(1) Analysis of crack properties
From the crack morphology, the crack originates from the stress concentration. Whether the root of spiral bevel gear or the threaded hole is the place where the stress of the part is most concentrated, which belongs to the stress concentration crack of heat treatment.
(2) Analysis of crack causes
The large and small cracks extending along the tooth root of spiral bevel gear have oxidation phenomenon, and the oxidation depth is slightly shallower than that of the workpiece surface, indicating that the crack is generated before the second carburization, not quenching crack, nor during the first heating carburization and slow cooling.
When the oxygen probe is found to have a problem for the first time, the workpiece is carburized for 3h under high temperature and high carbon potential controlled by the oxygen free probe, and the material 18crnimo7-6 is made of low carbon and high alloy. In this case, the carburized layer structure will inevitably produce coarse martensite structure and high residual austenite structure, forming a certain tensile stress on the workpiece surface.
After replacing with a new oxygen probe, the fracture morphology of observation point 3 of workpiece Fig. 1 (1000) ×） Fig. 2 fracture morphology of observation point 4 (1000) ×） During the heating process, the workpiece surface is coarse martensite structure and high residual austenite structure. Under the action of thermal stress and structural stress, the workpiece cracks at the stress concentration.
Re blanking with the original material to process the large and small spiral bevel gears. Turn them to the hot place
After treatment, it was processed according to the heat treatment process of 1.8mm carburizing layer. Carburizing → slow cooling → quenching → cleaning → tempering → shot peening. No abnormality was found in the inspection of three pairs of spiral bevel gears. The metallographic structure was qualified, and the depth of effective hardening layer was qualified, which was 1.713mm. This fully proves that the crack of spiral bevel gear is caused by excessive residual stress caused by heat treatment process defects, and the raw material is OK 。
(1) The cracks of three pairs of spiral bevel gears are caused by abnormal heat treatment process and excessive residual stress of heat treatment. After the heat treatment process is normal, similar accidents can be avoided.
(2) It is suggested that after the oxygen probe has problems in the future, the samples with the furnace shall be inspected first, the depth and structure of the infiltration layer shall be inspected, and then the heat treatment process shall be re formulated for rework.