Effects of Mo addition and austenitizing temperature on hardenability of low alloy B-added steels

The hardenability change by an addition of B to 0.15%C-Ti added steel was investigated considering the effect of Mo in order to understand one of the peculiar characteristics that austenitizing at higher temperatures reduces the hardenability effect of B. Hardenability monotonically increases with increasing 8 content up to the optimum B content without the effect of Mo. The optimum 8 content increases with increasing Mo; 6 ppm, 9 ppm; 13 ppm for 0% Mo, 0.25% Mo, 0.50% Mo. The addition of Mo retards the precipitation of M-23(CB)(6) and thus more B in solution that contributes to hardenability can exist along grain boundaries by Mo. The optimum B content is reduced-with increasing temperatures because much more B concentrates along grain boundaries through non-equilibrium segregation mechanism during cooling from an elevated temperature and thus precipitation of M-23(CB)(6) easily occurs.