The effects of austenitization temperature on the microstructural characteristics and mechanical properties of Cr–Mo–V hot work tool steels with different nitrogen contents

Abstract

Two kinds of Cr–Mo–V hot work tool steels with different nitrogen contents, i.e. High-N steel and Low-N steel, were heat treated to investigate phase transformation phenomena in the present study. Microstructure characterization revealed the existence of numerous particles within both steels, which could be further categorized as the following three types: large- (>150 nm), medium- (50–100 nm), and tiny-sized (<10 nm) particles. It should be noted that numerous nanometer-sized V (C, N) precipitates were randomly dispersed within the tempered martensite matrix. Fewer V (C, N) precipitates were found in the High-N steel than in the Low-N steel, which was consistent with the hardness test results. Microstructure analysis (EBSD) also revealed that the matrixes of both hot work tool steels consisted of primarily tempered martensite matrix with extremely low amounts of retained austenite, indicating that the retained austenite could be eliminated effectively by suitable tempering treatment, even in the High-N steel. On the other hand, from the impact toughness and hardness tests, it was found that a higher nitrogen content was beneficial in achieving higher impact toughness, especially at higher austenitizing temperatures, which could be ascribed to the smaller austenite grains in the High-N steel.