Cryogenic treatment of the hottest powder metallur

2022-08-14
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Cryogenic treatment of powder metallurgy mold

powder metallurgy is widely used in machinery and other industries as an important molding process. The quality and service life of powder metallurgy forming and shaping molds are important factors that determine the quality and service life of powder metallurgy parts. Therefore, improving the quality and service life of powder metallurgy molds is very important to expand the application range of powder metallurgy parts. Powder metallurgy dies are usually scrapped due to over tolerance caused by wear in the process of use. Generally, the metallographic structure of tool steel after heat treatment contains more residual austenite. Because residual austenite is a soft phase with low strength and hardness, and residual austenite is an unstable phase, it is easy to produce structural transformation and structural stress during mold service, resulting in early damage of tools and molds. Especially when the amount of residual austenite exceeds 5%, the strength and wear resistance of the material will be significantly reduced [1]. In view of the low service life of Gr15 steel powder metallurgy parts forming dies and reshaping dies, which are widely used and cheap in the powder metallurgy industry, cryogenic treatment is used to improve the wear resistance of steel and prolong the service life of powder metallurgy dies, so as to reduce the production cost of powder metallurgy parts and improve the quality of parts. 1 test method 1.1 mold treatment and sample preparation

take a batch of brake ring shaping molds with Gr15 material from a powder metallurgy factory, and immediately carry out cryogenic treatment after normal quenching heat treatment, and then carry out 150 ℃ low temperature tempering on the mold after cryogenic treatment. The cryogenic treatment process is -100 ℃, -120 ℃, -140 ℃, -160 ℃, -196 ℃ respectively, and the insulation time of cryogenic treatment is 2H. The sampling is directly cut from the die after cryogenic treatment, and the size of the metallographic sample is 12mm × 12mm × 10mm, the bending specimen is? 10mm × 140mm, the size of the notchless impact specimen is 10mm × 10mm × 55mm, residual austenite measurement and hardness measurement samples are the same as metallographic samples. 1.2 performance testing and wear morphology observation

the impact toughness of samples with different processes is measured on the impact testing machine, the bending stiffness is measured by the universal material testing machine, the residual austenite volume is measured by the X-ray diffractometer, and the hardness of each sample is measured by the durometer. Install the cryogenic treated plastic mold for production, compare the service life of the mold without cryogenic treatment with that treated by different cryogenic processes, and observe the wear morphology of the failed mold by scanning electron microscope. 2 test results and analysis

the properties of Gr15 steel after different cryogenic treatment processes are shown in Table 1. The table also lists the service life of the brake ring shaping die treated by different cryogenic processes (the number of shaping parts of a single die). Table 1gr15 steel properties and die life after different process treatment cryogenic treatment temperature (℃) residual austenite (%) hardness (HRC) impact toughness (j/cm2) bending strength (MPA) die service life (number of shaping parts) -100 ℃ 4.5-5.5 61 6.82 2572 4 ℃ 3-4 61 6.78 2608 4 ℃ 2-3 61.5 6.75 every year, more than 8million tons of plastic are dumped into the world's oceans 2635 ℃ 1.5-2.5 62 6.72 2637 6 ℃ 1-2 626 6 9000 ℃ 7-8 60.5 6.86 25013 000. It can be seen that there is still a lot of residual austenite in Gr15 steel after normal heat treatment, and cryogenic treatment is the most effective method to reduce residual austenite. When the cryogenic treatment temperature drops below -140 ℃, the amount of residual austenite has basically stabilized. This is because the temperature has decreased to a certain extent, the stress state of untransformed residual austenite has been close to the equiaxed state, and the residual austenite is difficult to shear deformation, resulting in the cessation of the transformation process from residual austenite to martensite. Therefore, a small amount of residual austenite still exists after the cryogenic treatment of -196 ℃. Due to the transformation from retained austenite to martensite, the hardness of the steel is slightly improved after cryogenic treatment. The impact toughness of Gr15 steel does not decrease with the increase of hardness, because the residual austenite is distributed among the martensite needles, so these newly transformed martensites are dispersed, fine and uniform, and a part of the residual austenite retained after cryogenic treatment is distributed around the martensite in the form of film, which can increase the difficulty of crack generation and propagation. Therefore, the impact toughness of steel after cryogenic treatment is almost unchanged. The strength of steel is improved after cryogenic treatment. This is because after cryogenic treatment, with the transformation of residual austenite to martensite, the structure is further refined and the hardness is improved. Because of the courage to take the lead, the overall strength of the steel is improved

the wear surface of the brake ring shaping die for powder metallurgy parts was observed by scanning electron microscope. It can be seen that the wear morphology of the die is full of a large number of peeling pits on the wear marks. The arrow in Figure 1 shows that the fatigue crack extends to the die surface, and the arrow in Figure 2 shows the peeling pit left after the wear debris peels off

fatigue cracks extending to the die surface 700 × Die wear surface wear debris peeling pit 400 × Combined with the stress condition of the mold in the working process, the following analysis can be made: because the reshaping mold is used to press the sintered powder metallurgy parts, the working pressure is very high, and the sintered powder metallurgy parts have high hardness, therefore, the worn surface of the reshaping mold is full of furrows. In addition, the working surface of the die bears a lot of extrusion force. Under the repeated action of this strong extrusion force, the soft phase (residual austenite) on the sub surface of the die is deformed repeatedly, producing a large number of dislocations and stacking at defects such as inclusions, so as to initiate cracks. The cracks continue to expand under the action of stress. When the crack length reaches the critical value, the material between the surface and the crack is sheared, producing flake like wear debris [2]. With the continuous peeling of wear debris, the wear of the mold is greatly accelerated, and finally the size of the plastic mold is out of tolerance and scrapped. After cryogenic treatment, the residual austenite content of the reshaping die is significantly reduced. On the one hand, the difficulty of ploughing is increased due to the increase of hardness and strength, thereby reducing the wear rate of the die; On the other hand, it increases the difficulty of crack initiation. It is necessary to limit the temperature gradient and temperature fluctuation in the experimental box. In addition, the microstructure is further refined after the transformation of residual austenite, which increases the energy required for crack propagation and increases the difficulty of crack propagation. From the aspect of mechanical properties, cryogenic treatment can not reduce the toughness of Cr15 steel, but improve the strength, that is, the comprehensive properties of the steel are improved, thus improving the ability of the steel to resist crack initiation and propagation. In this way, the speed of debris peeling is reduced, and the service life of the die is also improved. Therefore, cryogenic treatment not only improves the anti ploughing wear ability of the reshaping die, but also improves the anti fatigue wear ability of the die, which significantly improves the service life of the reshaping die, that is, the number of reshaping parts of a single die increases significantly when in use. Conclusion

the powder metallurgy shaping die bears strong cyclic extrusion stress and produces fatigue wear in the working process. After cryogenic treatment, the strength and toughness of the reshaping die are improved, which increases the difficulty of the formation and propagation of fatigue cracks, shortens the time of molding each component, reduces the rate of wear debris peeling, and greatly improves the service life of the die. On the other hand, the cryogenic treatment of powder metallurgy mold significantly reduces the residual austenite volume, increases the hardness and strength of the mold, improves the ability of the mold to resist ploughing wear, and also significantly improves the service life of the mold

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