Discussion on the failure of tower crane bolt disconnection

Generally, the structure of high-strength bolts requires tempered sorbite, which indicates that there is something wrong with the heat treatment process. When the bolt is heat treated, the effect of quenching and tempering is not achieved, so that it cannot have good comprehensive mechanical properties. There is a significant difference in the hardness of the surface of the sample, and the hardness of the tempered martensite ferrite region is significantly higher than that of the tempered sorbite region. During the dry fatigue process, there is friction between the two surfaces of the crack, which makes the surface bright, so no obvious initial step and fatigue striation are observed, but the surface has obvious secondary crack, which is the obvious feature of low cycle fatigue. . The presence of inclusions was also observed on the fracture surface, indicating that there is a problem with the quality of the steel.

Bolt fracture process analysis During the use of bolts, due to improper roots, micro-cracks, etc., the bolt material is mixed, these parts may form a source of fatigue cracks, and at the same time, the root of the thread has a relatively high stress concentration, plus the use of bolts Loose or pre-tightened in the middle, resulting in additional bending force, the second fatigue source, the fatigue cracks on both sides expand at the same time, so that the effective bearing area of ​​the bolt is continuously reduced, and finally the fracture occurs. Conclusions and recommendations (l) The bolt is a dual source fatigue fracture. (2) There are problems with the bolt heat treatment process and should be improved to ensure a qualified organization. (3) There are inclusions in the bolt material, and it is recommended to strictly control the quality of raw materials.