Investigation on the Interrelationship between the Chemical Composition, Heat Treatment Parameters and the Phase Transformation Process, Microstructure Evolution and the Mechanical Properties of Austempered Steel

The interrelationship between chemical composition, heat treatment parameters, and phase transformation, microstructural evolution and the mechanical properties of austempered steel was studied. Two samples of steel with different percentage composition of carbon of 0.56 and 0.76 were used for the study. They were austentized at the respective temperatures of 800⁰C, 840⁰C, 900⁰C and 960⁰C for 30 minutes. They were thereafter quenched using bitumen-palm kernel oil, and subjected to austempering isothermal heat treatment at 420⁰Cfor different time durations of 5, 15, 30, 45 and 60 minutes. The samples were tested for tensile strength, elongation, hardness and impact strength. They were also subjected to microstructural characterization to determine the phases in the microstructures and their effects on the properties of the developed materials. Results obtained revealed that the dominant phases in the microstructure were bainite, matensite and traces of retained austenite. It was found that decreasing austenitizing temperature yields finer grain structures with increase in tensile strength and elongation with decrease in hardness and relatively little effect on the impact strength. At any given austenitizing temperature, shorter austempering holding time yielded optimum properties in tensile strength and elongation while higher hardness values were associated with shorter holding time. These results proved that the process conditions have strong correlation with both the microstructures and the mechanical properties. It was concluded that the most promising microstructures with respect to excellent strength-ductility property are those obtained at the ausenitizing temperature range of 800 -840⁰C for the austempering time range of 5 – 30 minutes. These materials have potential for load bearing application while those austenitized within the range of 900 – 960⁰C using austempering time range of 5 – 15 minutes are candidate material for wear resistant application.

Keywords: Austempering, Austenitize, Composition, Heat treatment, Microstructure, Properties