Welding stainless steel

 　　When corrosion resistance is important, stainless steel is one of the most popular materials. Its abundant chromium and other alloy elements, as well as the structural characteristics of carbon steel, make it a very useful material for many projects. However, not all stainless steel is easy to weld, and it can be said that certain grades of stainless steel are not weldable at all. Therefore, to provide assistance, this article will focus on introducing some of the best grades of welded stainless steel.

　　austenitic stainless steel

　　Austenitic stainless steel can be welded together using many different welding processes. Some are more suitable for welding than others, such as 304, 308, 316, 321, and 347, all of which are weldable austenitic grades.

　　However, it should be noted that these grades may be subject to intergranular corrosion as their carbon content is relatively high. When chromium in stainless steel combines with carbon instead of forming a protective chromium oxide layer, intergranular corrosion occurs. Over time, these chromium carbides will corrode the area around the weld seam.

　　In order to reduce the risk of intergranular corrosion, alloys 304, 308, and 316 also have low-carbon forms. This is specified by the 'L' suffix, such as 304L. The lower carbon content in these grades allows chromium to form a protective chromium oxide layer, rather than being bound to carbon atoms. Other options include stable levels such as 321 or 347. This means using additives of titanium or niobium so that carbon can combine with chromium before it can. These two forms of intergranular corrosion prevention allow welding of austenitic stainless steel while reducing the risk of welding or Heat-affected zone failure.

　　Ferritic stainless steel

　　Generally speaking, ferritic stainless steel is the easiest stainless steel to weld. Although ferrite grades still contain chromium and other alloying elements, their content is reduced compared to austenitic stainless steel, making ferrite stainless steel easier to weld. Due to the low chromium content in ferritic stainless steel, intergranular corrosion is not a problem. This means that maintaining corrosion resistance is likely not an issue compared to applications using austenitic grades.

　　The low content of alloying elements also reduces the risk of thermal cracking during the welding process. If excessive heat input is used during welding, ferritic stainless steels may be affected by excessive grain growth in the Heat-affected zone. If this happens, there will be a loss of toughness and ductility. Ferritic stainless steel (such as 407 and 430) is an excellent choice for ferritic stainless steel in welding applications.

　　Martensite stainless steel

　　Due to the high carbon content, martensite stainless steel is more difficult to weld than ferrite or austenite. The increased carbon content, along with other alloying elements found in stainless steel, increases the chances of forming brittle microstructures. This may lead to weld cracking. In order to prevent cracking, some preventive measures must be taken. The hydrogen gas introduced into the weld seam during the welding process must be kept to a minimum to reduce the risk of hydrogen cracking. In addition, preheating and post weld heat treatment shall be used to reduce the brittleness of weld joints and Heat-affected zone.

　　The grades of martensite such as 403, 410 and 420 can be welded when proper heat treatment is carried out and proper filler metal is selected. However, stainless steel with martensite grade is considered almost impossible to weld. Care should be taken to avoid these when selecting martensite stainless steels for welding applications.

　　Other types of stainless steel

　　Duplex stainless steel is partially composed of austenite and partially ferrite. To achieve this mixed makeup, complex chemical components were used. Compared to other stainless steels, they typically have lower nickel and higher chromium content. They can be welded, but different types of duplex stainless steel must use different filler metals. For example, Grade 2205 requires welding with Grade 2209 filler metal.

　　Most of the strength and hardness of precipitation hardened stainless steel come from intermetallic precipitates that block the microstructure dislocations of the stainless steel. These precipitates are formed through special forms of heat treatment. When the heat generated by welding is applied to these materials, it can damage the original mechanical properties of precipitation hardened stainless steel. These materials must be heat treated again according to the material manufacturer's guidelines after welding. Common welding precipitation hardening stainless steels include 17-4PH and 17-7PH. It is important to choose the correct filler metal for all stainless steel (including these grades).