Isothermal Transformation of a CMnSi Steel Below the M_S Temperature

After applying an austenitizing heat treatment above A _c3 followed by quenching to a temperature below the martensite-start (M _S ) temperature, an isothermal transformation was observed by means of dilatation measurement in a low-alloyed, low-carbon steel. The precise nature of this isothermal transformation below the M _S temperature is still unclear. The present contribution is a comprehensive comparison of the main difference between the isothermal transformation below the M _S temperature and the athermal martensitic transformation using electron microscopy and internal friction measurements. The mechanical properties of the transformation product also were characterized. The observations revealed that the isothermal transformation product below the M _S temperature had its own characteristic microstructure with a Kurdjumov–Sachs orientation relationship with the parent austenite and without carbide precipitation.