Tempering refers to a heat treatment process in which the quenched or normalized steel castings are heated to a selected temperature lower than the critical point Ac1, and after holding for a period of time, they are cooled at an appropriate rate. Tempering heat treatment can transform the unstable structure obtained after quenching or normalizing into a stable structure to eliminate stress and improve the plasticity and toughness of steel castings. Generally, the heat treatment process of quenching and high temperature tempering treatment is called quenching and tempering treatment. The quenched steel castings must be tempered in time, and the normalized steel castings should be tempered when necessary.
The performance of steel castings after tempering depends on the tempering temperature, time and number of times. The increase of tempering temperature and the extension of holding time at any time can not only relieve the quenching stress of steel castings, but also transform unstable quenched martensite into tempered martensite, troostite or sorbite. The strength and hardness of steel castings are reduced, and the plasticity is significantly improved. For some medium alloy steels with alloying elements that strongly form carbides (such as chromium, molybdenum, vanadium and tungsten, etc.), the hardness increases and toughness decreases when tempering at 400℃-500℃. This phenomenon is called secondary hardening, that is, the hardness of the cast steel in the tempered state reaches the maximum. In actual production, medium alloy cast steel with secondary hardening characteristics needs to be tempered many times.
Low temperature tempering
The temperature range of low temperature tempering is 150℃-250℃. Low temperature tempering can obtain tempered martensite structure, which is mainly used for quenching high carbon steel and quenching high alloy steel. Tempered martensite refers to the structure of cryptocrystalline martensite plus fine granular carbides. The structure of hypoeutectoid steel after low temperature tempering is tempered martensite; the structure of hypereutectoid steel after low temperature tempering is tempered martensite + carbides + retained austenite. The purpose of low temperature tempering is to appropriately improve the toughness of quenched steel while maintaining high hardness (58HRC-64HRC), high strength and wear resistance, while significantly reducing the quenching stress and brittleness of steel castings.
Medium temperature tempering
The tempering temperature of medium temperature is generally between 350℃-500℃. The structure after tempering at medium temperature is a large amount of fine-grained cementite dispersed and distributed on the ferrite matrix, that is, the tempered troostite structure. The ferrite in the tempered troostite structure still retains the shape of martensite. The internal stress of steel castings after tempering is basically eliminated, and they have higher elastic limit and yield limit, higher strength and hardness, and good plasticity and toughness.
High temperature tempering
The high temperature tempering temperature is generally 500°C-650°C, and the heat treatment process that combines quenching and subsequent high temperature tempering is usually called quenching and tempering treatment. The structure after high temperature tempering is tempered sorbite, that is, fine-grained cementite and ferrite. The ferrite in the tempered sorbite is polygonal ferrite that undergoes recrystallization. Steel castings after high temperature tempering have good comprehensive mechanical properties in terms of strength, plasticity and toughness. High temperature tempering is widely used in medium carbon steel, low alloy steel, and various important structural parts with complex forces.