Article ID: TETSU-2023-092
The effect of boron addition on low cycle bending fatigue strength of carburized steel was investigated. The low-cycle bending fatigue strength of B-added steel was improved compared to SCM420. When the fatigue failure was divided into the crack initiation process and the crack propagation process, the addition of boron did not affect the crack initiation life, but improved the crack propagation life. The presence of boron in B-added steels was evaluated using TOF-SIMS and AES measurements. Grain boundary segregation of boron was not observed in the gas carburized surface layer, but precipitation of BN was observed. On the other hand, grain boundary segregation of boron was observed from the carburized surface layer toward the 400 μm core side. Therefore, it is considered that the grain boundary strengthening effect of boron was not obtained in the carburized surface layer, and the crack initiation life did not change.
In order to clarify the reason why the crack propagation life is improved by boron addition, the initial crack observation after crack initiation and the crack propagation test was carried out. From the observation of the pop-in crack, it was found that the pop-in crack length was shorter in the B-added steel than in the SCM420 steel. On the other hand, the crack propagation test showed no difference in crack propagation rate between SCM420 and B-added steel. Therefore, the decrease in pop-in cracks due to the addition of boron is considered to be the main factor for the improvement in crack propagation life.
