Initiation and propagation of cracks under simulated primary water conditions and different slow strain rates have been studied for an austenitic 304-type stainless steel. Two surface finishes were used to better understand the conditions that trigger stress corrosion cracking (SCC). The main objective is to identify the mechanism(s) that govern the initiation and propagation of SCC and the influence of microstructure. Crack morphology, stress localisation and local chemical composition were characterized for all samples studied. The characterization methodology includes scanning electron microscopy (SEM), 3D focused ion beam (FIB), Transmission Kikuchi Diffraction (TKD), and analytical scanning transmission electron microscopy (STEM).
Slow strain rate tensile (SSRT) tests were conducted on conventional and tapered samples manufactured from forged Type 304L stainless steel to assess the stress corrosion cracking (SCC) behaviour in simulated PWR primary water. Several testing and microstructural parameters were investigated in order to explore the conditions under which crack initiation might occur. Surface preparation appeared to play a very important role on SCC initiation whereby the machined surfaces were the least susceptible to SCC initiation whilst oxide polishing suspension (OPS) polished surfaces were more susceptible. On the machined surfaces the cracks were always transgranular (TG) in nature and associated with the machining marks. Conversely, on fine polished surfaces with oxide polishing suspension the crack morphology was mainly intergranular in nature, although minor transgranular cracking was observed. The regions in the proximity of the δ-ferrite/austenite interface were shown to be very susceptible to SCC initiation especially on the OPS polished surfaces and this was attributed to the strain localization upon dynamic deformation. Furthermore, intragranular inclusions appeared to dissolve and act as initiation sites for transgranular cracking to occur. The roles of strain rate, dynamic deformation and microstructure on the initiation of SCC are also discussed.
