the family tree of stainless steel

8/13/2019 The Family Tree of Stainless Steel

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The Family Tree of Stainless Steel

FIRST BRANCH

Chromium Containing

As we mentioned, to be a stainless, the iron base must contain at least 10.5% Cr.

and the carbon content is less that 1%. These two things made stainless "Steel"

totally different from mild "Steel."

The basic stainless with 1 to 1!% chromium are called "artensitic" #based on

the structure$ and hae the following characteristics&

Are magnetic

Can be hardened by "heat treatment"

'ae "(oor" welding characteristics

Common Uses:

)nife blades

Surgical instruments

*asteners

Shafts

S(rings

Common Grades:


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Stainless is designated by three different systems

1. etallurgical structure + artensitic

. rade& -10 #most used$, -0 #cutlery$, --0C #for ery high hardness$

. /nified umbering System /S& S-1000, S-000, S--00-

Second Branch

Chromium Containing

The second branch also contain 1 to 1!% chromium /T has a 2346 carbon

leel #less than 0.%$. Since the carbon is low, these grades hae a different

metallurgical structure and are called "*erritic" stainless steels. They hae the

following characteristics&

Are magnetic

CA3T be hardened by "heat treatment" #always used in the annealed orsoftened condition$

4eldability is still (oor

Common Uses:

Automotie e7haust and fuel lines

Architectural trim

Coo8ing utensils

an8 aults

Common grades:


1. etallurgical structure + *erritic

. rade& -09 #high tem(erature$, -0 #most used$

. /nified umbering System /S& S-0900, S-000

Third Branch

Nicel Containing:

4hen nic8el is added and the chromium leel is increased, the structure changes

again and it is called "Austenitic" and they hae the following characteristics&

Are 3T magnetic

CA3T be hardened by "heat treatment" /T CA be hardened by cold

wor8ing

'ae the "ST" corrosion resistance

Can be easily welded

'ae e7cellent cleanability and hygiene characteristics

'ae e7ce(tional resistance to both high and low tem(erature


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Common Uses:

)itchen sin8s

Architectural a((lications such as roofs and gutters, doors and windows,tubular frames

*ood (rocessing e:ui(ment

6estaurant food (re(aration areas

Chemical essels

3ens

'eat e7changers

Common Grades:


1. etallurgical structure + Austenitic

. rade& 0- #most used$, 10 #for high tem(erature$, 1; #for better

corrosion resistance$, 1< #for best corrosion resistance$

. /nified umbering System /S& S0-00, S1000, S1;00, S1C #see heat

resisting stainless steels$

They are suitable only for low concentrations of reducing acid #Su(er

Austenitics are aailable for higher acid leels$

?n cerices and shielded areas, there might not be enough o7ygen to

maintain the (assie o7ide film and creice corrosion might occur #Su(er

Austenitics, @u(le7 and Su(er *erritic are aailable in these situations$

ery high leels of halide ions, es(ecially the chloride ion can alsobrea8down the (assie surface film

#Su(er Austenitics and @u(le7 are aailable to withstand these conditions$

Fourth Branch

"u#le$ Stainless Steels

4hen the chromium content is high #1! to ;%$ and the nic8el content is low #-

to


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'igh resistance to stress corrosion crac8ing

?ncreased resistance to chloride ion attac8

ery weldable

'ae higher tensile and yield strengths than Ausenitic or *erritic stainless

steels

Common Uses:

Sea water a((lications

'eat e7changers

@esalination (lants

*ood (ic8ling (lants

Common Grades:

1. etallurgical structure + @u(le7

. rade& 05

. /nified umbering System /S& S1!0

Chemical Com#osition

Chemical Com#osition %

#a7 unless noted$

Stainless C n B S Si Cr i o

-10 0.15 1.00 0.0-0 0.00 0.500 11.50+

1.00

-0 0.1 1.00 0.0-0 0.00 1.000 1;.00+1!.00

0.


