Heat treatment manufacturers talk about how to improve heat treatment deformation and cracking of stamping dies?

There are many reasons that affect the deformation and cracking of stamping die during heat treatment, mainly related to the original structure, chemical composition of steel, structural shape and section size of parts, heat treatment process and other factors. Cracking is often preventable, but heat treatment deformation is always unavoidable. In practice, due to the difference in cross-sectional size, the structural shape of stamping die parts, and the different heating and cooling rates in the process of heat treatment (heating - heat preservation - cooling), under the combined effect of thermal stress, tissue stress and phase transformation volume changes, It causes the volume of the part to expand or contract, resulting in deviations in size and shape, deformation, and even cracking.

1. Preparatory heat treatment

For the stamping die forgings of eutectoid steel, normalizing treatment should be carried out first, and then spheroidizing annealing should be carried out to eliminate the reticulated secondary cementite in the forgings, refine the grains, eliminate the internal stress, and prepare the structure for the subsequent heat treatment. Prepare. Before quenching of stamping die parts, low temperature tempering (stabilization treatment) should be carried out. For some concave dies with complex shapes and high precision requirements, they should be quenched and tempered before finishing after rough machining to reduce quenching deformation, try to avoid cracking tendency, and prepare for heat treatment.

2. Optimize the quenching and tempering process

1. Control of tempering treatment

After the mold parts are taken out from the coolant, they should not stay in the air for a long time, and should be placed in a tempering furnace for tempering treatment in time. During tempering, low temperature tempering embrittlement and high temperature tempering embrittlement 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 quenched parts

Quenching and tempering are important links that affect the heat treatment deformation or cracking of stamping die parts. For the parts of the quenched important mold parts (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 parts symmetrical and the internal stress balanced. Commonly used protection methods are as follows: a. packing method; b. filling method; c. blocking method.

3. Determination of heating temperature

If the quenching heating temperature is too high, the austenite grains are coarse, and it will cause oxidation and decarburization, and the tendency of parts deformation and cracking will increase. Within the specified heating temperature range, a low quenching heating temperature will cause the inner hole of the part to shrink and the hole 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 expand and the pore size will become larger, so the lower limit of the heating temperature should be selected.

4. Improvement of heating method

For some small stamping convex and concave dies or slender cylindrical parts (such as small punches), it can be preheated to 520--580 ℃ in advance, and then put into a medium temperature salt bath furnace and heated to the quenching temperature, which is better than directly using an electric furnace or The deformation of the parts heated and quenched in the reverberatory furnace is significantly reduced, and the cracking tendency can be controlled. Especially for high-alloy steel mold parts, the correct heating method is: first preheat (temperature is 530--560 ℃), and then rise 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, the choice of coolant

For alloy steel, the method to reduce quenching deformation is isothermal quenching or graded quenching using a hot bath of potassium nitrate and sodium nitrite. This method is especially suitable for processing stamping dies with complex shapes and accurate dimensions. For some porous mold parts (such as porous concave molds), the isothermal quenching time should not be too long, otherwise the pore size or hole spacing will become larger. If the cooling shrinkage in the oil and the cooling expansion in the nitrate are used, and the dual-medium quenching is reasonably applied, the deformation of the parts can be reduced.

6. Quenching treatment before wire cutting

For some stamping die parts processed by wire cutting, step quenching and multiple tempering (or high temperature tempering) heat treatment processes should be used before wire cutting to improve the hardenability of the parts and make their internal stress distribution tend to be uniform, and in a state of low internal stress. The smaller the internal stress, the smaller the tendency of deformation and cracking after wire cutting.

7. Choice of coolant

For alloy steel, the method to reduce quenching deformation is isothermal quenching or graded quenching using a hot bath of potassium nitrate and sodium nitrite. This method is especially suitable for processing stamping dies with complex shapes and accurate dimensions. For some porous mold parts (such as porous concave molds), the isothermal quenching time should not be too long, otherwise the pore size or hole spacing will become larger. If the cooling shrinkage in the oil and the cooling expansion in the nitrate are used, and the dual-medium quenching is reasonably applied, the deformation of the parts can be reduced.

8. Optimization of cooling method

When the parts are taken out of the box-type carburizing furnace and put into the coolant, they should be properly pre-cooled in the air, and then put into the coolant for quenching. This is one of the effective methods to reduce the quenching deformation of the parts and prevent the cracking tendency of the parts. one. After the mold parts are put into the coolant, they should be rotated properly, and the direction of rotation has changed, which is conducive to maintaining a uniform cooling rate for the parts, which can significantly reduce deformation and prevent cracking.