Study of microstructure uniformity and impact toughness enhancement mechanism of G13Cr4Mo4Ni4V bearing steel

Various heat treatments were adopted to control the microstructure in G13Cr4Mo4Ni4V bearing steel (termed as "Common", 1LTT, 1200 and 1200 (1LTT)). The distribution of alloy elements and the homogenization behavior of carbides and ferrites were studied through optical microscope, scanning electron microscopy, electron probe micro-analysis and first principles methods. The mechanisms of microstructure evolution and impact energy enhancement were explored during the heat treatments. The results showed that more complex homogenization treatments were necessary to improve the homogenous of the microstructure for steel with poorer uniformity. The 1200 (1LTT) homogenization treatment effectively reduced the content of ferrites, homogenized the microstructure and improved the impact energy from 16 J to 224 J for steel I with the poorest uniformity. 1LTT treatment was employed to stabilize the impact energy of higher homogeneous steel II. The steel III with the best uniformity can obtain high impact energy (271.25 J) using the "Common" treatment. G13Cr4Mo4Ni4V steel exhibited light and dark areas after corrosion due to the existence of ferrites and uneven distribution of carbon element. The light areas contained a large number of ferrites which are mainly rich in Mo, V, Cr and poor in C and Ni. Steel I with the content of ferrite higher than 2.84% (adopting "Common" or 1LTT treatment) can achieve high impact property using 1200 (1LTT) treatment by reducing the ferrite. The difference in deformation ability between ferrites and matrix and the aggregated carbides are the main reasons for the decrease in impact property.