Improvement Methods for Heat Treatment Deformation and Cracking of Stamping Dies

There are many reasons that affect the heat treatment deformation and cracking of stamping dies, which are mainly related to many factors such as the original organization, the chemical composition of the steel, the heat treatment process, the structural shape and size of the parts.

Generally speaking, cracking can often be prevented, but heat treatment deformation is always difficult to avoid. In engineering practice, due to differences in cross-sectional dimensions, the structure and shape of stamping die parts, and different heating and cooling rates during heat treatment (heating-insulation-cooling), the combined effects of thermal stress, organizational stress, and phase change volume change will cause the volume of the parts to expand or shrink, resulting in certain deviations and deformations in the size and shape of the die, and even cracking.

1. Preparatory heat treatment

For stamping die forgings made of eutectoid steel, it is recommended to first perform normalizing treatment and then spheroidizing annealing. The purpose is to eliminate the reticular secondary cementite in the forging, refine the grains, eliminate its internal stress, and prepare the organization for subsequent (or final) heat treatment.

For stamping die parts, low-temperature tempering heat treatment (i.e. stabilization treatment) should be carried out before quenching. For some die parts with complex shapes and high precision requirements, quenching and tempering treatment should be adopted after rough machining and before fine machining. The purpose is to reduce the quenching deformation of the parts, avoid cracking tendency as much as possible, and make organizational preparations for the final heat treatment process.

2. Optimize quenching and tempering treatment process

2.1. Control of tempering treatment

After the mold parts are taken out of the coolant (coolant), they should not stay in the air for a long time. They should be placed in the tempering furnace in time for tempering heat treatment. During tempering treatment, low-temperature tempering brittleness and high-temperature tempering brittleness should be avoided. For some mold parts with high precision requirements, it is recommended to perform multiple tempering treatments after quenching. The purpose is to eliminate internal stress, reduce part deformation, and avoid cracking tendency.

2.2. Protection of quenched parts

Quenching and tempering treatment is an important link that affects the heat treatment deformation or cracking of stamping die parts. For mold parts that are important for quenching (such as punches, dies, etc.), some effective protective measures must be taken for parts that are prone to deformation or cracking, so as to make the shape and cross-section of the heat-treated parts symmetrical and balance their internal stress. Commonly used protection methods include filling method, bundling method and plugging method.

2.3. Determination of heating temperature

If the quenching heating temperature is too high, the austenite grains will be coarse, and oxidation and decarburization will occur, thereby increasing the tendency of parts to deform and crack. Within the specified heating temperature range, if the quenching heating temperature is too low, the inner hole of the part will shrink and the aperture size will become smaller. Therefore, it is recommended to use the upper limit of the heating temperature specification; for alloy steel parts, if the heating temperature is too high, it will cause the inner hole of the part to expand and the aperture size will become larger. Therefore, it is recommended to use the lower limit of the heating temperature specification.

2.4. Improvement of heating method

For some small stamping convex and concave dies, or slender cylindrical parts (such as small punches, etc.), they can be preheated to a certain temperature in advance, such as 520-580℃, and then placed in a medium-temperature salt bath furnace and heated to the quenching temperature.

Compared with direct heating and quenching in an electric furnace or reverberatory furnace, the deformation of the parts is significantly reduced, and the cracking tendency can be controlled. Especially for those high-alloy steel mold parts, the correct heating method is recommended: first preheat (temperature to 530-560℃), and then heat to the quenching temperature. During the heating process, the time of the high temperature section should be shortened as much as possible, the purpose is to reduce the quenching deformation of the parts and avoid the production of small cracks.

2.5. Selection of coolant

For alloy steel parts, the best way to reduce quenching deformation is to use the isothermal quenching method or the graded quenching method of potassium nitrate and sodium nitrite hot bath.

This method is particularly suitable for processing stamping dies with complex shapes and high dimensional accuracy requirements. For some porous mold parts (such as porous concave molds, etc.), the isothermal quenching time should not be too long, otherwise it will cause the aperture or hole spacing of the parts to become larger; if the characteristics of cooling and shrinking in oil or cooling and expanding in nitrate are adopted, the reasonable use of dual-medium quenching can greatly reduce the deformation of the parts.

2.6. Optimization of cooling method

When the parts are taken out of the heating furnace and placed in the coolant (coolant), they should be placed in the air, pre-cooled for a period of time, and then placed in the coolant for quenching.

This is one of the common and effective methods to reduce the quenching deformation of the parts and prevent the tendency of the parts to crack. After the mold parts are placed in the coolant (coolant), they should be rotated appropriately, and the direction of rotation should be changed. This is more conducive to maintaining a uniform cooling rate at the parts, which can significantly reduce the deformation of the parts and prevent the tendency to crack.

2.7. Quenching treatment before wire cutting

For some stamping die parts that need to be processed by wire cutting, a heat treatment process of graded quenching and multiple tempering (or high temperature tempering) should be used before wire cutting. The purpose is to improve the hardenability of the parts and make the internal stress distribution more uniform and in a state of smaller internal stress. Generally speaking, the smaller the internal stress, the smaller the tendency of deformation and cracking of the parts after wire cutting.