design guidelines

021214 Design Guidelines 1/24/03 9:08 AM

CONTENTS lntroduction ............................................1ldentification of Stainless Steel ...............1Guidelines for Selection..........................5Corrosion Resistance..............................5

Material Selection .............................5Mechanical & Physical Properties..........9

Austenitic ..........................................9Ferritic ...............................................9Martensitic ......................................11Precipitation Hardening..................12

Properties..............................................14Thermal Stability....................................14Low-Temperature Mechanical

Properties........................................16Heat Transfer Properties.......................17Sizes, Shapes, and Finishes.................18Fabrication ............................................18Hot Forming ..........................................18Cold Forming ........................................25Machining .............................................27Joining...................................................28

Welding...........................................28Soldering.........................................29Brazing............................................29Fastening ........................................29

Surface Protection & Cleaning .............30Appendix A

Corrosion Characteristics...............31Appendix B

Figures ............................................35References ...........................................52

The Specialty Steel lndustry of North America (SSINA) and the individualcompanies it represents have madeevery effort to ensure that the information presented in this handbook is technically correct. However, neither the SSlNA norits member companies warrants theaccuracy of the information contained inthis handbook or its suitability for anygeneral and specific use, and assumesno liability or responsibility of any kind inconnection with the use of this information.The reader is advised that the materialcontained herein should not be used orrelied on for any specific or generalapplications without first securingcompetent advice.

11-02-18

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1

INTRODUCTIONStainless steels are iron-base alloys containing 10.5% or more

chromium. They have been used for many industrial, architectural,chemical, and consumer applications for over a half century. Currentlythere are being marketed a number of stainless steels originallyrecognized by the American lron and Steel lnstitute (AISI) as standardalloys. Also commercially available are proprietary stainless steelswith special characteristics. (See Appendix A.)

With so many stainless steels from which to choose, designersshould have a ready source of information on the characteristics andcapabilities of these useful alloys. To fill this need, the Committee ofStainless Steel Producers initially prepared this booklet. The datawas reviewed and updated by the Specialty Steel lndustry of NorthAmerica (SSINA). Written especially for design engineers, it presentsan overview of a broad range of stainless steels — both standard andproprietary — their compositions, their properties, their fabrication,and their use. More detailed information on the standard grades,with special emphasis on the manufacture, finish designations anddimensional and weight tolerances of the product forms in whichthey are marketed, is contained in the lron and Steel Society of theAlME (the American lnstitute of Mining, Metallurgical and PetroleumEngineers) “Steel Products Manual — Stainless and Heat ResistingSteels.” The AlME undertook the publication, updating and sale ofthis manual after the AlSl discontinued publication in 1986.

Reference is often made to stainless steel in the singular sense asif it were one material. Actually there are well over 100 stainless steelalloys. Three general classifications are used to identify stainlesssteels. They are: 1. Metallurgical Structure; 2. The AlSl numberingsystem: namely 200, 300, and 400 Series numbers; 3. The UnifiedNumbering System, which was developed by American Society forTesting Materials (ASTM) and Society of Automotive Engineers (SAE)to apply to all commerical metals and alloys.

There are also a number of grades known by common names thatresemble AlSl designations and these are recognized by ASTM.These common names, which are neither trademarks nor closelyassociated with a single producer, are shown and identified in thetables. These common (non-AISI) names also appear in the ASTMspecification. Nearly all stainless steels used in North America haveUNS designations.

On the following pages there is a description of these classifications.Tables 1-5 list stainless steels according to metallurgical structure:austenitic, ferritic, martensitic, precipitation hardening, and duplex.

3050 K Street, N.W.Washington, D.C. 20007TEL: (202) 342-8630 or (800) 982-0355FAX: (202) 342-8631http://www.ssina.com

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Austenitic stainless steels (Table 1)containing chromium and nickel areidentified as 300 Series types. Alloyscontaining chromium, nickel andmanganese are identified as 200 Seriestypes. The stainless steels in the austeniticgroup have different compositions andproperties, but many common character-istics. They can be hardened by coldworking, but not by heat treatment. In theannealed condition, all are essentiallynonmagnetic, although some maybecome slightly magnetic by coldworking. They have excellent corrosionresistance, unusually good formability,and increase in strength as a result ofcold work.

Type 304 (sometimes referred to as18-8 stainless) is the most widely usedalloy of the austenitic group. It has anominal composition of 18% chromiumand 8% nickel.

