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 M23(C,B)6 and M2B were observed. The transition from M23(C,B)6 to M2B caused by the increase in holding time at 923 K. By contrast, in Mo-B combined steel, no M2B was observed regardless of the holding time. Mo addition suppresses not only the M23(C,B)6 formation but also the M2B formation. M2B contains larger amounts of B than M23(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 M2B precipitates. The effect of Mo for B containing steel suppresses the precipitation of M23(C,B)6 and M2B and increases more segregated B in austenite grain boundary that contributes to γ to α transformation.