Stainless steel forgings are widely used in the defense industry, chemical industry, petroleum industry and power sector, and many products not only require corrosion resistance but also require high strength, so most stainless steels are used after forging. Compared with carbon steel, stainless steel has different characteristics: low thermal conductivity, narrow forging temperature range, strong overheating sensitivity, high resistance at high temperature, low plasticity, etc., which have brought many difficulties to forging production, and different types The stainless steel forging process is also different. The following introduces the types and forging process characteristics of commonly used stainless steel.
(1) Martensitic stainless steel Forging process characteristics of martensitic stainless steel:
1) Martensitic (20Crl3, 40Crl3, 14Crl7Ni2, etc.) stainless steel undergoes an allotropic transformation in the process of heating and cooling. For this type of steel, there is no special requirement for the amount of deformation in the last fire.
2) The formation of ferrite should be avoided during forging and heating of martensitic stainless steel, because the appearance of ferrite will cause cracks in the forging, and it is necessary to avoid overheating caused by excessive heating of the metal. Surface decarburization of forgings can cause excessive ferrite formation, so surface decarburization should be minimized.
3) Martensitic stainless steel is prone to cracking after forging. This is because martensite and carbide structures will appear when air-cooled after forging, and the resulting internal stress is large, so it should be cooled slowly after forging, generally around 200°C Slow cooling in the sandpit or slag, and annealing should be carried out in time after being taken out from the sandpit to prevent breakage.
(2) Forging features of ferritic stainless steel forgings:
1) The recrystallization temperature of ferritic stainless steel is lower and the speed is faster, so the tendency of grain growth during plastic deformation is greater. Ferritic stainless steel grains grow faster at temperatures above 950°C.
2) The forging properties of ferritic stainless steel are limited by grain growth and structural weakening. For example, for American 405 steel (similar to 06Crl3AI), the appearance of a small amount of austenite will lead to the weakening of grain boundaries, so the final forging temperature should be strictly controlled.
3) In order to obtain a fine-grained structure, the compression amount of the last forging should not be lower than 12%~20%, and the final forging temperature should not be higher than 800°C. In order to avoid cold work hardening due to too low temperature, the final forging temperature should not be lower than 705°C.
Due to the poor thermal conductivity of ferritic stainless steel, when its surface defects are cleaned with a grinding wheel, local overheating may cause cracks. It is best to use air shovel cleaning or peeling to remove surface defects.
(3) Austenitic stainless steel 18-8 type austenitic stainless steel is often used to make boiler and steam turbine forgings that work for a long time below 610°C, as well as various forgings in chemical production. The forging process features:
1) The surface of 18-8 austenitic stainless steel is easy to carburize when heated in a coal furnace, so avoid contact with carbon-containing substances during heating, and use oxidizing media to heat to reduce carburization of steel and prevent Intergranular corrosion.
2) Austenitic steel should be heated slowly due to poor thermal conductivity at low temperatures, and the initial forging temperature should not be too high, as the grains will grow rapidly. Generally, choose 1150~1180°C.
3) The surface defects of the billet must be removed before forging and heating to prevent the forging from continuing to expand and causing the forging to be scrapped.
4) The final forging temperature should not be too low, and the σ phase will be precipitated by slow cooling at 700~900°C, and cracks will occur if continued forging.