Ferritic stainless steels (Table 2) arestraight-chromium 400 Series types thatcannot be hardened by heat treatment,and only moderately hardened by coldworking. They are magnetic, have goodductility and resistance to corrosion andoxidation. Type 430 is the general-purposestainless of the ferritic group.

Martensitic stainless steels (Table 3)are straight-chromium 400 Series typesthat are hardenable by heat treatment.They are magnetic. They resist corrosionin mild environments. They have fairlygood ductility, and some can be heattreated to tensile strengths exceeding200,000 psi (1379 MPa).

Type 410 is the general-purpose alloyof the martensitic group.

Precipitation-hardening stainlesssteels (Table 4) are chromium-nickeltypes, some containing other alloyingelements, such as copper or aluminum.They can be hardened by solutiontreating and aging to high strength.

Duplex stainless steels (Table 5) havean annealed structure which is typicallyabout equal parts of austenite and ferrite.Although not formally defined, it isgenerally accepted that the lesser phasewill be at least 30% by volume.

Duplex stainless steels offer severaladvantages over the common austeniticstainless steels. The duplex grades arehighly resistant to chloride stresscorrosion cracking, have excellent pittingand crevice corrosion resistance andexhibit about twice the yield strength asconventional grades. Type 329 and 2205are typical alloys.

With respect to the Unified NumberingSystem, the UNS designations areshown alongside each AlSl type number,in Tables 1-5, except for four stainlesssteels (see Table 4) for which UNSdesignations only are listed.

Table 1AUSTENITIC

STAINLESS STEELSEquivalent Equivalent

TYPE UNS TYPE UNS

201 S20100 310 S31000202 S20200 310S S31008205 S20500 314 S31400301 S30100 316 S31600302 S30200 316L S31603302B S30215 316F S31620303 S30300 316N S31651303Se S30323 317 S31700304 S30400 317L S31703304L S30403 317LMN S31726302HQ S30430 321 S32100304N S30451 330 NO8330305 S30500 347 S34700308 S30800 348 S34800309 S30900 384 S38400309S S30908

Table 3MARTENSITIC

STAINLESS STEELSEquivalent Equivalent

TYPE UNS TYPE UNS

403 S40300 420F S42020410 S41000 422 S42200414 S41400 431 S43100416 S41600 440A S44002416Se S41623 440B S44003420 S42000 440C S44004

Table 4PRECIPITATION HARDENING

STAINLESS STEELSUNS UNS

S13800 S17400S15500 S17700

Table 5DUPLEX

STAINLESS STEELSType/Name UNS

329 S329002205 S31803

2205 (hi N) S32205

Table 2FERRITIC STAINLESS STEELS

Equivalent EquivalentTYPE UNS TYPE UNS

405 S40500 430FSe S43023409 S40900 434 S43400429 S42900 436 S43600430 S43000 442 S44200430F S43020 446 S44600


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AUSTENITIC

304GeneralPurpose

202N & MnpartiallyreplacesNi

302BSi addedto increasescalingresistance



317MoreMo & Cradded forbettercorrosionresistance

316Mo addedto increasecorrosionresistance

309S309S

CR & Niincreasedfor hightemperature

308HigherCR & Niusedprimarilyfor welding

302Higher Cfor increasedstrength

305Ni increasedto lowerworkhardening

303S addedto improvemachining

301Cr & Nilowered toincreaseworkhardening

317LC reducedforwelding

316LC reducedforwelding

310S310S

Same as309 onlymore so

347Cbaddedto preventcarbideprecip..

321Ti addedto preventcarbideprecip.

304LC reducedevenfurther

384More Nito lowerworkhardening

303SeSe addedfor bettermachinedsurfaces

317LMNMo addedN added

316NN addedtoincreasestrength

314Si increasedfor highestheatresistance

348Ta & Corestrictedfor nuclearapplications

304NN addedto increasestrength

S30430Cu addedto improvecold working

316FS & Pincreased to improvemachining

330Ni addedto resistcarburization& thermalshock

Al — Aluminum P — PhosphorusC — Carbon S — SulfurCr — Chromium Se — SeleniumCb — Columbium Si — SiliconCo — Cobalt Ta — TantalumCu — Copper Ti — TitaniumMn — Manganese V — VanadiumMo — Molybdenum W — TungstenN — Nitrogen SCC — Stress - CorrosionNi — Nickel Cracking


