Formation Mechanism and Influence of Heating Temperature and Holding Time on Surface Decarburization of 40Cr Steel during Hot Forging Process

During hot forging and heat treatment of 40Cr steel, a decarburized layer emerges on the surface, affecting fatigue life. However, the decarbonization of 40Cr steel has not been reported so far. The experiment mainly studies the effect of holding temperature and time on decarbonization. The 40Cr steel undergoes complete decarburization at 700 and 750 °C while experiencing a combination of complete and partial decarburization at 800 °C, transitioning solely to partial decarburization above 850 °C. The decarburized layer thickness measured at 750 and 800 °C is 140 and 200 μm respectively; the total decarburization layer thickness exhibits an exponential increase with the temperature. The thickness of the completely decarburized layer, influenced by varying factors across temperature ranges, initially rises and subsequently declines with the escalation of temperature, reaching its maximum at ≈750 °C. The maximum value of complete decarburization is 146 μm. Simultaneously, the total depth of decarburization is found to be proportional to the square root of time, adhering to Fick's second law. The fitting result is expressed as x = 165.47 t 1/2 at 750 °C. Notably, the existence of the decarburized layer results in a decrement in both the tensile strength and yield strength of the component, albeit accompanied by a marginal improvement in ductility.