Basic knowledge of ferritic stainless steel forging
Pure chromium ferritic stainless steel has no significant increase in hardness after quenching. During the forging process, work hardening will occur and the hardening degree varies with temperature and shape variables. This kind of steel has no rigorous requirement for the cooling temperature after forging.
Ferritic stainless steel has a wide range of forging temperatures, but at higher temperatures, its range is limited to a certain extent due to the grain growth and the weak structure. For 06O13A1(405) steel, the final forging temperature must be strictly controlled. Generally, ferritic steels are forged at temperatures below 704 °C. For 16Cr25N (446) steel, the temperature must be lower than 871 °C when the total reduction is 10%, so as to refine the grain and let the steel have good toughness at room temperature. Ferritic stainless steel is preferably annealed after forging.
Compared with austenitic stainless steel, the forging properties of ferritic stainless steel are better since ferritic steel has higher recrystallization rate and lower recrystallization temperature, the tendency of grain growth during plastic deformation is greater. The upper limit temperature of forging should be strictly limited, generally, the initial forging temperature of the ferritic stainless steel is 1040 °C to 1120 °C. In order to obtain fine grain structure and prevent brittleness, the amount of deformation and the final forging temperature should be properly controlled. The minimum amount of deformation necessary to refine the grains depends on the temperature, which is about 5% at 700 °C, and about 10% to 15% at 800 to 900 °C. The final forging compression should not be lower than 12% - 20%, and the final forging temperature should not higher than 800 °C. In order to avoid cold work hardening due to the low temperature, the final forging temperature should not be lower than 705 °C.
Due to the poor thermal conductivity of ferritic stainless steel, when the surface defects are cleaned by a grinding wheel, local overheating can cause cracks, and it is necessary to remove surface defects by using methods like peeling. Ferritic stainless steel does not undergo a phase change when heated or cooled in a certain temperature range, and therefore cannot be strengthened by heat treatment. The purpose of the heat treatment is firstly, to eliminate the internal stress generated during cold deformation processing and welding, and to improve the processing performance. Secondly, the segregation generated during solidification of the casting is eliminated by heat treatment to obtain a single, uniform ferrite structure.
Characteristics summary of the ferritic stainless steel forging process
1. The ferritic stainless steel has a lower recrystallization temperature and a faster speed, so the tendency of grain growth during plastic deformation is greater.
2. Forging properties of ferritic stainless steel are limited by grain growth and weakening of the structure. For example, for 405 stainless steel, the presence of a small amount of austenite will lead to the weakening of the grain boundary, so the final forging temperature should be strictly controlled.
3. In order to obtain fine grain structure, the compression amount of the last forging should not be lower than 12%~20%, and the final forging temperature is not higher than 800 °C. In order to avoid work hardening due tolow temperature, the final forging temperature should not be lower than 705 °C.
Forging temperature range
1. The initial forging temperature of ferritic stainless steel should be lower than 1200 °C.
2. The final forging temperature of ferritic stainless steel is usually set at 720~800 °C in production, and is not allowed to be higher than 800 °C;
3. The grain growth of ferrite tends to be larger than that of austenite.
Ferritic stainless steel has a wide range of forging temperatures, but at higher temperatures, its range is limited to a certain extent due to the grain growth and the weak structure. For 06O13A1(405) steel, the final forging temperature must be strictly controlled. Generally, ferritic steels are forged at temperatures below 704 °C. For 16Cr25N (446) steel, the temperature must be lower than 871 °C when the total reduction is 10%, so as to refine the grain and let the steel have good toughness at room temperature. Ferritic stainless steel is preferably annealed after forging.
Compared with austenitic stainless steel, the forging properties of ferritic stainless steel are better since ferritic steel has higher recrystallization rate and lower recrystallization temperature, the tendency of grain growth during plastic deformation is greater. The upper limit temperature of forging should be strictly limited, generally, the initial forging temperature of the ferritic stainless steel is 1040 °C to 1120 °C. In order to obtain fine grain structure and prevent brittleness, the amount of deformation and the final forging temperature should be properly controlled. The minimum amount of deformation necessary to refine the grains depends on the temperature, which is about 5% at 700 °C, and about 10% to 15% at 800 to 900 °C. The final forging compression should not be lower than 12% - 20%, and the final forging temperature should not higher than 800 °C. In order to avoid cold work hardening due to the low temperature, the final forging temperature should not be lower than 705 °C.
Due to the poor thermal conductivity of ferritic stainless steel, when the surface defects are cleaned by a grinding wheel, local overheating can cause cracks, and it is necessary to remove surface defects by using methods like peeling. Ferritic stainless steel does not undergo a phase change when heated or cooled in a certain temperature range, and therefore cannot be strengthened by heat treatment. The purpose of the heat treatment is firstly, to eliminate the internal stress generated during cold deformation processing and welding, and to improve the processing performance. Secondly, the segregation generated during solidification of the casting is eliminated by heat treatment to obtain a single, uniform ferrite structure.
Characteristics summary of the ferritic stainless steel forging process
1. The ferritic stainless steel has a lower recrystallization temperature and a faster speed, so the tendency of grain growth during plastic deformation is greater.
2. Forging properties of ferritic stainless steel are limited by grain growth and weakening of the structure. For example, for 405 stainless steel, the presence of a small amount of austenite will lead to the weakening of the grain boundary, so the final forging temperature should be strictly controlled.
3. In order to obtain fine grain structure, the compression amount of the last forging should not be lower than 12%~20%, and the final forging temperature is not higher than 800 °C. In order to avoid work hardening due tolow temperature, the final forging temperature should not be lower than 705 °C.
Forging temperature range
1. The initial forging temperature of ferritic stainless steel should be lower than 1200 °C.
2. The final forging temperature of ferritic stainless steel is usually set at 720~800 °C in production, and is not allowed to be higher than 800 °C;
3. The grain growth of ferrite tends to be larger than that of austenite.
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