4

FERRITIC

430GeneralPurpose

446Crincreasedto improvescalingresistance

442Crincreasedto improvescalingresistance

429SlightlyLess Crfor betterweldability

405Lower CrAI addedto preventhardeningwhen colledfrom elevatedtemperatures

409Lower CrPrimarilyused forautomotiveexhaust

430FP & Sadded forimprovedmachining

434Mo addedfor improvedcorrosionresistancein auto trim

430F SeSe addedfor bettermachinedsurfaces

436Mo. Cb addedfor corrosion& heatresistance &improvedroping

MARTENSITIC

410GeneralPurpose

431Cr increasedNi addedfor corrosionresistanceGoodMechan.Properties

414Ni addedfor bettercorrosionresistance

403Selectqualityfor turbinesand highlystressedparts

420IncreasedC toimprovemechanicalproperties

416P & Sincreasedto improvemachining

440CC increasedfor highesthardnessCr increasedfor corrosionresistance

422Strength &toughness to1200F viaaddition ofMo, V, W

416 SeSe added for bettermachinedsurface

440BC decreasedslightly toimprovetoughness

420FP & Sincreasedto improvemachining

440ASame as440B onlymore so

Al — Aluminum P — PhosphorusC — Carbon S — SulfurCr — Chromium Se — SeleniumCb — Columbium Si — SiliconCo — Cobalt Ta — TantalumCu — Copper Ti — TitaniumMn — Manganese V — VanadiumMo — Molybdenum W — TungstenN — Nitrogen SCC — Stress - CorrosionNi — Nickel Cracking


5

GUIDELINES FOR SELECTION Stainless steels are engineering

materials with good corrosion resistance,strength, and fabrication characteristics.They can readily meet a wide range ofdesign criteria — load, service life, lowmaintenance, etc. Selecting the properstainless steel essentially meansweighing four elements. In order ofimportance, they are:

1. Corrosion or Heat Resistance —the primary reason for specifyingstainless. The specifier needs to knowthe nature of the environment and thedegree of corrosion or heat resistancerequired.

2. Mechanical Properties — withparticular emphasis on strength at room,elevated, or low temperature. Generallyspeaking, the combination of corrosionresistance and strength is the basis forselection.

3. Fabrication Operations — andhow the product is to be made is a third-level consideration. This includes forging,machining, forming, welding, etc.

4. Total Cost — in considering totalcost, it is appropriate to consider not onlymaterial and production costs, but thelife cycle cost including the cost-savingbenefits of a maintenance-free producthaving a long life expectancy.

CORROSION RESISTANCEChromium is the alloying element that

imparts to stainless steels their corrosion-resistance qualities by combining withoxygen to form a thin, invisible chromium-oxide protective film on the surface.(Figure 1. Figures are shown in AppendixB.) Because the passive film is such animportant factor, there are precautionswhich must be observed in designingstainless steel equipment, inmanufacturing the equipment, and inoperation and use of the equipment, toavoid destroying or disturbing the film.

In the event that the protective (passive)film is disturbed or even destroyed, itwill, in the presence of oxygen in theenvironment, reform and continue to givemaximum protection.

The protective film is stable andprotective in normal atmospheric or mildaqueous environments, but can beimproved by higher chromium, and bymolybdenum, nickel, and other alloyingelements. Chromium improves filmstability; molybdenum and chromiumincrease resistance to chloridepenetration; and nickel improves filmresistance in some acid environments.

Material SelectionMany variables characterize a

corrosive environment — i.e. chemicalsand their concentration, atmosphericconditions, temperature, time — so it isdifficult to select which alloy to usewithout knowing the exact nature of theenvironment, However, there areguidelines:

Type 304 serves a wide range ofapplications. It withstands ordinaryrusting in architecture, it is resistant tofood processing environments (exceptpossibly for high-temperature conditionsinvolving high acid and chloridecontents), it resists organic chemicals,dyestuffs, and a wide variety of inorganicchemicals. Type 304 L (low carbon)resists nitric acid well and sulfuric acidsat moderate temperature andconcentrations. It is used extensively forstorage of liquified gases, equipment foruse at cryogenic temperatures (304N),appliances and other consumerproducts, kitchen equipment, hospitalequipment, transportation, and waste-water treatment.

Type 316 contains slightly more nickelthan Type 304, and 2-3% molybdenumgiving it better resistance to corrosionthan Type 304, especially in chlorideenvironments that tend to cause pitting.Type 316 was developed for use insulfite pulp mills because it resistssulfuric acid compounds. Its use hasbeen broadened, however, to handlingmany chemicals in the processindustries.

Type 317 contains 3-4% molybdenum(higher levels are also available in thisseries) and more chromium than Type316 for even better resistance to pittingand crevice corrosion.

