Metallographic structure evaluation of failed gears is carried out according to national inspection standards.The metallographic observation is etched with 4% ethanol nitrate solution. The metallographic images of the axial and radial samples are shown in Figure 1 and Figure 2 respectively.

Metallographic structure of intermediate shaft sun wheel material shows that the gear has been tempered as a whole, but not surface hardening treatment, which is in accordance with the previous analysis results; the material has band segregation, and the metallographic structure of non-segregation zone is “tempered sorbite + fine block and needle ferrite”; there is widmanstatt structure, and there is a small amount of widmanstatt structure on the surface close to the wear area, which is brittle.The segregation zone has a metallographic structure of “tempered toroidal” mainly. The alloy element content in this area is high, so the hardenability is high. It is easy to form all martensite without ferrite precipitation in this area during quenching.

According to the national standard GB/T 13320-2007 and according to the grade 3 rating chart, the gear material is rated as grade 4. No metallographic defects such as decarbonization are found on the surface of the teeth.