Analysis and Solution of Fracture of Fastener Lock Nut

Manufacturing process and fracture diagram of fastener lock nut

During the manufacturing process of fastener lock nuts, the common form of failure is fracture. Under normal circumstances, the manufacturing process of fastener lock nuts is: material reform-cold heading-head,

chamfer, and center hole -Drilling the bottom hole of the thread-deburring-tapping-dephosphorization,

heat treatment-grinding thread diameter-rolling thread-(pickling) oxidation treatment-flaw detection, demagnetization-(pickling) oxidation treatment-coating, inspection,

packaging Library. Figure 1 below shows the broken parts, simulated broken parts and intact parts; Figure 2 shows the macro morphology of the fractured parts.

Analysis of the fracture problem of fastener lock nut

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.

(1) 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 metal material knowledge to carry out fracture analysis.

Figure 1 shows the macro morphology of the fracture of the fractured part. The fracture of the 1# part is relatively flat without obvious plastic deformation marks. The shear lip on the outer edge of the fracture is a fresh fracture, which is light gray, and the rest of the area is basically oxidized and black;

2# fracture The macro morphology of the part is similar to that of the 1# fractured part. There is no obvious plastic deformation mark on the fracture. The edge of the outer ring is a shear lip, which is light gray and fresh fracture. The surface of the remaining area is basically oxidized and blackened.

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By observing the microscopic morphology 2 of the black area of ​​the fracture of 1# piece, it can be seen that there is obvious oxidation and corrosion on the fracture surface, but rock sugar-like crystal morphology is still visible, and there are secondary cracks between the crystals.

Due to the oxidation corrosion phenomenon3 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 fracture4 is the morphology of dimples, and this area is the terminal zone.

By observing the microscopic morphology of the fracture in the black area of ​​the 2# fracture except for the edge shear lip, it can be seen that there are rock sugar-like intercrystalline fractures with secondary cracks between the crystals; the micro morphology of the outer edge of the fracture is the morphology of equiaxed dimples.

(2) Metallographic analysis

The metallographic analyzer was used to observe the metallographic structure near the core and fracture of the intact part and the 1# fractured part, and combined with heat treatment knowledge to conduct a comparative analysis of 5%.

(3) Hardness analysis

The hardness test is carried out on the intact and broken parts. The results are shown in Table 1. The surface hardness of the broken parts is too high, which does not meet the hardness requirements of 12.9 fasteners.

(4) Chemical composition analysis

The failed part was broken during assembly, and the section was obviously black and the fresh fracture area (as shown in Figure 2.Figure 4 and Figure 5). 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 are not in the Produced at the same time. The black area of ​​the fracture shows a fracture morphology along the product, accompanied by secondary intergranular cracks, which has the characteristics of hydrogen brittle fracture17. 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 surface oxidation treatment. The metallographic structure and chemical composition of the submitted parts are normal.

Using direct reading spectroscopy to analyze the chemical composition of the intact and broken parts, the results are shown in Table 2. According to “JISG4053-2008”, its chemical composition meets the requirements for SCM435 steel in the standard.

(5) Solution

The reason for the fracture of the parts submitted for inspection is the existence of original cracks, which are hydrogen-induced cracks. The hydrogen comes from pickling before oxidation and blackening. The following measures are recommended:

Use alkaline electrolysis to remove oil and rust. If pickling cannot be avoided, stress relief treatment should be taken after rolling the external thread, and hydrogen drive treatment should be carried out within four hours after surface treatment.