SCC of cold-worked austenitic stainless steels in PWR conditions

Abstract

Solution annealed austenitic stainless steels, essentially of type AISI 304(L) and AISI 316(L), were originally considered not susceptible to cracking by SCC in the reducing environment of PWRs primary water. However, Constant Extension Rate Tests (CERTs), performed on heavily cold worked austenitic stainless steels specimens have demonstrated that these materials can exhibit clear SCC cracking susceptibility with fast intergranular or transgranular crack growth. Careful examination of the austenitic stainless steels susceptibility conditions to cracking have shown that work hardening under restricted conditions can promote SCC in PWR conditions. The respective influence of various cold working modes (shot peening, cold rolling, fatigue work hardening, milling and tensile deformation) is investigated by means of screening CERTs performed essentially on 304L in simulated PWRs primary water at 320°C and 360°C. The effect of dissolved hydrogen level on the materials susceptibility to cracking is examined via dedicated tests at 320°C. For a given cold work hardening level, the susceptibility to crack initiation strongly depends on the cold working process and no propagation is observed for a hardness level lower than 300 10 HV(0.49N). The propagation of cracks appears to be strongly connected with dynamic loading conditions associated with CERT, tension tests and crack growth propagation tests performed on CT specimens under cyclic loading. Indeed, although shallow crack initiation is observed for constant load, crack propagation does not occur under strict static loading conditions during a long term 17,000 hours SCC test, even for initial material hardness levels higher than 450 HV(0.49N). On the basis of existing references, the presence of significant levels of atomic hydrogen inside coldworked austenitic stainless steels as a consequence of long duration exposure to reducing PWRs conditions can be assumed. The role of interstitial hydrogen on the process of cracking is questioned. Dedicated experiments carried out on 304L stainless steel have shown that atomic hydrogen present inside cold-worked austenitic stainless steels can under some conditions promote occurrence of limited H-induced embrittlement cracking phenomena in the low temperature range (tests performed at ambient temperature and 140°C). This process should be considered in the context of the evaluation of crack initiation susceptibility of 304L austenitic stainless steel exposed to primary water environment. Introduction Susceptibility of sensitized austenitic stainless steels to SCC is a well known phenomenon identified in BWR oxidizing environments. Long term research carried out in the case of BWR-type conditions have shown certain minimum levels of chloride and oxygen are required for SCC susceptibility [1 4]. The detrimental role of cold-work on the resistance of austenitic stainless steels to SCC was also clearly recognized in pure water BWR-type conditions in the presence of limited amounts of oxygen [5 9]. Cold-work was generally identified to increase Crack Growth Rates (CGR) and reduce the estimated crack initiation time for austenitic stainless steels tested in pure water containing oxygen at a temperature around 300°C. The electrochemical potential usually measured for austenitic stainless steels exposed to hydrogenated PWR conditions appears well below the critical potential recognized for SCC initiation and too low to promote SCC susceptibility of these materials in absence of a significant amount of specific impurities. Consequently, solution annealed austenitic stainless steels, essentially of type AISI 304(L) and AISI 316(L), are generally considered as immune to SCC in hydrogenated primary water and these materials are thus widely used in PWRs. Some rather recent data [10, 11] have however demonstrated that austenitic stainless steels specimens that were severely cold-worked (samples incorporated a cold deformed hump) were susceptible to SCC in hydrogenated PWR environment under CERTs conditions. Crack propagation was similarly observed on pre-strained, unsensitized austenitic stainless steels in pure water containing hydrogen despite the low electrochemical potential measured during the experiments [8]. The resistance of austenitic stainless steels to SCC in PWR conditions should be therefore re-evaluated with regard to the potential specific effects of cold-work. The objective of this paper is to detail the results obtained in the framework of the CEA-EDF research program dealing with SCC of cold-worked austenitic stainless steels. The main focus of this program is to assess SCC initiation susceptibility of steels considering the respective effects of cold-working mode and environmental parameters derived from standard PWR conditions. Experimental procedure