The Significance of Austenite/Ferrite/Martensite Ratio on Governing Impact Toughness of Fe-0.2C-6Mn-1.5Al Medium-Mn Transformation-Induced Plasticity Steel Studied by a Novel Heat Treatment

In this paper, the scientific challenges associated with obtaining high impact toughness in the medium-Mn TRIP steel, Fe-0.2C-6Mn-1.5Al, have been explored. This was enabled by a novel and effective heat treatment involving the combination of intercritical hardening and tempering to obtain high impact toughness. Electron microscopy and x-ray diffraction studies clearly underscored absence of TRIP effect in Fe-0.2C-6Mn-1.5Al medium manganese steel during impact and the volume fraction of austenite played a determining role in governing impact toughness. The highest impact toughness of 180.17 J/cm(2)was obtained when the steel was subjected to intercritical hardening temperature of 630 degrees C and low tempering temperature of 200 degrees C. The presence of martensite in the microstructure reduced the impact toughness on quenching from 670 to 700 degrees C. When the quenching temperature was increased to 750 degrees C, the impact toughness of steel was slightly increased, which is ascribed to the supersaturated carbon in martensite that precipitated as carbides. The fracture mode was ductile after intercritical hardening in the temperature range of 600-700 degrees C.