Effect of Molybdenum and Chromium on Hardenability of Low-Carbon Boron-Added Steels

The hardenability of low-carbon boron-added steels containing molybdenum or chromium was studied using dilatometry, thermodynamic calculations, and secondary ion mass spectroscopy (SIMS). The combined addition of boron and molybdenum was found to be more effective than that of boron and chromium in enhancing the hardenability of boron-added steels. In particular, the addition of 0.5 wt.% molybdenum to the boron-added steel almost completely suppressed the formation of polygonal ferrite even at a slow cooling rate of 0.5 degrees C/s. The synergistic effect of the combined addition of molybdenum and boron is thought to be due to both the suppression of M-23(C,B)(6) precipitation resulting from the deterioration of phase stability and the reduction of carbon diffusivity by the molybdenum addition.