The effect of boron and nitrogen on the low cycle fatigue and creep-fatigue behaviour for 316L stainless steel was investigated at 873K with a strain rate of $4 \times 10^{-3}$/sec in air atmosphere. Creep-fatigue tests were performed by applying 30 min hold times at the maximum strain in tension.
Under low cycle fatigue test, 316LN(i.e. N-alloyed steel) had longer fatigue lives than 316L+B(i.e. B-alloyed steel). From the result of the observation of microstructures after fatigue tests, there was little difference for fatigue behaviour between two alloys. So it can be concluded that because 316LN has higher ductility 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, the result was completely reversed compared with the result of low cycle fatigue, that is to say, 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 modified life prediction model which is based on the cavitational damage under creep-fatigue condition, the major damaging mechanism can be said the cavitation for the two alloys. In addition, from observation of fractured surface and P' value which is regarded as new cavity nucleation factor, it can be known that 316L+B has lower grain boundary carbide density than 316LN, which has been known to provide the beneficial site for the cavity nucleation. Therefore, the reason why 316L+B has longer creep-fatigue lives than 316LN can be explained that as the addition of boron retards significantly carbide nucleation and causes to be lower dislocation density than nitrogen near the grain boudary, 316L+B has lower grain boundary carbide density, which results in lowering cavity nucleation.
Finally, it can be said that the proper addition of boron and nitrogen is beneficial for structural materials which are subjected to complex loading such as creep-fatigue at high temperature, because the addition of boron and nitrogen can decrease carbide density by inhibiting cavity nucleation.
