Review of Phosphorus Segregation and Intergranular Embrittlement in Reactor Pressure Vessel Steels

Abstract

This paper presents a systematic review of the behavior of phosphorus (P), highlighting the implications of P segregation to grain boundaries under neutron irradiation. The review focuses on Mn-Mo-Ni steels employed in US pressurized water reactors (PWRs), and other PWRs worldwide. Segregation of P to grain boundaries in reactor pressure vessel (RPV) steels can occur during fabrication (especially during the slow cooling stage of a post-weld heat treatment), and as a result of in-service exposure to high operating temperature and irradiation. This segregation of P to grain boundaries can promote a change in the brittle fracture mode from transgranular (TGF) to intergranular (IGF), and a degradation in the mechanical properties. In US RPV steels, most data are on thermal aging of the heat-affected zone (HAZ). Studies in coarse-grained HAZ have shown that the embrittlement arising from segregation of P to grain boundaries is approximately linearly related to the proportion of the brittle fracture that is IGF, and/or the P concentration at the grain boundary. Data are sparse on the effect of irradiation at 288°C on P segregation, and on the contribution of IGF to the total shift in the 41J transition temperature, T41J. In general, the bulk P content appears to be less than about 0.028 wt% P, with base metals having lower levels than weldments. In addition, the consequences of vessel annealing are considered at temperatures around 475°C. It is certain that the annealing treatment will have the consequence of reducing the irradiation hardening, but may significantly increase the grain boundary phosphorus coverage and the likelihood of IGF.

Current understanding of radiation-induced degradation in light water reactor structural materials

“…MD is divided into two types: stable MD (SMD) such as interstitial clusters and unstable MD (UMD) such as small vacancy clusters that may redissolve at irradiation temperatures [48], although the exact nature of MD has not been identified. Radiation-enhanced segregation at GBs and its influence on intergranular fracture were discussed in the 1990s [49]. Further improvement of analysis techniques such as local electrode atom probe and PA with coincidence Doppler broadening (PA-CDB) [50] and application of these techniques to surveillance specimens have brought detailed information on microstructural evolution.…”

“…Since intergranular fracture was found in an irradiated high-P C-Mn weld [112], the influence of irradiation on P segregation and GB embrittlement has been studied. Knowledge on the segregation behavior and its influence on embrittlement, obtained until the early 2000s, has been summarized in the literature [49,114], and new studies have increased understanding of the critical condition for embrittlement due to P segregation. Major points are summarized below.…”

“…P segregation at GBs during thermal ageing is treated as thermal equilibrium segregation. Model estimation shows that the P concentration at GBs remains almost unchanged due to thermal aging alone for 60 years at around 300 • C in commercial low-alloy steels [49,115]. (2) Neutron irradiation significantly enhances P segregation at GBs.…”

Ab initio calculations and interatomic potentials for iron and iron alloys: Achievements within the Perfect Project

“…Additional information concerning P in Fe from ab initio and empirical potential calculations can be found in [35]. P is experimentally known to segregate at grain boundaries (GB) under irradiation and this process is believed to be partially responsible for RPV steel embrittlement, by promoting intergranular fracture [49]. Segregation under irradiation can occur as a consequence of purely kinetic effects, e.g.…”

Segregation behavior of phosphorus in the heat-affected zone of an A533B/A182 dissimilar weld joint before and after simulated thermal aging

“…Owing to its potential of causing temper embrittlement, creep cavitation and intergranular stress corrosion cracking, grain boundary segregation of phosphorus induced by long term aging has been suspected as a major cause of degradation of RPV steels, especially at locations such as the core shroud support where the irradiation rate is negligibly small. Equilibrium segregation occurs during aging [1][2][3][4][5][6], i.e., the saturated grain boundary concentration increases with increasing bulk P concentration and decreasing aging temperature, while the kinetics of segregation decrease with the increase of aging temperature.…”