Effect of phosphorus on the high temperature low cycle fatigue behavior in AISI 304L stainless steel was studied in three different heats. The strain wave form was a symmetrical triangular shape of tension and compression, the strain rate was $4\times10^{-3}/s$ or $4\times10^{-5}/s$. For the investigation of the phenomenon of creep-fatigue interaction, 30min hold at tensile peak strain was applied.
The microstructural observation was very distinctive. As the content of phosphorus increases, the density of grain boundary carbide decreases. And intragranular precipitates which are considered phosphide are observed at 0.20%-P specimen.
From the results of continuous fatigue test for three different heats, the effect of phosphorus on fatigue behavior is proven to be detrimental due to degradation of plastic ductility.
On the other hand, the results of creep-fatigue test for the three materials show that the effect of phosphorus is benificial for the life. Under superimposed creep-fatigue loading conditions, it is generally accepted that cavity nucleation and growth during hold time degrades the fatigue endurance and cavities are closely related to the grain boundary carbides. As a result, it can be considered that the addition of phosphorus lowers the nucleation site of cavity mainly by decreasing the number of grain boundary carbides. Otherwies, the effect of phosphorus segregation at the grain boundary which is decreased the growth rate of cavity was relatively small.
For 304L stainless steel, decreasing the strain rate the fatigue life was decreased markedly under continuous cycling. Due to oxidation and material flow effect the crack propagated transgranular at $4\times10^{-5}/s$. As it is reported very small cavities were observed, but cavitation damage did not affect to crack propagation.
