Geometrical size detection In order to understand the size of the broken part of the part, the semi-finished product after hot bending was tested. The test result: the diameter of the bend of the broken part is reduced to meet the requirement of not less than 20.5mm; the radius of the inner circle of the bend of the broken part is in the range of R22mm20mm Inside; the inner circle of the bend of the fractured part has different degrees of protruding folds. Analysis of the cause of fracture The torsion bar of the radiator is an elastic part. The structure after quenching and tempering should be troostite with good elasticity, and the general hardness is required to be in the range of 375495HB. From the test results, it can be seen that there are no problems with the chemical composition, hardness, and organization of the material, and the metallographic analysis did 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 smaller, and the rapid expansion zone is larger, which means that the part is fractured under the action of larger alternating load. It can be seen from the assembly of the components that in the locked state of the fixing mechanism, the part is subjected to static torsion, which will not cause the part to break quickly (the torsion angle is within its elastic range). However, statistics on the fracture of the part show that there are 5 fracture records for the part in the past 2 years, which shows that the alternating load of the alcohol solution erosion will occur and exist in the use of some vehicles. It can be seen that some vehicles have the problem of improper use of the locking device.
The source of the crack is on the surface of the part. There is no decarburization on both sides of the crack, and no problems such as oxidation are found, indicating that the crack is generated during use. The surface of the part is found to have an obvious 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 the carbon content in the surface and the inside of the structure, so that the surface of the part is under the action of a larger external force. Cracks occur. 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 part is 0.10mm, which is close to 0.125mm. Therefore, the decarburization of the part surface is a cause of cracks.
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Link to this article：Erosive alternating loads will occur and exist in the use of some vehicles
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