How to Weld Stainless Steel

Intro­duc­tion to Stain­less Steel

Before deal­ing with weld­ing stain­less steel, one should offer a loose descrip­tion of the mate­r­i­al. Stain­less steels rep­re­sent a class of iron-base mate­ri­als that have a cer­tain resis­tance to rust­ing and cor­ro­sion in some envi­ron­ments, due to the pres­ence of chromi­um in their com­po­si­tion. Chromi­um helps pro­duce a tough, imper­vi­ous lay­er of chromi­um oxide on the material’s sur­face, which shields the sur­face from rust and cor­ro­sion. One should be aware that the expres­sion ​“stain­less steel” rep­re­sents a huge class of dif­fer­ent mate­ri­als. It is not a tech­ni­cal term iden­ti­fy­ing any spe­cif­ic met­al and can­not be used for prac­ti­cal pur­pos­es like purchasing. 

Three Class­es

The three more gen­er­al class­es of stain­less steels – Austenitic, Fer­rit­ic, and Marten­sitic – are indi­cat­ed by ref­er­ence to their met­al­lur­gi­cal struc­ture. More specif­i­cal­ly, they use an iden­ti­fi­er that refers to the appear­ance of their micro-struc­ture, as seen under the micro­scope or by x‑ray dif­frac­tion. These micro-struc­tures may be present in cer­tain steel, so they are used to indi­cate the pre­vail­ing struc­ture in the stain­less steel. The prop­er­ties of each class can affect the weld­ing process in dif­fer­ent ways, so it is impor­tant to iden­ti­fy which type is being used ahead of time. 

1. Austenitic

When weld­ing stain­less steel, Austenitic stain­less steels are con­sid­ered the most eas­i­ly weld­ed of the three class­es. They are known as the ​“300 series”, which refers to a stan­dard clas­si­fi­ca­tion orig­i­nat­ed by the Amer­i­can Iron and Steel Insti­tute (AISI) and the Soci­ety of Auto­mo­tive Engi­neers (SAE). An impor­tant sub-class, known as ​“18/8”, has alloy­ing ele­ments that are 18 per­cent chromi­um and eight per­cent nickel. 

Main char­ac­ter­is­tics of Austenitic stain­less steels are as follows: 

• Not mag­net­ic or only slight­ly magnetic
• Not attacked by a 10 per­cent solu­tion of nitric acid (HNO3) in alcohol
• Does not hard­en by any heat treatment
• Strain hard­en­ing — Quite duc­tile and eas­i­ly deformable by mechan­i­cal work­ing which increas­es both hard­ness and strength
• Eas­i­ly weld­ed, with the nec­es­sary precautions
• Ther­mal con­duc­tiv­i­ty between one third and one half that of oth­er steels
• Coef­fi­cient of ther­mal expan­sion by 30 – 40 per­cent, even 50% at times

When weld­ing stain­less steels, the two last char­ac­ter­is­tics affect the out­come in var­i­ous ways, pro­duc­ing larg­er dis­tor­tion than as found in oth­er steels. 

Not all austenitic stain­less steels of the 300 series have equal weld­abil­i­ty. The addi­tion of sul­fur or sele­ni­um used to improve machin­abil­i­ty (as in Type 303) results in severe weld hot-crack­ing, which makes this par­tic­u­lar mate­r­i­al ​“non-weld­able”.

Use cau­tion when weld­ing austenitic stain­less steels. The cor­ro­sion resis­tant char­ac­ter­is­tics of these stain­less steels may be adverse­ly affect­ed by the sen­si­ti­za­tion process, which occurs at tem­per­a­ture inter­vals from 600 to 900 Cel­sius (1100 to 1650 Fahren­heit). This pro­motes the gath­er­ing of chromi­um car­bides at grain bound­aries and the par­al­lel loss of anti­cor­ro­sive chromi­um from the base metal. 

The above tem­per­a­ture range occurs nat­u­ral­ly, not in the weld zone where tem­per­a­ture is high­er and lasts only for a short time, but in the two strips of met­al on each side of the weld bead. This is the Heat Affect­ed Zone (HAZ) where the harm­ful effects take place. 

