Induced Embrittlement

Stainless steels are susceptible to embrittlement during thermal treatment or elevated-temperature service These thermally induced forms of embrittlement of stainless steels include sensitization, 475 °C (885 °F) embrittlement, and -phase embrittlement

Sensitization

Stainless steels become susceptible to localized intergranular corrosion when chromium carbides form at the grain boundaries during high-temperature exposure This depletion of chromium at the grain boundaries is termed

"sensitization" because the alloys become more sensitive to localized attack in corrosive environments

Austenitic Stainless Steels. At temperatures above approximately 1035 °C (1900 °F), chromium carbides are completely dissolved in austenitic stainless steels However, when these steels are slowly cooled from these high temperatures or reheated into the range of 425 to 815 °C (800 to 1500 °F), chromium carbides are precipitated at the grain boundaries These carbides contain more chromium than the matrix contains

The precipitation of the carbides depletes the matrix of chromium adjacent to the grain boundary The diffusion rate of chromium in austenite is slow at the precipitation temperatures; therefore, the depleted zone persists, and the alloy is sensitized to intergranular corrosion This sensitization occurs because the depleted zones have higher corrosion rates than the matrix in many environments Loss of toughness also results from sensitization

If the austenitic stainless steels are cooled rapidly to below approximately 425 °C (800 °F), the carbides do not precipitate, and the steels are immune to intergranular corrosion Reheating the alloys to 425 to 815 °C (800 to 1500 °F),

as for stress relief, will cause carbide precipitation and sensitivity to intergranular corrosion The maximum rate of carbide precipitation occurs at approximately 675 °C (1250 °F) Because this is a common temperature for the stress relief

of carbon and low-alloy steels, care must be exercised in selecting stainless steels to be used in dissimilar-metal joints that are to be stress relieved

Welding is the common cause of the sensitization of stainless steels to intergranular corrosion Although the cooling rates

in the weld itself and the base metal immediately adjacent to it are sufficiently high to avoid carbide precipitation, the weld thermal cycle will bring part of the heat-affected zone (HAZ) into the precipitation range Carbides will precipitate, and a zone somewhat removed from the weld will become susceptible to intergranular corrosion Welding does not always sensitize austenitic stainless steels In thin sections, the thermal cycle may be such that no part of the HAZ is at sensitizing temperatures long enough to cause carbide precipitation Once the precipitation has occurred, it can be removed by reheating the alloy to above 1035 °C (1895 °F) and cooling it rapidly

Susceptibility to intergranular corrosion in austenitic stainless steels can be avoided by controlling their carbon contents

or by adding elements whose carbides are more stable than those of chromium For most austenitic stainless steels, restricting their carbon contents to 0.03% or less will prevent sensitization during welding and most heat treatment This