Metallographic examination and analysis of driving gear of engine starting motor

1. Gear crack morphology

After cutting, grinding and polishing, the fault gear was detected under the optical microscope. The crack originated from the root of the gear. The crack is relatively straight, the starting part is wide, and the tail is sharp. There is a certain angle between the tail of the crack and the surface of the tooth root, as shown in Figure 1; According to the standard JB / t6141.3 “metallographic examination of carburizing heavy duty gears”, the tested microstructure after corrosion is fine acicular martensite + carbide with grade 2 martensite and grade 2 carbide, which is normal carburizing quenching tempering microstructure; No oxidative decarburization was found on both sides of the crack, as shown in Figure 2.

Overall crack morphology 25X Microstructure at crack 400X

2. Detection and analysis of non-metallic inclusions

If the non-metallic inclusions exceed the standard, the continuity of the matrix will be destroyed, and the stress concentration point and crack source will be easily produced. The existence of banded structure makes the structure of the material uneven and reduces the plasticity and impact toughness of the material. Therefore, the non-metallic inclusions and banded structure were detected.

The axial matrix of failure fracture gear was sampled longitudinally, and then the non-metallic inclusions and banded structure were detected and analyzed. According to GB / t10561-2005 “determination of content of non-metallic inclusions in steel standard grading diagram microscopic inspection method”, the items of non-metallic inclusions were detected and analyzed, and the grades were A0, B0, C0 and d1.0, which met the requirements of GB / T 5216 “guaranteed hardenability structural steel”.

According to GB / t34474.1 “evaluation of banded structure in steel”, the banded structure is tested and analyzed. The grade is grade 2. The banded structure is generally required to be no more than grade 2, and no abnormality is found.

3. Detection and analysis of surface and core tissues

According to JB / t6141.3 “metallographic examination of carburizing heavy duty gears”, the surface microstructure is fine acicular martensite + carbide, with grade 2 of martensite and grade 2 of carbide; The microstructure in the core is low carbon martensite + ferrite, grade 3-4. No abnormality is found in the surface and core, which belongs to normal carburizing quenching tempering structure.