Analysis of the Causes of the Tripping of the Thread of 45# Steel Nut
During the test of the nut load test in the laboratory, the inspectors found that a number of nut threads had tripped. Figure 1 shows the appearance of the nut after tripping.
Figure 1 Macro view of the thread of the 45# steel nut showing the tripping phenomenon
The batch of nuts is M20, the performance grade is 8 grades, the material is 45 steel, and it is quenched and tempered.
1. Physical and chemical test
(1) Component analysis
The chemical composition of the nut was tested using an ARL 4460 direct reading spectrometer. The results are shown in Table 1.
|
Element |
C |
Si |
Mn |
S |
P |
Cr |
Ni |
Cu |
|
Standard |
0.42~0.50 |
0.17~0.37 |
0.50~0.80 |
≤0.035 |
≤0.035 |
≤0.25 |
≤0.30 |
≤0.25 |
|
Measured |
0.43 |
0.22 |
0.63 |
0.029 |
0.018 |
0.08 |
0.02 |
0.01 |
The test results are in compliance with the chemical composition requirements of 45 steel in GB/T699-2015 “Quality Carbon Structural Steel” standard.
（2）Metallographic and mechanical analysis
Fig. 2 is a metallographic photograph of the threaded edge of the nut. A decarburized layer having a depth of about 0.15 mm is clearly present at the edge of the thread, and a deformed layer when the thread is tripped is apparent in the upper portion of the photograph. Figure 3 is a metallographic photograph of the core of the nut, organized into a sheet of pearlite and a white network of ferrite, with a small amount of widmanstatten structure.
Figure 2 Metallographic photo of the edge of the thread
Figure 3 Metallographic photo of the nut core
A micro Vickers hardness test was performed on the core of the nut, and the load was 300 g, and the core hardness was 324 HV0.3.
2. Analysis and discussion
The hardenability of #45 steel is relatively low, but the M20 nut of this batch of 45 steel is small in size, can be completely hardened under normal quenching and tempering treatment, and obtains uniform metallographic structure inside and outside by high temperature tempering. Better comprehensive mechanical properties of tempered sorbite structure.
The metallographic observation shows that the tripped nut is not a sorbite structure, but a ferrite and pearlite structure, but the hardness has reached 324HV0.3, indicating that the nut has not undergone quenching and tempering during the heat treatment process. Poor mechanical properties and insufficient resistance to deformation.
The thread of the nut is generally processed after the quenching and tempering treatment, and the metallographic photograph of the thread edge of Fig. 2 shows that there is a uniform decarburization layer on the thread surface, indicating that the nut is first processed and then heat treated to make the thread surface There is a layer of low strength decarburization layer. The organization of the nut base is not up to the requirements and the decarburization layer on the threaded surface is the main cause of the batch of nuts being tripped.
The piping solution provider: www.tourochina.com
Author: Zhou Zhen
Source: Metal Processing (Hot Processing)
