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Bearing premature failure analysis (ii)

Bearing premature failure analysis (ii)


3. Result analysis

1. The grinding quality of bearing early failure can decide from the above test results, the influence of the grinding crack on one side of the raceway grinding burn is ring and split, a direct cause of the premature bearing failure, a crack in this example reveals macro morphology and distribution feature of grinding burn, the original position of raceway partial or race R not round, make on one side of the raceway grinding allowance increases, when the coarse grinding phase feeding, grinding speed, the whole race is uneven grinding.If the grinding wheel is not dressed in time, the grinding debris will be embedded or stuck, resulting in the blockage of the grinding wheel pores and passivation, which will lead to the increase of the grinding wheel imbalance degree, the increase of spindle vibration, and even the vibration of the machine tool and grinding chatter.Apparently the grinding force changes periodically to form intermittent burn bands.

In this way, the rate of grinding force and grinding force will be increased, producing a higher grinding heat.When the surface temperature of grinding area is up to or over Acm, the surface of raceway will be reaustenitized.At this time, the volume expands, while the low temperature part of the inner layer hinders its expansion, resulting in the thermal stress of surface compression while the inner layer is extension.When the grinding wheel is separated from the grinding surface, the thin layer of austenitized surface will be re-cooled for a second time under the condition of external cooling fluid (even if it is air-cooled) to form a high hardness secondary quenching structure, namely "grinding white layer".At the same time due to grinding heat inward and its adjacent layer diffusion effect, to form a layer of low hardness of black "tempering layer", the volume shrinkage, so the grinding surface layer is produced in high temperature gradient, which caused a series of changes in the microstructure and mechanical properties. "department of grinding white layer" in compressive stress state, while "tempering layer" in the maximum tensile stress state, so the core of crack is the most easy to the initiation, especially "grinding white layer" and "tempering layer, the boundary surface, because here is the most obvious mutation in organizational change, the most concentrated tensile stress, the strength of the weakest,When the residual tensile stress is greater than the breaking strength of the material, the crack expands in the direction of radius and cracks appear on the surface, that is, the grinding crack.

2. Characteristics of secondary quenching of white layer tissue

As the white layer is a kind of multiphase and highly dispersed structure with the coexistence of austenite, martensite and carbide, there is a coherent pseudo-equilibrium system between austenite and martensite, and the lattice constants are different, so when the formation of martensite, the lattice of the secondary austenite has a strong dark change, which leads to the increase of the structure hardness of the white layer.Because the surface energy on the austenitic-martensite interface tends to zero, under the action of corrosives, the organization does not produce electrochemical galvanic cells, so the white layer is difficult to corrosion, but it is formed in grinding conditions of the white layer.

3. Effect of steel quality and quenching and tempering microstructure quality on induced grinding crack

In the state of quenching and tempering, the black and white concentration of martensite matrix is different, the high macroscopic hardness value and the loose defects are also a potential factor which can not be neglected.Black concentration difference appeared in the original organization exists serious banded carbide, the resulting wear between the belt and carbon concentration of segregation, so after quenching increases the overheating tendency between the belt and Ms point of the difference in height, caused quenching organization and the distribution of residual stress nonuniformity and increase matrix brittleness, weakened the grain and grain, grain and matrix, and the joint force between matrix and matrix. Although the ring has been oil at low temperature tempering, but because of the insufficient tempering or the martensitic matrix under the condition of the tempering, with its high drawability of reason, so that the quenching martensite tempered transformation is incomplete,Still maintain high hardness, high internal stress quenching state.All these factors are conducive to the grinding stress concentration, not easy to disperse, easy to superposition with the residual stress of the matrix, especially in the super-grade belt carbide accompanied by network, strip carbide, grinding stress concentration is the most easy to cause the surface of the parts to form peeling and distributed strip and residual network cracks.Obvious loose defects are not allowed in finished bearing rings.It will reduce the density of ring, enrichment of the internal micro pore, low melting point impurity, gas and nonmetallic inclusions. It like banded carbide, dividing matrix continuity effect, cause the composition segregation, matrix embrittlement, greatly reduce the comprehensive mechanics performance of the ring, and increase the frictional resistance and notch sensitivity when grinding, grinding stress concentration, easy to induce grinding crack.

4. Characteristic analysis of grinding crack

Grinding crack is a kind of typical surface crack, which is very fragile and extremely dangerous in engineering. It is the most dangerous one among all the cracks.Often once the external force action to make the metal internal structure of the movement of the direction, sometimes even in a small external force action, as long as the external force action time is sufficient, will also produce cracks or parts fracture., and sometimes there is no external force due to the metal internal space and product diffusion, the release of residual stress and residual austenite of martensite phase transformation expansion stress, the movement of atoms is aggravated, as long as under certain temperature conditions, through the surface of some gap, such as grinding cracks, burns, craze, typing mark, mark, acute Angle, osteoporosis and local tensile stress, high hole, etc., can also cause crack (crack) itself, forming split (crack, casting off, take off the shoulder, and take off the edge), causing large flake pieces.

The most common macroscopic forms of grinding cracks are several parallel lines at right angles to the grinding direction and tortoise shell.The macroscopic shape of the grinding crack revealed in this example is special and parallel to the grinding direction.Because of this, the ring is delayed in the process of breaking.Or the crack is not fully formed, but the high tensile stress is concentrated in this part.Or the crack has been formed because it is so fine that even magnetic inspection cannot show it, but under the action of hot acid corrosion, the grinding crack develops along the direction of maximum tensile stress, which is similar to stress corrosion crack (SCC), and further expands the crack.

Undoubtedly, the vibration caused by the knock (impact) of the 6205 bearing when installed and the elastic (plastic) deformation caused by the expansion force between the shaft and the sleeve provide the "natural" external force and time for the collapse of the ring. When the collapse occurs, it rapidly expands into the secondary crack in a vertical way.

Four, conclusion

(1) the poor uniformity and density of the materials used for the rings is the congenital factor causing the rings to crack.

(2) under the action of intense grinding heat and coolant, the surface thin layer instantaneously forms the secondary dog-fire grinding white layer, which generates high grinding tensile stress and causes grinding cracks and grinding burns.

(3) insufficient ring tempering results in high hardness and brittleness of the martensite matrix, high residual stress, and inhomogeneity of the microstructure, which can easily promote stress superposition and induce grinding cracks and grinding burns.

(4) it can be considered that the premature failure of 6205 bearing is the result of the combined action of metallurgical defects, heat treatment defects, grinding cracks and other factors of the material, while the inhomogeneity of the microstructure (the difference of cross section microstructure) and the local tensile stress on the surface are the fundamental causes of the fracture.

(5) the vibration and expansion force when the bearing is installed provides the "natural" external force and time for the cracking, causing the ring to suddenly occur brittle cracking block.

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