In a sen­si­tized joint, the chromi­um, which is the main ​“stain­less” ingre­di­ent, becomes sequestered or tak­en out of play and local­ly unavail­able for the pro­tec­tive action. If not addressed cor­rect­ly, weld­ing stain­less ¹⁸⁄₈ steels may cause the loss of their pro­tec­tive prop­er­ty along sen­si­tized paths, and the weld­ed mate­r­i­al becomes prone to inter­gran­u­lar attack in a cor­ro­sive environment. 

There are three strate­gies that can be employed to cut down on the adverse effects of the sen­si­ti­za­tion process in series 300 stain­less steels. One is to use a very low car­bon ver­sion (i.e. 304L where L stands for low-car­bon) where not much car­bon is avail­able for mak­ing chromi­um carbides. 

Anoth­er strat­e­gy is to use a dif­fer­ent type of base met­al that includes tita­ni­um (type 321) or columbi­um (type 347), which will form tita­ni­um or columbi­um car­bides, caus­ing the car­bon to become unavail­able for chromi­um dur­ing the sen­si­ti­za­tion process. This leaves the chromi­um free to per­form its anti-cor­ro­sive tasks. 

Note: The filler met­al for this mate­r­i­al, if required, should always be columbi­um. Why? Because tita­ni­um is reac­tive and is not read­i­ly recov­ered dur­ing depo­si­tion. This means it would not be avail­able when need­ed most. Columbi­um how­ev­er is not reac­tive. It will stay put dur­ing the melt­ing process, and when the mate­r­i­al is heat­ed to the sen­si­ti­za­tion tem­per­a­ture range, it will do the job of pro­duc­ing columbi­um car­bides, sav­ing the day.

The third strat­e­gy is to per­form a solu­tion heat treat­ment at ele­vat­ed tem­per­a­ture (1050 Cel­sius or 1900 Fahren­heit), which would repair cor­ro­sion sus­cep­ti­bil­i­ty. This strat­e­gy puts in a sol­id solu­tion of chromi­um car­bides, which orig­i­nat­ed dur­ing the weld­ing sen­si­ti­za­tion process of reg­u­lar ¹⁸⁄₈ stain­less steel. This process, how­ev­er, con­tends with prob­lems like heavy oxide for­ma­tion if not per­formed in a vac­u­um or oth­er pro­tec­tive atmos­phere, free from con­t­a­m­i­nates. Type 309 and 310, used for ele­vat­ed tem­per­a­ture appli­ca­tions, and type 316 or bet­ter type 316L used for enhanced cor­ro­sion resis­tance, are gen­er­al­ly not prone to sen­si­ti­za­tion and are used with filler wires of sim­i­lar composition. 

2. Fer­rit­ic

The sec­ond class of stain­less steel is called fer­rit­ic stain­less steel. This steel is fer­ro­mag­net­ic, but can­not be hard­ened by heat treat­ment. This is a com­mon type of stain­less steel used in car exhaust com­po­nents. A lim­it­ed amount of fer­rit­ic struc­ture, when present in an oth­er­wise main­ly austenitic struc­ture, is con­sid­ered ben­e­fi­cial in that it reduces the chances of hot crack­ing. Weld­ing stain­less fer­rit­ic steels can read­i­ly be per­formed using arc weld­ing process­es, either with fer­rit­ic or austenitic filler met­al. A post weld heat treat­ment may be need­ed to improve properties. 

3. Marten­sitic

Marten­sitic stain­less steels are mag­net­ic and ful­ly able to be hard­ened through heat treat­ment. Weld­ing stain­less steel of this type is not rec­om­mend­ed, although fea­si­ble with spe­cial tech­niques. Weld­ing may pro­duce cracks, espe­cial­ly if car­bon con­tent is not suf­fi­cient­ly low. Pre­heat and post heat treat­ments may be necessary. 

One Final Class 

To com­plete the weld­ing stain­less steel overview, one should men­tion a fourth class of mate­ri­als not list­ed above – pre­cip­i­ta­tion hard­en­able (PH) stain­less steels, which are quite read­i­ly weld­able. How­ev­er pre­cise instruc­tions should be fol­lowed con­cern­ing heat treat­ments in order to devel­op the required properties.

Weld­ing Process­es for Stain­less Steel 

There are many dif­fer­ent types of weld­ing that can be used when weld­ing stain­less steel. They all have their advan­tages and dis­ad­van­tages, and they all require spe­cif­ic instruc­tions to ensure a prop­er, strong weld every time. 

