Bearing parts in the manufacturing process, to go through forging, rolling, punching, turning, grinding, heat treatment and other procedures, may appear all kinds of defects. The following are the five common defects in the bearing manufacturing process.
1, forging defects – forging folding
Due to uneven cutting material, burr, flying edge and other reasons, it is easy to form folding on the surface, which is characterized by thick folding, irregular shape, easy to appear on the surface of parts.
It is better to use fluorescent magnetic powder for flaw detection to make the defect display more clear and intuitive. Forged and folded magnetic marks and the surface into a certain Angle of the line, groove and fish scale sheet.
The defect section was made into metallographic samples and observed under a microscope. The defect tail was round and blunt, both sides were smooth, and there was obvious oxidation phenomenon. No material inclusions and other foreign bodies were found in the defect. After the cold acid corroded the metallographic specimen, the defect part and its two sides had serious decarburization and oxidation. The surface morphology of the defect layer was observed, and the plastic deformation trace was obvious, without tearing fracture morphology. Through microhardness testing and metallographic observation, carburizing hardening phenomenon existed in different degrees on the surface of defect layer. In conclusion, the defect should have existed before heat treatment and quenching, and communicated with the outside world, and was judged to be forged folding.
2, forging defects – forging overburn
Overheat will occur when the forging heating temperature is too high and the holding time is too long. In serious cases, grain boundary oxidation or even melting will occur. Microscopic observation shows that the metal grain boundary of the surface layer is oxidized and cracked with sharp Angle. Moreover, grain boundaries also began to melt in areas where the internal composition of the metal was more segregated, and in severe cases, pointed caves formed. The overburned material is forged in this defect state, subjected to heavy hammer forging, punching and grinding, and the defect will tear here, forming a larger defect. The severely overburned surface of the forging is like orange peel with fine cracks and thick oxide peel.
It is advisable to use fluorescent magnetic powder for flaw detection to make the defect display clearer. The pitting holes are caused by overburning defects in forging.
Microscopic observation of the metallographic samples made along the defect section showed that the holes were distributed on the surface and the secondary surface. The holes were angular in some parts, with different sizes and no bottom in depth. There were fine cracks at the edges and grain boundary oxidation in some areas. In addition, the fracture surface was observed after being smashed along the defect holes. It was found that the fracture was stone-shaped, and a large number of holes and micro-cracks were distributed on it.
3. Quenching crack
In the quenching process, when the quenching temperature is too high or the cooling rate is too fast, the internal stress is greater than the fracture strength of the material, there will be quenching cracks.
Fluorescent magnetic particle inspection should be used to improve sensitivity and reliability. Magnetic marks of quenching defects are generally oblique, circular, dendritic or reticular, with a wide initial position and gradually thinning along the extension direction.
Basically along the circumferential direction distribution, tail tapering. After cutting the crack to make metallographic sample, it can be seen that the crack is very deep, basically perpendicular to the outer surface, and no material inclusion and other foreign bodies are found in the crack. It was observed that the fracture was brittle and the surface of the fracture was obviously pyrochromatic.
4, grinding defects
In the quenching process, when the quenching temperature is too high or the cooling rate is too fast, the internal stress is greater than the fracture strength of the material, there will be quenching cracks.
Fluorescent magnetic particle inspection should be used to improve sensitivity and reliability. Magnetic marks of quenching defects are generally oblique, circular, dendritic or reticular, with a wide initial position and gradually thinning along the extension direction.
Basically along the circumferential direction distribution, tail tapering. After cutting the crack to make metallographic sample, it can be seen that the crack is very deep, basically perpendicular to the outer surface, and no material inclusion and other foreign bodies are found in the crack. It was observed that the fracture was brittle and the surface of the fracture was obviously pyrochromatic.
5. Defects of raw materials
In the grinding process of bearing parts, grinding cracks are easy to occur due to too much feed of grinding wheel, runout of sand wheel shaft, insufficient cutting fluid supply and dull grinding grain of grinding wheel. In addition, during heat treatment, the quenching temperature is too high, resulting in overheating of the parts, coarse grains, more residual austenite volume, mesh and coarse particles.
The magnetic marks of grinding defects are generally netted, radial, parallel linear or cracked. The magnetic marks are thin and sharp, with a clear outline and a large number of them, which are usually perpendicular to the grinding direction. Magnetic marks are mostly concentrated in the middle part, along the circumferential direction, in the shape of a long line or dendritic, partial bifurcation, magnetic marks convergence.
It was observed that the crack section was fine and perpendicular to the surface. No material inclusion, oxide scale and other foreign bodies were found in the crack section.
Post time: Jun-20-2022