On 2018-08-08 14:48:54
How to prevent deformation during heat treatment
If in the process of heat treatment, improper operation will lead to the thick and even deformation of the workpiece, and then to the safety of production threats. How to avoid deformation of workpiece during heat treatment is particularly important. The main measures are as follows:
Heat treatment processing analysis heat treatment how to prevent deformation?
1. Influence of heating speed
The deformation of mould after heat treatment is generally thought to be caused by cooling, which is not correct. Mold, especially complex mold, the accuracy of processing technology on the mold deformation is often a greater impact, for some mold heating process comparison can be clearly seen, heating speed is faster, often produce greater deformation.
1) the reason for deformation any metal heating expansion, because the steel heating, the same mold, the temperature of each part is not uniform (that is, heating is not uniform) will inevitably cause the mold expansion of each part is not consistent, resulting in the formation of internal stress due to uneven heating. At the temperature below the phase change point of the steel, the non-uniform heating mainly produces thermal stress, and beyond the phase change temperature, the non-uniform heating will also produce the time inequality of the tissue transformation, which not only produces the tissue stress. Therefore, the faster the heating speed, the greater the temperature difference between the mold surface and the core, the greater the stress, the greater the deformation of the mold after heat treatment.
2) preventive measures? For complex mold heating below the phase change point should be heated slowly, generally speaking, mold vacuum heat treatment deformation is much smaller than the salt bath furnace heating quenching. ? Preheat is used, for low alloy steel mold can use a preheat (550-620oc); Secondary preheating (550-620oc and 800-850oc) shall be used for high alloy rigid dies. Shenzhen mold heat treatment processing shenzhen mold heat treatment processing
2. Influence of heating temperature
Some manufacturers in order to ensure that the mold to achieve a higher hardness, it is necessary to increase the quenching heating temperature. However, the production practice shows that this method is not appropriate. For the complex mold, the heating and quenching is also carried out at the normal heating temperature. The heat treatment deformation after heating at the upper allowable temperature is much larger than that after heating at the lower allowable temperature.
(1) the deformation reason is known to all, the higher the quenching heating temperature, the larger the grain size of the steel, because the larger the grain size can increase the hardenability, the greater the stress generated during the quenching cooling. Moreover, since most of the complex molds are made of medium and high alloy steel, if the quenching temperature is high, the residual austenitic volume in the tissues will increase due to the low Ms point, which will increase the deformation of the molds after heat treatment.
(2) preventive measures under the condition of ensuring the technical conditions of the mold to choose a reasonable heating temperature, as far as possible to choose the lower limit of the quenching heating temperature, in order to reduce the stress during cooling, so as to reduce the complex heat treatment deformation.
Residual austenite effect
After quenching and low-temperature tempering, the length, width and height of some high-alloy die steels, such as Cr12MoV steel die, all shrink, which is caused by excessive austenitic residual after quenching.
1. Reasons for deformation
Because alloy steel (such as Cr12MoV steel) contains a large amount of residual austenite after quenching, various structures in the steel have different specific volume, the specific volume of austenite is the smallest, which is the main reason for the reduction of the volume of high-alloy steel mold after quenching and tempering at low temperature. The specific volumes of the various structures of steel decrease in the following order: martensite - tempered sostenite - pearlite - austenite.
2. Preventive measures
(1) reduce the quenching temperature appropriately. As described above, the higher the quenching heating temperature is, the larger the residual austenitic volume will be. Therefore, it is important to choose the appropriate quenching heating temperature to reduce the die shrinkage. Generally, under the condition of ensuring the technical requirements of the mold, the comprehensive performance of the mold should be considered and the quenching heating temperature of the mold should be reduced appropriately.
(2) some data show that after quenching of Cr12MoV steel mold, the residual austenitic volume of tempering at 500oC is less than half of that at 200oC, so the tempering temperature should be appropriately raised on the premise of ensuring the technical requirements of the mold. The production practice shows that the deformation of tempering die at 500oC for Cr12MoV steel die is the minimum, while the hardness is not reduced much (2~3HRC).
(3) cold treatment after quenching is the best process to reduce the residual austenitic volume, and also to reduce the deformation of the mold and the size change during stable use.