Status of Research on Environmentally Assisted Cracking in LWR Pressure Vessel Steels

This paper reviews collaborative work that has the objective of defining, from first principles, the environmentally assisted crack growth rates in the Type A533B or A508 low-alloy steel/water system at 288°C under static and cyclic loading conditions. These theoretical rates are then used to assess the validity of the current ASME XI life evaluation code. The investigations, which were conducted by members of the International Cyclic Crack Growth Rate Group, have centered around (a) defining a working hypothesis for environmentally assisted cracking, (b) determining the nature and magnitude of crack tip environments and reaction rates that are pertinent to the crack advance hypotheses, (c) quantitatively validating a hypothesis by comparing observed and theoretical values, and (d) using the qualified mechanism to evalute the validity of current life-evaluation codes for environmentally assisted crack propagation. It is concluded that, on the basis of the bulk of present data, the slip dissolution (film rupture) model seems to be quantitatively the most valid crack advance mechanism for this system at 288°C. However, under certain system conditions, it is possible that an additional advance component due to environmentally assisted cleavage may become significant. Regardless of these nuances, however, it is apparent that the current ASME XI code is probably conservative for extended cyclic loading conditions, and that a time-based (rather than a cyclic-base) code would give a more realistic assessment of the structural integrity for the expected range of load/time histories in pressure vessels.