Type 430 has lower alloy content thanType 304 and is used for highly polishedtrim applications in mild atmospheres. Itis also used in nitric acid and foodprocessing.

Type 410 has the lowest alloy contentof the three general-purpose stainlesssteels and is selected for highly stressedparts needing the combination ofstrength and corrosion resistance, suchas fasteners. Type 410 resists corrosionin mild atmospheres, steam, and manymild chemical environments.

2205 may have advantages over Type304 and 316 since it is highly resistant tochloride stress corrosion cracking and isabout twice as strong.

Table 6 lists the relative corrosionresistance of the AlSl standard numberedstainless steels in seven broad categoriesof corrosive environments. Table 7 detailsmore specific environments in whichvarious grades are used, such as acids,bases, organics, and pharmaceuticals.

The above comments on the suitabilityof stainless steels in various environmentsare based on a long history of successfulapplication, but they are intended only asguidelines. Small differences in chemicalcontent and temperature, such as mightoccur during processing, can affectcorrosion rates. The magnitude can beconsiderable, as suggested by Figures 2and 3. Figure 2 shows small quantities ofhydrofluoric and sulfuric acids having aserious effect on Type 316 stainless steelin an environment of 25% phosphoricacid, and Figure 3 shows effects oftemperature on Types 304 and 316 invery concentrated sulfuric acid.

Service tests are most reliable indetermining optimum material, andASTM G 4 is a recommended practicefor carrying out such tests. Tests shouldcover conditions both during operationand shutdown. For instance, sulfuric,sulfurous and polythionic acid conden-sates formed in some processes duringshutdowns may be more corrosive thanthe process stream itself. Tests shouldbe conducted under the worst operatingconditions anticipated.

Several standard reference volumesdiscuss corrosion and corrosion control,including Uhlig’s Corrosion Handbook;LaQue and Copsons’ Corrosion ResistanceOf Metals and Alloys; Fontana andGreens’ Corrosion Engineering; A Guideto Corrosion Resistance by ClimaxMolybdenum Company; the CorrosionData Survey by the National Associationof Corrosion Engineers; and the ASMMetals Handbook. Corrosion data,specifications, and recommendedpractices relating to stainless steels arealso issued by ASTM. Stainless steelsresist corrosion in a broad range ofconditions, but they are not immune toevery environment. For example, stainlesssteels perform poorly in reducingenvironments, such as 50% sulfuric and hydrochloric acids at elevatedtemperatures. The corrosive attackexperienced is a breakdown of theprotective film over the entire metal surface.

Such misapplications of stainlesssteels are rare and are usually avoided.The types of attack which are more likelyto be of concern are pitting, creviceattack, stress corrosion cracking, andintergranular corrosion, which arediscussed in Appendix A.


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Table 6

Relative Corrosion Resistance of AISI Stainless Steels (1)

Mild Atmos- Atmospheric ChemicalTYPE UNS pheric and –––––––––––––––––––––––– ––––––––––––––––––––––––––––––––––

Number Number Fresh Water Industrial Marine Miild Oxidizing Reducing

201 (S20100) x x x x x202 (S20200) x x x x x205 (S20500) x x x x x301 (S30100) x x x x x302 (S30200) x x x x x

302B (S30215) x x x x x303 (S30300) x x x

303 Se (S30323) x x x x304 (S30400) x x x x x304L (S30403) x x x x x

(S30430) x x x x x304N (S30451) x x x x x305 (S30500) x x x x x308 (S30800) x x x x x309 (S30900) x x x x x

309S (S30908) x x x x x310 (S31000) x x x x x

310S (S31008) x x x x x314 (S31400) x x x x x316 (S31600) x x x x x x316F (S31620) x x x x x x316L (S31603) x x x x x x316N (S31651) x x x x x x317 (S31700) x x x x x x317L (S31703) x x x x x321 (S32100) x x x x x329 (S32900) x x x x x x330 (N08330) x x x x x x347 (S34700) x x x x x348 (S34800) x x x x x384 (S38400) x x x x x403 (S40300) x x405 (S40500) x x409 (S40900) x x410 (S41000) x x414 (S41400) x x416 (S41600) x

416 Se (S41623) x420 (S42000) x420F (S42020) x422 (S42200) x429 (S42900) x x x x430 (S43000) x x x x430F (S43020) x x x

430F Se (S43023) x x x431 (S43100) x x x x434 (S43400) x x x x x436 (S43600) x x x x x

440A (S44002) x x440B (S44003) x440C (S44004) x442 (S44200) x x x x446 (S44600) x x x x x

(S13800) x x x x(S15500) x x x x x(S17400) x x x x x(S17700) x x x x x

* The “X” notations indicate that a specific stainless steel type may be consideredas resistant to the corrosive environment categories.

