Stainless steels represent a class of iron base materials which have a certain resistance to rusting and corrosion in some environments, due to the presence in their composition of at least 12% of the element Chromium. The reason of this behavior is that chromium helps produce on the material's surface a tough and impervious layer of chromium oxide, which is the shield protecting the surface from rust and corrosion.
One should be aware of the fact that the expression stainless steel represents a huge class of different materials. It is not a technical term; therefore it cannot identify any specific metal, and cannot be used for any practical purpose such as purchasing.

ThreeClasses
The three more general classes of stainless steels, Austenitic, Ferritic and Martensitic, are indicated by reference to their metallurgical structure. More specifically they are called by using an identifier which refers to the appearance of their micro-structure as seen under the microscope or by another very special technique called x-ray diffraction. This of course has influence on their welding-stainless properties. The microstructures identified by those names may be present at the same time in various proportions in steel. Therefore they are used to indicate the prevailing structure.

Austenitic
Austenitic stainless steels are considered the most weldable. They are known as the 300 series, which refers to a standard classification originated by the AISI (American Iron and Steel Institute) and by the SAE (Society of Automotive Engineers). An important sub-class is also known as "18/8" (meaning that the important alloying elements of these steels are about 18% Chromium and 8% Nickel).
Main characteristics of Austenitic stainless steels:

Not magnetic or only slightly magnetic.
Not attacked by a 10% solution of Nitric Acid (HNO3) in alcohol.
Unhardenable by any heat treatment.
Quite ductile and easily deformable by mechanical working which increases both hardness and strength. This characteristic is called strain hardening.
Easily welded, with the needed precautions.
Thermal conductivity only between one third and one half that of other steels.
Coefficient of thermal expansion by 30-40%, even 50% greater.
In welding stainless, these two last characteristics variously affect the outcome, producing larger distortion than other steels.
Not all austenitic stainless steels of the 300 series are equally weldable. The addition of sulfur or selenium used to improve machinability (type 303) results in severe weld hot-cracking, which makes this particular material "non-weldable".

Avoiding WeldingProblems
The corrosion resistant characteristics of stainless steels may be adversely affected by the sensitization process occurring in a certain temperature interval from about 600-900 degrees Celsius (1100-1650 degrees Fahrenheit) which promotes the precipitation (gathering) of chromium carbides at grain boundaries and the parallel loss of anti-corrosive chromium from the base metal.
This temperature range naturally occurs at the welding stainless location, where temperature is higher and lasts only for a short time, but in two strips of metal on both sides of the weld bead. This is the so called 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 loose the loss of their protective property along sensitized paths. The welded material becomes prone to intergranular attack in a corrosive environment.

Three Strategies toOpposeWeakening
First, use a very low carbon version (i.e. 304L where L stands for low-carbon) so chromium carbides are not created.
Secondly, use a different type of base metal including an amount of titanium (type 321) or columbium (also known as niobium - type 347) which tends to form readily titanium carbides (or columbium carbides) (and by this action the carbon becomes unavailable for chromium) at sensitization temperatures leaving the chromium free to perform its anticorrosive task.
Note that the filler metal for this material, if required, should be always of type 347. Why? Because titanium (in type 321) being reactive, is not readily recovered during deposition, so that it would not be available when it is needed most. Columbium however is not reactive, it will stay put through melting, and, when the material is heated to the "sensitization" temperature, will do its job of producing columbium carbides in preference to chromium carbide, and so it will save the day.
The third strategy is performing a solution heat treatment at elevated temperature (1050 degrees Celsius or 1900 degrees Fahrenheit), for repairing a condition of corrosion susceptibility. This puts again in solution (called solid solution) the chromium carbides originated during Welding-stainless sensitization of regular 18/8 stainless (like types 302 or 304). This process however must contend with problems of heavy oxide formation if not done in vacuum or protective atmosphere, and of distortions.
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.
 
Ferritic
Ferritic stainless steels are ferromagnetic but they cannot be hardened by heat treatment. A limited amount of ferritic structure, when present in an otherwise mainly austenitic structure, reduces the chances of hot cracking. Welding stainless ferritic steels can be performed using arc processes, either with ferritic or with austenitic filler metal, except that a post weld heat treatment may be needed for improving properties. Car exhaust components may be made out of these materials.

Martensitic
Martensitic stainless steels are magnetic and fully hardenable by heat treatment. Welding-stainless 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 postheat may be necessary.

Cleaning: Not only must stainless steels be cleaned of external surface dirt, oil, grease or paint before welding, but the naturally forming chromium oxide layer should be removed with a stainless steel wire brush as well.

Processes
Friction welding stainless steels presents almost no problems, except for the free cutting types that should not be welded at all. It is used for Welding-stainless steel not only to itself but also to quite different materials like copper or aluminum and other combinations. One should always be aware of the type and material condition before welding and of the effects of heat near the joint and that some elements (sulfur, selenium) or very high hardness originating while welding could compromise the final soundness of welded joints.
Resistance process for welding stainless is currently used, with due adaptations deriving from differences in high electrical resistance and low thermal conductivity, high coefficient of thermal expansion, higher melting temperature and higher strength at elevated temperature. Electrode force is more elevated, while time and current are less than for low carbon steels. Resistance welding stainless Austenitic steel of the 300 series is readily performed. Ferritic steels are also welded. But martensitic, hardenable, stainless steels are problematic as the welds result brittle if not softened adequately by a post weld tempering treatment.
Gas Welding can be used for Welding-stainless steel but the use of a proper flux is required. This makes the process much less attractive than Gas Tungsten Arc Welding (see in the following) unless there is no other choice, because of the imperative need to eliminate all traces of residual flux on the part after welding: this introduces an additional operation which might increase the cost.
Arc Welding is commonly used for welding stainless steel with due attention being paid to the class and to the condition of the material and to the influence of the process on such consequences as sensitization or deformations.
Electron Beam welding of stainless steels produces good results even in deep welds. As usual, the remarkably high depth to width ratio permits to join configurations not possible with other means. The heat input being low and the heat affected zone of limited extent, there is often no remarkable damage to the mechanical properties so that further heat treatment is not required.
Laser Beam welding is performed on stainless steels, with the usual precautions needed to insulate the weld from air and to limit the damage to properties obtained by heat treatment.

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