Effect of Molybdenum Contents on Hardenability and Precipitation Behaviors in B-added Steels Austenitized at High Temperature

The effect of molybdenum (Mo) contents on hardenability and precipitation behaviors in Mo-B simultaneously added steels were investigated placing a focus on high austenitizing temperature. The hardenability of 0.5 Mo - 11 ppm B steel austenitized at 1150 degrees C was decreased compared with that austenitized at 950 degrees C, whereas 1.0 % Mo - 10 ppm B and 1.5 Mo - 9 ppm B steels were less affected by high austenitizing temperature than 0.5 % Mo - 11 ppm B steel. The Fe-23(C, B)(6) precipitation by increasing austenitizing temperature was also revealed to be suppressed in 1.5 Mo - 9 ppm B steel. These results indicate that the improved effect of the Mo addition on hardenability by retarding the precipitation of Fe-23(C, B)(6) still appear in B-added steels austenitized at high temperature. Furthermore, Fe-23(C, B)6 precipitation start temperature was increased in Mo-B added steels austenitized at 1150 degrees C. This result implies that non-equilibrium B segregation mechanism during cooling from high austenitizing temperature enhances the amount of segregated B on grain boundaries leading to the promotion of the borides precipitation at high temperature austenitizing region. However, Mo is presumed to fix a part of thermal vacancies as Mo-V complex resulting in the suppression of non-equilibrium B segregation to grain boundaries during cooling, which is speculated to inhibit the Fe-23(C, B)(6) precipitation. Thus, the effect of Mo-B combined addition on hardenability was presumably maintained even in high austenitizing temperature region.