Two‐Stage Isothermal Transformation of Austenite into Nanobainite in a Chemically Inhomogeneous Medium‐Alloy Steel Containing 0.5 wt% C

The effect of chemical segregation on the microstructure of isothermally heat‐treated nanobainitic steel containing 0.5C‐2.0Mn‐1.6Si‐1.6Cr‐0.55Mo (wt%) is investigated using dilatometry, light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy. The characteristic temperatures Ac 1 , Ac 3 , and M S are determined for the experimental steel and a part of the time–temperature–transformation diagram is developed within the temperature range of 205–255 °C using various cooling rates of dilatometric specimens from the austenitization temperature. The Si, Mo, Cr, and Mn contents in the segregation zones are analyzed using energy‐dispersive spectroscopy (EDS)‐SEM. The average difference in M S values between zones depleted and enriched in alloying elements is ≈25 °C, whereas the maximum observed gap between the highest M S in the depleted zones and the lowest M S in the enriched zones reached 90 °C. The relationship between the microstructure and mechanical properties is analyzed, and heat‐treatment parameters that limit the negative impact of chemical segregation on the strength‐to‐plasticity ratio are proposed. A two‐stage isothermal heat treatment at temperatures of 235 and 225 °C produced a favorable combination of mechanical properties of the investigated steel: tensile strength of 2.0 GPa, yield strength of 1.3 GPa, and total tensile elongation of 14%.