Effect of silicon on stability of austenite during isothermal annealing of low-alloy steel with medium carbon content in the transition region between pearlitic and bainitic transformation

In a vast majority of steels, a prerequisite to successful heat treatment is the phase transformation of initial austenite to the desired type of microstructure which may consist of ferrite, pearlite, bainite, martensite or their combinations. Diffusion plays an important role in this phase transformation. Together with enthalpy and entropy, two thermodynamic quantities, diffusion represents the decisive mechanism for the formation of the particular phase. The basis of diffusion is the thermally-activated movement of ions of alloying and residual elements. It is generally known that austenite becomes more stable during isothermal treatment in the transitional region between pearlitic and bainitic transformation. This is due to thermodynamic processes which arise from the chemical composition of the steel. The transformation of austenite to pearlite or bainite is generally accompanied by formation of cementite. The latter can be suppressed by adding silicon to the steel because this element does not dissolve in cementite, and therefore prevents its formation. The strength of this effect of silicon depends mainly on the temperature of isothermal treatment. If a steel with a sufficient silicon content is annealed at a temperature, at which silicon cannot migrate by diffusion, cementite cannot form and austenite becomes stable for hours. (paper)