Stainless steel is made by adding alloys, namely chromium and nickel, to steel. Depending on their composition, stainless steels are divided into four main types: ferritic, martensitic and precipitation hardening, duplex and austenitic. Nickel adds carbon, manganese and nitrogen to form austenitic stainless steel. Chromium plus silicon, molybdenum and niobium form ferritic stainless steels. The structure of stainless steel welds can be largely predicted based on their chemical composition. Due to their different microstructures, alloy groups have different welding properties and are susceptible to different welding defects.
Austenitic stainless steels are 200 and 300 series stainless steels. They have high corrosion resistance, are highly formable, but are prone to stress cracking. They are considered the easiest stainless steel series to weld. Relatively little trouble is experienced in producing satisfactory physical properties of the welded joint, and mechanical properties are given due consideration. Austenitic alloys are also commonly used in welded fabrication as they can be easily welded using any arc welding process. They exhibit good toughness because they are not hardenable when cooled and do not require pre-welding or post-weld heat treatment.
Ferritic stainless steels are 400 series. Compared to austenitic stainless steels, they have lower ductility and lower corrosion resistance, but higher resistance to stress corrosion cracking. Compared to austenitic stainless steels, ferritic stainless steels are generally considered to have poor weldability because the brittleness and poor ductility of these materials limit their use in welding conditions. Ferritic stainless steels become fully ferritic at elevated temperatures, undergoing rapid grain growth, resulting in brittle heat-affected zones in the finished product. They have reduced formability, susceptibility to embrittlement, susceptibility to thermal cracking, and adverse effects on their mechanical properties (toughness and ductility) when welded. If welded, ferritic stainless steels are generally welded in thin parts, with a thickness of at least 6mm, and the loss of toughness is small. Thinker sections (> 1/4") have a higher risk of cracking during fusion. When welding ferritic stainless steels, filler metals that match or exceed the chromium content of the base alloy should be used; Types 409 and 430 are commonly used as fillers, and austenitic types 309 and 312 are used for different joints.
Martensitic stainless steels are 400 and 500 series. These alloys have higher strength, wear and fatigue resistance than austenitic and ferritic stainless steels, but lower corrosion resistance. Martensitic steel becomes hard and brittle as it cools, making it a wear-resistant material, but it is more difficult to weld due to its tendency to weld cracks as it cools. However, martensitic stainless steels can be successfully welded if careful measures are taken to avoid cracking in the heat affected zone. The filler metal used should generally match the chromium and carbon content of the base martensitic metal. Type 410 fillers can be used to weld 402, 410, 414 and 420 sections. Austenitic types 308, 309 and 310 are also used when welding martensitic steels to themselves or to different metals.
Precipitation hardening stainless steels contain both chromium and nickel, providing the best combination of martensitic and austenitic stainless steels. These steels are similar to martensitic grades, known for their ability to achieve high strength through heat treatment, while having the corrosion resistance of austenitic stainless steels. Precipitation hardened steel can be easily welded using a similar procedure to 300 stainless steel. In particular, PH grades 17-4 are commonly used for soldering (filler 17-7 PH is recommended) and can be successfully soldered without preheating. As with many other alloys, it is difficult to achieve the same weld mechanical properties as the base metal in the precipitation hardening series, even when matched fillers are used. Heat treatment after welding can be used to help the weld metal achieve properties similar to the base metal.
