Discussion on the assembly and function of introducing large hardness bolts
Materials and Test Methods Test materials were tested using American imported bolts used in stamping dies for aluminum cans. The dimensions of the three bolts are as shown. Three domestic materials, 40Cr, 42CrMo and 45 steel, were selected for comparative tests. Imported bolt size specifications Large bolt (mm) Bolt (mm) Small bolt (mm) Outer diameter test In order to select the most suitable heat treatment process, the following three heat treatment processes are used for three domestic materials: quenching + medium temperature tempering, quenching + high temperature tempering, austempering, etc.
The heat treatment process selected for the test is as follows: heat treatment process of 40Cr material: 850 oil quenching + 450 tempering; 850 oil quenching + 550 tempering; 850 heating + 300 austempering. Heat treatment process of 42CrMo material: 840 oil quenching + 450 tempering; 840 oil quenching + 550 tempering; 840 heating + 280 austempering. Heat treatment process of No. 45 steel: 840 water quenching + 400 tempering; 840 water quenching + 520 tempering; 840 heating + 280 austempering.
Tensile test The tensile test was carried out on an INSTRON 1185 universal material testing machine. Because of the limited size of the bolt, only non-standard tensile specimens can be used. The dimensions of the plate-like tensile specimen are shown in the figure as shown. The microscopic analysis test of the notched tensile specimen size firstly analyzed the chemical composition of the imported bolts, and used the XL30ESEM-TMP type environmental scanning electron microscope to study the metallographic structure of the three bolts and the metallurgical materials after several heat treatments. Tissue and tensile fractures were observed.
Hardness test This test uses the Rockwell hardness test method to measure the hardness of three different gauge bolts and the hardness of domestically produced materials after heat treatment. Test results and analysis Anatomical test results of imported bolts and analysis Chemical composition analysis The chemical composition of imported bolts was analyzed by chemical method. The composition of the three bolts was basically the same. The results were as follows: C (0.360.42%); Cr (0.900.99%); Mn (0.430.55%); Si (0.200.23); Ni (0.450.66). According to the above results, the imported bolts are medium carbon tempered alloy steel.
Tensile test results and analysis Due to dimensional limitations, tensile tests were carried out only on imported large bolts with a diameter of 12.5 mm and imported intermediate bolts with a diameter of 10.5 mm. test results. It can be seen that the strength of the imported bolt is high, but the elongation is low. The strength of the large bolt is as high as 1350 MPa, and the strength of the middle bolt is 1013 MPa. The difference between the strength of the large and medium bolts is due to the use of non-standard specimens containing different threads. Its tensile strength is greater than 1000MPa.
The tensile test results of the bolts and the results of the analysis of the hardness test are as shown. The hardness of the imported bolts of different sizes has reached 3940HRC. The hardness of the imported bolts has been proved that high-strength bolts are easy to produce when the HRC is higher than 40. Brittle break <3>. The hardness of the imported bolts is skillfully controlled at around 40HRC.
Since the small bolt size is too small, the tensile test cannot be performed. However, from the hardness test results, the strength of the small bolt is basically the same as that of the large bolt, and the strength values ​​of the three bolts are not much different, within the error tolerance. This difference is an error caused by heat treatment or measurement.
Test results of domestic bolt materials Tensile test results are tensile test results of three domestic materials after heat treatment by different processes. From the results of the tensile test, it can be concluded that the tensile strength of the 42CrMo material is significantly higher than the other two materials. And its plasticity is better than the other two materials. In particular, its notch performance is significantly better than other materials.
The hardness test results and analysis are the hardness test results of 40Cr and 42CrMo steel after heat treatment by different processes. It can be seen that the hardness value of 40Cr steel after isothermal quenching treatment and medium temperature tempering treatment is close to 40HRC, and the hardness value of 42CrMo steel after medium temperature tempering treatment is closer to the hardness of imported bolts. The microstructure analysis of the microstructure of domestically produced materials shows that the metallographic structure of 40Cr and 42CrMo steel after medium temperature tempering is a small uniform tempered sorbite as shown.
The comparison between the test results of imported bolts and domestic materials is compared with the microstructure and properties of imported bolts. The results show that the three domestic materials after austempering are bainite, although their notched tensile strength and hardness are the highest, but the gap The sensitivity is also high; the three domestic materials after high temperature tempering are tempered sorbite, the strength is also above 1000 MPa, the elongation is also the highest, but the hardness value is lower, the HRC is between 2935, the fracture morphology It is dimple-shaped; only 40Cr and 42CrMo steel after medium temperature tempering is tempered sorbite, the strength is higher than the tempering state, and the hardness value HRC is between 3839, the notch sensitivity is small, the shape of the tensile fracture The appearance is a dimple-like fracture, which is closest to the imported bolt. Considering the organization and performance of imported bolts and domestic materials, it can be concluded that the microstructure and properties of 40Cr and 42CrMo after quenching are the closest to imported high-strength bolts.
Conclusion 1) Microscopic analysis shows that the imported high-strength bolts are quenched and tempered and the structure is tempered sorbite; the tensile strength is above 1000 MPa, and the elongation is relatively small; It is concluded that the fractures of bolts of different sizes exhibit deeper dimples. The analysis shows that the material of the imported high-strength bolt is medium carbon tempered alloy steel. 2) The microstructure, structure and properties of 45 domestic steel, 40Cr and 42CrMo alloy steels were analyzed. The results show that the microstructure and properties of 40Cr and 42CrMo alloy steels are closest to imported bolts after quenching and medium temperature tempering.
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