The heat treatment of Cr12MoV steel is characterized by that it can be quenched at different heating temperatures (980-1100 ℃), the martensitic transformation point MS can be changed in a large range, and the content of retained austenite can also be changed greatly after quenching. Tempering has the phenomenon of secondary tempering hardening. The tempering hardness before hardening peak value can also meet the use requirements. Therefore, the tempering temperature range changes greatly.
(1) annealing
① Cr12MoV forging residual heat treatment: after forging according to the reasonable process, the billet shall stay at the final forging temperature of 900 ℃ for quenching and cooling immediately, oil cooling or fan cooling. Note: when the internal and external temperature of the workpiece reaches 200-300 ℃, it will be tempered at 750 ℃ for 2-4h and then cooled by furnace or air. Instead of the usual spheroidizing annealing process, it saves energy, shortens the production cycle, refines the grains, makes the corners of carbides rounder and the grains smaller, improves the strength and toughness of the mould, and improves the service life of the mould.
② Spheroidizing annealing: generally, isothermal spheroidizing annealing process is adopted, with heating temperature of 850-870 ℃, heat preservation after through burning for 2-4h, isothermal temperature of 720-740 ℃, heat preservation for 3-4h, air cooling after furnace cooling to 500 ℃, and annealing hardness of 207-255hbs.
③ Pre conditioning process: in high alloy steel, due to the high quenching temperature, the pre conditioning temperature can be 10-30 ℃ lower than the final heat treatment temperature of the mold. The tempering temperature is 720-750 ℃, high temperature tempering, holding for 2-4h, furnace cooling or air cooling, and finishing can be carried out after treatment. The tempering treatment shall be sufficient, and two times of tempering treatment can also be used.
(2) Types of quenching and tempering
① Low quenching and low return process: the heating temperature is 950-1020 ℃ and the tempering temperature is 180-200 ℃. The hardness of this process can be 58-62hrc, which is not applicable to the workpiece requiring wire cutting. The grinding feed during grinding must be ≤ 0.02mm, otherwise it is easy to form grinding cracks.
② Medium quenching and medium return process: heating temperature 1030 ℃, oil cooling quenching, tempering temperature 400-520 ℃, hardness control ≤ 59HRC, especially suitable for wire cutting or large grinding amount of workpiece.
③ High quenching and high recovery process: heating temperature 1050-1080 ℃, oil cooling quenching, tempering temperature 500-520 ℃.
(3) Selection of quenching and tempering process
① Cr12MoV graded quenching (to reduce deformation and prevent cracking): the heating temperature is 1020 ℃, after heat preservation, it is put into 260-280 ℃ nitrate furnace for classification for 3-10min, and then transferred to MS - (10-20) ℃ nitrate furnace for 5-10min and air cooling. Or it can be directly quenched in a salting furnace at 160-180 ℃ for 5-10 minutes and then air cooled. Air cooling to about 120 ℃ is transferred to tempering process.
② Cr12MoV isothermal quenching (to increase strength and toughness): the heating temperature is 1020 ℃, after heat preservation, put it into the nitrate furnace with MS - (10-20) ℃ for 3-10min, turn to 260-280 ℃ for 2-3H air cooling, and then turn to the tempering process at 120 ℃.
③ Cooling quenching of Cr12MoV steel (reducing quenching deformation): the forming die of bakelite powder made of Cr12MoV steel has complex shape, strict dimensional deformation requirements and certain toughness, but the hardness requirements are generally 45-50hrc. If 1020 ℃ is used for heating and quenching, high temperature tempering must be used, so the deformation is difficult to control. After being heated at 880 ℃, the oil is cooled to 150-200 ℃ and immediately transferred to 300 ℃ for 3-4h, and then tempered at 200 ℃. The deformation of the die is very small, the toughness is good, and the hardness is about 48 HRC. The defect is that there is a small amount of Troostite in the heat treated tissue.
④ Selection of tempering temperature of Cr12MoV Steel: 1020-1050 ℃ for quenching and heating, 180-200 ℃ for high hardness; 400-420 ℃ for preventing wire cutting cracking. Due to the deformation of quenching and cooling, 480 ℃ tempering can make the size shrink a little; 510 ℃ tempering can make the size swell a little.
⑤ Deep cooling treatment of die: improve wear resistance and dimensional stability.
The quenched die is put into liquid nitrogen for 1-2 h for cryogenic treatment, and then tempered, or cryogenic treatment can be carried out after tempering. After cryogenic treatment, the hardness of Cr12MoV die is increased by 1-2 HRC.
(4) Compensation of modified layer after wire cutting of Cr12MoV mould
The surface of the quenched and tempered die has a modified layer of about 0.05mm after wire cutting. There are many micro cracks within the outermost layer of 0.01mm. The hardness of the quenched layer and the heat affected zone is relatively low, about 52-55hrc. The solution is grinding by grinder. It can also be used to grind off micro cracks and make up for them by vacuum pulse nitriding to improve the hardness.
On 2018-08-08 11:45:46
