Method for preventing deformation and cracking of die casting die heat treatment
In the production and processing of die casting die parts, because of its complex shape and structure, there are obvious differences in the section size of each part, so in the heat treatment, the heating and cooling rates of each part are also different. This may lead to distinct thermal stresses, structural stresses and phase change volumes in various parts of the part. It causes abnormal expansion or shrinkage of the volume of the parts, which makes the size and shape of the parts deviate greatly, even causes cracking.
The causes of heat treatment deformation and cracking of die casting die are various, including the chemical composition and original structure of steel, the structural shape and section size of parts and heat treatment process. In actual production, deformation is often not completely eradicated, can only minimize the extent of its occurrence, but as long as appropriate means, cracking can be completely avoided.
Preparatory heat treatment
The so-called preparatory heat treatment is relative to the final heat treatment, which is to add a preparatory heat treatment step before the final heat treatment, this step can provide a good mechanical properties or microstructure for the final heat treatment. Common preparatory heat treatment processes include annealing, normalizing and quenching and tempering.
For the preparation heat treatment of eutectoid steel stamping die, the emphasis is to eliminate the network secondary cementite, refine the grain and internal stress in the forging. The specific process is normalizing and spheroidizing. For stamping die parts, low temperature tempering should be used for stabilization. For those moulds with complex shape and high precision, due to the higher possibility of deformation and cracking during heat treatment, proper quenching and tempering should be carried out after roughing and before finishing, so as to prepare for the final heat treatment and avoid cracking as much as possible.
Quenching heating mode and parts protection
Quenching and tempering are most likely to cause deformation and cracking of parts. For some small die-casting molds, slender cylindrical parts or high alloy steel mold parts, direct heating should be avoided. Instead, they should be preheated to 520 to 580 degrees Celsius before being heated to the quenching temperature in a medium temperature salt bath furnace. Practice has proved that the deformation of the parts heated by this method is obviously smaller than that heated and quenched directly in electric furnace or reverberatory furnace, and the cracking can be basically avoided.
In quenching, if the heating temperature is too high, the austenite parts will coarse grains, and easily cause oxidation, decarbonization and other phenomena, resulting in deformation and cracking of the parts; and if the temperature is low, it will cause shrinkage of the parts, and the size of the pores will become smaller. Therefore, the upper limit of temperature should be selected for quenching in the range of heating temperature. For alloy steel, the heating temperature is too high, it will cause the expansion of the inner hole, the pore size becomes larger, the best choice is the lower limit of allowable temperature.
In addition, during quenching and tempering, it is necessary to take effective measures to protect the parts which are prone to deformation and cracking, so that the shape and cross-section are symmetrical and the internal stress is balanced. Especially for those parts with complex shapes. The usual protection methods include baling, filling and blockage.
Optimization of cooling mode and selection of coolant
When the die casting die parts are heated, they should not be put into the coolant directly after being removed from the furnace, which will easily lead to deformation and cracking due to excessive local temperature difference. The correct method is to pre-cool the parts in the air and then quench them in the coolant. In order to ensure the uniform cooling speed of each part of the parts, the coolant should be put into the appropriate rotation, and the rotation direction is preferably not fixed.
The choice of coolant is also important. For alloy steels, isothermal quenching or stepwise quenching with potassium nitrate and sodium nitrite hot bath is an effective method to reduce deformation, especially for die-casting dies with complex shapes and precise size requirements. Some porous die parts have the characteristics of cooling shrinkage in oil and cooling expansion in nitrate. The deformation of parts caused by quenching can also be reduced by rational use of two different media.
Control of tempering treatment
Die casting die parts should not stay in the air too long after quenching in coolant, but should be put into tempering furnace in time for tempering treatment to eliminate the internal stress of the parts, reduce the deformation and cracking tendency. Especially for some die-casting die parts which need to be machined by WEDM, the hardenability of the parts can be effectively improved by adopting stepwise quenching and multiple tempering heat treatment before WEDM, so that the internal stress distribution is uniform and the deformation and cracking are not easy to occur. During tempering process, low temperature temper brittleness and high temperature temper brittleness must be avoided.
