Microstructure Refinement and Crack Initiation in the Forging Process

摘要

In this paper, the type and morphology of defects in heavy ingots were detected mainly on the basis of experimental results, which were obtained by metallographic analysis of the materials dissected from one 380 ton 30Cr2Ni4MoV ingot. Then the evolution of typical defects in the forging process was investigated at temperatures ranging from 900°C to 1200°C and reduction ratios ranging from 0% to 50%. It was shown that coarse grains and inclusions were the main types of defects. In the forging process, the refinement of coarse grains and crack initiation at inclusions were summarized respectively with respect to temperature, reduction ratio and multi-pass forging. The microstructure evolution of 30Cr2Ni4MoV steel is accompanied by the formation of fine grains on condition that multi-pass forging or an optimal reduction ratio of 10% is adopted at temperatures lower than 1000°C, while temperatures higher than 1100°C can lead up to coarse grains. Besides, cracks originate from CaO and FeS inclusions at 900°C and 1000°C related to intense shear stress and melting of the FeS-Fe eutectics distributing along grain boundaries.