Effects of cold work and stress on oxidation and SCC behavior of stainless steels in PWR primary water environments
Fracture and Reliability Research Institute, Tohoku University
2 The Kansai Electric Power Co. Inc.
3 Mitsubishi Heavy Industries, Ltd. (Japan)
Recently intergranular stress corrosion cracking (IGSCC) in austenitic stainless steel 316L of Primary Loop Recirculation (PLR) piping and also a nozzle, and several core shrouds (CS) have been experienced in BWRs [1-4]. PWSCC incidences in nickel base alloys of Vessel Head Penetrations (VHP), Bottom Mount Instrumentation (BMI) nozzle and Steam Generator (SG) inlet nozzles in PWR were also reported [5-7]. Therefore, a great concern has been focused and an extensive research and development have been done on these SCC phenomena from a point of view of structural integrity of reactor components.
In addition to those SCC incidences in BWR and PWR, there is one unique SCC incidence in 316 stainless steel in PWR plants [8-9]. The mode of this cracking is intergranular and cracks were found in weld HAZ of 316 stainless steel welded to a low alloy steel of SG nozzle with nickel base alloy 82 where PWSCC were also observed. Preliminary root cause analysis implied that the cracked 316 HAZ has a highly hardened not only at surface by machining but also at the region of a far distant from an inner surface of 316 pipe, probably due to a weld shrinkage strain as have been observed in PLR piping and Core shroud cracks in BWR. In laboratory tests [10, 11], there are many well-known evidences that strain hardened austenitic stainless steels such as 304, 304L, 316, 316L and 347, show SCC susceptibility without any sensitization of grain boundary when they are hardened by any sources during fabrication or under an operational condition.
An extensive metallographic and TEM analysis has been performed on this cracked 316 HAZ sample as a part of failure analysis to characterize the microstructure and oxides morphology, compositional profiles as well as their crystal structures near the cracked surface of 316 HAZ [8, 9]. Typical strain hardening was measured on the sample due to weld shrinkage and also due to surface grinding. In addition to these analyses, an extensive surface oxides analysis has been performed by Raman Spectroscopy (RS) on a break-opened IGSCC surfaces and a cross-section sample where oxides can be observed in the cracks of concerns. In order to identify the oxides by RS, an extensive laboratory analysis of oxides formed in well-known environmental conditions in the laboratory was performed to generate a set of reference database of oxides by Raman spectroscopy. Based upon the observation of these oxides on/in the cracks, the crack growth history during the plant operation will be discussed in connection to a residual stress distribution at HAZ and oxides variation on/in the cracks. Possible time dependence of crack growth rate with crack growth in components will be discussed based upon the evidences observed about oxides. Finally, an importance of surface integrity assessment in SCC initiation and propagation will be emphasized from a point of view of oxidation localization which can be promoted by strain (dislocation density), straining and stress, which play an crucial role in an oxidation due to accelerated mass transfer in oxides as well as underlying metallic materials. Especially, preferential oxidation along slip bands suggests that oxygen diffusion in such a region with a high dislocation density is faster than the other region. This fact implies that grain boundary can also be a preferential path of oxidation as has been observed by TEM, TOFSIMS and 3D-APT. This localization of oxidation and acceleration will be discussed based upon an analysis of an profile development at an oxide/metal interface which is recently proposed by Y. Takeda et al.  based upon some statistical analysis of an oxide/metal interface profile and a concept of surface integrity will be emphasized for an quantitative prediction of SCC initiation.
© SFEN 2011