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, K-t, 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 H-2 gas mixture and the resulting carburized case depth was approximately 40 gm. As normally observed, the fatigue strengths of both the untreated and carburized specimens decreased with increasing K-t All carburized notched specimens showed higher fatigue strength than the untreated ones, and the extent of increase in fatigue strength decreased with increasing K-t and then saturated at high K-t values. In the carburized notched specimens with K-t = 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 K-t = 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. (c) 2005 Elsevier B.V. All rights reserved.