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Cause of Quenching Cracks in 50CrMo4

After the quenching and tempering treatment of 50CrMo4 steel, the inspectors found several small crack defects when visually inspecting the surface quality of 50CrMo4 steel. Within the range of 10 mm ~ 20 mm from the outer circular surface. The cracks are distributed along the fiber direction of the 50CrMo4 steel ring gear, with different lengths, fine width, and shallow depth. Most of them can be removed by processing, and some cracks remain, which can only be scrapped.

Although we have taken a series of measures to prevent quenching cracks, such as pre-cooling and quenching, cooling and quenching, sub-temperature quenching, shortening the quenching cooling time, increasing the final cooling temperature of quenching, and timely tempering after quenching to eliminate quenching stress. The quenching of 50CrMo4 steel ring gear cracks are still not completely eradicated, and occasionally occur during production.

The cooling characteristics of the two are slightly different, and the cooling rate of the quenching liquid is basically the same as that of tap water, which is the root cause of quenching cracks in the quenching and tempering of 50CrMo4 steel rings. The purity of 50CrMo4 steel in the area near the inner hole of the ring forging is low, and there are metallurgical defects such as metal inclusions. These defects will also become the crack source of quenching cracks. Under the action of quenching stress, metallurgical defects expand and extend to form quenching cracks, which is one of the reasons why quenching cracks often occur near the end face of the inner hole.

The macroscopic morphology of the quenching crack of the 50CrMo4 steel ring gear. The metallographic observation of the quenching crack part shows that most of the non-metallic inclusions are distributed along the crack extension direction of the 50CrMo4 steel, and there are serious oxidation decarburization and crystallization around the crack. Grain coarsening, this kind of crack existed before quenching and tempering treatment, but it was not found during processing. It was further expanded under the action of quenching and tempering quenching stress.