Heat treatment of die casting mold parts:
1. The quenching equipment is a vacuum quenching furnace with high pressure and high flow rate.
(1) Before quenching: heat balance method is adopted to improve the overall consistency of mold heating and cooling. All thin-walled holes, grooves, and cavities that affect this point must be filled and blocked, and the mold can be heated and cooled as much as possible; at the same time, pay attention to the furnace installation method to prevent the die-casting mold from being caused by high temperatures. Deformation caused by its own weight.
(2) The heating of the mold: During the heating process, it should be heated slowly (200℃/h heating), and a two-stage preheating method should be adopted to prevent the rapid heating from causing excessive temperature difference between the inside and outside of the mold, causing excessive thermal stress. At the same time, the phase change stress is reduced.
(3) Quenching temperature and holding time: the lower limit of the quenching heating temperature should be used, and the soaking time should not be too short or too long. The soaking time is generally determined by the wall thickness and hardness.
(4) Quenching and cooling: using pre-cooling, and by adjusting the air pressure and wind speed, the cooling rate is effectively controlled to maximize the ideal cooling. Namely: After pre-cooling to 850℃, increase the cooling rate and quickly pass the nose of the “C” curve. When the mold temperature is below 500℃, the cooling rate will gradually decrease. When the temperature is below the Ms point, the cooling method of approximately isothermal transformation is adopted to maximize Minimize quenching deformation. When the mold is cooled to about 150°C, turn off the cooling fan and let the mold cool naturally.
2. Annealing includes two parts: spheroidizing annealing after forging and stress relief annealing during mold making. Its main purpose is to improve the crystalline structure in the raw material stage; facilitate processing and reduce hardness; prevent deformation and quenching cracks after processing to remove internal stress.
(1) Spheroidizing annealing. After the die steel is forged, the internal structure of the steel becomes unstable crystals, and it is difficult to cut with high hardness. The steel in this state has high internal stress, is easily deformed and quenched after processing, and has poor mechanical properties. In order to crystallize carbides To become a spheroidized stable structure, spheroidizing annealing is required.
(2) Stress relief annealing. Machining die steel with residual stress will deform after machining. If there is still stress after machining, great deformation or quenching cracks will occur during quenching. To prevent these problems from occurring, stress relief annealing must be carried out.
Generally, three times of stress relief annealing are carried out during the mold making process:
(1) When cutting away more than 1/3 of the raw material volume or deep processing the raw material thickness 1/2, the machining allowance is left 5-10mm, and the first stress relief annealing is performed.
(2) When there is a margin (2-5mm) for finishing, perform the second stress relief annealing.
(3) After the mold test, the third stress relief annealing is carried out before quenching.
3. When the tempered and quenched mold is cooled to about 100°C, it must be tempered immediately to prevent further deformation or even cracking. The tempering temperature is determined by the working hardness, generally three times of tempering.
4. Nitriding treatment Generally, die-casting molds can be used after quenching and tempering (45～47HRC), but in order to improve the wear resistance, corrosion resistance and oxidation resistance of the mold, prevent mold sticking, and extend the life of the mold, it is necessary Carry out nitriding treatment. The depth of the nitride layer is generally 0.15 to 0.2 mm. After nitriding, it needs to be polished to remove the white layer (thickness about 0.01mm).
5. A few explanations
(1) The heat treatment deformation of the mold is caused by the combined action of phase transformation stress and thermal stress, and is affected by many factors. Therefore, under the premise of correct selection of materials, attention should be paid to the forging of the blank, and the method of six-sided forging should be adopted, and the upsetting should be repeated. At the same time, in the design stage of the mold, it is necessary to pay attention to make the wall thickness as uniform as possible (open process holes when the wall thickness is uneven); for molds with complex shapes, use the mosaic structure instead of the overall structure; for thin-walled molds , Sharp corners of the mold, it is necessary to use fillet transition and increase the fillet radius. During the heat treatment, it is necessary to make a good data record, the amount of deformation in each direction of length, width, and thickness, and the heat treatment conditions (furnace installation method, heating temperature, cooling rate, hardness, etc.), to accumulate experience for the heat treatment of the mold in the future.
(2) There are generally two technological processes for the processing of die-casting molds, both of which are determined according to actual conditions. The first type: general die-casting mold. Forging → spheroidizing annealing → rough machining → first stress relief annealing (leave a margin of 5～10mm) → rough machining → second stress relief annealing (leave a margin of 2～5mm) → finishing → third Secondary stress relief annealing (after mold trial, before quenching) → quenching → tempering → clamp repair → nitriding. The second type: particularly complex and easily deformed molds after quenching. Forging → spheroidizing annealing → rough machining → the first stress relief annealing (leave a margin of 5-10mm) → quenching → tempering → mechanical and electrical machining → the second stress relief annealing (leave a margin of 2-5mm )→Machine and electrical machining→Third stress relief annealing (after mold trial)→Clamping→Nitriding.