Influence of two-step austempering at different temperatures on mechanical and microstructural properties of AISI 9254 high silicon steel

Abstract

This study was undertaken to understand the effects of two-step austempering treatment on an AISI 9254 high silicon steel towards tailoring the properties as desired while simultaneously employing the benefits of high and low austempering temperatures. The samples were initially austenitized at 850°C for 20 min, followed by austempering in a salt bath at the temperatures of 250–270–290°C for 20 min during the first stage. Subsequently, a second stage austempering was carried out by raising the temperature of the salt bath to 300°C at an average heating rate of 0.5°C/min, and the samples were kept in the salt bath for achieving a total austempering time of 120 min including the heating time. A conventional single-stage austempering was also conducted for comparison purposes, in which the austenitization temperature, the austempering temperatures and total time (stage I and stage II, i.e. 120 min) were kept the same for the benchmark samples. In the characterization studies, tensile test, hardness test, XRD analysis, optical microscope and field emission-scanning electron microscope (FE-SEM) equipped with EBSD detector were utilized. The findings of this study indicated that lowering the austempering temperature resulted in refining the structure with a decrease in the amount of austenite. According to the carbon content analysis through XRD patterns, the two-step austempering processes appeared to have considerably increased the carbon content of the austenite irrespective of austempering temperature. The best ultimate tensile strength (U.T.S) of 2194 MPa was achieved in the conventionally austempered sample at the lowest temperature of 250°C, while the best yield strength (Y.S.) of 1753 MPa was reached in the stepped austempered sample initially at 250°C followed by 300°C. In general, two-step austempering process led to a higher yield strength while affecting the ultimate tensile strength and total elongation depending on the austempering temperature.