This list is suggested as a guideline only and does not suggest or imply awarranty on the part of the Specialty Steel Industry of the United States or any of

the member companies. When selecting a stainless steel for any corrosiveenvironment, it is always best to consult with a corrosion engineer and, if possible,conduct tests in the environment involved under actual operating conditions.


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Table 7

Where Different Grades Are Used (15)

Environment Grades Environment Grades

AcidsHydrochloric acid Stainless generally is not recommended except when solutions are very dilute and at room temperature.

“Mixed acids” There is usually no appreciable attack on Type 304 or 316 as long as sufficient nitric acid is present.

Nitric acid Type 304L or 430 is used.

Phosphoric acid Type 304 is satisfactory for storing cold phosphoric acid up to 85% and for handling concentrations up to 5% in some unitprocesses of manufacture. Type 316 is more resistant and is generally used for storing and manufacture if the fluorinecontent is not too high. Type 317 is somewhat more resistant then Type 316. At concentrations up to 85%, the metaltemperature should not exceed 212 F (100 C) with Type 316 and slightly higher with Type 317. Oxidizing ions inhibitattack and other inhibitors such as arsenic may be added.

Sulfuric acid Type 304 can be used at room temperature for concentrations over 80%. Type 316 can be used in contact with sulfuricacid up to 10% at temperatures up to 120 F (50 C), if the solutions are aerated; the attack is greater in airfree solutions.Type 317 may be used at temperatures as high as 150 F (65 C) with up to 5% concentration. The presence of othermaterials may markedly change the corrosion rate. As little as 500 to 2000 ppm of cupric ions make it possible to useType 304 in hot solutions of moderate concentration. Other additives may have the opposite effect.

Sulfurous acid Type 304 may be subject to pitting, particularly if some sulfuric acid is present. Type 316 is usable at moderateconcentrations and temperatures.

BasesAmmonium hydroxide, Steels in the 300 series generally have good corrosion resistance at virtually all concentrations and temperatures in sodium hydroxide, weak bases, such as ammonium hydroxide. In stronger bases, such as sodium hydroxide, there may be some attack, caustic solutions cracking or etching in more concentrated solutions and at higher temperatures. Commercial purity caustic solutions may

contain chlorides, which will accentuate any attack and may cause pitting of Type 316 as well Type 304.

OrganicsAcetic acid Acetic acid is seldom pure in chemical plants but generally includes numerous and varied minor constituents. Type 304 is

used for a wide variety of equipment including stills, base heaters, holding tanks, heat exchangers, pipelines, valves andpumps for concentrations up to 99% at temperatures up to about 120 F (50 C). Type 304 is also satisfactory for contactwith 100% acetic acid vapors, and — if small amounts of turbidity or color pickup can be tolerated — for roomtemperature storage of glacial acetic acid. Types 316 and 317 have the broadest range of usefulness, especially if formicacid is also present or if solutions are unaerated. Type 316 is used for fractionating equipment, for 30 to 99%concentrations where Type 304 cannot be used, for storage vessels, pumps and process equipment handling glacialacetic acid, which would be discolored by Type 304. Type 316 is likewise applicable for parts having temperatures above120 F (50 C), for dilute vapors and high pressures. Type 317 has somewhat greater corrosion resistance than Type 316under severely corrosive conditions. None of the stainless steels has adequate corrosion resistance to glacial acetic acidat the boiling temperature or at superheated vapor temperatures.

Aldehydes Type 304 is generally satisfactory.

Amines Type 316 is usually preferred to Type 304.

Cellulose acetate Type 304 is satisfactory for low temperatures, but Type 316 or Type 317 is needed for high temperatures.

Citric, formic and Type 304 is generally acceptable at moderate temperatures, but Type 316 is resistant to all concentrations attartaric acids temperatures up to boiling.

Esters From the corrosion standpoint, esters are comparable with organic acids.

Fatty acids Up at about 300 F (150 C), Type 304 is resistant to fats and fatty acids, but Type 316 is needed at 300 to 500 F (150 to260 C) and Type 317 at higher temperatures.

Paint vehicles Type 316 may be needed if exact color and lack of contamination are important.

Phthalic anhydride Type 316 is usually used for reactors, fractionating columns, traps, baffles, caps and piping.

