Failure Analysis on Cracking of Outer Ring of Tapered Roller Bearing Made of Carburized Steel
1. Outer ring cracking macroscopic features
In addition to a through crack that is parallel to the axis, the outer surface of the outer ring of the carbonized steel tapered roller bearing has two scattered fine cracks originating from the large crack. Except for the two ends and the middle non-work area, there are two large wear zones on the ferrule, which have different bright white gloss bands. A number of "scale"-shaped small straight cracks are distributed from the edge of the wear zone about 20 mm from the end face, and the direction is parallel with the large crack. The longest is about 40 mm, and most of them are 5-10 mm, as shown in Fig. 1 and Fig. 2. These characteristics indicate that the large crack that penetrates is developed by one of these fine cracks.
2. Fracture scanning electron microscopy
The macroscopic features of the fracture of the original large crack in the outer ring are shown in Figure 3. The brittle fracture feature is exhibited. The fatigue source characteristics can be seen at the fracture corresponding to the "scale" crack zone on the outer surface of the outer ring fracture, as shown in Fig. 4. According to this, it can be judged that the crack of the ferrule is fatigue and brittle fracture.
It was found by scanning electron microscopy that the fatigue source region was located in the white bright band on the outer surface of the ferrule. As shown in Fig. 5 and Fig. 6, it can be seen from the fracture structure of different magnification that the white bright region is located on the surface of the carburized and quenched layer. That is, near the outer surface of the ferrule. The cracking of the carburized quenching layer below the fatigue zone is characterized by cleavage cracking, as shown in Fig. 7, indicating that a one-time rapid fracture occurs shortly after fatigue cracking. The fracture structure of the ferrule core is mainly due to the dimple, which is caused by the soft slat martensite structure of the heart, as shown in Fig. 8.
A large number of small cracks on the outer surface of the outer ring are finely cracked. The details under the scanning electron microscope are shown in Fig. 9. It can be seen that these small cracks are parallel to the main crack that penetrates, and are completely perpendicular to the wear direction on the outer ring surface. The microhardness test of the wear zone of the metallographic sample made in parallel with the small cracking showed that the hardness value of the section at a depth of about 0.1 mm below the wear zone was higher than that of the carburized quenched layer (see Figure 10). This indicates that the wear zone on the outer surface of the ferrule has a hardening phenomenon. The hardness of the wear hardened layer is 923 HV and 941 HV, and the hardness of the carburized and quenched layer is 730 HV and 719 HV.
3. Metallographic examination
The outer surface wear area has a layer of corrosion-resistant white bright area of about 0.05mm thick, and the normal carburizing and quenching layer under the white light area, that is, fine needle-shaped martensite structure (see Figure 11), the core of the ring is lath Quenched martensite structure (see Figure 12).
4. Chemical composition analysis
The energy spectrum analysis is shown in Figure 13. The composition analysis is shown in the attached table. Both the energy spectrum and the chemical analysis indicate that the chemical composition of the ferrule material meets the requirements of the G20CrNi2Mo standard.
5 Conclusion
(1) The chemical composition of the cracking ferrule material, the carburizing heat treatment process and the metallographic structure are normal.
(2) The cracking of the ferrule is a brittle fracture caused by fatigue, and the fatigue source is located in the friction damage hardening zone on the outer surface of the ferrule.
(3) The surface of the bearing has grinding burn or secondary hardening during the grinding process. The bearing has an eccentric sliding wear phenomenon in the running outer ring, which causes the outer ring of the bearing to harden and crack in the friction damage zone. At the same time, the ferrule is subjected to a large pressure, and a large alternating tensile stress is generated in the direction around the ferrule, which promotes the fine cracking of the surface of the wear zone and causes the fatigue source to sprout, and finally causes the ferrule to penetrate and crack.
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