Keywords: Quenching, quenching process, quenching cooling, quenching heating temperature, quenching insulation, heating temperature, complete quenching, incomplete quenching, martensite, quenching medium,

Application of Quenching Process

The quenching process has been widely used in modern mechanical manufacturing industry. Important parts in machinery, especially steel components used in automobiles, airplanes, and rockets, are almost all quenched. To meet the diverse technical requirements of various parts, various quenching processes have been developed. For example, according to the parts to be treated, there are overall, local quenching, and surface quenching; According to whether the phase transformation is complete during heating, there are complete quenching and incomplete quenching (for hypoeutectoid steel, this method is also known as subcritical quenching); According to the content of phase transformation during cooling, there are graded quenching, isothermal quenching, and underspeed quenching.

The process includes three stages: heating, insulation, and cooling. Taking quenching of steel as an example, the principles for selecting process parameters in the three stages mentioned above will be introduced.

Quenching heating temperature

Based on the critical point of phase transformation in steel, small and uniform austenite grains should be formed during heating, and a fine martensitic structure should be obtained after quenching.

The principle of selecting quenching temperature also applies to most alloy steels, especially low-alloy steels. The heating temperature of hypoeutectoid steel is 30-50 ℃ above the Ac3 temperature. From the graph, it can be seen that the state of steel at high temperature is in the single-phase austenite (A) zone, hence it is called complete quenching. If the heating temperature of hypoeutectoid steel is higher than Ac1 and lower than Ac3, then at high temperatures, some of the pre eutectoid ferrite has not completely transformed into austenite, which is called incomplete (or subcritical) quenching. The quenching temperature of hypereutectoid steel is 30-50 ℃ above the Ac1 temperature, which is in the dual phase zone of austenite and cementite (A+C). Therefore, the normal quenching of hypereutectoid steel still belongs to incomplete quenching, and the microstructure of cementite distributed on the martensitic matrix is obtained after quenching. This organizational state has high hardness and high wear resistance. For hypereutectoid steel, if the heating temperature is too high and the eutectoid carbides dissolve too much or even completely, the austenite grains will grow and the austenite carbon content will increase. After quenching, the coarse martensitic structure increases the internal stress and microcracks in the quenched microstructure of the steel, leading to an increased tendency for deformation and cracking of the parts; Due to the high concentration of austenite carbon, the martensite point decreases and the amount of residual austenite increases, resulting in a decrease in the hardness and wear resistance of the workpiece. In actual production, the selection of heating temperature should be adjusted according to specific circumstances. If the carbon content in hypoeutectoid steel is the lower limit, the upper temperature limit can be selected when the furnace load is large and the depth of the quenching layer of the parts needs to be increased; If the shape of the workpiece is complex and the deformation requirements are strict, a lower temperature limit should be used.

Quenching and insulation

The quenching and insulation time is determined by various factors such as equipment heating method, part size, steel composition, furnace load, and equipment power. For overall quenching, the purpose of insulation is to make the internal temperature of the workpiece uniform and tend to be consistent. The holding time for various types of quenching ultimately depends on obtaining a good quenching heating structure in the required quenching area.

Heating and insulation are important links that affect the quality of quenching, and the microstructure obtained by austenitization directly affects the properties after quenching- The austenite grain size of general steel parts is controlled at level 5-8.

quench cooling 

To transform the high-temperature phase austenite in steel into the low-temperature metastable phase martensite during the cooling process, the cooling rate must be greater than the critical cooling rate of the steel. During the cooling process of the workpiece, there is a certain difference in the cooling rate between the surface and the core. If this difference is large enough, it may cause the part above the critical cooling rate to transform into martensite, while the core below the critical cooling rate cannot transform into martensite. To ensure that the entire cross-section is transformed into martensite, a quenching medium with sufficient cooling capacity needs to be selected to ensure a sufficiently high cooling rate at the center of the workpiece. However, the cooling rate is high, and the uneven thermal expansion and contraction inside the workpiece may cause internal stress, which may cause deformation or cracking of the workpiece. Therefore, it is necessary to consider the two conflicting factors mentioned above and choose a reasonable quenching medium and cooling method.

The cooling stage not only ensures that the parts have a reasonable structure and achieve the required performance, but also maintains the dimensional and shape accuracy of the parts, which is a key link in the quenching process.

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