The results are as follows

(1) Through spectral analysis, the chemical composition of the sample is more than the requirements of GB / t5216-2014. In sample 1, the chemical composition was analyzed by three times direct reading spectrum. The content of C element (mass fraction,%) was 1.5, 0.5 and 0.47, and the corresponding content of S element was 0.114, 0.062 and 0.043, which all exceeded the standard requirements.

(2) Through metallographic and electron microscope analysis, the structure near the crack is normal. The over burning is a regional defect, and the cracks in sample 1 and 2 are not caused by over burning.

(3) A large number of MNS inclusions were found in samples 1 and 2, which were distributed at the regional segregation. The existence of Mn inclusions destroys the continuity of the base metal. Under the action of stress, the close position of MNS inclusions and adjacent base metal is easy to produce stress concentration and form microcracks, which is easy to produce defects in some subsequent processes (quenching).

(4) Dendrite segregation and shrinkage cavity, local carbide segregation and solidified liquidus carbide were found in the sample 2, which indicated that the raw material contained as cast microstructure, which would become the crack source and form cracks under the action of thermal stress, structural stress and deformation stress produced by subsequent forging and heat treatment.