Strengthening mechanisms influenced by silicon content in high temperature tempered martensite and bainite

Abstract

Strengthening mechanisms influenced by silicon (Si) content during tempering of martensite and bainite between 500 and 650 °C were investigated. Microstructural features such as dislocation density, subgrain size, cementite size, and microalloy precipitate volume fraction and size were evaluated in low and high Si alloys heat treated to form bainite or martensite and tempered at 500 or 650 °C for 1 h. Higher Si contents increase the hardness of both bainite and martensite more after tempering at 500 °C, as compared to 650 °C. The increase in Si content leads to a greater increase in hardness in bainite compared to martensite after tempering at 500 °C. A strength model was used to calculate the increase in hardness from the quantitatively measured microstructural features affected by Si content. The strength model suggests that after tempering at 650 °C, increased Si content primarily increases the hardness by solid solution strengthening. However, after tempering at 500 °C, increased Si contents result in higher dislocation densities in martensite and bainite. A recovery model based on solute drag was evaluated and the results indicate that increases in Si content can delay recovery and result in higher dislocation densities after tempering at 500 °C. Increases in Si content also lead to finer cementite sizes in bainite after tempering at 500 °C. A cementite coarsening model was evaluated, and the results indicate that cementite refinement in bainite tempered at 500 °C cannot be accounted for by Si diffusion controlled coarsening alone. The lower transformation temperature employed to form high Si bainite, indirectly influenced by Si decreasing the martensite start temperature, is suggested to result in refinement of both cementite and subgrain size after tempering at 500 °C. Microalloy precipitation is not significantly affected by Si content in the present study.