Which elements in the composition of stainless steel have an impact on the microstructure

The common metallographic structures of stainless steel are martensite, austenite and ferrite. The elemental composition of the steel has some different effects on these metallographic structures. For instance, for chromium-manganese martensitic stainless steel, the elements that have an impact on its metallographic structure mainly include chromium, carbon and molybdenum; while for chromium-manganese-nickel martensitic stainless steel, the elements that have an impact mainly include nickel, molybdenum, aluminum, cobalt, nitrogen and titanium. 

In martensitic chromium-nickel stainless steel, the chromium and carbon present in the material interact with each other, resulting in the formation of a relatively stable r-phase region and a stable a+r-phase region at high temperatures. An increase in carbon content will expand the r-phase region. However, as the chromium content increases, the solubility limit of carbon also decreases. Therefore, adding nickel to martensitic chromium-nickel stainless steel solves the problem of martensitic stainless steel, which sacrifices hardness to increase corrosion resistance at the cost of losing its quenching ability. However, the nickel content in the steel should not be too high, otherwise, due to the expansion of the austenite phase region by nickel and the decrease in Ms temperature, the stainless steel will turn into austenitic stainless steel, completely losing its quenching ability. 

The elements that affect the microstructure of ferritic stainless steel mainly include chromium, molybdenum, carbon, nitrogen and nickel. In addition, some ferritic stainless steels also contain elements such as titanium, niobium and copper, which also have certain effects on the metallographic structure. The purpose of adding chromium and molybdenum is to accelerate and promote the formation and precipitation of the α phase, making the ferrite grains larger. 

The main elements that affect the microstructure of austenitic stainless steel are carbon, chromium, nickel, molybdenum, nitrogen, copper, silicon and manganese, etc. When producing easily machinable austenitic stainless steel, sulfur is sometimes added as an element. The role of carbon in austenitic stainless steel is to form, stabilize and expand the austenite zone. Its ability to form austenite is much stronger than that of nickel. 

Although carbon is a useful element in austenitic stainless steel, it is also a harmful element. On one hand, as a interstitial element, carbon can significantly enhance the strength of austenitic stainless steel through solid solution strengthening, and also improve its corrosion resistance in high-concentration chloride corrosive media. On the other hand, under certain conditions, carbon can form Cr23C6, which will greatly reduce the corrosion resistance of austenitic stainless steel. The role of chromium in austenitic stainless steel is basically the same as its role in ferritic stainless steel.