Research on cryogenic treatment technology of powder metallurgy parts shaping die

        Powder metallurgy parts are more and more widely used in machinery and other industries due to their inherent advantages. The production process of powder metallurgy parts is generally as follows: powder compaction - high temperature pouring - shaping. The reshaped powder metallurgy parts can be directly equipped with a little machining. Therefore, there are strict requirements on the dimensional tolerances of the reshaped parts. In order to ensure the dimensional requirements of the powder metallurgy parts, the dimensional tolerances of the female mold and the mandrel of the shaping die are required. The requirements are more stringent, such as the size of the shaping mandrel for a certain bushing is Ф20+0.025. The female die and the mandrel are subjected to a large extrusion force during the shaping process, and the working conditions are very bad, and it is easy to be scrapped due to the excessive size caused by severe wear. Therefore, improving the wear resistance of powder metallurgy parts shaping molds, thereby improving the life of shaping molds, is of great significance to the quality of powder metallurgy parts and reducing the production cost of powder metallurgy parts. This paper intends to discuss the cryogenic treatment process to improve the life of powder metallurgy parts shaping die through the cryogenic treatment of powder metallurgy parts shaping mandrel.

1 Test method
Take a batch of machined 132 bushing shaping mandrels from a powder metallurgy factory, and the mandrel material is GCr15. After normal quenching and low temperature tempering at 150℃, the mandrel rod should be cryogenically treated as soon as possible. 1h, 2h, 4h, and 8h 4 times. The molds after cryogenic treatment were all tempered at a temperature lower than 150 °C, and the holding time was 1 h, and their hardness was measured respectively. The cryogenically treated mandrel and the same batch of heat-treated mandrels without cryogenic treatment were installed on the machine to measure their life, and the wear patterns of the molds with different treatment processes were observed with a low magnification magnifying glass.

2 Test results and analysis

2.1 Test results

Table 1 Life and hardness of mandrel after different treatments

It can be seen from Table 1 that the service life of the powder metallurgy parts shaping mandrel is significantly improved after cryogenic treatment. The lower the cryogenic temperature and the longer the holding time, the more parts are reshaped before the mold fails, that is, the longer the service life of the mold. However, when the holding time is the same, the lifespan of the shaping mandrel at -160℃ and -196℃ is not much different. At the same processing temperature, when the holding time exceeds two hours, the mold life jumps up, and then increases smoothly with the extension of the holding time.

2.2 Analysis of test results

Observing the surface of the failed shaping mold, it was found that the surface of the mold was covered with spalling pits and many small furrows. This is due to the fact that during the shaping process of the mold, the powder density of the 132 bushing of the shaped powder metallurgy part is high, reaching 7.0g/cm3, and the hardness after sintering is also high, and the mandrel and the female mold are greatly affected during the shaping process. Under the repeated extrusion of the die, the residual austenite in the subsurface of the die is repeatedly deformed, resulting in dislocation accumulation and initiation of cracks. During the repeated extrusion process, the cracks expand until the surface is peeled off, forming Severe spall pit (1). In addition, the impurities in the powder—a variety of hard phase particles, plough on the surface of the mold during the mold shaping process, so that the surface of the mold is covered with furrows. Because the impurity particles are small, the furrows formed are shallow. The mandrel is scrapped due to oversize due to the combined action of spalling and ploughing.

The microstructure of the mandrel made of GCr15 die steel undergoes the following changes during the cryogenic treatment (2) (3), and the amount of retained austenite decreases. The alloying elements in the retained austenite, especially the carbon element, play a role in strengthening the retained austenite, so more energy is required to promote the shearing mechanism to generate martensite. Cryogenic treatment is to use the degree of supercooling to increase martensite. The driving force for bulk transformation, as the cryogenic temperature decreases, the degree of subcooling increases, and the martensitic transformation becomes more complete. In the cooling process, the transformation speed of retained austenite to martensite does not depend on time, but is related to the degree of supercooling, that is, the amount of retained austenite is only related to the cryogenic temperature. In addition, cryogenic treatment can also precipitate fine carbides and refine the structure.

Cryogenic treatment reduces the amount of retained austenite and increases the difficulty of crack initiation in the subsurface layer of the mandrel during extrusion. At the same time, due to the decrease in the amount of retained austenite, the precipitation of fine carbides and the refinement of the structure, the difficulty of ploughing is increased. . Therefore, after the mandrel is cryogenically treated, the ability to resist fatigue wear and abrasive wear is increased, thereby improving the life of the mandrel. If the cryogenic treatment temperature is high (higher than -160°C), the degree of undercooling is small, the energy for the transformation of retained austenite to martensite is insufficient, and the amount of retained austenite is large, which means that the hardness of the mold is almost unchanged. The mold life is not much improved. Cryogenic treatment at a lower temperature, as long as the holding time is enough to make the mandrel cool through (not less than 2h), the transformation of retained austenite to martensite is relatively complete, but the amount of fine carbides precipitated in martensite varies with the increased slowly with the extension of the holding time. Therefore, mold life also increases slowly with holding time. If it is considered that with the extension of the holding time, the cost of the cryogenic treatment of the mold also increases, then the most suitable holding time is 2 to 4 hours.

3 Conclusion
The failure mechanism of powder metallurgy parts shaping die is fatigue wear and abrasive wear. Cryogenic treatment of the mold can greatly improve its life. The best cryogenic temperature is -160℃～-196℃. The longer the holding time is, the longer the life of the mold will be. However, considering both the cost and the lifespan, it is appropriate to take 2 to 4 hours.