Method for improving deformation and cracking during heat treatment of stamping die

Stamping dies are difficult to manufacture due to the large variety of specifications, complex shapes and low surface roughness values. The deformation of the mold after heat treatment will seriously affect the quality and service life of the mold. Once cracked during the heat treatment, the mold will be scrapped. Therefore, reducing and preventing heat treatment deformation and cracking of the mold is an important research topic for the majority of stamping die heat treatment workers. We briefly analyze the causes of common deformation and cracking defects in the heat treatment process of stamping dies, and propose ways to improve them. 1. Preliminary heat treatment For eutectoid steel stamping die forgings, 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, and eliminate internal stress. (Or finally) heat treatment to prepare the organization. Before quenching, stamping die parts should be tempered at low temperature (stabilization treatment). For some concave molds with more complex shapes and high precision requirements, quenching and tempering treatment should be carried out before finishing after rough machining to reduce quenching deformation, try to avoid cracking tendency, and prepare for final heat treatment. 2. Optimize the quenching and tempering treatment process 1. Tempering treatment control After the mold parts are taken out of the coolant, they should not stay in the air for a long time, and should be put into the tempering furnace for tempering treatment in time. 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 quenched parts Quenching and tempering are important links that affect the deformation or cracking of the heat treatment of stamping die parts. For hardening important mold parts (such as convex molds, concave molds) that are prone to deformation or cracking, effective protective measures should be taken to make the shape of the parts symmetrical with the cross-section, and the internal stress is balanced. The commonly used protection methods are as follows: bundling method, filling method, and blocking method. 3. Determination of heating temperature Excessive heating temperature for quenching makes the austenite grains coarser, and will cause oxidation and decarburization, and the tendency of parts to deform and crack increases. 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; and for alloy steel, the higher heating temperature will cause the inner hole to expand and the pore size will increase, 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℃, and then heated to the quenching temperature in a medium temperature salt bath furnace , Compared with the direct use of electric furnace or reverberatory furnace, the deformation of the quenched parts 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 increase 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 best way to reduce quenching distortion is to use potassium nitrate and sodium nitrite hot bath austempering or stage quenching. This method is especially suitable for processing complex shapes and precise dimensions. Stamping die. For some porous mold parts (such as porous die), the austempering time should not be too long, otherwise it will cause the hole diameter or hole distance to become larger. If the characteristics of cooling shrinkage in oil and cooling expansion in nitrate are used, dual-medium quenching can be used reasonably to reduce part deformation. 6. Quenching treatment before wire cutting For some stamping die parts processed by wire cutting, stepped 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 them The internal stress distribution tends to be uniform and in a state of relatively small internal stress. The smaller the internal stress, the smaller the tendency for deformation and cracking after wire cutting. 7. Optimization of the cooling method. Before the parts are taken out of the heating furnace and put into the coolant, they should be placed in the air for proper pre-cooling, and then quenched in the coolant, which is effective to reduce the quenching deformation of the parts and prevent the tendency of the parts to crack One of the methods. After the mold parts are put into the coolant, they should be rotated appropriately, and the direction of rotation has been changed, so that the parts can maintain a uniform cooling rate, which can significantly reduce deformation and prevent cracking tendencies. Recommended article: Inquiry into the problem of drawing mold surface strain and preventive measures. Previous article: Do these points to effectively prevent the deformation of stamping gears