What should I do if the heat treatment is deformed?
The method of mechanical or local heating is used to cause local micro-plastic deformation of the deformed workpiece, accompanied by the release and redistribution of residual internal stress to achieve the purpose of correcting deformation. Commonly used mechanical correction methods include cold pressure correction, hot pressure correction before quenching to room temperature, pressure tempering correction, "hot spot" correction using localized heating of oxidized acetylene flame or high frequency, hammer correction, etc. . Mechanically calibrated parts may partially recover to their original deformation and create new deformations due to the attenuation and release of residual stress during use, placement, or finishing. Therefore, it is best not to perform mechanical correction for workpieces and precision parts that are subjected to high loads. When mechanical correction is necessary, the plastic strain to be corrected should exceed the plastic strain of the heat treatment deformation, but the corrected plastic deformation must be controlled within a small range, generally greater than 10 times the elastic ultimate strain, and less than the conditional strength limit. one. The correction should be carried out as soon as possible after quenching, and the residual stress treatment should be carried out after the correction. Correction of heat-treated deformed workpieces requires skilled operators and labor hours. Therefore, calibration automation is an important task for heat treatment workers.
2. Heat treatment correction
For workpieces that are out of tolerance due to heat expansion or shrinkage deformation, the deformation can be corrected by appropriate heat treatment. Commonly used heat treatment correction methods are:
(1) Shrinking the workpiece with expansion and deformation by heating and quenching at ac1 temperature
The workpiece does not undergo a phase change in the change in volume of the structure. Therefore, no tissue stress is generated, and only thermal stress due to the difference in the amount of heat shrinkage between the core portion and the surface is generated. When the surface of the workpiece is rapidly cooled, the surface of the workpiece is subjected to compressive stress, so that the workpiece is subjected to plastic shrinkage deformation along the direction of the dominant stress. This is the mechanism of heat treatment shrinkage treatment. The chemical composition of steel is different, its heat conduction and thermal expansion coefficient are different. After heating at ac1 temperature, the plasticity and yield strength of steel are also different. The plastic shrinkage deformation effect that can be achieved by thermal stress is not the same. Generally, carbon steel and The shrinkage effect of low alloy steel is obvious, and the shrinkage effect of high carbon high alloy steel is relatively poor.
The heating temperature of the shrinkage treatment should be selected according to ac1, and the principle of not quenching when chilling in water should be ensured. For carbon steel with poor austenite stability, the temperature slightly higher than ac1 can be used to take advantage of the phase transition in the phase transition temperature zone. Superplasticity achieves maximum shrinkage.
The heating temperature of various types of steel is:
Carbon steel ac1—20⌒ac1+20°C low alloy steel ac1—20⌒ac1+10°C;
Low carbon high alloy steel (1cr13, 2cr13, 18cr2ni4wa, etc.) ac1—30⌒ac1+10°C;
Austenitic heat and corrosion resistant steel 850～1000°C;
The heating time should ensure that the workpiece is fully heat-permeable, and it is best to cool it with brine. Heated quenching and shrinking treatment at ac1 temperature, can shrink the workpieces of various shapes, such as the inner and outer circles of the ring-shaped workpiece, the hole, the hole size and the outer dimension of the flat workpiece, the length of the shaft workpiece and some A workpiece that requires partial shrinkage, etc.
(2) swelling the shrink-deformed workpiece by the method of quenching and expanding
Mainly suitable for workpieces with simple shapes. The principle is to increase the specific volume when martensite transformation occurs in the surface layer of the workpiece during quenching, to apply tensile stress to the core portion which has not undergone martensite transformation or unhardenability, and to achieve the workpiece along the tensile deformation of the core. The purpose of swelling in the direction of stress. For the workpieces made of low-medium carbon steel and low-medium-carbon alloy structural steel, when the water is quenched by the upper limit of the conventional quenching heating temperature, the direction of the dominant stress is swollen by 0.20 to 0.50 in the case of hardening or semi-hardening of the workpiece. %. The workpiece with a simple shape can be quenched 1 or 2 times after heating orthonormally at a temperature slightly higher than ac1. Crmn, 9crsi, gcr15, crwmn and other hypereutectoid alloy tool steel parts, in the original un-quenched condition, can be heated according to the upper limit heating temperature of the conventional heat treatment specification, and as far as possible to harden or obtain a deep hardened hard layer, The workpiece is swelled by 0.15 to 0.20% in the direction of the dominant stress. After quenching, it should be tempered by 240~280c. The quenching deformation of this kind of steel mainly depends on the specific volume of martensite transformation during quenching, so the deformation of the large deformation is limited and there is danger of quenching.
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