Types of Stainless Steel
Stainless steels are created from the addition of alloys, namely chromium and nickel, to steel. Depending on their composition, stainless steels are classified in to different grades that fall into four main types: ferritic, martensitic and precipitation hardening, duplex, and austenitic. Nickel plus carbon, manganese and nitrogen forms austenite stainless steel. Chromium plus silicon, molybdenum and niobium forms ferrite stainless steel. The structure of welds in stainless steels can be largely predicted based on their chemical composition. Because of their different microstructures, the alloy groups have both different welding characteristics and are susceptible to different welding defects.
Austenitic stainless steels are 200 and 300 series stainless. They have high corrosion resistance and are highly formable but are prone to stress cracking. They are considered to be the most easily weldable family of the stainless steels. Comparatively little trouble is experienced in making satisfactory welded joints in their physical characteristics and mechanical properties are given proper consideration. Austenitic alloys are also commonly used for welded fabrications because they can be readily welded using any of the arc welding processes. They exhibit good toughness because they are non-hardenable on cooling, and there is no need for pre- or post-weld heat treatment. Austenitic stainless steels 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: sheet 302: balls, sheet 303: hex bar, rectangle bar, round bar, square bar 304: round bar, channel, sheet/plate, expanded sheet, perforated sheet, textured sheet, tread sheet, threaded rod, rectangle tube, round tube, square tube 304/304L: angle, hex bar, rectangle bar, round bar, square bar, pipe, round tube 316: balls, round bar, threaded rod, rectangle tube 316/316L: angle, hex bar, rectangle bar, round bar, square bar, pipe, sheet/plate, round tube, square tube 321: hex bar, round bar 347: hex bar, round bar N50: round bar N60: round bar
Ferritic stainless steels are 400 series. They have lower ductility and lower corrosion resistance than the austenitic grades but offer high resistance to stress corrosion cracking. The ferritic stainless steels are generally considered to have poor weldability when compared to the austenitic stainless steels, as the brittleness and poor ductility of these materials limit their applications in the welded condition. Ferritic stainless steels become fully ferritic at high temperatures and undergo rapid grain growth, which leads to brittle, heat affected zones in the fabricated product. They possess reduced formability, susceptibility to embrittlement, susceptibility to hot cracking, and an adverse effect on their mechanical properties (toughness and ductility) when welded. If welding, ferritic stainless steels are generally welded in thin sections, most less than 6mm in thickness where any loss of toughness is less significant. Thinker sections (>1/4 inches) 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; types 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: sheet 420: rectangle bar, square bar 440: balls 440C: rectangle bar, round bar, square bar
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: round bar 13-8: round bar 15-5: hex bar, round bar, sheet 17-4: round bar, rectangle bar, square bar, sheet
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.