Effect of temperature on high cycle fatigue behaviour in 18Cr–2Mo ferritic stainless steel

Fully reversed axial fatigue tests have been performed using smooth specimens of 18Cr–2Mo ferritic stainless steel (type 444) at ambient temperature, 673 K and 773 K in laboratory air, in order to understand the effect of temperature on high cycle fatigue behaviour. Fatigue strength significantly decreased with increasing temperature. When characterized in terms of fatigue ratio, fatigue strength still decreased at elevated temperatures compared with at ambient temperature. At all temperatures studied, cracks were generated at the specimen surface due to cyclic slip deformation, but crack initiation occurred much earlier at elevated temperatures than at ambient temperature. Subsequent small crack growth was considerably faster at elevated temperatures even though difference in elastic modulus was taken into account, indicating the decrease in the intrinsic crack growth resistance. Fractographic analysis revealed some brittle features in fracture surface near the crack initiation site at elevated temperatures, which was more pronounced and extensive at 773 K, indicating that embrittlement has occurred at the temperature range studied. It was believed that such embrittlement was responsible for the observed decrease in both crack initiation resistance and small crack growth resistance.