Processing and characterization of high strength dual-phase steel by two-step intercritical heat treatment process

Abstract

A simple approach of two-step intercritical heat treatment has been employed to study the effect of heat treatment on the evolution of microstructures and their effect on the mechanical properties of alloy steel (AISI 1012). The selected steel samples were directly placed in the preheated furnace and were progressively heat treated in two steps, intercritically between the Ac1–Ac3 temperature range. Immediate water quenching (preheated at 30 ℃) was carried out after heat treatment cycles. The processed steels were characterized by examining the X-ray diffraction patterns, microstructures, Vickers microhardness, and tensile strength. The normalized X-ray diffraction results of heat-treated steels revealed the substantial growth in the martenistic phases. The microstructures of heat-treated steel revealed the formation of needle-shape-like structures, which corresponds to the martenistic phase. The increased formation of martenistic phase due to the intercritical heat treatment process improved the overall microhardness (from 188 ± 9 HV of the parent steel to 412 ± 32 HV for 800 ℃ heat-treated steel) up to 2.2 times. The presence of soft and ductile (ferritic and pearlite) phases simultaneously with tough and strong (martenistic) phase allowed the improvement in the ultimate tensile strength. In comparison to parent steel with tensile strength of 510 ± 15 MPa, the intercritical heat treatment steel at 800 ℃ revealed 169.6% higher tensile strength of 1375 ± 35 MPa. However, percentage elongation was reduced by 60%, i.e. from 13 ± 1% for parent steel to 5.2 ± 2% of intercritical heat treatment steel (processed at 800 ℃). An overall study revealed that by a proper intercritical heat treatment process, dual-phase steels with better structure–properties correlation can be obtained for industrial applications.