In the production and processing of die-casting mold parts, due to the complex shape and structure, there are obvious differences in the cross-sectional size of each part, so when heat treatment is performed, the heating and cooling rates of each part are also different. This situation can lead to the formation of distinct thermal stresses, tissue stresses, and phase-transition volumes in various parts of the part. It causes abnormal expansion or contraction of the volume of the part, resulting in a large deviation in its size and shape, and even cracking.

 There are many reasons for the heat treatment deformation and cracking of die casting molds, including the chemical composition and original structure of the steel, the structural shape and section size of the parts, and the heat treatment process. In actual production, deformation is often impossible to completely eliminate, and the degree of its occurrence can only be reduced as much as possible, but as long as appropriate measures are taken, cracking can be completely avoided.

 

 The so-called preparatory heat treatment is relative to the final heat treatment, that is, a preparatory heat treatment step is added before the final heat treatment, which can provide good machinability or microstructure for the final heat treatment. Common preparatory heat treatment processes include annealing, normalizing, quenching, and tempering.

 For the preparatory heat treatment of eutectoid steel stamping dies, the focus is on eliminating reticulated secondary cementite, grain refinement, and internal stress in forgings. The specific process is to carry out normalizing treatment first, and then carry out spheroidizing annealing. For stamping die parts, it should be stabilized by low-temperature tempering. For those molds with complex shapes and high precision requirements, due to the higher possibility of deformation and cracking during heat treatment, appropriate quenching and tempering treatment should be carried out after the rough machining is completed and before the finishing machining is started. The heat treatment is used to prepare the organization to avoid the occurrence of cracking as much as possible.

 

 Quenching and tempering are the processes most prone to deformation and cracking of parts. For some small die-casting molds, slender cylindrical parts or high-alloy steel mold parts, etc., the quenching method of direct heating should be avoided, but it should be preheated to 520 to 580 degrees Celsius, and then placed in a medium temperature salt bath furnace. Heat to quenching temperature. The practice has proved that the deformation of parts using this heating method is significantly smaller than that of direct heating and quenching in an electric furnace or reverberatory furnace, and cracking can be basically avoided.

 During quenching, if the heating temperature of austenitic parts is too high, the grains will be coarse, and it is easy to cause oxidation, decarburization, and other phenomena, resulting in deformation and cracking of the parts; and if the temperature is too low, the inner holes of the parts will shrink and the size of the pore size will shrink. become smaller. Therefore, within the allowable range of heating temperature, try to select the upper limit of temperature for quenching. For alloy steel, the heating temperature is too high, which will cause the inner hole to expand and the pore size to become larger. It is best to choose the lower limit of the allowable temperature.

 In addition, when quenching and tempering, it is necessary to take measures to effectively protect the parts that are prone to deformation and cracking, so that the shape and section are symmetrical, and the internal stress is balanced. This is especially true for parts with complex shapes. Commonly used protection methods include baling, filling, and plugging.

 

 When the die-casting mold parts are heated, they should not be directly put into the coolant after being taken out of the furnace, which will easily cause the local temperature difference to be too large and cause deformation and cracking. The correct method is to pre-cool the parts in the air first, and then put them into the coolant for quenching. In order to ensure the uniform cooling speed of each part of the part, it should be rotated properly after the coolant is put in, and the rotation direction is preferably not fixed.

 The choice of coolant is equally important. For alloy steels, isothermal quenching or graded quenching using potassium nitrate and sodium nitrite hot baths is an effective method to reduce deformation, especially for die-casting molds with complex shapes and precise dimensions. Some porous mold parts have the characteristics of cooling and shrinking in oil and cooling and expansion in nitrate. The rational use of two different media can also reduce the deformation of parts caused by quenching.

 

 After the die-casting mold parts are quenched in the coolant, they should not stay in the air for too long but should be placed in a tempering furnace for tempering treatment in time to eliminate the internal stress of the parts and reduce the tendency of deformation and cracking. Especially for some die-casting mold parts that need to be processed by wire cutting, graded quenching, and multiple tempering heat treatments are used before wire cutting processing, which can effectively improve the hardenability of the parts, make the internal stress distribution uniform, and be less prone to deformation and cracking. During the tempering process, it is necessary to avoid the occurrence of low-temperature tempering embrittlement and high-temperature tempering embrittlement.