Deformation and cracking of stamping die during heat treatment

There are many factors affecting the deformation and cracking of stamping die, which are mainly related to the original structure, the chemical composition of steel, the structural shape and section size of parts, the heat treatment process and other factors. Cracking is often preventable, but heat treatment deformation is always unavoidable.

In practice, the section size of the difference, the structure of the stamping die parts shape, in the process of heat treatment (heating, heat preservation, cooling) for heating and cooling rate, the thermal stress and organization stress and phase change to a combination of volume change, cause parts volume expansion or contraction, so that the size and shape deviation, deformation, and even cause cracking.

Preliminary heat treatment

For the stamping die forgings of eutectoid steel, normalizing treatment and spheroidizing annealing should be carried out to eliminate the meshed secondary cementite in the forgings, refine grain size, eliminate internal stress, and prepare for the subsequent (or final) heat treatment. Stamping die parts should be tempered at low temperature (stabilization) before quenching. For some concave die with complex shape and high precision requirements, the quenching and tempering treatment should be carried out first before finishing after rough machining, in order to reduce quenching deformation, avoid cracking tendency as far as possible, and prepare for the final heat treatment.

Optimize quenching and tempering process

1. Control of tempering treatment

After the mould parts are taken out from the coolant, it is not suitable to stay in the air for a long time. During tempering treatment, low tempering brittleness and high tempering brittleness should be avoided. For some precision mold parts, quenching after using multiple tempering treatment, in order to eliminate the internal stress, reduce the deformation, avoid cracking tendency.

2. Protection of quenching parts

Quenching and tempering are the important steps that affect the heat treatment deformation or cracking of stamping die parts. For quenching important die parts (such as punch, die) prone to deformation or cracking parts, should take effective protection measures, and strive to make the shape and section of parts symmetrical, internal stress balance. The commonly used protection methods are as follows: a. B. filling method; C. blocking method.

3. Determination of heating temperature

The high quenching temperature makes the austenite grain coarse, and causes oxidation and decarburization, and the tendency of deformation and cracking increases. In the specified heating temperature range, the lower quenching temperature will cause the shrinkage of the inner hole of the part, the smaller the aperture size. Therefore, the upper limit of the heating temperature specification should be selected. However, for alloy steel, the heating temperature is too high, it will cause the expansion of the inner hole, the aperture size of the larger, so should choose the heating temperature of the lower limit is appropriate.

4. Improvement of heating method

For some small punch punch punch or slender cylindrical parts (such as small punch), can be preheated to 520--580℃, and then put into the middle temperature of the salt bath furnace heating to quenching temperature, than the direct use of electric furnace or reflection furnace heating quenching parts deformation significantly reduced, and can control the cracking tendency. In particular, high alloy steel mold parts, the correct heating method is: preheat (the temperature is 530--560℃), and then to the quenching temperature. In the heating process, the high temperature period should be shortened as far as possible to reduce quenching deformation and avoid the production of small cracks.

5. Choice of coolant

For alloy steel, the best way to reduce quenching deformation is to use potassium nitrate and sodium nitrite hot bath isothermal quenching or graded quenching, this method is particularly suitable for the complex shape, size requirements of precision stamping die. Some porous mold parts (such as porous die), the isothermal quenching time should not be too long, otherwise it will cause the aperture or hole spacing. If the characteristics of cooling shrinkage in oil and cooling expansion in nitrate are taken advantage of, the reasonable application of dual media quenching can reduce the deformation of parts.

6. Quenching treatment before wire cutting

For some die parts processed by wire cutting, the process of graded quenching and multiple tempering (or high temperature tempering) should be used before the processing of wire cutting, in order to improve the hardenability of parts, and make its internal stress distribution tends to be uniform, and in a state of less internal stress. The smaller the internal stress, the smaller the tendency of deformation and cracking after linear cutting.

7. Optimization of cooling mode

Before the parts are taken out from the heating furnace and put into the coolant, they should be placed in the air to be precooled properly and then put into the coolant to be quenched. After the coolant is put into the mold parts, they should be properly rotated, and the rotation direction is changed, which is conducive to the parts parts to maintain a uniform cooling rate, can significantly reduce the deformation and prevent cracking tendency.

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