Magnetic Particle Inspection and Failure Analysis of Common Defects in Bearings

Magnetic Particle Inspection and Failure Analysis of Common Defects in Bearings

Due to the irregularity of the cutting, burrs, flashing, etc., it is easy to form a fold on the surface, which is characterized by a coarse folding, an irregular shape, and easy to appear on the surface of the part.

It is best to use fluorescent magnetic powder for flaw detection to make the defect display clearer and more intuitive. The forged folded magnetic marks are generally linear, grooved and scaly in shape at an angle to the surface.

The defect cross section was made into a metallographic sample and observed under a microscope. The defect tail was blunt, smooth on both sides, and there was obvious oxidation. As shown in Fig. 2, no foreign matter such as material inclusions were found in the defect. After cold metal corrosion, metallographic samples were observed, the defects and their sides were severely decarburized and oxidized. The surface morphology of the defect delamination was observed, and the plastic deformation marks were obvious, and there was no tear-like fracture morphology. After the microhardness test and metallographic observation, there are different degrees of carburization and hardening on the surface of the defect layer. In summary, it shows that the defect should exist before the heat treatment quenching, and communicate with the outside world to determine that the defect is forged folding.

The forging heating temperature is too high, the holding time is too long to produce overheating, and in the severe case, the grain boundary is oxidized or even melted. In the microscopic observation, not only the surface layer of the metal layer is oxidized and cracked, but also the sharp corners are formed. Moreover, in the region where the internal component of the metal is segregated, the grain boundary begins to melt, and in severe cases, a sharp-angled cavern is formed. The over-fired material is forged in such a defect state, subjected to forging, punching, and grinding of the weight, where the defect is torn and a larger defect is formed. Forging severely over-fired surface forms such as orange peel with fine cracks and thick scales.

Fluorescent magnetic powder should be used for flaw detection to make the defect display clearer. The pitting hole is caused by the forging burn defect.

The metallographic specimens taken along the defect section were observed under the microscope. It can be seen that the pores are distributed on the surface and the subsurface. The localities are sharp-angled, different in size, deep in the bottom, and there are fine cracks on the edges. Some areas have appeared. The grain boundary oxidation phenomenon, the pore morphology is shown in Figure 4. Another fracture hole was observed along the defect hole, and the fracture surface was observed. The fracture was a stone-like fracture, and a large number of holes and fine cracks were distributed thereon.