Processing of bearing ring fracture, crack failure analysis

Bearing in the superfinishing process was conducted few bearing ring fracture, but there will be a lot of crack of set of bearing ring, and in the place has more bearing ring appeared in the process of the crack. This is how to return a responsibility? The bearing network ( Hereinafter referred to as:) According to the understanding of the bearing ring processing knowledge, to share the bearing ring in the process of fracture failure analysis and introduced the reasons. 1, bearing ring fracture samples bearing ring fracture failure is shown in figure 1, the first cut a fracture the bearing ring fracture failure sample, to observe the fracture appearance characteristics. Other bearing ring with crack nondestructive testing in the position of the corresponding crack after use marker pen mark sat down, along the marked the location of the sampling, keep will crack is included and happened to be in surface rendering. Figure 1 bearing outer ring fracture, macro, macro and micro analysis diagram 2 for bearing failure of outer ring fracture scanning electron microscopy (sem) pictures, and the steps of enlarge images. It can be seen that the main crack source for the steps of the facade, exists in the facade is more more smooth corrugated area and holes, and on the lower level of two internal cracks are found more obvious. Figure 2 bearing ring fracture morphology of figure 3 is not given several microscopic morphology of crack fracture. Crack depth of 0. 5 - 1 mm, with an average depth of about 0. Basic perpendicular to 8 mm, and extends to the surface. Will crack sample along the direction of crack depth to make metallographic specimen, polishing and etching through electronic microscope play 1 stroke after the microstructure of the crack, in figure 3, figure 4 shows the microstructure of the listed three crack, crack can be found from figure 4 basic inward extension, perpendicular to the surface for bending, serrated, also have discontinuous phenomenon, and in some areas there is micro holes, cracks for the intergranular crack, crack near the organization the same as the matrix organization, but a bit deep color. Figure 5 in figure 4, the last a picture amplification, can see clearly from the picture the crack tip exist a certain amount of microscopic holes in the organizations. In figure 3 bearing ring every crack appearance figure 4 the microstructure of the crack in figure 3 5 crack tip point of scanning mirror according to the above analysis and delayed fracture characteristics, basic can determine the cracking phenomena belong to the category of hydrogen induced delayed fracture, rule out the possibility of quenching crack, belongs to a kind of grinding cracking phenomenon. Grinding is one of the important process of miniature bearing processing. In general, grinding surface when the temperature is very high, many experiments also prove that grinding zone temperature can rise to 400 ~ 1000 c. At the same time, the cooling fluid and the workpiece under the action of its thermal conductivity, the workpiece surface would be rapid cooling, similar to a kind of instantaneous heat treatment process, usually call this thin layer of grinding badness layer, the secondary hardening layer is formed after quenching, while on the surface of workpiece times due to grinding heat is not completely lost, in turn to the surface layer second quenching and tempering) was studied. When the tempering temperature material close to the final tempering temperature, the corrosion of the organization will and center to normal microstructure close, but the color will be slightly different. Grinding heat temperature on the surface of the parts, and under the action of workpiece surface and the coolant quenching, this process can produce very large thermal stress. Due to grinding metamorphic layer is very shallow, the maximum residual stress also can appear within the scope of the grinding surface is smaller, so the grinding cracks are generally lighter. In addition, the metal material in the processing and manufacturing process is invaded by hydrogen. Research shows that the hydrogen content of ordinary steel in ( 5 ~ 10) ×10- Can produce hydrogen induced cracks at more than 4%, but for high mild steel, even if the hydrogen in the steel, including minimum 1 x 10 - 4%, also can produce hydrogen induced cracks. In grinding process, due to stress the role of the original hydrogen atoms in a lattice can spread by focusing on the crack tip and stress concentration, and defects such as priority and dislocation in the product and the location, the interaction of the hydrogen atoms and dislocation, the dislocation line by pinning live, can't free activities, so that the substrate become fragile. In plus or residual tensile stress under the action of hydrogen atom by reducing the adhesion strength of the material, or change the surface energy of plastic deformation behavior and other possible ways, lead to material embrittlement, someone through design test also proved that the grinding crack formation and propagation of hydrogen for GCr15 has a promoting effect. In general, only more than 45 HRC hardness of metal materials there is a possibility of grinding crack, and to achieve the required hardness, its microstructure. General should be tempered martensite, bainite and so on. Because there are a great quantity of crystal defect (quenching martensite Dislocation, etc. ) , and the residual stress after quenching, increase the hydrogen absorption effect, thus quenching martensite ( Low temperature tempering martensite) Is the most sensitive microstructure of hydrogen embrittlement. In addition, this paper used the bearing products of cryogenic treatment process. Cryogenic treatment is equivalent to the secondary hardening, its main function is to material of retained austenite into martensite under lower temperature, also easy to produce grinding crack. 3, the results ( 1) Cracking of the bearing components of hydrogen induced delayed cracking, belongs to a kind of grinding crack. ( 2) For hydrogen induced delayed fracture this grinding cracking pattern, need to adjust the relevant parameters in the process of grinding, on the one hand, reducing grinding heat as far as possible a large number of accumulation, such as control of a single feeding, etc. ; Is another aspect to consider in cryogenic treatment increase the tempering process, before or after cryogenic treatment promptly carry out higher temperature tempering processing, specific process parameters and progress of research is still needed.