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What is the Cause of the Fracture of the Fastener Lock Nut?

Nov. 19, 2020

As a Railway Fastening System Exporter, let me share with you.

The cause of the fracture of the locking nut of the fastener may come from the aspects of material, organization, or strength. To this end, according to its process, combined with the knowledge of materials, analysis of metallography and hardness using magnifying glass, microscope and other aspects.

Fracture analysis

With the help of a low-power magnifying glass and a high-position display mirror to observe the macroscopic and microscopic morphology of the fracture, combined with engineering practical experience and the use of metal materials and microscopic morphology, combined with engineering practical experience and the use of knowledge of metal materials for fracture analysis.

Rail Anchor Bolt

Rail Anchor Bolt

The macroscopic appearance of the fractured part is relatively flat, without obvious plastic deformation marks. The outer edge of the fracture is a fresh fracture, which is light gray, and the rest of the area is basically oxidized and blackened. The macro morphology of the fractured part is similar to that of the fractured part. The fracture surface has no obvious plastic deformation marks. The outer ring edge is a shear lip, which is light gray and is a fresh fracture. The surface of the remaining area is basically oxidized and blackened.

By observing the microscopic morphology of the black area of the fracture, it can be seen that there is obvious oxidation and corrosion on the cross section, but rock sugar-like crystal morphology is still visible, and there are secondary cracks between the crystals. Due to the presence of oxidation corrosion on the section, it is impossible to determine whether there is chicken claw-shaped hairline on the surface; the microscopic morphology of the outer edge of the fracture is a dimple morphology, and this area is the terminal zone.

By observing the microscopic morphology of the fracture in the black area of the fracture, except for the edge shear lip, it can be seen that there are rock sugar-like fractures along the crystals, and there are secondary cracks between the crystals; the micro morphology of the outer edge of the fracture is the morphology of equiaxed dimples.

Chemical composition analysis

The failed part was fractured during assembly. The section showed two areas of obvious black and fresh fracture. The surface of the black area was seriously oxidized, and the surface of the fresh fracture was not oxidized. This shows that the two areas did not occur at the same time. The black area of the fracture shows a fracture morphology along the grade, accompanied by secondary intergranular cracks, which is characterized by hydrogen embrittlement fracture. The freshly fractured area at the outer edge of the cross-section has a dimple morphology. The surface of the fracture along the grain has been basically oxidized and corroded, and only the edge termination area does not have a large number of obvious oxidation corrosion products, indicating that there are cracks in the submitted parts before the later surface oxidation treatment. The metallographic structure and chemical composition of the submitted parts are normal.

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