Modelling and simulation of isothermal and continuous-heating surface decarburization behaviour of Fe-0.6C-1.8Si-0.8Mn spring steel

Abstract Steel surface decarburization leads to insufficient hardness in the surface area due to carbon depletion, reducing its fatigue life significantly. Various models have been proposed to simulate the decarburization behaviour. However, the influence of steel oxidation is either completely ignored or calculated separately, and the concentration-dependent carbon diffusivity in austenite is seldom considered. In this research, a numerical decarburization-oxidation integrated model based on the control volume method considering concentration-dependent carbon diffusivity was proposed. Isothermal and continuous-heating decarburization tests were conducted for a Fe-0.6C-1.8Si-0.8Mn spring steel in the air. Under isothermal holding between 700–1000 °C for 30–60min, the decarburization ferrite thickness rises with the increase of temperature, levelling off around 800–825 °C, and then starts to decrease. During continuous heating, the decarburization ferrite thickness increases at first to 76.1  μm at 840 °C and then starts to decrease to 61.6 μm after holding at 900 °C for 300s, finally rising to 65.8 μm after holding at 900 °C for 900s. Predictions of the proposed decarburization model were compared with experimental results, and a good match can be found between them. The significance of considering oxidation and concentration-dependent carbon diffusivity during modelling was emphasised as well.