The difference of structure shape, section size, heat and cooling rate in different heat treatment process, the combination of thermal stress and microstructure stress and the phase change of volume change, cause part volume expansion or contraction, such size and shape deviation, deformation, even cracking reason. There are many factors affecting the deformation and cracking of stamping die during heat treatment, which are mainly related to the chemical composition of steel, the size and shape of the original structure, the components and heat treatment process. Heat treatment deformation is inevitable, but can prevent the generation of cracks.
Preliminary heat treatment: stamping die forgings, eutectoid steels are normalized, and then spheroidized annealing is performed to eliminate the forging in the secondary cementite mesh, refine the grain, eliminate internal stress, and prepare the tissue for subsequent treatment. Cold tempering should be carried out before stamping. For some complex shapes and high-precision molds, quenching and tempering treatment should be carried out before rough machining is completed to reduce quenching deformation and avoid cracking tendency, and to prepare for the final heat treatment.
Optimization of quenching and tempering process: protection of quenching parts: quenching and tempering are important factors affecting the heat treatment deformation or cracking of stamping parts. For the parts which are easy to be deformed or cracked, effective protective measures must be taken to equalize the shape, section symmetry and internal stress of the parts. Commonly used protection methods are: package method, completion method, block method.
The improvement of heating mode: for some small stamping dense or slender cylindrical parts, preheat to 520-580 ℃, add the medium temperature salt bath furnace heating quenching temperature, than directly using electric furnace and reverberatory furnace heating quenching parts deformation decrease obviously, and can control the cracking tendency. In particular, the high alloy steel mold parts, the correct heating method is: preheating, and then rise to the quenching temperature. In the heating process, the high temperature period should be shortened to reduce quenching deformation and avoid fine cracks.
Determination of heating temperature: the quenching temperature is too high, making the austenite grains thick, resulting in oxidation and decarbonization, parts deformation and cracking tendency increase. In a certain range of heating temperature, quenching temperature reduction will lead to the parts of the hole shrinkage, pore diameter smaller. Therefore, the upper limit of heating temperature should be selected. In the case of alloy steel, the heating temperature is high, resulting in the expansion of the inner hole, the pore diameter. Therefore, the lower limit of heating temperature should be selected.
The best way to reduce the quenching deformation is to adopt potassium nitrate and sodium nitrite constant temperature quenching or intermittent quenching to choose the cooling liquid of alloy steel. Some porous mold parts (such as porous mold), isothermal quenching time should not be too long, otherwise it will make the hole or hole spacing. If cooling shrinkage oil is used, it is reasonable to adopt double medium quenching to reduce the deformation of parts by taking advantage of the cooling expansion characteristics of nitrate.
Optimization of cooling method: when the parts from the furnace before cooling, should be placed in the appropriate air pre-cooling, and then into the cooling quenching, quenching to reduce the parts deformation and cracking prone to prevent parts one of the effective methods. After entering the coolant, the mold parts should be properly rotated and the rotation direction should be changed, which is conducive to maintaining uniform cooling speed of the components, significantly reducing deformation and preventing cracking tendency.
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