Heat treatment has a great impact on the service life of die-casting molds. According to relevant information, due to improper heat treatment, the early failure of die-casting molds accounts for about 44% of the entire die-casting mold accidents. The stress generated when the steel is quenched is actually the result of the superposition of the thermal stress during the cooling process and the structural stress during the phase transformation. After the steel is quenched, it is in a state of high stress and low toughness, with high hardness and strength, and is relatively brittle, which actually cannot be used. The quenching stress is the cause of deformation and cracking, and leads to the reduction of fatigue strength and impact toughness. For this reason, the steel must be tempered after quenching to remove stress, stabilize the structure and improve toughness.
Due to the large internal stress generated during the machining of the mold, in order to prevent the superposition of the internal stress during the heat treatment and cause the deformation and cracking of the die casting mold, the mold is required to be subjected to a stress relief annealing treatment after machining and before quenching.
The annealing temperature is 600-650°C, and the holding time is calculated according to the thickness of each 25mm holding for 1 hour.
1. The quenching process adopts multi-stage preheating and strictly controls the heating rate
Compared with low alloy steel, medium carbon high alloy steel has poor thermal conductivity. Therefore, multi-stage preheating (2 to 3 stages) should be used for quenching heating, which is 300 ° C, 650 ° C, and 850 ° C preheating. At the same time, the heating rate should follow the principle of slow heating (100-200℃/h) to reduce the thermal stress generated during the heating process. In the high-temperature heating stage, a faster heating rate (10-15°C/h) can be used to shorten the high-temperature heating time and prevent coarse grains.
2. Prevent the workpiece from overheating
If the temperature in the heating stage exceeds the normal quenching temperature (including the failure of the instrument or the placement of the workpiece close to the heating element, etc.), with the increase of the quenching temperature and the extension of the holding time, the grains generally coarsen, the brittleness of the structure increases, and the residual austenite increases, the spherical carbide is transformed into a polygonal carbide, and there is a network structure, and the die-casting mold is prone to cracking during use.
3. Selection of quenching heating medium and cooling medium
In today's world, large and complex precision molds are mostly heat treated in vacuum or protective atmosphere to prevent oxidation and decarburization of the mold surface, especially the vacuum high-pressure air-cooled quenching process is widely used. However, it should be noted that the air cooling speed cannot be too low, then the pressure of nitrogen is required to be sufficient. Usually, the surface cooling rate of the workpiece is required to be greater than or equal to 28 °C/min during the process of cooling the workpiece from the quenching temperature (1020-1050 °C) to 538 °C. There are also requirements for the cooling rate of the workpiece core to be greater than or equal to 28°C/min.
If the cooling rate is too slow in this temperature range, carbides or other transformation products will be precipitated along the grain boundaries in the microstructure after quenching, thereby reducing the toughness of the steel, increasing the brittleness, and prematurely cracking during use. In addition, if conditions permit, graded isothermal quenching can be used for particularly complex large-scale die-casting molds, which can effectively reduce mold deformation and avoid cracking.
4. Tempering process
Temper in time after quenching. After quenching, the workpiece should be tempered immediately after cooling to 90 ~ 70 ℃, because the workpiece is in a state of high stress and low plasticity after quenching, which is easy to cause cracking; the tempering should be sufficient. After quenching of large and complex die-casting molds, it is usually tempered three times, and the tempering time for each time is calculated according to the effective thickness of the workpiece for 1 hour per 25mm, but not less than 4 hours. The purpose is to reduce tissue transformation stress and stabilize dimensions.