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Ho& is it made'

Stainless steel is (roduced in an electric arc furnace where carbon electrodes

contact recycled stainless scra( and arious alloys of chromium #and nic8el,

molybdenum etc. de(ending on the stainless ty(e$. A current is (assed through

the electrode and the tem(erature increases to a (oint where the scra( and alloysmelt. The molten material from the electric furnace is then transferred into an

A3@ #Argon 37ygen @ecarboni=ation$ essel, where the carbon leels are

reduced #remember stainless has a much lower carbon leel than mild steel$ and

the final alloy additions are made to ma8e the e7act chemistry. 7hibit 1 shows

the (rocess from melting and casting either into ingots or continually cast into a

slab or billet form. Then the material is hot rolled or forged into its final form.

Some material receies cold rolling to further reduce the thic8ness as in sheets or

drawn into smaller diameters as in rods and wire.

ost stainless steels receie a final annealing #a heat treatment that softens thestructure$ and (ic8ling #an acid wash that remoes furnace scale from annealing

and hel(s (romote the (assie surface film that naturally occurs$.

!IF( C)C!(

The fact that stainless steel has a great resistance to corrosion means that using

stainless will result in a ery long life com(ared to mild steel. Structures made

from stainless steel will last many times the normal life #well oer 100 years in

most cases$. So, while stainless steel is (robably more e7(ensie to buy in the

beginning ++ because it lasts a long time, it is usually chea(er in the long run

because there is little or no maintenance and re(air costs. A@, once the useful

life is oer, stainless steel is 100% 6CC2A2. Scra( stainless steel is

recharged into the electric furnaces for re+melting bac8 into stainless steel.

Stainless steel is a true "full life cycle" material.

*echanical +ro#erties

#Annealed condition$

Tensile Strength ield Strength longation 'ardness


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Stainless 8si Ba 8si Ba

-10


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Im#act resistance

++the austenitic microstructure of the 00 series (roides high toughness, from

eleated tem(eratures to far below free=ing, ma8ing these steels (articularly

suited to cryogenic a((lications.

!ong term .alue

++when the total life cycle costs are considered, stainless is often the least

e7(ensie material o(tion

0elding Stainless Steel

The stainless (ro(erties of stainless steels are (rimarily due to the(resence of chromium in :uantities greater than roughly 1 weight

(ercent. This leel of chromium is the minimum leel of chromium toensure a continuous stable layer of (rotectie chromium+rich o7ide forms

on the surface. The ability to form chromium o7ide in the weld regionmust be maintained to ensure stainless (ro(erties of the weld region afterwelding. ?n commercial (ractice, howeer, some stainless steels are soldcontaining as little as 9 weight (ercent chromium and will rust at ambient

tem(eratures.

Stainless steels are generally classified by their microstructure and areidentified as ferritic, martensitic, austenitic, or du(le7 #austenitic andferritic$. The microstructure significantly affects the weld (ro(erties and

the choice of welding (rocedure used for these stainless steel alloys. ?naddition, a number of (reci(itation+hardenable #B'$ stainless steels e7ist.

Breci(itation+hardenable stainless steels hae martensitic or austeniticmicrostructures.

?ron, carbon, chromium and nic8el are the (rimary elements found instainless steels and significantly affect microstructure and welding. 3ther

alloying elements are added to control microstructure or enhance material(ro(erties. These other alloys affect welding (ro(erties by changing thechromium or nic8el e:uialents and thereby changing the microstructureof the weld metal. enerally, 00 and 00 series alloys are mostly

austenitic and -00 series alloys are ferritic or martensitic, but e7ce(tionse7ist.

Stainless steels are subDect to seeral forms of locali=ed corrosie attac8.The (reention of locali=ed corrosie attac8 is one of the concerns whenselecting base metal, filler metal and welding (rocedures when fabricatingcom(onents from stainless steels.

Stainless steels are subDect to weld metal and heat affected =one crac8ing,the formation of embrittling second (hases and concerns about ductile tobrittle fracture transition. The (reention of crac8ing or the formation ofembrittling microstructures is another main concern when welding or

fabricating stainless steels.