Soaps Type 304 is used for parts such as spray towers, but Type 316 may be preferred for spray nozzles and flake-drying beltsto miniimize offcolor products.

Synthetic detergents Type 316 is used for preheat, piping, pumps and reactors in catalytic hydrogenation of fatty acids to give salts ofsulfonated high molecular alcohols.

Tall oil (pump and Type 304 has only limited usage in tall-oil distillation service. High-rosin-acid streams can be handled by Type 316Lpaper industry) with a minimum molybdenum content of 2.75%. Type 316 can also be used in the more corrosive high-fatty acid streams

at temperatures up to 475F (245 C), but Type 317 will probably be required at higher temperatures.

Tar Tar distillation equipment is almost all Type 316 because coal tar has a high chloride content; Type 304 does not have adequate resistance to pitting.

Urea Type 316L is generally required.

Pharmaceuticals Type 316 is usually selected for all parts in contact with the product because of its inherent corrosion resistance andgreater assurance of product purity.


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9

MECHANICAL AND PHYSICAL PROPERTIES (Room Temperature)

Austenitic Stainless Steels The austenitic stainless steels cannot

be hardened by heat treatment but canbe strengthened by cold work, and thusthey exhibit a wide range of mechanicalproperties. At room temperature,austenitic stainless steels exhibit yieldstrengths between 30 and 200 ksi (207-1379 MPa), depending on compositionand amount of cold work. They alsoexhibit good ductility and toughnesseven at high strengths, and this goodductility and toughness is retained atcryogenic temperatures. The chemicalcompositions and nominal mechanicalproperties of annealed austeniticstainless steels are given in Table 8.

The difference in effect of cold work ofTypes 301 and 304 is indicated by thestress strain diagrams in Figure 11.

Carbon and nitrogen contents affectyield strength, as shown by thedifferences among Types 304, 304L,and 304N. The effect of manganese andnitrogen on strength can be seen bycomparing Types 301 and 302 againstTypes 201 and 202.

Figures 12, 13, 14, and 15 illustrateother effects of small compositionchanges. For example, at a givenamount of cold work, Types 202 and 301exhibit higher yield and tensile strengthsthan Types 305 and 310.

Austenitic stainless steels which canbe cold worked to high tensile and yieldstrengths, while retaining good ductilityand toughness, meet a wide range ofdesign criteria. For example, sheet andstrip of austenitic steels — usually Types301 and 201 — are produced in thefollowing tempers:

In structural applications, thetoughness and fatigue strength of thesesteels are important. At roomtemperature in the annealed condition,the austenitic steels exhibit Charpy V-notch energy absorption values inexcess of 100 ft.-lb. The effect of coldrolling Type 301 on toughness isillustrated in Figure 16. This shows Type301 to have good toughness even aftercold rolling to high tensile strengths.

Fatigue or endurance limits (in bending)of austenitic stainless steels in theannealed condition shown in Table 9 areabout one-half the tensile strength.

New Design SpecificationUntil recently, design engineers

wanting to use austenitic stainless steelsstructurally had to improvise due to thelack of an appropriate design specification.The familiar American lnstitute for SteelConstruction and AlSl design specificationsfor carbon steel design do not apply tothe design of stainless steel membersbecause of differences in strengthproperties, modulus of elasticity, and theshape of the stress strain curve. Figure17 shows that there is no well-definedyield point for stainless steel.

Now the American Society of CivilEngineers (ASCE), in conjunction withthe SSINA, has prepared a standard(ANSI/ASCE-8-90) “Specification for theDesign of Cold-Formed Stainless SteelStructural Members.” This standardcovers four types of austenitic stainlesssteel, specifically Types 201, 301, 304and 316, and three types of ferriticstainless steels (See Ferritic sectionbelow). This standard requires the use ofstructural quality stainless steel asdefined in general by the provisions ofthe American Society for Testing andMaterials (ASTM) specifications.

Some of the physical properties ofaustenitic stainless steels are similar tothose of the martensitic and ferriticstainless steels. The modulus of elasticity,for example, is 28 x 106 psi (193 GPa)and density is 0.29 Ib. per cu. in. (8060Kg/m3). The physical properties ofannealed Type 304 are shown in Table 10.

Ferritic Stainless SteelsFerritic stainless steels contain approxi-

mately 12% chromium (and up). Thechemical composition of the standardgrades are shown in Table 11 along withnominal mechanical properties. Alsoseveral proprietary grades (see AppendixA) have achieved relatively widecommercial acceptance.

