Experimental and modelling research on the influence of different pre-decarburization microstructures of Fe-0.6C-1.8Si-0.8Mn spring steel

Abstract

Steel surface decarburization is an important defect, deteriorating the component fatigue life significantly. Although there have been great efforts over decades to investigate the steel decarburization behaviour and related factors, one important aspect is the influence of pre-decarburization microstructures. During steel production, after continuous casting, if hot charging is not applicable, steel billets/blooms are normally stacked and cooled down before reheating and hot-rolling. During the continuous casting and subsequent stacking processes, steel billet/bloom surfaces are exposed to the air at relatively high temperatures, resulting in various decarburization microstructures. These decarburization microstructures could have different impacts on the subsequent reheating decarburization behaviour. However, research on this aspect is quite limited in the literature. In this research, the total decarburization depth (TDD) evolution of a Fe-0.6C-1.8Si-0.8Mn spring steel after isothermal holding and air-cooling was investigated, and various pre-decarburization microstructures were generated and subjected to a continuous heating decarburization test to study the influences of the pre-decarburization microstructures on the subsequent continuous heating decarburization behaviour. In the meanwhile, the experimental results were compared with the predictions of a decarburization model, and the model calculation results were used to explain the possible reasons for the influences of different pre-decarburization microstructures. The results of this research indicate that the higher the pre-decarburization TDD, the severer the final TDD after continuous heating, and the larger the contribution of the pre-decarburization TDD to the final TDD.