0elding Austenitic Stainless Steels


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?deally, austenitic stainless steels e7hibit a single+(hase, the face+centered cubic #fcc$ structure, that is maintained oer a wide range of

tem(eratures. This structure results from a balance of alloying additions,(rimarily nic8el, that stabili=e the austenite (hase from eleated tocryogenic tem(eratures. ecause these alloys are (redominantly single

(hase, they can only be strengthened by solid+solution alloying or by wor8hardening. Breci(itation+strengthened austenitic stainless steels will bediscussed se(arately below.

The austenitic stainless steels were deelo(ed for use in both mild andseere corrosie conditions. Austenitic stainless steels are used at

tem(eratures that range from cryogenic tem(eratures, where they e7hibithigh toughness, to eleated tem(eratures, where they e7hibit good

o7idation resistance. ecause the austenitic materials are nonmagnetic,they are sometimes used in a((lications where magnetic materials are notacce(table.

The most common ty(es of austenitic stainless steels are the 00 and 00

series. 4ithin these two grades, the alloying additions ary significantly.*urthermore, alloying additions and s(ecific alloy com(osition can hae amaDor effect on weldability and the as+welded microstructure. The 00series of alloys ty(ically contain from ! to 0 weight (ercent i and from

1; to 5 weight (ercent Cr.

A concern, when welding the austenitic stainless steels, is thesusce(tibility to solidification and li:uation crac8ing. Crac8s can occur inarious regions of the weld with different orientations, such as centerline

crac8s, transerse crac8s, and microcrac8s in the underlying weld metalor adDacent heat+affected =one #'AE$. These crac8s are (rimarily due, to

low+melting li:uid (hases, which allow boundaries to se(arate under thethermal and shrin8age stresses during weld solidification and cooling.

en with these crac8ing concerns, the austenitic stainless steels aregenerally considered the most weldable of the stainless steels. ecause of

their (hysical (ro(erties, the welding behaior of austenitic stainlesssteels is different than the ferritic, martensitic, and du(le7 stainless steels.*or e7am(le, the thermal conductiity of austenitic alloys is roughly half

that of ferritic alloys. Therefore, the weld heat in(ut that is re:uired to

achiee the same (enetration is reduced. ?n contrast, the coefficient ofthermal e7(ansion of austenite is 0 to -0 (ercent greater than that of

ferrite, which can result in increases in both distortion and residualstresses, due to welding. The molten weld (ool of the austenitic stainlesssteels is commonly more iscous, or sluggish, than ferritic and martensiticalloys. This slows down the metal flow and wettability of welds in

austenitic alloys, which may (romote lac8+of+fusion defects when (oorwelding (rocedures are em(loyed.

0elding Ferritic Stainless Steels

*erritic stainless steels com(rise a((ro7imately half of the -00 seriesstainless steels. These steels contain from 10.5 to 0 weight (ercent


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chromium along with other alloying elements, (articularly molybdenum.*erritic stainless steels are noted for their stress+corrosion crac8ing #SCC$

resistance and good resistance to (itting and creice corrosion in chlorideenironments, but hae (oor toughness, es(ecially in the weldedcondition.

?deally, ferritic stainless steels hae the body+centered cubic #bcc$ crystal

structure 8nown as ferrite at all tem(eratures below their meltingtem(eratures. any of these alloys are subDect to the (reci(itation ofundesirable intermetallic (hases when e7(osed to certain tem(eratureranges. The higher+chromium alloys can be embrittled by (reci(itation of

the tetragonal sigma (hase, which is based on the com(ound *eCr.

olybdenum (romotes formation of the com(le7 cubic chi (hase, whichhas a nominal com(osition of *e;Cr1o10. mbrittlement increaseswith increasing chromium (lus molybdenum contents. ?t is generally

agreed that the seere embrittlement which occurs u(on long+terme7(osure is due to the decom(osition of the iron+chromium ferrite (hase

into a mi7ture of iron+rich al(ha and chromium+rich al(ha+(rime (hases.This embrittlement is often called "al(ha+(rime embrittlement."Additional reactions such as chromium carbide and nitride (reci(itation

may (lay a significant role in the more ra(id, early stage !!5 F*

embrittlement.

