The role of ferrite in Type 316H austenitic stainless steels on the susceptibility to creep cavitation

An ex-service Type 316H stainless steel which was subsequently aged at 500 °C for ~22×103 h was found to contain approximately 2% mixed (δ and α) ferrite distributed in localised regions of the microstructure. Preferred creep cavitation at boundaries was associated with these ferrite regions. Creep cavities associated with the austenite−austenite−ferrite boundary junctions, showed a lenticular morphology while austenite−austenite grain boundary creep cavities had a more spherical morphology. Details of the microstructure of these localised regions are described together with the creep cavity distribution. It has been established the extended heat treatment post-service did not result in any significant sintering of creep induced cavities. The results are discussed with respect to the contribution of the ferrite on the nucleation and growth of the creep cavities and the influence on the overall creep behaviour. An evaluation of the impact of low volume fractions of widely dispersed ferrite on the creep life-time of Type 316H stainless steel is discussed.