Effect of Mo on γ to α transformation and precipitation behavior in B-added steel

Abstract

Effect of the combined addition of molybdenum (Mo) and boron (B) on austenite (γ) to ferrite (α) transformation and precipitation behavior were investigated using low-alloy steels. B-added steel and Mo-B combined steel were held at 923 K (γ region) in order to precipitate boride. B content as precipitates increased and γ to α transformation was promoted with holding time at 923 K. In B-added steel, both M 23 (C,B) 6 and M 2 B were observed. The transition from M 23 (C,B) 6 to M 2 B caused by the increase in holding time at 923 K. By contrast, in Mo-B combined steel, no M 2 B was observed regardless of the holding time. Mo addition suppresses not only the M 23 (C,B) 6 formation but also the M 2 B formation. M 2 B contains larger amounts of B than M 23 (C,B) 6. B content as precipitates in Mo-B combined steel was much lower than that in B-added steel due to the suppression of M 2 B precipitates. The effect of Mo for B containing steel suppresses the precipitation of M 23 (C,B) 6 and M 2 B and increases more segregated B in austenite grain boundary that contributes to γ to α transformation.