Tempering process after steel quenching

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After steel is quenched, it usually needs to undergo tempering treatment afterward. This is because after quenching, steel has high hardness and strength, but poor toughness; moreover, the quenching stress is high, making it prone to cracking. Therefore, after steel is quenched, it must be tempered immediately to improve its properties and prevent the components from deforming or cracking. Depending on the heat treatment temperature, tempering is generally divided into three categories: low-temperature tempering, medium-temperature tempering, and high-temperature tempering. Low-temperature tempering. When the tempering temperature is below 150–250°C, martensite decomposes to form an α phase along with ε carbides that are dispersed throughout the material; these ε carbides appear as black needle-like particles. Residual austenite also decomposes, resulting in an α phase along with similarly dispersed ε carbides. All of these forms are referred to as tempered martensite. See Figure 1. Tempered martensite has relatively high strength and hardness, and its toughness is also improved. Figure 1 It should be noted that hypoeutectoid steel has a relatively high martensite transformation temperature; the transformation begins below 400°C, which allows for self-tempering during the subsequent cooling process. Medium-temperature tempering. The tempering temperature is 350–500°C, and the microstructure under an optical microscope is tempered martensite. The quenching stress is essentially eliminated, resulting in good strength, toughness, and elastic limit, making it commonly used for springs. See Figure 2. Figure 2 High-temperature tempering. The tempering temperature is 500–650°C; under an optical microscope, the microstructure consists of tempered sorbite, as shown in Figure 3. Quenching followed by high-temperature tempering is commonly also referred to as quenching and tempering treatment. Components that have undergone quenching and tempering treatment exhibit good overall mechanical properties and are widely used. Figure 3 Cooling method for tempering. It is generally cooled naturally in the air. The advantages of air cooling are simplicity, low cost, and easy operation. For components with complex structures or large sizes, slow cooling in the furnace is employed to eliminate stresses and prevent deformation and cracking. For components prone to type II temper brittleness, rapid cooling is required, using water or oil cooling, in order to prevent the occurrence of temper brittleness. Temper brittleness is divided into two categories: first-type temper brittleness and second-type temper brittleness. Type 1 temper embrittlement occurs at tempering temperatures in the range of 250–400°C, while type 2 temper embrittlement takes place at temperatures between 400–650°C. The first type of temper brittleness is independent of the cooling rate. The way to avoid this is to avoid tempering within the brittle temperature range. The second type of temper brittleness is related to the cooling rate. The solution is to perform rapid cooling after tempering, in order to prevent impurity elements from concentrating at the grain boundaries.