Tempering process of stainless steel precision casting
The tempering of stainless steel precision casting has […]
The tempering of stainless steel precision casting has two main purposes: one is to eliminate the stress generated during quenching; the two feet promote the decomposition and transformation of martensite and retained austenite metastable structure after quenching.
The tempering temperature is lower than the eutectoid transformation temperature, usually about 650 ° C, and the holding time is not less than 2 hours. As the tempering temperature increases, martensite precipitates carbon from the supersaturated state to form dispersed fine carbides. Martensite eventually forms a bulk ferrite. Therefore, the hardness of "stainless steel precision cast" steel will gradually decrease as the tempering temperature increases.
A problem that cannot be ignored in tempering is: the brittleness problem in the tempering process, the temper brittleness in the low temperature zone of 300 °C ~ 400 °C, and the temper brittleness in the high temperature of 500 °C ~ 650 °C. When a carbon atom is precipitated, a thin layer of carbide is formed at the grain boundary, and an effect called low temperature temper brittleness is produced. This carbide is called an e-carbide, and its structure and composition are different from those of the carbide, resulting in a decrease in impact toughness, which is called low temperature temper brittleness. When the concave fire continues to heat up and form a stable tempering structure, the temper brittleness does not appear after the secondary tempering treatment, so it is also called "irreversible temper brittleness". At 450 ° C ~ 650 ° C high temperature temper brittleness and cold, there are many slow cooling temper brittleness, no temper brittle rapid cooling. High-temperature temper brittleness is reversible. As long as the tempered brittle steel is removed and heated to 600 °C" (: above, then slowly cooled, temper brittleness still occurs. Therefore, temper brittleness may be associated with precipitation of low-melting brittle compounds at grain boundaries. Relevant. Adding molybdenum to some alloy steels can eliminate temper brittleness, ie low temperature temper brittleness. Experts generally believe that molybdenum may form complex carbides with certain alloying elements, offsetting the adverse effects of carbides.
According to the different requirements of casting performance, the tempering temperature is divided into low temperature tempering, medium temperature tempering and high temperature tempering.
(1) Low temperature tempering tempering temperature 100 °C ~ 250 °C, using martensite structure, and a part of carbon martensite supersaturated, hard alloy martensite. This steel has a very small carbon content and a small supersaturation of carbon in martensite. Therefore, the tempered martensite itself has good toughness, and the precipitation and stability of the carbide further increase the strength and hardness of the steel.
(2) The intermediate temperature tempering temperature is 300 °C ~ 500 °C tempering, and the tempered structure is tempered and has good strength and toughness. This tempering process is widely used in low alloy steels.
(3) Tempering at a high temperature tempering temperature of 550 ° C to 650 ° C, tempering and dispersing, fine-grained sorbite. The quenching + high temperature tempering process is called tempering and quenching. Suitable for low alloy steel, multi-element strong carbide molding, with high strength and toughness.