Powder metallurgy is widely used in machinery and other industries as an important molding process. The quality and life of powder metallurgy forming and shaping molds are important factors that determine the quality and lifespan of powder metallurgy parts. Therefore, improving the quality and life of powder metallurgy molds is very important for expanding the application range of powder metallurgy parts.
In the process of using powder metallurgy moulds, it is usually due to wear that causes the size to be out of tolerance and scrapped. After the heat treatment of the tool steel, the metallurgical structure generally contains a lot of retained austenite. Because the retained austenite is a soft phase, the strength and hardness are low, and the residual austenite is also an unstable phase. In the process, organizational changes and tissue stress are prone to occur, causing early damage to the tooling. Especially when the amount of retained austenite exceeds 5%, the strength and wear resistance of the material are significantly reduced.
For cryogenic Gr15 steel powder metallurgy parts that are widely used in the powder metallurgy industry, molds and molds have a low life expectancy. Cryogenic treatment is used to increase the wear resistance of the steel to extend the life of the powder metallurgy mold, thereby reducing powder metallurgy parts. Production costs and improve part quality.
After the normal heat treatment of Gr15 steel, there are still many residual austenite, cryogenic treatment is the most effective method to reduce the residual austenite. When the cryogenic treatment temperature drops below -140°C, the amount of retained austenite is basically stable. This is because the temperature is reduced to a certain degree, the untransformed retained austenite stress state is close to the isometric state, and the retained austenite is retained. It is difficult for shear deformation to occur, resulting in the stoppage of retained austenite to martensite. Therefore, there is still a small amount of retained austenite after cryogenic treatment at -196°C. Due to the transformation of retained austenite to martensite, the hardness of the steel increases slightly after cryogenic treatment.
The impact toughness of Gr15 steel does not decrease with increasing hardness. This is because residual austenite is distributed between martensite. Therefore, these newly transformed martensite are more dispersed, finer and more uniform, and they remain after cryogenic treatment. Some of the retained austenite is distributed around the martensite in a thin film shape, which can increase the difficulty of crack generation and expansion. Therefore, after cryogenic treatment, the impact toughness of the steel is almost unchanged. After cryogenic treatment, the strength of the steel has increased. This is because the cryogenic treatment changes with the retained austenite to martensite, the structure is further refined, and the hardness increases, so that the overall strength of the steel is improved.
Since the shaping mold is used to press the sintered powder metallurgy parts, the working pressure is high, and the hardness of the powder metallurgy parts after sintering is high. Therefore, the wear surface of the shaping mold is covered with furrows. In addition, the working surface of the mold is subjected to a large squeezing force. Under the repeated action of this powerful squeezing force, the subsurface soft phase (retained austenite) of the mold undergoes repeated deformation, resulting in a large number of dislocations and inclusions. Defects occur at other defects, causing cracks. Cracks continue to expand under stress. When the crack length reaches a critical value, the material between the surface and the crack is sheared off, resulting in flake-like wear debris. As the grinding debris is continuously peeled off, the wear of the mold is greatly accelerated, and eventually the size of the orthopedic mold is exceeded and scrapped.
After the cryogenic treatment of the plastic mold, the amount of retained austenite is significantly reduced. On the one hand, the difficulty of plowing is increased due to the increase of hardness and strength, which reduces the wear rate of the mold; on the other hand, it increases the difficulty of crack initiation, plus After the retained austenite transforms, the microstructure is further refined, which increases the energy required for crack propagation and increases the difficulty of crack propagation.
From the analysis of mechanical properties, cryogenic treatment makes the toughness of Cr15 steel not decrease, but the strength increases, that is, the comprehensive performance of the steel is improved, thereby improving the ability of the steel to resist crack initiation and expansion, thus reducing the flaking of the grinding debris. Speed, which also increases the life of the mold.
Therefore, the cryogenic treatment not only improves the ability of the shaping mold to resist the abrasion of the plough, but also improves the ability of the mold to resist fatigue and wear, thereby remarkably improving the life of the shaping mold, that is, the number of the individual mold shaping parts is significantly increased in use.
In the process of powder metallurgy, the orthopedic molds are subjected to strong cyclic compression stress and fatigue wear. The deep cooling treatment of the plastic mold improves the toughness, increases the difficulty in the formation and expansion of fatigue cracks, and reduces the rate of wear debris removal, thereby greatly increasing the service life of the mold. On the other hand, the cryogenic treatment of powder metallurgy molds significantly reduces the amount of retained austenite, increases the hardness and strength of the mold, and improves the ability of the mold to resist plough wear. This also significantly increases the life of the mold.
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