1 failure parts macroscopic examination
1.1 Roller
Shown in the cylindrical surface visible peeling of the pits of different sizes, and the end face of the damaged, rubbed off a lot. Roller in the pits at the cut diameter longitudinal direction after observation profile the lower right corner of the inverted triangle hole, which is the premises of the original crack start peeling caused. Upper right corner has a column extending from the end face of the crack, about 4.5mm, the cracks in the central branching crack thin straight energetic, trailing tapered (see Figure 2), has the stress cracking characteristics.
1.2 The bearing outer ring of debris
Be able to see some of the fatigue fracture morphology in the outer ring of debris. You can see a parallel with the surface cracks in the debris from the surface 0.7mm at intermittent cracks, twists and turns, the end of tapering. That the part of the cracks, the cracks also has the characteristics of stress cracks.
2 non-metallic inclusion rating
1.5 (1) Roller: sulfide l-level, point-like and spherical oxide.
(2) bearing outer ring: 1.5 sulfide a point-like, spherical oxide.
3 Microstructure examination
3.1 Roller
3.1.1 Heart Ministry Organization
The organization of the core part of the tempered the hidden needle martensite, granular carbide and retained austenite.
Roller original organization 4% slightly acid alcohol etching × 500
3.1.2 surface pits surrounding tissue
Observe the microstructure at the roller surface spalling pits formed found: pit bottom is a white layer, and its width is about 0.18 ~ 0.5mm, and a length of 15mm. The white layer has three transverse cracks. Pit at the bottom of the outer layer close to the white layer has a width of 0.40 ~ 0.50mm dark areas, dark areas is the roller after the original organization. The white bright areas and dark interval organizations quenched martensite, dark area for tempering hidden needle fine needle martensite and a small amount of granular carbide + retained austenite.
3.1.3 microhardness
To reflect the difference between the three different colors of tissue hardness outside to the inside of the roller profile microhardness testing, intuitively reflect changes in microhardness indentation size: the smallest indentation in the white layer the dark areas indentation, slightly lighter color of the original tissue indentation center. The average hardness value of the three zones were: the white bright areas HV0.11264, the dark areas HV0.1551, the original organization HV0.1800.
Roller pit at the bottom of the white bright areas, dark areas, the original organization and microhardness indentation × 200 4% slightly acid alcohol etch
3.2 bearing outer ring microstructure
The bearing outer ring microstructure of martensite + for the tempering hidden needle a small amount of the granular carbide and retained austenite.
3.3 The surface hardness
3.3.1 Roller
The hardness of most of the points on the roller surface between HRC60 ~ 61, but also in some places the longitudinal line hardness between both HRC56 ~ 57.
3.3.2 The bearing outer ring of debris hardness
Test, the bearing outer ring of debris inside surface hardness HRC58 ~ 59.
4 roller surface pit causes
We can infer from the white bright area quenched martensite, the rollers during operation, because each other too far, due to the friction generated a large amount of heat, is extruded parts to form a high-temperature region, due to the presence of a lubricating oil, Therefore, on the surface of high-temperature parts re-quenched, quenched martensite (corrosion was bright white). Accepted tempering due to the role of local high temperatures, so close to the surface of the sub-surface, a result of the tempering temperature is higher than the temperature of the original roller heat treatment tempering, resulting in sub-surface hardness decreased, easily peeling, flaking after the formation of the pit, the bright white area at the bottom of the pit we see actual peeling of left.
As a result, we can easily conclude: the roller on the cracks due to the roller assembly tight running hit pressure, so that the local organization into huge organizational stress, and therefore has the characteristics of stress cracks . Makes the organization into the bearings inside the roller volume becomes larger, the bearing outer ring surface to withstand a great deal of pressure stress, and thus the final stretch and split.
 



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