Thermal Stability of Resistance to Propagation of Mechanically Small Fatigue-cracks in an Fe-N Binary Ferritic Steel

Abstract

We investigated the effect of solute nitrogen on threshold stress intensity factor range, ΔK_th, of the growth of small cracks using a water-quenched Fe-0.011N (wt.%) binary alloy, in terms of strain-age hardening. Fatigue tests were carried out for micro-notched specimens at 20°C and 160°C at a frequency of 30 Hz with a stress ratio of –1. The nitrogen effect on ΔK_th at room temperature was significant, but smaller than the carbon effect. On the other hand, the thermal stability of the strain aging effect on ΔK_th was higher in the Fe-0.011N steel than in Fe-C steels containing supersaturated carbon, because the nitrogen solubility above room temperature is higher than the carbon solubility in ferritic steels.