Effect of Molybdenum Content on the Combined Effect of Boron and Molybdenum on Hardenability of Low-Carbon Boron-Added Steels

Abstract

Addition of Mo to boron-alloyed steel improves the hardenability by suppressing the precipitation of Fe₂₃(C, B)₆; this is known as the combined Mo-B effect. However, the maximum Mo content for the combined effect to occur is still unclear because previous studies on this effect mainly investigated steels with a Mo content of less than 0.80%. Therefore, in this study, 0.15% C steels containing more than 0.80% Mo were investigated to determine the maximum content required for the occurrence of the combined Mo-B effect. The combined effect increased with increasing Mo content up to 0.75%, after which it decreased. The optimum B content decreased from 12 to 11 ppm with increasing Mo content from 1.0% to 1.5%. In 1.0%Mo-20 ppm B steel and 1.5%Mo-20 ppm B steel, Mo₂FeB₂ precipitated instead of Fe₂₃(C, B)₆. Thermodynamic calculations revealed that the temperature at which Mo₂FeB₂ precipitation started increased with increasing Mo content in 20 ppm B steel. Moreover, Mo₂FeB₂ could precipitate even at a reheating temperature of 950°C. Thus, it is suggested that the maximum Mo content for the combined Mo–B effect on hardenability is determined by the precipitation of Mo₂FeB₂ mainly during reheating.