Types of Stainless Steel
The 200 and 300 series are austenitic. They have high corrosion resistance and are highly formable but are prone to stress cracking. Also considered the most easily weldable family of the stainless steels.
They exhibit good toughness because they are non-hardenable on cooling, and there is no need for pre or post-weld heat treatment. Austenitic alloys are normally welded with fillers with matching composition to the base material; in specific cases, type 308 filler is used for alloys 302 and 304 and type 347 filler used for 321.
Austenitic Stainless Available at Online Metals
301, 302, 303, 304, 316, 321, 347, N50 and N60
All ferritic alloys are in the 400 family, but not all 400s are ferritic. They have lower ductility, are more brittle, prone to hot cracking, and posses lower corrosion resistance than the austenitic grades. But they offer higher resistance to stress corrosion cracking. This type is generally considered to have poor weldability.
Ferritic stainless steels become fully ferritic at high temperatures and undergo rapid grain growth. This leads to brittle, heat affected zones in the fabricated product. This is what gives this type its negative welding qualities.
When ferritic alloys are being welded, it is generally welded in thin sections less than 6mm thick. At this size, any loss of toughness is less significant. Thinker sections (0.25″+) have a higher risk of cracking during fusion. When welding the ferritic stainless steels, filler metals should be used which match or exceed the chromium level of the base alloy. 409 and 430 are commonly used as fillers, and austenitic types 309 and 312 for dissimilar joints.
Ferritic Stainless Available at Online Metals
Martensitic stainless are 400 and 500 series. These alloys have higher strength, wear resistance and fatigue resistance than the austenitic and ferritic grades but a lower corrosion resistance. Martensitic steel becomes hard and brittle upon cooling, making it a great material for wear resistance but more difficult to weld as it has a tendency toward weld cracking on cooling. However, martensitic stainless steel can be successfully welded if careful precautions are taken to avoid cracking in the heat affected zone. The filler metals used should generally match the chromium and carbon content of base martensitic metal. Type 410 filler can be used to weld types 402, 410, 414 and 420 steels. Austenitic types 308, 309 and 310 are also used when welding martensitic steels to themselves or dissimilar metals.
Martensitic Stainless Available at Online Metals
410, 420, 440
Precipitation-hardening stainless steels contain both chromium and nickel, which provide an optimum combination of the properties of martensitic and austenitic grades. Similar to martensitic grades, these steels are known for their ability to acquire high strength through heat treatment, while possessing the corrosion resistance of austenitic grades. Precipitation-hardening steels can be readily welded using procedures similar to those used for the 300 stainless steels. Grade 17-4 PH in particular is commonly used for welding (filler 17-7 PH is recommended), and can be successfully welded without preheating. As with many other alloys, achieving the same mechanical properties in the weld as in parent material is difficult in the precipitation-hardening series, even when utilizing a matching filler. Heat treating after welding can be used to help the weld metal achieve close to the similar properties as the parent metal.
Precipitaion-Hardening Stainless Available at Online Metals
455, 13-8, 15-5, 17-4
Duplex stainless steels are “duplex” because they possess a two-phase microstructure consisting of grains of both ferritic and austenitic stainless steel. These steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades. They do, however, share a comparable corrosion resistance to the austenitic steels. Modern duplex steels are readily weldable, but the procedure, especially maintaining the heat input range, must be strictly followed. Thanks to the greater complexity of the material’s chemical composition, too much heat also adversely affects duplex stainless steels. Likewise, because Duplex alloys have portions of both austenitic and ferritic stainless, selecting a filler metal is slightly more challenging. Many types of duplex stainless base metals are not available as filler metals due to the fact that filler metal cools much more quickly than the base metal.