The reasons that affect the deformation and cracking of metal stamping die are various, mainly related to the original structure, the chemical composition of the steel, the structural shape and section size of the part, and the heat treatment process. Cracking is often preventable, but heat treatment deformation is always difficult to avoid.
In practice, the difference in cross-sectional dimensions, the structural shape of the stamping die parts, and the rate of heating and cooling during heat treatment (heating-insulation-cooling) are different under the combined effects of thermal stress, tissue stress and phase change volume. Causes the volume of the part to expand or contract, which causes the size and shape to deviate, deform, and even cause cracking.
First, the preliminary heat treatment
For stamping die forgings of eutectoid steel, normalizing treatment should be carried out first, followed by spheroidizing annealing to eliminate the reticulated secondary cementite in the forging, refining the grains, eliminating internal stress, and for subsequent (or final) Heat treatment is prepared for the organization. Before quenching the stamped die parts, low temperature tempering (stabilization) should be performed. For some concave molds with complex shapes and high precision requirements, before finalizing after rough machining, the quenching and tempering treatment should be carried out to reduce the quenching deformation, avoid the tendency of cracking as much as possible, and prepare the structure for the final heat treatment.
Second, optimize the quenching and tempering treatment process
1. Control of tempering
After the mold parts are taken out from the coolant, it is not suitable to stay in the air for a long time. It should be placed in the tempering furnace for tempering. When tempering, low temperature temper brittleness and high temperature temper brittleness should be avoided. For some mold parts with precision requirements, multiple tempering treatments are used after quenching to eliminate internal stress, reduce deformation and avoid cracking tendency.
2. Protection of hardened parts
Quenching and tempering are important steps that affect the heat treatment deformation or cracking of stamping die parts. For parts of the mold parts that are important for quenching (such as punches and dies) that are prone to deformation or cracking, effective protective measures should be taken to make the shape and section of the part symmetrical and the internal stress balanced. Commonly used protection methods are as follows: a. baling method; b. filling method; c. blocking method.
3. Determination of heating temperature
When the quenching heating temperature is too high, the austenite grains are coarse, and oxidation and decarburization are caused, and the tendency of deformation and cracking of the parts is increased. In the specified heating temperature range, the quenching heating temperature is low, which causes the inner hole of the part to shrink and the pore size to become smaller. Therefore, the upper limit of the heating temperature specification should be selected; for alloy steel, if the heating temperature is too high, the inner hole will be expanded and the pore size will become larger. Therefore, the lower limit of the heating temperature should be selected.
4. Improvement of heating method
For some small stamping embossing dies or elongated cylindrical parts (such as small punches), it can be preheated to 520--580 °C before being heated in a medium temperature salt bath furnace to quenching temperature, than directly using an electric furnace or The deformation of the reheating and quenching parts of the reverberatory furnace is significantly reduced, and the tendency of cracking can be controlled. Especially for high-alloy steel mold parts, the correct heating method is: preheating (temperature 530--560 °C), and then rising to quenching temperature. During the heating process, the high temperature period should be shortened as much as possible to reduce the quenching deformation and avoid the production of small cracks.
5. Coolant selection
For alloy steels, the best way to reduce quenching deformation is to use isothermal quenching or fractional quenching of potassium nitrate and sodium nitrite hot baths. This method is especially suitable for processing stamping dies with complex shapes and precise dimensions. Some porous mold parts (such as porous die), the austempering time should not be too long, otherwise it will cause the aperture or hole distance to become larger. If the characteristics of cooling shrinkage in oil and cooling and expansion in nitrate are utilized, the double-medium quenching can be reasonably applied to reduce the deformation of the part.
6. Quenching treatment before wire cutting
For some wire-cutting stamping parts, prior to wire-cutting, staged quenching and multiple tempering (or high-temperature tempering) heat treatment processes should be used to improve the hardenability of the parts and to make the internal stress distribution uniform. In a state of less internal stress. The smaller the internal stress, the smaller the tendency to deform and crack after wire cutting.
7. Optimization of cooling method
Before the parts are taken out of the furnace and placed in the coolant, they should be placed in the air for proper pre-cooling and then quenched in the coolant. This is one of the effective ways to reduce the quenching deformation of the parts and prevent the parts from cracking. After the mold part is placed in the coolant, it should be rotated properly and the direction of rotation changed. This will help maintain a uniform cooling rate of the part, which can significantly reduce the deformation and prevent the tendency of cracking.