Effect of carburizing on notch fatigue behaviour in AISI 316 austenitic stainless steel

The present paper describes the fatigue behaviour of carburized notched specimens in AISI 316 austenitic stainless steel. Cantilever-type rotary bending fatigue tests have been performed using carburized notched specimens with three different stress concentration factors, Kt, of 2.08, 3.55 and 6.50 and the effects of carburizing on fatigue strength and notch sensitivity were discussed. Carburizing was performed at a temperature below 773 K for 35 h in a CO and H2 gas mixture and the resulting carburized case depth was approximately 40 μm. As normally observed, the fatigue strengths of both the untreated and carburized specimens decreased with increasing Kt. All carburized notched specimens showed higher fatigue strength than the untreated ones, and the extent of increase in fatigue strength decreased with increasing Kt and then saturated at high Kt values. In the carburized notched specimens with Kt = 2.08 and 3.55, fatigue cracks initiated at the surface of the notch root when applied stress was high, but underneath the carburized case (subsurface) when applied stress was low. On the contrary, the carburized notched specimens with Kt = 6.50 showed surface crack initiation at the notch root regardless of applied stress level. Furthermore, it was indicated that both the untreated and carburized specimens had significantly low notch sensitivity, with a slight increase by carburizing.