Fric­tion Welding 

Fric­tion weld­ing stain­less steels presents almost no prob­lems, except for the free cut­ting types of stain­less steel that should not be weld­ed at all. It is used for weld­ing stain­less steel not only to oth­er stain­less steel work pieces, but also to dif­fer­ent met­als like cop­per and alu­minum. One should always be aware of the mate­r­i­al type and con­di­tion before weld­ing, as well as the effects of heat near the joint. Some ele­ments like sul­fur or sele­ni­um can com­pro­mise the final sound­ness of weld­ed joints. 

Resis­tance Welding 

Resis­tance weld­ing can be used in most stain­less steels. 300 series austenitic steel of the 300 series can read­i­ly use resis­tance weld­ing, as can fer­rit­ic steels. How­ev­er, marten­sitic stain­less steels can pose a prob­lem because the weld result is brit­tle, if not soft­ened ade­quate­ly by a post weld tem­per­ing treatment. 

The resis­tance weld­ing process is cur­rent­ly used on stain­less steel with adap­ta­tions to deal with the dif­fer­ences in the elec­tri­cal resis­tance and low ther­mal con­duc­tiv­i­ty, as well as high coef­fi­cient ther­mal expan­sion, high­er melt­ing tem­per­a­ture, and high strength at ele­vat­ed tem­per­a­tures. Elec­trode force is more ele­vat­ed, while time and cur­rent are less for low car­bon steels. 

All stain­less steels must not only be cleaned of dirt, oil, grease, or paint before the resis­tance weld­ing process (or any weld­ing process), but they must also be cleaned of the nat­u­ral­ly form­ing chromi­um oxide lay­er. This has to be removed with a stain­less steel wire brush. 

Arc Weld­ing

Arc weld­ing can be used when weld­ing stain­less steel, as long as the prop­er flux is used. This makes the process much less viable to TIG weld­ing, unless there is no oth­er choice avail­able. TIG weld­ing of stain­less steel requires elim­i­nat­ing all traces of resid­ual flux on the part after the weld­ing process, which length­ens the oper­a­tion and increas­es costs. Arc weld­ing is com­mon­ly used, pay­ing close atten­tion to the class and the con­di­tion of the mate­r­i­al being weld­ed, as well as keep­ing an eye on sen­si­ti­za­tion and deformations. 

All types of arc process­es can be employed for weld­ing stain­less steels with due atten­tion to joint shape, dimen­sions and prepa­ra­tion. In par­tic­u­lar Shield­ed Met­al Arc Weld­ing (SMAW) is wide­ly used because of its flex­i­bil­i­ty. It should be not­ed that elec­trodes come in two types con­cern­ing the cov­er com­po­si­tion, which may influ­ence the choice of the cur­rent employed. 

There are many filler met­als that can be used dur­ing the arc process. The clas­si­fi­ca­tion for stain­less steel filler met­als can be found in the Amer­i­can Weld­ing Society’s AWS A5.9/A5.9M:2006 – Spec­i­fi­ca­tion for Bare Stain­less Steel Weld­ing Elec­trodes and Rods. 

Elec­tron Beam Welding 

Elec­tron beam weld­ing (EBW) of stain­less steels is read­i­ly per­formed with good results, even in very deep welds. The remark­ably high depth to width ratio per­mits EBW to join con­fig­u­ra­tions not pos­si­ble with oth­er means. With the heat input being low and the heat affect­ed zone hav­ing lim­it­ed extent, there is often no remark­able dam­age to the mechan­i­cal prop­er­ties so that fur­ther heat treat­ment is not required. 

Laser Weld­ing

Laser weld­ing can also be used when weld­ing stain­less steels, as long as pre­cau­tions are in plate to insu­late the weld­ing from the air and to lim­it the dam­age prop­er­ties obtained dur­ing the heat­ing treatment. 

In the end, stain­less steel weld­ing is not a com­pli­cat­ed process at all. It just takes some atten­tion to detail when it comes to work piece mate­ri­als, filler met­als, and the type of weld­ing being used. If all of that is up to par, you can be suc­cess­ful weld­ing your stain­less steel parts. 

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