| Abstract Scope |
Addition of Ni in medium-Mn steels enhances austenite stability due to two reasons (i) Unlike Mn, Ni doesn’t form carbides (ii) Ni opens the austenite phase-field. However, addition of Ni increases overall content of austenite stabilisers too. In the present work, three different compositions Fe-(6, 5, 4)Mn-(0, 1, 2)Ni-3Al-1Si-0.2C have been processed through casting → hot-forging → hot-rolling → heat-treatment (HT) route while keeping (Mn + Ni) content constant, i.e., 6Mn0Ni, 5Mn1Ni & 4Mn2Ni. Although the process parameters have been kept same, fraction of retained austenite vary significantly after HT. Alloy containing 6Mn0Ni provides more amount of retained austenite (> 10 volume% difference in case of 750 °C & 800 °C HT for 10 min) as compared to 5Mn1Ni and 4Mn2Ni compositions at all HT temperatures. Mechanical properties of HT processed steels at 750 °C for 2, 10, 30 and 60 minutes have been investigated by uniaxial tensile test. Product of strength and total elongation is more than 50GPa% in case of 6Mn0Ni and 4Mn2Ni alloys. Whereas, 5Mn1Ni shows reduced values at around 45GPa%. Hence, influence of Ni on the kinetics of austenite reversion and its chemical stability have been understood through diffusion simulation. Subsequently, its effect on the mechanical stability of austenite have been quantified. To comprehend the mechanical stability of austenite interrupted tensile tests have been conducted and advanced characterisation techniques are performed to explain the underlying mechanisms. |