PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Heat Treatment to Improve the Life of Cr12MoV Steel Die

Cr12MoV steel is one of the widely used cold work die steels. The steel has the advantages of good hardenability, high hardness, wear resistance, and small heat treatment deformation. It is often used to make cold-working dies that bear heavy loads and have large-scale production batches and complex shapes, such as cold-sensitive cold-extrusion dies for cold stamping and printing. However, the obvious disadvantage of this steel is its high brittleness, which often leads to early failure of the mold.. Therefore, how to improve its strength and toughness and prevent premature fracture and failure of the mold is a problem that is often encountered and needs to be solved. The mold failure analysis shows that the heat treatment factor has the greatest influence, accounting for about 50%. Based on the existing research, this paper analyzes and reviews the vacuum heat treatment process and measures to improve the die life of the various factors that affect the properties of Cr 12MoV steel in heat treatment production.

1 Improved preparatory heat treatment

Cr 12MoV belongs to high-carbon high-chromium ledeburite steel with high carbide content, and is often distributed unevenly in the form of bands or networks. The steel matrix has a large splitting effect, which often becomes the source of fatigue fracture. After forging, the carbides are crushed and the segregation situation is effectively improved, but its shape is not ideal, and the hardness after forging is also high. Therefore, spheroidizing annealing is often used as a preparatory heat treatment for Cr12MoV steel after forging to obtain uniform and fine spherical carbides, reduce hardness, improve cutting performance, and prepare tissue for subsequent vacuum quenching. When the effect of the conventional spheroidizing annealing process is not ideal, post-forging quenching and tempering treatment can be used, that is, a short stay after forging to allow the austenite to recover and start recrystallization, and then immediately vacuum quenching and vacuum tempering at 700 ~ 750°C. Or add a conditioning process before finishing. Spheroidizing annealing or cyclic spheroidizing annealing can also be carried out directly by using the waste heat after forging.

The general quenching temperature of Cr12MoV steel is 1000 ~ 1040°C, while the quenching and tempering vacuum quenching temperature can reach 1120°C. On the one hand, the high temperature promotes the complete dissolution of smaller carbides, and on the other hand, it also promotes the sharpness of large carbides. The local dissolution of the corner; moreover, the carbides dissolved into the matrix are uniformly dispersed and precipitated again during the subsequent high-temperature tempering process, which improves the shape, size and distribution of the carbides, which is conducive to improving the strength and toughness of the mold.

2 Improve vacuum quenching and vacuum tempering

The final hardening treatment of Cr 12MoV steel is divided into “primary hardening treatment” and “secondary hardening treatment”. Primary hardening refers to low temperature quenching (~1000°C) and low temperature tempering (~200°C); secondary hardening refers to high temperature quenching (~1100°C) and high temperature tempering (~500°C). Research and practice have shown that Cr 12MoV steel has high hardness (60~62 HRC), fine wear-resistant grains, good toughness, small quenching deformation, and moderate amount of retained austenite (A : ) after primary hardening treatment, which is the best choice. conventional toughening treatment process.

After forging and pretreatment of Cr 12MoV steel, the particle size, shape and distribution of carbides are improved, but some carbides are still large in size and have edges and corners, which reduces the fracture toughness of the steel. If the primary hardening treatment is fully preheated at a specific temperature such as about 800 °C, the unevenly distributed carbides, especially large sharp-edged carbides, will be continuously decomposed and diffused, which is conducive to the formation of a large number of highly dispersed nucleation centers. Subsequent vacuum quenching is conducive to the formation of highly dispersed fine-grained carbides, which can effectively improve the strength and toughness of the steel and the service life of the die. For example, Cr12MoV steel is preheated at a specific temperature, vacuum quenched at 1000°C, and vacuum tempered at 200°C. The life of the drawing die is nearly doubled. Cr 12MoV steel is preheated at 800~850℃, heated and quenched at 1020~1050℃, and tempered twice at 160~180℃ for low temperature, so the effect is better.

If the Cr12MoV steel is not modified for forging, solid solution double refinement treatment is adopted, that is, secondary preheating at 500 ℃ and 800 ℃, solution treatment at 1100 ~ 1150 ℃, quenching into hot oil or austempering, and high temperature vacuum tempering at 750 ℃. After machining, heat the oil at 960°C and then carry out the final heat treatment, which can also refine the carbides, round the edges and corners, and refine the grains.

The lack of strength and toughness of Cr 12MoV steel is also related to the high quenching temperature and large austenite grains, which make the martensite coarse after vacuum quenching. If the Cr12MoV steel is heated at low temperature and austempered, an appropriate amount of substructure B is formed first, which splits the original austenite grains and refines the subsequently formed acicular M; The toughness of the interface itself is also good, and the stress concentration can be relieved by its own plastic deformation, which helps to reduce the initiation and propagation of cracks, reduces the probability of brittle fracture, and increases the strength and toughness. For example, Cr12MoV steel is pre-nitrided at 520 °C for 12 h, nitrogen expanded at 1000 °C for 10 min, austempering at 260 °C for 4 h, and tempered in vacuum at 230 °C for 2 h. Appropriate amount of complex phase structure of B and M, the microhardness of the nitrogen-expanded layer reaches 900 ~ 1000 HV, and the hardness transition of the core surface is uniform, which can increase the life of the mold by an order of magnitude. For Cr 12 steel, a multiphase strengthening and toughening treatment process of preheating at 850°C, heating at 1030°C, austempering at 280°C, and vacuum tempering at 200°C was also proposed. When isothermal, 7% ~ 10% of B and M complex phase structure can be obtained, which has the best combination of wear resistance and impact toughness, so that the life of Cr 12 steel bicycle cold heading die can be increased by 3 ~ 4 times. It also has reference significance for Cr12MoV steel.

3 vacuum furnace selection

The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tools and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU company focuses on 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.