The ferritic stainless steels hae higher yield strengths and lowerductilities than austenitic stainless steels. 2i8e carbon steels, and unli8eaustenitic stainless steels, the ferritic stainless alloys e7hibit a transition

from ductile+to+brittle behaior as the tem(erature is reduced, es(eciallyin notched im(act tests. The ductile+to+brittle transition tem(erature

#@TT$ for the ultrahigh+(urity ferritic stainless steels is lower than thatfor standard ferritic stainless steels. ?t is ty(ically below roomtem(erature for the ultrahigh+(urity ferritic stainless steels. ic8eladditions lower the @TT and there by slightly increase the thic8nesses

associated with high toughness. eertheless, with or without nic8el, theferritic stainless steels would need engineering reiew for anything otherthan thin walled a((lications as they are (rone to brittle failure.

0elding *artensitic Stainless Steels

artensitic stainless steels are considered to be the most difficult of thestainless steel alloys to weld. 'igher carbon contents will (roduce greaterhardness and, therefore, an increased susce(tibility to crac8ing.

?n addition to the (roblems that result from locali=ed stresses associated

with the olume change u(on martensitic transformation, the ris8 ofcrac8ing will increase when hydrogen from arious sources is (resent in

the weld metal. A com(lete and a((ro(riate welding (rocess is needed to(reent crac8ing and (roduce a sound weld.

artensitic stainless steels are essentially alloys of chromium and carbonthat (ossess a body+centered cubic #bcc$ or body+centered tetragonal


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#bct$ crystal structure #martensitic$ in the hardened condition. They areferromagnetic and hardenable by heat treatments. Their general

resistance to corrosion is ade:uate for some corrosie enironments, butnot as good as other stainless steels.

The chromium content of these materials generally ranges from 11.5 to1! weight (ercent, and their carbon content can be as high as 1. weight

(ercent. The chromium and carbon contents are balanced to ensure amartensitic structure after hardening. artensitic stainless steels arechosen for their good tensile strength, cree(, and fatigue strength(ro(erties, in combination with moderate corrosion resistance and heat

resistance.

The most commonly used alloy within this stainless steel family is ty(e-10, which contains about 1 weight (ercent chromium and 0.1 weight(ercent carbon to (roide strength. olybdenum can be added to

im(roe mechanical (ro(erties or corrosion resistance. ic8el can beadded for the same reasons. 4hen higher chromium leels are used to

im(roe corrosion resistance, nic8el also seres to maintain the desiredmicrostructure and to (reent e7cessie free ferrite. The limitations onthe alloy content re:uired to maintain the desired fully martensiticstructure restrict the obtainable corrosion resistance to moderate leels.

0elding "u#le$ Stainless Steels

@u(le7 stainless steels are two (hase alloys based on the iron+chromium+nic8el system. @u(le7 stainless steels usually com(rise a((ro7imately

e:ual (ro(ortions of the body+centered cubic #bcc$ ferrite and face+centered cubic #fcc$ austenite (hases in their microstructure and generallyhae a low carbon content as well as, additions of molybdenum, nitrogen,tungsten, and co((er. Ty(ical chromium contents are 0 to 0 weight

(ercent and nic8el contents are 5 to 10 weight (ercent. The s(ecificadantages offered by du(le7 stainless steels oer conentional 00 series

stainless steels are strength, chloride stress+corrosion crac8ing resistance,and (itting corrosion resistance.

@u(le7 stainless steels are used in the intermediate tem(erature rangesfrom ambient to seeral hundred degrees *ahrenheit #de(ending on

enironment$, where resistance to acids and a:ueous chlorides isre:uired. The weldability and welding characteristics of du(le7 stainlesssteels are better than those of ferritic stainless steels, but generally not asgood as austenitic materials.

