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The carbon structural steel forgings suddenly break during operation, and the fracture is close to the outer end of the bearing main journal and the root of the crank. The fracture is quite similar to the crankshaft fracture of the above example. The macroscopic inspection of the fracture did not reveal any signs of welding at the source of the fracture, but the fracture morphology was identical to the two typical components described above. Since there is no microscopic inspection of the fracture, it is impossible to qualitatively explain which manufacturing process the crack source is formed, but it can be said that there is a defect that forms fatigue fracture at the upper right, which causes the rice spindle to break during operation. The large reciprocating compressors used in ammonia production have relatively low rotational speeds, and their main components should not be continuously broken.
In the analysis of large compressor component fracture accidents, a common feature is that the fracture of the component under the low stress state has no obvious deformation and is a brittle fracture. This is caused by the fatigue of the metal under the alternating stress, which indicates fatigue. The fracture occupies a high ratio in the fracture accident of the component. When most metal components, especially the shaft-fatigue fracture, the macroscopic fracture is composed of three parts: the fatigue source zone, the crack extension zone and the overload tear zone. The crack extension zone presents a shell-like pattern or a beach-like smooth and shiny one is the most Characteristic, and these fatigued lines are substantially parallel to each other and perpendicular to the direction of crack propagation, which provides a valuable identification for our analysis of the cause of the fracture.
It is very important to avoid fatigue fracture or to prolong the fatigue life of the component and to eliminate the artificially generated fatigue source. During the visit to the manufacturing plant, we found many appearance defects of large forgings, and repair welding without any measures, which was doubled due to the uneven temperature gradient and the defects in the welding process. Increased stress concentration, and fatigue cracks are most sensitive to these stress concentrations, resulting in a significant reduction in the fatigue strength of the component, which causes serious problems in the use process. Therefore, for medium carbon steel materials, the welding should be avoided. If it is necessary to weld, the welding process should be strictly controlled, and the stress-relieving heat treatment and necessary inspection should be carried out after welding. At the same time, special attention should be paid to the stress concentration of components, and the acceptance of product quality should be strengthened to fundamentally eliminate the occurrence of fatigue fracture.
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