EAC Crack Initiation in Ni-based Dissimilar metal Welds using Doped Steam Test

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 localization is not widely familiar as a precursor for failure and its mechanisms are still not fully known. If strain localization 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 localization investigations are also performed on specimens from Super Slow Strain Rate Test 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 on sensitized Type 304 austenitic stainless steel either with or without cold deformation in simulated BWR environment. To increase the understanding of IGSCC and the role of localization of deformation, versatile techniques like FE-SEM/EBSD and TEM were used to characterize the materials after the extremely slow strain rate tests. Local variation in the amount of surface cold-work seems to affect crack initiation. During crack growth, strain localization occurs at grain boundaries ahead of the crack tips. Local variations of grain size also affect strain localization