Fracture failure analysis of ferrous powder metallurgical gear

Gear is a key structural part in modern transmission technology, its main function is to transmit torque reliably and without noise, and its size accuracy, hardness and strength have high requirements.In the past, powder metallurgy was limited to low strength applications due to inherent porosity of materials and alloying methods.In recent years, new alloying methods have significantly increased the density and strength of powder metallurgical parts, making them close to the level of forged gear material, thus enabling powder metallurgical gears to compete with high-performance forged gears, and have the characteristics of low noise and self-lubrication.In addition, powder metallurgy is a near net shape processing method, which has the advantages of high material utilization and good economic index.

The performance of powder metallurgical gears is closely related to the manufacturing process, and the gear performance produced by different processes and technical routes varies greatly.Yu Xiquan and others prepared high strength bevel gears by hot-pressing process and improving sintering temperature. However, the method of characterizing product quality improvement by mechanical properties was one-sided and the mechanism of single factor influence was not deeply studied.Gu Wenjin and his colleagues overcome the problem of gear cracking by improving the forming die and increasing the bevel angle, and successfully prepared camshaft timing gear. However, their research is limited to optimizing the mold and did not improve the material.Ding Xia et al. combined traditional sintering and heat treatment processes into one. Helical gears for electronic parking were prepared by sintering hardening process, which reduced the process and had certain economic benefits. However, the internal stress of gears prepared by this method was higher.Based on the results of fracture analysis, the sintering process was improved to deal with the fracture of a certain powder metallurgical gear, and the impact test was used to verify the effect of the process improvement.The results show that the powder metallurgical gears prepared by the improved process have good properties.

(1) From the fracture analysis, it is found that a lot of spherical iron carbons exist in the teeth and teeth of original process gear, which seriously reduces the bonding force between the powders.Therefore, the main reason for tooth fracture failure is that insufficient sintering temperature causes insufficient diffusion and welding between powders.

(2) After improving the sintering process, the number of pearlites in powder metallurgical structure is 60, the density is increased from 6.82 g/cm3 to 7.11 g/cm3, and the impact toughness is improved from 9.8 J/cm2 to 15.2 J/cm2, with an increase of about 55%. The fracture failure mode becomes cleavage fracture.

(3) The test results show that properly increasing the sintering temperature and prolonging the sintering time can significantly improve the diffusion, melting and recrystallization effects between the powders, improve the compactness and impact toughness of the structure, and make the structure plastic.