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Analysis of non -normal breaks in galvanized hexagonal bolts

Analysis of non -normal breaks in galvanized hexagonal bolts

Hexagonal bolts, specifications of M12 × 70, performance level of 8.8, 35K material, and surface galvanized on the surface. Normal breaks occur when the bolt is out of the factory test.


Test process and results

For fissure checks, the broken bolt fracture is located under the head, which is V-shaped. The difference between the two sides is obvious.

Fracture bolt macro appearance

The fracture bolts are mainly divided into two parts (A, B). The section of the part of the fracture A section is seriously corroded. The section of the part of the B is fiber metal.



(1) Scan electron microscope observation

Observation of the broken bolt of the fracture bolt of the head of the fracture bolt, there are obvious color contrasts on both sides of the fracture. The surface A surface is severe and the color is dark. The color of the B area is lighter.


Electronic image


The results of scanning electron microscopy image and spectrum analysis of the section of the A area: Energy spectrum analysis, the outer edge of the interoperability of the section of the A area contains more Zn and CL elements, as shown in Figure 13-4.

Area A Face Orthopedia and Energy Basement Analysis

The micro-appearance in the interior of the section of the A area can show that the surface is severe oxidation, as shown in Figure 13-5.

Figure 13-5 A micro-appearance in the interior

Observation of scanning electron microscopy image in area B, micro -appearance in area B, and area B in area B, can be seen in a large number of axis tough nests, see Figure 13 = 6.


(2) Chemical analysis

Use a direct reading spectrum method to analyze the bolt of the bolt. The analysis results are analyzed. The chemical composition of the bolt meets the requirements of the SWRCH35K steel in the JIS G3507.1-2005 standard.

(3) hardness testing

The surface hardness gradient and the cardiac hardness test of the bolt are performed, and the surface hardness is 205 ~ 300 hv10; the cardiac hardness is 301 ~ 306HV10; the bolt hardness meets the relevant provisions of the standard GB/T 3098.1-2010, and the results are shown in Figure 13-7.

Figure 13-7 Hardness gradient detection value


(4) Gold phase inspection

The gold-phase tissue of the broken bolt of the fracture bolt is to recover the Soviet Union, see Figure 13-8.


At the screw of the broken bolt, there is a slightly decarburization on the surface of the thread, as shown in Figure 13-9.

Figure 13-8 Bolt Bolt Heart Gold Phase Figure 13-9 Thread Terminal Gold Phase Organization

Analysis and discussion


The bolt material is qualified. The gold phase tissue of the broken bolt of the bolt is to ignit the Soviet Union. The thermal treatment temperature is basically no problem, but the heat treatment and protection atmosphere is not well controlled. The gradient is slightly lower than the hardness of the heart. Bolt fracture is not directly related to material, heat treatment, and mild decarburization.

There are two areas of the broken bolt, one of which is severely oxidized and corrosive, and the other is a fresh fracture. There are elements such as Zn and CL that have been infiltrated when electroplated in the edge of the severe oxidation. The bolts have not been exposed to corrosion containing CL after production, and can only be exposed to Zn and CL when plating. Zn and CL were found at the edge of the bolt fracture, indicating that there were already cracks before the electroplating. During the electroplating process, ZN and CL were infiltrated from the crack. Inside.

The broken position of the broken bolt is at the end of the thread, and the thread is located under the bolt head. The stress concentration is concentrated here. During the cooling process of heat treatment, the stress concentration produces quenching cracks.

Due to the existence of quenching cracks, the crack was oxidized during the fire, and Zn and CL were infiltrated from the crack during the electroplating process.

The crack is deeper, which reduces the bolt bearing area, which causes a break in advance during the installation test.

Conclusion and revelation

(1) There is a deeper quenching crack under the bolt head, causing it to break in advance during the installation test.

(2) The thread of the whole thread bolt is located under the bolt head. Whether it is a roller or rubbing thread, the stress concentration of the screw tail of the screw under the bolt head is obvious. Processing residual stress to prevent production quenching cracks at the stress concentration during heat treatment.

Fracture bolt macro appearance

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