Influence of Material, Environment and Strain Rate on Environmentally Assisted Cracking of Austenitic Nuclear Materials (DEFSPEED): Deformation localisation and EAC in inhomogeneous microstructures of austenitic stainless steels

Inhomogeneous microstructures, e.g. grain size, dislocation density etc., always occur in welded structures. Varying manufacturing methods leading to complex strain paths result in highly varying cold work and consequent residual strains. The role of strain localisation is probably playing a key role as a precursor for crack initiation but its mechanisms are still not fully known. If strain localisation occurs by a creep mechanism, the incubation time for crack initiation can be very long, as frequently observed in NPPs. EBSD employed to measure strain distributions in a Type 304 austenitic stainless steel weld shows a high variation in residual strain distribution, which was verified by hardness measurements as well as with residual stress measurements. Strain localisation investigations were also performed on specimens from Super Slow Strain Rate Test (SSSRT) using a very slow strain rate of 1 o 10-8 s-1. This is in the creep strain rate range, where diffusion along dislocation cores and grain boundaries occur together with grain boundary sliding. SSSRT's were performed in simulated BWR environment on sensitized Type 304 and non-sensitised Type 316L austenitic stainless steel either with or without cold deformation. Local variation in the amount of surface cold-work affects crack initiation. Local variations of grain size also affect strain localisation. During crack growth, strain localisation occurs at grain boundaries ahead of the crack tips.