|DINESH Kumar||Indian Institute of technology Kharagpur|
Increasing demand for automotive vehicles with reduced weight, improved crashworthiness and passengers safety has draw attention of researchers in the field of new Twinning Induced Plasticity (TWIP) steels. This work evaluates the effect of grain size on the formation of ε martensite in forged steel, annealed at different temperatures for 10 min than quenching in Ar medium. Test has been performed on two type dilatometer samples-one solid samples and other hollow samples. During quenching of solid samples only ε martensite is found to be formed but in case of hollow samples α΄ martensite is also appeared. Due to cooling rate variation, fraction of martensite varies with temperatures. Microstructure analysis has been done through optical microscopy, SEM and TEM. TEM analysis and SAD pattern clearly shows the presence of ε and α΄ martensite in steel. XRD analysis has been done for identification of martensite phases and phase fraction measurement via direct comparison method. Grain size has major effect on the formation of martensie phases as the grain size increases fraction increases. Further tensile testing of these quenched tensile specimens shows the formation of α΄ martensite along with ε- martensite. It was found that mechanical property getting poorer with increase in the fraction of α΄ martensite during tensile testing. Tensile properties of annealed specimen at 1000 °C found to be better than comprasion with other samples annealed at 800 °C and 1200 °C.