The reasons for the deformation and cracking of metal stamping dies are multi-faceted, mainly related to the original structure, the chemical composition of steel, the structural shape and cross-sectional dimensions of parts, heat treatment processes and other factors. Cracking is often preventable, but heat treatment deformation is always difficult to avoid.

In practice, the differences in cross-sectional dimensions, the structural shapes of stamping die parts, and the heat treatment (heating - holding - cooling) process, due to the different rates of heating and cooling, under the combined effect of thermal stress, organizational stress and phase transformation volume changes, cause the volume of the parts to expand or contract, thereby leading to deviations in size and shape, deformation, and even cracking.

I. Pre-heat treatment

For the stamping die forgings of eutectoid steel, normalizing treatment should be carried out first, followed by spheroidizing annealing to eliminate the network secondary cementite within the forgings, refine the grains, eliminate internal stress, and prepare the microstructure for subsequent (or final) heat treatment. Before quenching the stamping die parts, low-temperature tempering (stabilization treatment) should be carried out first. For some dies with complex shapes and high precision requirements, quenching and tempering treatment should be carried out before fine machining after rough machining to reduce quenching deformation, avoid cracking tendency as much as possible, and prepare the microstructure for the final heat treatment.

Second, optimize the quenching and tempering treatment processes

Control of tempering treatment

After the mold parts are taken out of the coolant, they should not be left in the air for a long time. They should be promptly placed in the tempering furnace for tempering treatment. During tempering treatment, low-temperature tempering brittleness and high-temperature tempering brittleness should be avoided. For some mold parts with high precision requirements, multiple tempering treatments are adopted after quenching to eliminate internal stress, reduce deformation and avoid the tendency of cracking.

2. Protection of quenched parts

Quenching and tempering treatment are important links that affect the deformation or cracking of stamping die parts during heat treatment. For the parts of important quenched molds (such as punches and dies) that are prone to deformation or cracking, effective protective measures should be taken to ensure that the shape and cross-section of the parts are symmetrical and the internal stress is balanced. The commonly used protective methods are as follows: a. Bundling method; b. Filling method; c. Blocking method.

3. Determination of heating temperature

If the quenching heating temperature is too high, it will cause coarse austenite grains and lead to oxidation and decarburization, increasing the tendency of parts to deform and crack. Within the specified heating temperature range, if the quenching heating temperature is too low, it will cause the inner hole of the part to shrink and the hole diameter size to decrease. Therefore, the upper limit of the heating temperature specification should be selected. For alloy steel, if the heating temperature is too high, it will cause the inner hole to expand and the pore size to increase. Therefore, it is advisable to select the lower limit value of the heating temperature.

4. Improvement of heating methods

For some small stamping punch and die or slender cylindrical parts (such as small punches), they can be preheated to 520- 580℃ in advance, and then placed in a medium-temperature salt bath furnace to be heated to the quenching temperature. Compared with directly heating and quenching parts in an electric furnace or a reverberation furnace, the deformation of the quenched parts is significantly reduced, and the cracking tendency can be controlled. Especially for high alloy steel mold parts, the correct heating method is: preheat first (at a temperature of 530- 560℃), and then raise to the quenching temperature. During the heating process, the high-temperature period should be shortened as much as possible to reduce quenching deformation and avoid the production of small cracks.

5. Selection of coolant

For alloy steel, the most effective method to reduce quenching deformation is isothermal quenching or step quenching using a hot bath of potassium nitrate and sodium nitrite. This method is particularly suitable for processing stamping dies with complex shapes and high dimensional requirements. For some multi-hole mold parts (such as multi-hole dies), the isothermal quenching time should not be too long; otherwise, it will cause the hole diameter or hole spacing to increase. If the characteristics of cooling shrinkage in oil and cooling expansion in nitrate are utilized and dual-medium quenching is reasonably applied, the deformation of parts can be reduced.

6. Quenching treatment before wire cutting

For some stamping die parts processed by wire cutting, a heat treatment process of step quenching and multiple tempering (or high-temperature tempering) should be adopted before wire cutting processing to improve the hardenability of the parts and make the internal stress distribution tend to be uniform and in a state of relatively small internal stress. The smaller the internal stress is, the lower the tendency of deformation and cracking after wire cutting will be.

7. Optimization of the cooling method

Before the parts are taken out of the heating furnace and placed in the coolant, they should be appropriately pre-cooled in the air and then quenched in the coolant. This is one of the effective methods to reduce the quenching deformation of the parts and prevent the tendency of parts to crack. After the mold parts are placed in the coolant, they should be rotated appropriately, and the direction of rotation should be changed. This is conducive to maintaining a uniform cooling rate in the part area, which can significantly reduce deformation and prevent the tendency of cracking