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Vacuum Heat Treatment Process of Cr12MoV Steel Die
1 Steel mold vacuum heat treatment technology
A factory is producing the lower die of a certain mold, the size is 14×100×140 (mm), the hole tolerance of 6~12mm is required to be ±0.01 mm, the hole spacing tolerance is ±0.02mm, and the warpage of the plane is > 0.02 mm, the hardness is HRC50 ~ 55, and the material is Cr12MoV. Because of the strict tolerance requirements, not only the machining accuracy should be strictly required during cold working, but also strict anti-deformation measures should be taken during vacuum heat treatment. In production practice, the following measures are taken to effectively control the deformation of the mold.
The material requires forging, so the forging temperature is required to be high. The initial forging temperature of Cr12MoV steel is 1100 ~ 1150 ℃, and the final forging temperature is 880 ~ 900 ℃. During forging, when the billet is at 1150 °C, it should be lightly hit to prevent forging cracking; it should be hit hard in the temperature range of 1000 ~ 1100 °C; in the temperature range of 900 ~ 1000 °C, due to the reduction of plasticity, it should be lightly hit, that is, at different temperatures stage, master the light-heavy-light forging method. Due to the complex chemical composition of Cr12MoV steel, high recrystallization temperature, and low initial forging temperature, it is easy to cause cold hardening of the material, so the final forging temperature of Cr12MoV is below the Acm line and 50 ~ 100 ℃ above the A1 line. If the forging temperature is selected above the Acm line, the secondary network cementite will precipitate along the grain boundary during the cooling process after forging, which will greatly reduce the mechanical properties of the forging, such as the Acm line and the A1 line. Due to the mechanical damage of plastic deformation, the precipitated secondary network cementite is dispersive. The actual forgeable temperature range of Cr12MoV steel is narrow (about 250 ℃), so sometimes it takes several forgings to forge a qualified product. Special attention should be paid to crushing carbides as much as possible during forging, and making the metal fibers flow perpendicular to the large plane, which is more conducive to controlling and adjusting the deformation of the workpiece during heat treatment.
1.2 Vacuum annealing treatment after forging
The annealing after forging must be spheroidized annealing. Because complete annealing will make Cr12MoV form network carbide, and it can still be maintained in the final vacuum quenching and vacuum tempering process, which will increase its brittleness and cannot be used. After spheroidizing annealing, due to the spheroidization of carbides in the steel, it can not only improve the plasticity, toughness, and machinability, but also reduce the deformation and cracking tendency of the final heat treatment. The microstructure after spheroidizing annealing is sorbite type pearlite + granular carbide, and the hardness is HB206 ~ 260.
1.3 Pretreatment before finishing
First, the thickness of the mold is roughed to 16mm, and the finishing amount is 2mm. After quenching and tempering, a uniform and fine sorbite structure can be obtained. This not only ensures that the final quenching has uniform hardness, but also helps to reduce the deformation of the workpiece after quenching, and increases the dimensional stability of the mold.
1.4 The following points should be paid special attention to during final quenching
(1) In order to prevent the oxidation and decarburization of the workpiece during quenching, first add a deoxidizer to the salt furnace, thoroughly remove the slag and then heat it.
(2) In order to prevent deformation caused by heating, a segmented heating method is adopted. In the past, direct oil quenching was adopted after heating, and the deformation was large, and the hole diameter, hole distance and warpage did not meet the requirements. , First pre-cooling in a medium temperature salt furnace and then isothermal quenching in a 260 ℃ nitrate furnace for 3 hours at a medium temperature, the pore diameter and hole distance can meet the requirements, but the warpage degree is > 0.03mm, which cannot meet the requirements of the drawings.
Finally, the following process is adopted: after the workpiece is heated, it is first pre-cooled in a medium temperature salt furnace at 820 ° C for 2 minutes, and then the workpiece is placed between two steel plates that have been preheated to 350 ~ 400 ° C, and then pressurized on a press and cooled to At room temperature, after measurement, not only the hole diameter and hole distance of the workpiece are guaranteed, but also the warpage degree is also required.
With the correct selection of vacuum quenching temperature, retained austenite can partially or even completely offset the size increase caused by martensitic transformation during quenching, resulting in minimal or no deformation. When heated at 1025±5℃, the dissolving amount of alloy carbide (Cr, Fe)7C3 in austenite and the concentration of C and Cr in austenite can make Cr12MoV steel obtain a better combination of strength and plasticity , Not only the comprehensive mechanical properties are better, but also in the subsequent vacuum tempering process, the size of the mold can be controlled by changing the vacuum tempering temperature. If the heating temperature is too high, the content of alloying elements in the austenite will increase and the Ms point will decrease, which will lead to the increase of retained austenite, and the hardness of the workpiece will drop sharply. necessary. The determination of the vacuum tempering temperature should also be determined according to the amount of retained austenite after vacuum quenching, which is used to control and adjust the size of the mold. ; If the aperture is reduced, tempering at 420 ~ 520 ℃ can be used to increase the size. It should be noted that it must be tempered twice, and the next tempering temperature can be determined according to the actual size after each tempering, so as to control the deformation of the mold to a minimum.
By improving the forging process, changing the vacuum heat treatment vacuum annealing process after forging, and changing the final heat treatment process, the hole spacing, hole diameter, plane warpage and hardness of the die can meet the requirements of the drawings. Deformation, cracking and other phenomena, the service life of the mold has been greatly improved.
Selection of vacuum heat treatment equipment: RVA series vacuum annealing furnace produced by SIMUWU is a high-quality product for vacuum heat treatment process. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.
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