1. The influence of heating speed
The deformation of the mold after heat treatment is generally considered to be caused by cooling, which is incorrect. For molds, especially complex molds, the correctness of the processing technology often has a greater impact on the deformation of the mold. A comparison of some mold heating processes can clearly show that the heating speed is faster and often large deformations.
(1) Causes of deformation Any metal will expand when heated. Because steel is heated, the uneven temperature of each part (that is, uneven heating) in the same mold will inevitably cause inconsistent expansion of each part in the mold The formation of internal stress due to uneven heating. At the temperature below the transformation point of steel, uneven heating mainly produces thermal stress. If the heating exceeds the transformation temperature, uneven heating will also produce unequal time of the transformation of the structure, which will result in structure stress. Therefore, the faster the heating rate, the greater the temperature difference between the mold surface and the core, the greater the stress, and the greater the deformation of the mold after heat treatment.
(2) Preventive measures The complicated mold should be heated slowly when heating below the phase transition point. Generally speaking, the deformation of the mold in vacuum heat treatment is much smaller than that in the salt bath furnace. ‚Using preheating, for low-alloy steel molds, one preheating (550-620ºC); for high-alloy steel molds, two preheating (550-620ºC and 800-850ºC).
2. The influence of heating temperature
In order to ensure that the mold reaches a higher hardness, some manufacturers believe that it is necessary to increase the quenching heating temperature. However, production practice shows that this approach is inappropriate. For complex molds, heating and quenching are also performed at the normal heating temperature. The heat treatment deformation after heating at the allowable upper limit temperature is greater than the heat treatment deformation heated at the allowable lower limit temperature. Much larger.
(1) The cause of deformation It is well known that the higher the quenching heating temperature, the more the grains of steel will grow. As larger grains can increase the hardenability, the greater the stress generated during quenching and cooling. Furthermore, since complex molds are mostly made of medium and high alloy steel, if the quenching temperature is high, the amount of retained austenite in the structure will increase due to the low Ms point, which will increase the deformation of the mold after heat treatment.
(2) Preventive measures In the case of ensuring the technical conditions of the mold, the heating temperature should be selected reasonably, and the lower limit quenching heating temperature should be selected as far as possible to reduce the stress during cooling, thereby reducing the complicated heat treatment deformation.