According to the metallographic structure, stainless steel is divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, and duplex (ferrite in austenitic matrix) stainless steel:
Taking chromium as the main alloy element, the Cr content is generally between 13% and 30%. It has good corrosion resistance to oxidizing medium and air oxidation resistance at high temperature, and can also be used as heat-resistant steel. This kind of steel has poor welding performance. When the chromium content is greater than 16%, the as-cast structure is coarse, and long-term heat preservation between 400-525°C and 550-700°C will result in a “475°C” brittle phase and σ phase, which makes the steel brittle. The brittleness at 475℃ is related to the ordering phenomenon of Cr-containing ferrite. The brittleness of 475℃ brittle phase and σ phase can be improved by heating above 475℃ and then quickly cooling.
Brittleness at room temperature and brittleness in the heat-affected zone after welding are also one of the basic problems of ferritic stainless steel castings. Vacuum refining, adding trace elements (such as boron, rare earth and calcium, etc.) or austenite forming elements (such as Ni, Mu , N, Cu, etc.) to be improved. In order to improve the mechanical properties of the weld zone and the heat affected zone, a small amount of Ti and Nb are usually added to prevent the grain growth of the heat affected zone. Commonly used ferritic steels are ZGCr17 and ZGCr28. This kind of steel has low impact toughness and is replaced by austenitic stainless steel castings containing high nickel in many occasions. Ferrite steels with Ni content exceeding 2% and N content exceeding 0.15% have good impact properties.