The influence of nitrogen on the low cycle fatigue and creep-fatigue properties for Type 304L stainless steel was investigated with a strain rate of $4\times10^-3/\sec$ in air atmosphere. Low cycle fatigue tests were conducted at temperature of 298K, 573K, 673K, 773K and 873K. And creep-fatigue tests were also performed by applying 20 min hold times at the maximum strain in tension at 873K.
Under low cycle fatigue test 304NG(i.e. N-alloyed steel) had shorter fatigue lives than 304L at 873K and at the rest of temperatures two alloys showed almost similar fatigue resistance. 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 304NG has lower ductility than 304L due to the nitrogen addition, 304NG shows lower fatigue resistance than 304L at 873K.
When the creep deformation was introduced by imposing tensile hold time, result was completely reserved compared with the result of low cycle fatigue at 873K, that is to say 304NG had longer creep-fatigue lives than 304L. From the result of microstructure observed and the fact that the experimental life is in good agreement the predicted life from 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 two alloys, and also from observation of fractured surface and P' value which is regarded as new cavity nucleation factor, it can be known 304NG has lower grain bounary carbide density than 304L that has been known to provide beneficial site for cavity nucleartion. So the reason why 304NG has longer creep-fatigue lives than 304L can be explained that as 304NG has a smaller content of carbon and the nitrogen retards carbide nucleation at grain boundary by inhibiting diffusion of Cr to grain boundary, 304NG has lower grain boundary carbide density, which results in lowering cavity nucleation.
Finally it can be said that the addition of nitrogen is beneficial for real structural materials which are subjected to complex loading such as creep-fatigue, because the addition of nitrogen can decrease carbide density by lowering carbon content and nitrogen is believed to inhibit cavity nucleation.