A suitable welding (rocess is needed to obtain sound welds. @u(le7stainless steel weldability is generally good, although it is not as forgiingas austenitic stainless steels. Control of heat in(ut is im(ortant.Solidification crac8ing and hydrogen crac8ing are concerns when welding

du(le7 stainless steels, but not as significant for some other stainlesssteel alloys.


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Current commercial grades of du(le7 stainless steels contain between and ; weight (ercent chromium, - to < weight (ercent nic8el, u( to -.5

weight (ercent molybdenum, as well as some co((er, tungsten, andnitrogen. odifications to the alloy com(ositions hae been made toim(roe corrosion resistance, wor8ability, and weldability. ?n (articular,

nitrogen additions hae been effectie in im(roing (itting corrosionresistance and weldability.

The (ro(erties of du(le7 stainless steels can be a((reciably affected bywelding. @ue to the im(ortance of maintaining a balanced microstructureand aoiding the formation of undesirable metallurgical (hases, the

welding (rocedures must be (ro(erly s(ecified and controlled. ?f thewelding (rocedure is im(ro(er and disru(ts the a((ro(riate

microstructure, loss of material (ro(erties can occur.

ecause these steels derie (ro(erties from both austenitic and ferritic

(ortions of the structure, many of the single+(hase base materialcharacteristics are also eident in du(le7 materials. Austenitic stainless

steels hae good weldability and low+tem(erature toughness, whereastheir chloride SCC resistance and strength are com(aratiely (oor.*erritic stainless steels hae good resistance to chloride SCC but hae

(oor toughness, es(ecially in the welded condition. A du(le7

microstructure with high ferrite content can therefore hae (oor low+tem(erature notch toughness, whereas a structure with high austenite

content can (ossess low strength and reduced resistance to chloride SCC.

The high alloy content of du(le7 stainless steels also ma8es them

susce(tible to the formation of intermetallic (hases from e7tendede7(osure to high tem(eratures. Significant intermetallic (reci(itation may

lead to a loss of corrosion resistance and sometimes to a loss oftoughness.

@u(le7 stainless steels hae roughly e:ual (ro(ortions of austenite andferrite, with ferrite being the matri7. The du(le7 stainless steels alloying

additions are either austenite or ferrite formers. This is occurs bye7tending the tem(erature range oer which the (hase is stable. Amongthe maDor alloying elements in du(le7 stainless steels chromium andmolybdenum are ferrite formers, whereas nic8el, carbon, nitrogen, and

co((er are austenite formers.

Com(osition also (lays a maDor role in the corrosion resistance of du(le7stainless steels. Bitting corrosion resistance can be adersely affected.To determine the e7tent of (itting corrosion resistance offered by thematerial, a (itting resistance e:uialent is commonly used.

0elding +reci#itation-Hardena/le Stainless Steels

Breci(itation+hardening #B'$ stainless steels are iron+chromium+nic8elalloys. They generally hae better corrosion resistance than martensitic

stainless steels. The high tensile strengths of the B' stainless steels isdue to (reci(itation hardening of a martensitic or austenitic matri7.


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Co((er, aluminum, titanium, niobium #columbium$, and molybdenum arethe (rimary elements added to these stainless steels to (romote

(reci(itation hardening.

Breci(itation+hardening stainless steels are commonly categori=ed into

three ty(es martensitic, semiaustenitic, and austenitic based on theirmartensite start and finish #s and f$ tem(eratures and the resulting

microstructures. The issues inoled in welding B' steels are different foreach grou(.

?t is im(ortant to understand the microstructure of the (articular ty(e ofalloy being welded. Some of the B' stainless steels solidify as (rimary

ferrite and hae relatiely good resistance to hot crac8ing. ?n other B'stainless steels, ferrite is not formed, and it is more difficult to weld thesealloys without hot crac8ing.

Ty#ical A##lications

T)+( US(

012

2ightweight structural com(onents and

(anels in trans(ort ehicles. Architecturalframewor8 and (anelling

0-G0-2

Coo8ware, sin8s, cutlery, cateringe:ui(ment, hos(ital e:ui(ment, food H

beerage e:ui(ment, abattoir e:ui(ment,

(harmaceutical e:ui(ment, cryogenic,(i(ewor8, tan8s and (rocess essels for alarge ariety of corrosie li:uids.