Three ferritic stainless steels, namelyTypes 409, 430 and 439 are included inthe ASCE “Specification for the Design ofCold-Formed Stainless Steel StructuralMembers.” Designers should be aware

of two notations in this specification:(1) The maximum thickness for Type

409 ferritic stainless used in the standardis limited to 0.15 inches.

(2) The maximum thickness for Type 430and 439 ferritic stainless steels is limitedto 0.125 inches.

This is in recognition of concerns forthe ductile to brittle transitiontemperature of the ferritic stainless steelsin structural application. It should benoted that these alloys have been usedin plate thickness for other applications.

Generally, toughness in the annealedcondition decreases as the chromiumcontent increases. Molybdenum tends to increase ductility, whereas carbontends to decrease ductility. Ferriticstainless steels can be used for structuralapplications (as noted above), as well assuch traditional applications as kitchensinks, and automotive, appliance, andluggage trim, which require goodresistance to corrosion and bright, highlypolished finishes.

When compared to low-carbon steels,such as SAE 1010, the standard numberedAlSl ferritic stainless steels, (such asType 430) exhibit somewhat higher yieldand tensile strengths, and low elongations.Thus, they are not as formable as thelow-carbon steels. The proprietary ferriticstainless steels, on the other hand, withlower carbon levels have improvedductility and formability comparable withthat of low-carbon steels. Because of thehigher strength levels, the ferritic stainlesssteels require slightly more power to form.

Micro cleanliness is important to goodformability of the ferritic types becauseinclusions can act as initiation sites forcracks during forming.

Type 405 stainless is used where theannealed mechanical properties andcorrosion resistance of Type 410 aresatisfactory but when better weldability isdesired. Type 430 is used for formedproducts, such as sinks and decorativetrim. Physical properties of Type 430 areshown in Table 10. Types 434 and 436are used when better corrosionresistance is required and for relativelysevere stretching.

For fasteners and other machined parts,Types 430F and 430F Se are often used,the latter being specified when forming isrequired in addition to machining.

Types 442 and 446 are heat resistinggrades.

Type 409, which has the lowestchromium content of the stainless steels,is widely used for automotive exhaustsystems.

Tensile YieldStrength Strength

Temper Minimum Minimumksi MPa ksi MPa

1/4-Hard 125 862 75 5171/2-Hard 150 1034 110 7583/4-Hard 175 1207 135 931Full-Hard 185 1276 140 965

Table 9TYPICAL ENDURANCE LIMITS OFANNEALED CHROMIUM-NICKEL

STAINLESS STEEL SHEET (2)AISI EnduranceType limit, ksi MPa

301 35 241302 34 234303 35 241304 35 241316 39 269321 38 262347 39 269


10

5.5


11

Martensitic Stainless SteelsThe martensitic grades are so named

because when heated above their criticaltemperature (1600F or 870C) and cooledrapidly, a metallurgical structure known asmartensite is obtained. In the hardenedcondition the steel has very high strengthand hardness, but to obtain optimumcorrosion resistance, ductility, and impactstrength, the steel is given a stress-relieving or tempering treatment (usuallyin the range 300-700F (149-371C)).

Tables 12, 13 and 14 give the chemicalcompositions and mechanical propertiesof martensitic grades in the annealed andhardened conditions.

The martensitic stainless steels fall intotwo main groups that are associated withtwo ranges of mechanical properties:low-carbon compositions with a maximumhardness of about Rockwell C45 and thehigher-carbon compositions, which canbe hardened up to Rockwell C60. (Themaximum hardness of both groups in theannealed condition is about RockwellC24.) The dividing line between the twogroups is a carbon content ofapproximately 0.15%.

In the low-carbon class are Types 410,416 (a free-machining grade) and 403 (a“turbine-quality” grade). The properties,performance, heat treatment, andfabrication of these three stainless steelsare similar except for the bettermachinability of Type 416.

On the high-carbon side are Types440A, B, and C.

Types 420, 414, and 431, however, do not fit into either category. Type 420has a minimum carbon content of 0.15%and is usually produced to a carbonspecification of 0.3-0.4%. While it will notharden to such high values as the 440types, it can be tempered withoutsubstantial loss in corrosion resistance.Hence, a combination of hardness andadequate ductility (suitable for cutlery orplastic molds) is attained.

Types 414 and 431 contain 1.25 —2.50% nickel, which is enough to increasehardenability, but not enough to makethem austenitic at ambient temperature.The addition of nickel serves two purposes:(1) it improves corrosion resistancebecause it permits a higher chromiumcontent, and (2) it enhances toughness.

