Revised creep fracture maps of Type 316 stainless steel and their mechanistic perspective

Abstract

Creep fracture maps have been studied for Type 316 stainless steel as a common material for high-temperature structural applications. Creep failure metrics relevant to design (e.g. creep ductility) are sensitive to the creep failure mode. Current fracture maps are often deficient in accurately capturing the transition between failure mechanisms. This study rederives the creep fracture maps for Type 316 by capturing the sensitivity of fracture mechanisms to the microstructural state, its characteristic deformation response and associated microstructural features. Diffusive and dislocation creep rate laws are coupled to mechanistic models for the key fracture mechanisms – wedge cracking, intergranular cavitation and transgranular fracture. The mechanistic failure models embed assumptions supported by experimental findings. Examples include intergranular cavitation considering nucleation at grain-boundary particles and ledges; grain boundary sliding rates sensitive to the interface character, and transgranular void growth at intragranular second phases. A mechanistic model for transgranular fracture is derived which describes growth of transgranular voids in the presence or absence of particles. The modelling framework is exercised to predict failure times tf for any given applied stress (σ) and creep fracture mechanism regimes for annealed Type 316 at 500–900 °C. The models predict correctly fracture mechanisms ranges and trends in tf-σ as compared to an experimental database. Comprehensive examples supported by data are provided on the sensitivity of creep fracture to the presence of populations of embrittling phases affecting intergranular cavitation, and the shift between transgranular (ductile) and wedge cracking (brittle) fracture in the presence of impurities. The importance of the findings in understanding the underlying creep fracture mechanisms are discussed in relation to failure time extrapolations, creep-fatigue failure mechanisms and the link between creep failure mechanisms and creep crack growth.