The magnetic powder flaw detection and common defects of bearing failure analysis knowledge - bearing

Bearing parts in the process of manufacturing, through forging, grinding expansion, punching, cutting, grinding, heat treatment process, various kinds of defects may occur. Common defects as follows: 1, forging defects forging folding as blanking, burr, flash, easily formed in surface fold, its characteristic is folding the bulky, irregular shape, easy to appear in the surface of parts. Had better use fluorescent magnetic particle inspection, make the defects showed more clear and intuitive. Forging folding the magnetic mark of general linear with surface at an Angle, the trench and scales flake, as shown in figure 1. Defect section made from metallographic specimen under the microscope, defect tail obtuse, smooth on both sides, have obvious oxidation phenomenon, as shown in figure 2, inclusion defect was found in material such as foreign body distribution. After the cold acid etching metallographic specimen observation, defective parts and both sides have serious decarburization and oxidation; Observation defect layer in surface morphology, the plastic deformation trace is obvious, without tearing fracture morphology. After microhardness test and metallographic observation, defects in layered surface there are varying degrees of carburizing and hardening phenomenon. In analysis, which indicates that this defect should be existed before the heat treatment quenching, and communicates with the outside world, determine the defects for forging folding. Figure 1 figure 2 forging folding defect section metallographic topography forging burnt forging heating temperature is exorbitant, heat preservation time is too long cause overheating, serious when grain boundary oxidation even melting. Microscopic observation not only surface layer of metal grain boundary is presented oxidative cracking Angle; And the areas with severe internal metal composition segregation, grain boundary are also beginning to melt, serious when will form the Angle of the cave. Burnt under the material in this kind of defect status of forging processing, by a heavy hammer forging, punching and run over expansion, would have a tear in the defect place, forming a larger defects. Forging badly burnt surface morphology such as orange peel, the distribution of small cracks and thick oxide skin. Appropriate USES fluorescent magnetic particle inspection, make the defects showed more clear. Pitting holes in forging defects caused by burning, as shown in figure 3. Defects along the cross section of preparing metallographic specimen under the microscope, visible holes on the surface and sub-surface all have distribution, local Angle shaped, size is differ, bottomless, edge has a small crack distribution, was in the grain boundary area of the oxidation phenomenon, hole shape as shown in figure 4. After another along the defect hole in smashing the fracture observation of fracture surface, visible fracture in rock fracture and its distribution on a large number of holes and micro crack. Figure 3 forging any temperature figure 4 hole defects in section 2, in the process of quenching, quenching crack morphology when the quenching temperature is too high or cooling speed is too fast, internal stress is greater than the fracture strength of materials, quenching cracks will occur. Appropriate fluorescent magnetic particle detection is used to improve the sensitivity and reliability. Quenching defect magnetic image generally taking diagonal shape, circular arc form, dendritic or mesh, starting site wide, with tapering extension direction, as shown in figure 5. The basic distribution along the circumferential direction, the tail nozzle. Cut out crackle made after metallographic specimen observation, crackle, deep basic perpendicular to the surface, which found no material with foreign body distribution. Along the crack place to hit after fracture observation, fracture to brittle fracture, fracture surface has obvious heat tint, as shown in figure 6. Figure 5 quenching crack figure 6 quenching crack fracture morphology 3, grinding defects in quenching process, when the temperature of the quenching cooling too fast, too high or internal stress is greater than the fracture strength of materials, quenching cracks will occur. Appropriate fluorescent magnetic particle detection is used to improve the sensitivity and reliability. Quenching defect magnetic image generally taking diagonal shape, circular arc form, dendritic or mesh, starting site wide, with tapering extension direction, as shown in figure 7. The basic distribution along the circumferential direction, the tail nozzle. Cut out crackle made after metallographic specimen observation, crackle, deep basic perpendicular to the surface, which found no material with foreign body distribution. Along the crack place to hit after fracture observation, fracture to brittle fracture, fracture surface has obvious heat tint, as shown in figure 8. Figure 7 quenching crack figure 8 quenching crack fracture morphology 4 bearing parts, raw material defects in the grinding, beating too much due to the grinding wheel feed, spindle, cutting fluid supply is not sufficient, and grinding wheel grinding grain of blunt, such as grinding cracks are easy to make the parts. In addition, the heat treatment quenching temperature is too high and parts of the organization is overheating, coarse grains, residual austenite size, mesh and more massive particles. Grinding defects of the magnetic mark general net-like, radial, parallel to the line or crack, magnetic image, sharp, outline is clear, a number, usually, perpendicular to the direction of grinding as shown in figure 9. Magnetic mark more concentrated distribution in the middle area, along the circumferential direction, a long linear or dendritic, local bifurcation, magnetic marks of convergence. After making crack section metallographic specimen observation, crack is fine, perpendicular to the surface, which did not see materials mixing, scale and other foreign body distribution, appearance is shown in figure 10. Figure 9 raw materials crack figure 10 crackle inclusions morphology of raw material