Martensitic stainless steels are subjectto temper embrittlement and should notbe heat treated or used in the range of800 to 1050F (427-566C) if toughness isimportant. The effect of tempering in thisrange is shown by the graph in Figure18. Tempering is usually performedabove this temperature range.

lmpact tests on martensitic gradesshow that toughness tends to decreasewith increasing hardness. High-strength(high-carbon) Type 440A exhibits lowertoughness than Type 410. Nickelincreases toughness, and Type 414 hasa higher level of toughness than Type410 at the same strength level.

Martensitic grades exhibit a ductile-brittle transition temperature at whichnotch ductility drops very suddenly. Thetransition temperature is near roomtemperature, and at low temperatureabout -300F (-184C) they become verybrittle, as shown by the data in Figure 19.This effect depends on composition,heat treatment, and other variables.

Clearly, if notch ductility is critical atroom temperature or below, and thesteel is to be used in the hardenedcondition, careful evaluation is required.If the material is to be used much belowroom temperature, the chances are thatquenched-and-tempered Type 410 willnot be satisfactory. While its notch ductilityis better in the annealed condition downto -100F (-73C), another type of stainlesssteel is probably more appropriate.

The fatigue properties of themartensitic stainless steels depend onheat treatment and design. A notch in astructure or the effect of a corrosiveenvironment can do more to reducefatigue limit than alloy content or heattreatment.

Figure 20 gives fatigue data for Type403 turbine quality stainless at three test temperatures. The samples weresmooth and polished, and theatmosphere was air.

Another important property is abrasionor wear resistance. Generally, the harderthe material, the more resistance toabrasion it exhibits. In applications wherecorrosion occurs, however, such as incoal handling operations, this generalrule may not hold, because the oxide filmis continuously removed, resulting in ahigh apparent abrasion/corrosion rate.

Other mechanical properties ofmartensitic stainless steels, such ascompressive yield shear strength, aregenerally similar to those of carbon andalloy steels at the same strength level.

Room-temperature physical propertiesof Type 410 are shown in Table 10. Theproperty of most interest is modulus ofelasticity. The moduli of the martensiticstainless steels (29 x 106 psi) (200 GPa)are slightly less than the modulus ofcarbon steel (30 x 106 psi) (207 GPa) butare markedly higher than the moduli ofother engineering materials, such asaluminum (10 x 106 psi) (67 GPa).

The densities of the martensiticstainless steels (about 0.28 Ib. per cu.in.) (7780 Kg/m3) are slightly lower thanthose of the carbon and alloy steels. As a result, they have excellent vibrationdamping capacity.

The martensitic stainless steels aregenerally selected for moderate resistanceto corrosion, relatively high strength, andgood fatigue properties after suitable heattreatment. Type 410 is used for fasteners,machinery parts and press plates. Ifgreater hardenability or higher toughnessis required, Type 414 may be used, andfor better machinability, Types 416 or416 Se are used. Springs, flatware, knifeblades, and hand tools are often madefrom Type 420, while Type 431 is frequentlyused for aircraft parts requiring high yieldstrength and resistance to shock. Cutleryconsumes most of Types 440A and B,whereas Type 440C is frequently used forvalve parts requiring good wear resistance.

High-carbon martensitic stainlesssteels are generally not recommendedfor welded applications, although Type410 can be welded with relative ease.Hardening heat treatments should followforming operations because of the poorforming qualities of the hardened steels.


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Precipitation HardeningStainless Steels

The principle of precipitation hardeningis that a supercooled solid solution(solution annealed material) changes itsmetallurgical structure on aging. Theprincipal advantage is that products canbe fabricated in the annealed conditionand then strengthened by a relativelylow-temperature 900-1150F (482-620C)treatment, minimizing the problemsassociated with high-temperaturetreatments. Strength levels of up to

260 ksi (1793 MPa) (tensile) can beachieved — exceeding even those ofthe martensitic stainless steels — whilecorrosion resistance is usually superior— nearly equal to that of Type 304stainless. Ductility is similar tocorresponding martensitic grades at thesame strength level. Table 15 shows thechemical composition and the nominalmechanical properties of four AlSlstandard precipitation hardeningstainless steels in solution treated andage hardened conditions.

Precipitation hardening stainlesssteels have high strength, relatively goodductility, and good corrosion resistanceat moderate temperatures. They areutilized for aerospace structuralcomponents, fuel tanks, landing gearcovers, pump parts, shafting, bolts,saws, knives, and flexible bellows-typeexpansion joints.

Physical properties of UNS S13800 areshown in Table 10.


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