| The effect of boron and nitrogen on the low cycle fatigue and creep-fatigue behavior of 316L stainless steel was investigated at 873K. Under low cycle continuous fatigue conditions, 316LN (i.e., nitrogen-alloyed steel) had longer fatigue lives than 316L+B (i.e., boron-alloyed steel). Because 316LN has higher ductility and lower strength at 873K than 316L+B due to the nitrogen addition, 316LN shows better fatigue resistance than 316L+B. When the creep deformation was introduced by imposing tensile hold time, 316L+B had longer creep-fatigue lives than 316LN. From the result of microstructure observation and the fact that the experimental life is in good agreement with the predicted life from the life prediction model which is based on the grain boundary cavitational damage under creep-fatigue condition, the major damaging mechanism is the cavitation for the two alloys. In addition, from observation of the fractured surface and the cavity nucleation factor (P`), which is regarded as a new material constant, 316L+B as a lower grain boundary carbide density than 316LN, which has been known to provide a beneficial site for cavity nucleation. Therefore, the reason for the longer creep-fatigue life of 316L+B than 316LN can be explained by the fact that as the addition of boron significantly retards carbide nucleation and growth near the grain boundary, 316L+B has a lower grain boundary carbide density, which results in a lower cavity nucleation. |
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