Dissolution and precipitation behaviors of boron bearing phase and their effects on hardenability and toughness of 25CrMoNbB steel

In order to improve the hardenability and toughness of train axles for high-speed railway, Nb (0.04 wt%) and B (20 ppm and 36 ppm) were added in 25CrMo steel. The dissolution and the precipitation behaviors of the B bearing phase in the different austenitizing processes and their effects on hardenability and toughness have been studied to clarify the influence mechanisms. Thermo-Calc was used to calculate the precipitation temperature, the quantity and the composition of the B bearing phase; SEM, TEM and EDS were used to investigate the morphology and the type of the B bearing phase; SIMS was used to analyze the distribution of B in steel. The results showed that the B bearing precipitates were found in the forms of different phases such as BN and M2B mainly containing Fe, Mo, Cr and B elements due to high Cr and Mo contents in steels. The excessive B atoms segregating at the austenite grain boundaries promote M2B precipitation not only during austenitizing to deteriorate the hardenability significantly, but also during tempering to deteriorate toughness obviously. Therefore, the B content in steels should be controlled below 20 ppm, and at the same time, the austenitizing temperature and the holding time should also be optimized to avoid the adverse effect of M2B on hardenability and impact toughness. In addition, the hardenability and the impact toughness can be improved by a double quenching process in which both the austenitizing temperatures are low.