There are many reasons that affect the deformation and cracking of stamping die during heat treatment. Heat treatment deformation is often unavoidable, but cracking can be prevented.
I. preparation for 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 network secondary cementite in the forgings, refine the grain, eliminate the internal stress, and prepare the microstructure for the subsequent (or final) heat treatment. Low temperature tempering (stabilization treatment) should be carried out before stamping die parts are quenched. For some dies with complex shapes and high precision requirements, quenching and tempering treatment should be carried out before finishing after rough machining to reduce quenching deformation, avoid cracking tendency as far as possible, and prepare for the final heat treatment.
Second, optimize the quenching and tempering process
1. Protection of quenching parts
Quenching and tempering is an important process that affects the heat treatment deformation or cracking of stamping die parts. Effective protection measures should be taken for the parts which are easy to be deformed or cracked, such as punch and die, to make sure the shape and section of parts are symmetrical and the internal stress is balanced. Commonly used protection methods are as follows: 1. (2) filling method; The plugging method.
2. Quenching treatment before linear cutting
In order to improve the hardenability of the parts, the internal stress distribution tends to be uniform and the parts are in the state of small internal stress. The smaller the internal stress, the smaller the tendency of deformation and cracking after linear cutting.
3. Improvement of heating method
For some small stamping intensive or slender cylindrical parts (such as small punch), prior to preheat to 520-580 ℃, and then into the medium temperature salt bath furnace heating and quenching temperature, than the direct use of heating furnace and reverberatory furnace quenching parts deformation decrease obviously, and can control the cracking tendency. Especially high alloy steel mold parts, the right heating mode is: the first preheating temperature (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 the quenching deformation and avoid the production of small cracks.
4. Determination of heating temperature
Quenching heating temperature is too high, making the austenite grain size, and will cause oxidation, decarbonization phenomenon, parts deformation and cracking tendency increase. Within the specified heating temperature range, the low quenching temperature will cause the shrinkage of the inner hole of the parts and the reduction of the pore size. Therefore, the upper limit of the heating temperature specification should be selected. However, for alloy steel, if the heating temperature is too high, the inner hole will expand and the hole size will become larger. Therefore, the lower limit of heating temperature should be selected.
5. Selection of coolant
For alloy steel, the best way to reduce the quenching deformation is to use potassium nitrate and sodium nitrite hot bath isothermal quenching or stepwise quenching, which is especially suitable for stamping dies with complex shapes and precise size requirements. Some porous mold parts (such as porous concave die), isothermal quenching time should not be too long, otherwise it will cause the hole diameter or hole spacing. If the characteristics of cooling shrinkage in oil and cooling expansion in nitrate are taken into account, the deformation of parts can be reduced by proper quenching with dual media.
6. Optimization of cooling mode
Before the parts are removed from the heating furnace and put into the coolant, they should be placed in the air for proper pre-cooling and then put into the coolant for quenching. This is one of the effective methods to reduce the quenching deformation of parts and prevent the cracking tendency of parts. After cooling agent is put into the mold part, it should be rotated properly, and the rotation direction is changed, which is conducive to the parts to maintain uniform cooling rate, can obviously reduce the deformation and prevent cracking tendency.
7. Control of tempering treatment
After the mold parts are removed from the coolant, they should not stay in the air for a long time and should be put into the tempering furnace in time. Low tempering brittleness and high tempering brittleness should be avoided during tempering treatment. In order to eliminate the internal stress, reduce the deformation and avoid the tendency of cracking, several tempering treatments are adopted after quenching for some precision parts.
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