Influence of single- and multi-stage austempering treatments on microstructure and mechanical properties of AISI 4140 steel

Abstract

The present study examined the impact of single- and multi-stage austempering treatments on microstructural changes and the mechanical properties of AISI 4140 steel. All specimens underwent an annealing treatment before undergoing austempering treatments. The single-stage austempering treatment employed three distinct temperatures: 312, 362 and 412 °C. The multi-austempering employed three steps of austempering treatment, commencing at 312 °C and progressing in increments of 50 °C for an hour each, up to a maximum temperature of 412 °C. The mechanical properties were determined using hardness, impact and tensile testing. Furthermore, the microstructure alterations and phases identified in heat-treated specimens were characterised using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS) and X-ray diffraction (XRD) patterns. The mechanical properties of AISI 4140 steel subjected to single-stage and multi-stage austempering treatments exhibited a considerable enhancement compared to the annealed steel. The microstructure of annealed steel is predominantly coarse ferrite and pearlite. The austempering treatment forms a bainitic structure, retained austenite structures and a small number of martensitic structures within the steel's microstructure. In other words, the mechanical properties of AISI 4140 steel concerning austempering treatments are correlated with the microstructural changes resulting from different austempering temperatures.