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Heat Treatment Process of Cr12MoV Steel Cold Extrusion Die
Cr12MoV steel is one of the most commonly used mold steels in my country. Because of its high hardness, high strength, and small volume deformation during heat treatment, it is mostly used for high-load, high-precision, and long-life cold-deformed molds. At present, there are many factors that cause mold damage, but the main reason is the unreasonable forging process or heat treatment process. For Cr12MoV such high-carbon and high-chromium steel, the unevenness of carbides and reasonable heat treatment process are the decisive factors affecting the service life of the die, which is especially prominent for the high-load cold extrusion die. To this end, improve the carbon in Cr12MoV steel. The distribution of materials and the use of a reasonable mold heat treatment process are key measures to improve the service life of the mold.
1. Working Characteristics and Failure Analysis of Cold Extrusion Die
During cold extrusion, the metal undergoes plastic deformation under three-way uneven pressure. This mold has to withstand a large unit pressure, generally higher than 1000MPa. At the same time, the metal will produce thermal effects during the deformation process, which will increase the temperature of the workpiece and the mold. High, sometimes up to 200°C or more; during cold extrusion, the concave die bears tangential tensile stress, while the condition of the convex die varies depending on the extrusion process. The convex die bears compressive stress during forward extrusion; In the process of extrusion and compound extrusion, the punch is first subjected to compressive stress (the bending stress is still present under eccentric load), and then to a certain degree of tensile stress in the return stroke. In the process of extrusion, the punch is subjected to tension and compression. Variable stress, at this time the tensile stress is about 30 times the compressive stress.
According to the working characteristics of cold extrusion, the common failure modes of the punch are “split” rupture and “cap off” rupture. “Cleavage” fracture has a lot to do with the distribution of carbides; “cap off” fracture occurs when the part is removed from the punch. When the upper and lower parts are taken off, the end of the punch is broken due to the tensile stress on the working end of the punch. In addition, there are failure modes such as bending, upsetting and deformation, abrasion and damage.
2. Forging of Cold Extrusion Punch
The cold extrusion punch is made of Cr12MoV steel, which is a high carbon and high chromium cold work die steel. Its hardness (1800HV) is higher than cementite and M6C carbide, and it has high wear resistance. However, in the as-cast state, the carbides in the steel are distributed in a star network and large blocks, which makes the strength and toughness of the steel very low. From the failure analysis of the cold extrusion punch, it is found that the failure of the punch has a great relationship with the distribution of carbides. This kind of net-like and massive carbides cannot be changed by heat treatment. Only by forging can these carbides be broken and distributed along the direction of metal deformation.
2.1 Forging Process
Due to the high carbon content and alloying elements of Cr12MoV steel, the thermal conductivity is poor, the forging temperature is narrow, the heating speed should not be too fast, the heating should be uniform, and the heating temperature should not be too high; the lightness and weight of the forging should be controlled appropriately, and the central part should be ensured The carbides are crushed; the heating fire rate can be based on the production experience. The first three fires are three upsetting and three drawing, and the last fire is trimming; increase the deformation of the last time, the total forging ratio should be greater than 6, and the convexity of Cr12MoV steel The level of mold carbide should not be greater than level 2.
Table 1 Cr12MoV steel forging process specification
2.2 Operation Process
Before forging, the hammer drill should be preheated to 100~200℃. At the same time, according to the size of the blank section and the technical requirements of the parts, the method of cross upsetting or three-way upsetting should be adopted. The number of upsetting should be based on the level of uneven carbide. Depending on the requirements of the uneven level of carbide for forgings. During the operation, the forging method of “two light and one heavy” should be strictly implemented to ensure the crushing of carbides while preventing cracks.
2.3 Cooling after Forging
After forging, the forging should be immediately buried in dry asbestos ash or dry sand and slowly cooled to 100~150℃, and then taken out; after forging, the small forging can be kept in the furnace and then slowly cooled to about 100℃ after the furnace is air-cooled.
3. Heat Treatment Process of Cold Extrusion Punch
3.1 Spheroidizing Annealing Process
The cold extrusion punch of Cr12MoV steel adopts spheroidizing annealing process. The process is shown in Figure 1.
Figure 1 Spheroidizing annealing process of Cr12MoV steel
After annealing, its structure is fine-grained pearlite and carbide, and its hardness is below 255HBS. If you want to further reduce the hardness, you can add a high temperature tempering and keep it at 760~790℃ for 2~3 hours.
3.2 Quenching and Tempering
According to the working conditions and performance of the cold extrusion punch, it should have high strength and certain toughness after quenching and tempering. The Cr12MoV steel is heated at 1020~1030℃, quenched in stages, and then tempered twice at a temperature of 160~180℃. The process is shown in Figure 2. In order to prevent cracking, the mold surface should be tempered for the first time before it is cooled to room temperature. The first tempering should be cooled to room temperature before the second tempering.
Experiments show that the hardness of Cr12MoV steel after quenching at 1020~1030℃ can reach 62~65HRC, and the grain size can reach 12 grades, which can be obtained after quenching. Higher strength and better toughness. Practice has proved that after the process is adopted, the toughness is improved under the premise of ensuring the strength, and the fracture rate caused by insufficient heat treatment toughness is significantly reduced.
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