Measures to Improve Deformation and Cracking of Stamping Dies During Heat Treatment

I. Preliminary Heat Treatment
For hypereutectoid steel stamping die forgings, normalizing should be performed first, followed by spheroidizing annealing to eliminate the network-like secondary cementite within the forgings, refine the grain structure, relieve internal stresses, and prepare the microstructure for subsequent heat treatment. Before quenching the stamping die parts (such as concave dies), low-temperature tempering should be conducted first. For dies with more complex shapes and high precision requirements, quenching and tempering treatment should be carried out after rough machining and before finish machining to reduce quenching deformation, minimize the tendency for cracking, and prepare the microstructure for the final heat treatment.

II. Optimization of Quenching and Tempering Processes

1. Protection of Parts During Quenching
   Quenching and tempering are critical steps that affect the deformation or cracking of stamping die parts during heat treatment. For areas of critical die parts prone to deformation or cracking during quenching, effective protective measures should be taken to ensure symmetrical part shapes and cross-sections, as well as balanced internal stresses.

2. Improvement of Heating Methods
   For small stamping punches and dies or slender cylindrical parts, preheating to 520–580°C before placing them in a medium-temperature salt bath furnace for heating to the quenching temperature can significantly reduce deformation compared to direct heating in an electric or reverberatory furnace. This method also helps control the tendency for cracking. Especially for high-alloy steel die parts, the correct heating method involves preheating first, then raising the temperature to the quenching level. The duration of high-temperature exposure should be minimized during heating to reduce quenching deformation and avoid the formation of microcracks.

3. Determination of Heating Temperature
   Excessively high quenching temperatures coarsen austenite grains and cause oxidation and decarburization, increasing the tendency for deformation and cracking. Within the specified heating temperature range, if the quenching temperature is too low, the internal holes of the part may shrink, reducing the bore size. Therefore, the upper limit of the heating temperature range should be selected for carbon steels. For alloy steels, higher heating temperatures may cause expansion of internal holes and an increase in bore size, so the lower limit of the heating temperature range is preferable.

4. Selection of Cooling Media
   For alloy steels, the best method to minimize quenching deformation is isothermal quenching or martempering in a hot bath of potassium nitrate and sodium nitrite. This method is particularly suitable for stamping dies with complex shapes and precise dimensional requirements. For some porous die parts, the isothermal quenching time should not be too long, as it may cause an increase in hole diameter or pitch. Utilizing the characteristics of shrinkage during oil cooling and expansion during nitrate salt cooling, and applying dual-medium quenching appropriately, can reduce part deformation.

5. Optimization of Cooling Methods
   Before placing parts into the cooling medium after removal from the heating furnace, they should be appropriately air-cooled first. This is one of the effective methods to reduce quenching deformation and prevent cracking. After placing the die parts into the cooling medium, they should be rotated appropriately, with changes in rotation direction, to ensure uniform cooling rates across all parts of the component. This significantly reduces deformation and prevents cracking.

6. Control of Tempering Process
   After removal from the cooling medium, die parts should not be left in the air for too long but should be promptly placed into a tempering furnace for tempering. During tempering, low-temperature and high-temperature temper brittleness should be avoided. For die parts with high precision requirements, multiple tempering treatments after quenching can help relieve internal stresses, reduce deformation, and minimize the tendency for cracking.

7. Heat Treatment Before Wire Cutting
   For stamping die parts processed by wire cutting, graded quenching and multiple tempering treatments should be applied before wire cutting to improve the hardenability of the parts, ensure uniform distribution of internal stresses, and maintain a state of low internal stress. The lower the internal stress, the less the tendency for deformation and cracking after wire cutting.