Weldability Details for Different Stainless Steel Types

Stainless steel is made by adding alloys, namely chromium and nickel, to steel. According to the composition of stainless steel, stainless steel is divided into four main types: ferritic, martensitic and precipitation hardening, duplex and austenitic. Nickel plus carbon, manganese and nitrogen form austenitic stainless steels. 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 characteristics and are susceptible to different welding defects.

Austenitic stainless steels are 200 and 300 series stainless steels. They are highly corrosion resistant, highly formable, but prone to stress cracking. They are considered the easiest stainless steel series to weld. Relatively little trouble has been gone through in producing a satisfactory welded joint with physical characteristics and due consideration has been given to the mechanical properties. Austenitic alloys are also commonly used in welded fabrication because they can be easily welded using any arc welding process. They exhibit good toughness because they are non-hardenable on cooling and do not require pre-welding or post-weld heat treatment.
Ferritic stainless steels are the 400 series. They have lower ductility and lower corrosion resistance than austenitic stainless steels, but higher resistance to stress corrosion cracking. Ferritic stainless steels are generally considered to have poor weldability compared to austenitic stainless steels, as the brittleness and poor ductility of these materials limit their application in welding conditions. Ferritic stainless steels become fully ferritic at high 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 it is welded, ferritic stainless steel is generally welded in the thin part, the thickness is at least not more than 6mm, and the loss of toughness is small. Thinker sections (> 1/4 inch) 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 the 400 and 500 series. These alloys have higher strength, wear and fatigue resistance but lower corrosion resistance than austenitic and ferritic stainless steels. Martensitic steel becomes hard and brittle as it cools, making it a wear-resistant material, but more difficult to weld due to its tendency to crack the weld as it cools. However, martensitic stainless steels can be successfully welded if care is taken to avoid cracks in the heat-affected zone. The filler metal used should generally match the chromium and carbon content of the base martensitic metal. Type 410 filler can be used to weld Type 402, 410, 414 and 420 steels. Austenitic types 308, 309 and 310 are also used when welding martensitic steels to themselves or to dissimilar 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 and are known for their ability to achieve high strength through heat treatment, while having the corrosion resistance of austenitic stainless steels. Precipitation hardened steels can be easily welded using similar procedures as 300 stainless steel. A PH grade of 17-4 in particular is 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 using matching fillers. Post-weld heat treatment can be used to help the weld metal achieve properties similar to the base metal.