09G09S10G10S

'igh tem(erature o7idation #scaling$resistance, good high tem(eraturestrength. *urnace (arts, muffles, radianttubes

1;G1;21


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-0

Sin8s, washtroughs, trim for domestic

e:ui(ment, 8itchen and cafeteria utensils,cutlery.

C61

3re cars, freight cars, bus chassis, busframes, chutes, launders, buntons,

coneyor e:ui(ment and systems, tan8s.eneral material handling e:ui(ment

(articularly wet sliding abrasionconditions. Structural a((lications incorrosie industries, ladders, wal8ways,cable rac8s

Facts A/out Stainless

Conditions &hich fa.our use of stainless steel

Corrosie nironment

ery low #cryogenic$ tem(eratures (reent brittleness

'igh tem(eratures + (reent scale maintain strength

'igh strength s mass

'ygienic conditions re:uired + easy cleanibility

Aesthetic a((earance + no rust, thus no (aint necessary

o contamination + (reents catalytic reactions

@ischarge slideability from ho((ers

4et abrasion resistance

on+magnetic (ro(erties of austenitic grades

1ey considerations in &oring stainless steels

1N20 TH( *AT(RIA!

)nowledge im(roes decision ma8ing, aoids (roblems and saescosts

1N20 TH( GRA"( 2F *AT(RIA!

Correct material selection is ital + ris8 ta8ing is costly

1N20 TH( "(SIGN

ood design ensures saings for fabricator and user 1N20 SURFAC( FINISH(S

ood finishes (erform well, loo8 good and (romote sales

A++!) G22" H2US(1((+ING

ood house8ee(ing saes rectification costs A++!) ACCURAT( I"(NTIFICATI2N

2ost identity can (roe costly

A++!) +R2"UCTI2N +!ANNING

Blanning saes costs and (romotes :uality A++!) 1N20!("G(

6emember that additional information if always aailable through

SASS@A

2ther factors


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Thermal conducti.ityAll stainless steels hae a much lower conductiity than that of carbon

#mild$ steel. #Blain chromium grades I+ 1G and austenitic grades I+1G-$ This must be borne in mind for any o(eration which inoles hightem(erature, e.g. effects during welding #control of heat in(ut$ longer

times re:uired for heating to attain a uniform tem(erature for hot wor8ing

($#ansion coefficientBlain chromium grades hae an e7(ansion coefficient similar to carbon#mild$ steels, but that of the austenitic grades is I+ 1 1G timeshigher. The combination of high e7(ansion and low thermal conductiity

means that (recautions must be ta8en to aoid aderse effects, e.g.during welding use low heat in(ut, dissi(ate heat by use of co((er bac8ing

bars and use ade:uate Digging. This factor must also be considered incom(onents, which use a mi7ture of materials, e.g. a heat e7changer witha mild steel shell and austenitic grade tubes.

+assi.e film3#assi.ity

Stainless steels rely on a ery thin surface (assie film for their corrosionresistance. ?t is ital to maintain and (resere the integrity of the (assiefilm.

Aoid mechanical damage and contamination

6e(air any affected areas #e.g. high tem(erature scale adDacent to

a weld, mechanically damaged or ground areas$, by (assiation

only or by both (ic8ling H (assiation nsure a constant and sufficient aailability of o7ygen at the

surface of the stainless steel

Gailing3#icu#3sei4ingStainless Steels hae a tendency to gall, (ic8+u( or sei=e. To aoid this

ta8e (recautions such as *or surfaces e7(eriencing relatie motionminimise the load, ensure no heat build u(, 8ee( free of grit orcontaminants, use lubricants or surface coatings

3n threaded com(onents the threads must hae a high degree of surface

finish, com(onents should hae an intermediate to free fit, aoid oertor:ue and contamination of threads.

the family tree of stainless steel

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