In order to understand the size of the broken part of the part, the size of the semi-finished product after hot bending is tested. The test result: the diameter of the bend at the fracture site is reduced to meet the requirement of not less than 20.5mm; the radius of the inner circle of the fracture is in the range of R22mm20mm. Inside; the inner circle of the fracture site has different degrees of protrusion folds. Analysis of the cause of the fracture The radiator torsion bar is an elastic part. After quenching and tempering, the structure should be a good elastic yttrium body. The general hardness requirement is in the range of 375495HB. From the test results, it is known that there is no problem in the chemical composition, hardness and structure of the material. Further, the metallographic analysis does not find quenching defects such as coarse grains, indicating that the quenching and tempering heat treatment process is normal. The macroscopic fracture shows that the fatigue zone with fatigue arc is small and the rapid expansion zone is large, which indicates that the part is broken by the large alternating load. From the assembly of the component, the part is subjected to static torsion when the fixing mechanism is locked, and this static torque does not cause the part to break rapidly (the torsion angle is within its elastic range). However, the statistics of the fracture of the part show that the part has 5 fracture records in 2 years, which indicates that the alcohol solution etched alternating load will occur and exist in the use of some vehicles. It can be seen that some vehicles have problems with improper use of the locking device.
The crack source originated on the surface of the part, there was no decarburization on both sides of the crack, and no problems such as oxidation were found, indicating that the crack was generated during use. The surface of the part has been found to have a significant decarburization layer, which not only reduces the surface fatigue strength of the material, but also causes tensile stress on the surface of the part due to the difference in carbon content in the surface structure, so that the surface of the part is affected by a large external force. Cracks are generated. According to the data, taking 60Si2Mn as an example, even a very slight decarburization layer (0.125mm) will cause the fatigue strength to drop sharply from 570N/mm to 350N/mm. The decarburization layer of the fractured piece is 0.10mm, which is close to 0.125mm. Therefore, decarburization of the surface of the part is a cause of crack generation.
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