Magnetic Evaluation of Mechanical Properties of High Silicon Quenched and Tempered SteelJ N Mohapatra* D. Satish Kumar and G. Balachandran

Abstract A low carbon (0.17% C) TRIP assisted steel with 1.35%Si, 1.72%Mn, microalloyed with Nb and Ti was subjected to two different hardening and tempering heat treatments. One set of samples were quenched in water after austenitization at 920 oC to generate > 98% martensite while another set was austenitized in the inter critical temperature of 810 oC to generate about 50% ferrite along with > 48% martensite with retained austenite in both the sets, tempered in the temperature range between 150 and 650 oC. Samples hardened and tempered from 920 oC show a fully tempered martensite microstructure while the samples hardened from 810 oC, shows significant ferrite coexisting with tempered martensite. In both conditions, superior mechanical properties is observed when the tempering temperature is less than 300 oC, where the tensile toughness exceeded 20 GPa.%. The introduction of intercritical ferrite in tempered martensite enhanced the ductility of the steel significantly. High Si in the steel, shifts the tempered martensite embrittlement to higher temperatures and the Si suppresses the carbide formation and retains austenite laths, within the tempered martensite that enhances properties. Magnetic hysteresis loop measurements were carried out at each stage and magnetic coercivity was correlated with the tensile strength showed a strong correlation indicating precise non-destructive magnetic evaluation of tensile strength through a portable magnetic hysteresis loop tester.