New insights from crystallography into the effect of refining prior austenite grain size on transformation phenomenon and consequent mechanical properties of ultra-high strength low alloy steel

Abstract

Based on new insights from crystallography, this study aims to establish the relationship between prior austenite grain size and mechanical properties and enhance our understanding of Hall-Petch relationship. The refinement of prior austenite grains was achieved by decreasing the austenitizing temperature (from 920 ℃ to 880 ℃) and quenching. In addition, samples subjected to 880 ℃ heat treatment and quenching produced a significantly higher percentage of martensite. Electron backscattered diffraction (EBSD) used to characterize the crystallographic characteristics indicated that the steel subjected to 920 ℃ heat treatment and quenched had larger prior austenite grains, belonging to the transformation of Bain group. After 880 ℃ heat treatment and quenching, the prior austenite grains were smaller and more uniform, which belonged to the transformation dominated by CP (close-packed plane) group. The transformation from Bain group to CP group was related to transformation driving force, and resulted in increase in the density of high angle grain boundaries (DHAGBs). Using thermal expansion approach to measure the initial martensite transformation temperature (Ms temperature), the samples heat treated and quenched at 920 ℃ and 880 ℃ showed Ms temperature of 400 ℃ and 427 ℃, respectively, implying that the phase transformation driving force was increased by refining the prior austenite grain. Charpy impact energy test at −40 ℃ suggested that after 880 ℃ heat treatment and quenching, the Charpy energy increased from 46 J to 92 J, consistent with the results of EBSD.