As an important forming process, powder metallurgy is widely used in machinery and other industries. The quality and life of powder metallurgy molds are important factors that determine the quality and life of powder metallurgy parts. Therefore, improving the quality and life of powder metallurgy molds is very important to expand the application range of powder metallurgy parts. In the process of using powder metallurgy molds, it is usually out of tolerance due to wear. Generally speaking, the metallographic structure of tool steel after heat treatment contains a large amount of retained austenite. Since retained austenite is a soft phase with low strength and hardness, it is also an unstable phase, which is prone to structural transformation and stress during mold use, leading to early damage to the mold. Especially when the amount of retained austenite exceeds 5%, the strength and wear resistance of the material will be significantly reduced.

In view of the low-cost, widely-used Gr15 steel powder metallurgy parts forming die and the short service life of the powder metallurgy industry, the service life of the powder metallurgy die is extended. The wear resistance of steel is improved by cryogenic treatment, thereby improving the quality of parts and reducing The production cost of powder metallurgy parts. After normal heat treatment, there is still more retained austenite in Gr15 steel. Cryogenic treatment is a good way to reduce retained austenite. The amount of retained austenite is basically stable, when the cryogenic treatment temperature drops below -140. This is because the temperature drops to the point where the stress state of the untransformed retained austenite is close to the equiaxed state, and the retained austenite is not prone to shear deformation, leading to the cessation of the transformation process of retained austenite to martensite. Therefore, there is still a small amount of retained austenite after -196 cryogenic treatment. Due to the transformation of retained austenite to martensite, the hardness of the steel is slightly increased after cryogenic treatment. The impact toughness of Gr15 steel does not decrease with the increase in hardness. Because the retained austenite is distributed between the martensite, the newly transformed martensite is dispersed, fine and uniform, and a part of the retained austenite remains after cryogenic treatment. The tensite is distributed around the martensite in the form of a thin film, which will increase the difficulty of crack generation and propagation. Therefore, after cryogenic treatment, the impact toughness of steel hardly changes. After cryogenic treatment, the strength of the steel is increased. This is due to the transformation of retained austenite to martensite, further refinement of the microstructure and increased hardness, thereby increasing the overall strength of the steel.

Because powder metallurgy molds are used to press and sinter powder metallurgy parts, the working pressure is high, and the hardness of sintered powder metallurgy parts is high, so the wear surface of the forming mold is full of grooves. In addition, the working surface of the die is subjected to a large extrusion force. Under the repeated action of this strong extrusion force, the soft phase of the subsurface layer of the die is repeatedly deformed, resulting in a large number of dislocations and blockages at defects such as inclusions, resulting in cracks. Cracks continue to expand under pressure. When the length of the crack reaches a critical value, the material between the surface and the crack is cut off, producing flake wear debris. With the continuous peeling of the wear debris, the wear of the mold is greatly accelerated, and then the sizing mold is out of tolerance and scrapped. After cryogenic treatment, the retained austenite content of the forming die is significantly reduced. On the one hand, due to the increase in hardness and strength, the difficulty of ploughing increases, thereby reducing the wear rate of the mold; on the other hand, the difficulty of crack initiation is increased. After the transformation of retained austenite, the microstructure is further refined, which increases the crack growth rate. The required energy increases the difficulty of crack propagation.

From the perspective of mechanical properties, cryogenic treatment does not reduce the toughness of Cr15 steel, but improves the strength, that is, improves the overall performance of the steel, thereby improving the ability of the steel to resist crack initiation and propagation, thereby reducing the spalling of wear debris. Speed, thereby extending the service life of the mold. Therefore, cryogenic treatment not only improves the resistance to furrow wear of the powder metallurgy mold, but also improves the fatigue resistance of the forming mold, thereby significantly prolonging the life of the forming mold, that is, when in use, the number of forming parts of a single mold increases significantly. .

The powder metallurgy mold is subjected to strong cyclic extrusion stress during the working process, resulting in fatigue wear. After cryogenic treatment, the strength and toughness of the forming die are improved, the difficulty in the formation and propagation of fatigue cracks is increased, and the spalling rate of wear debris is reduced, thereby greatly extending the service life of the die. On the other hand, after cryogenic treatment, the amount of retained austenite is significantly reduced, the hardness and strength of the mold are increased, the ability of the mold to resist plowing wear is improved, and the life of the mold is greatly extended.