STP1405: Review of Phosphorus Segregation and Intergranular Embrittlement in Reactor Pressure Vessel Steels

    English, CA
    Technical Staff, AEA Technology, Nuclear Science, Harwell, Didcot, Oxfordshire

    Ortner, SR
    Technical Staff, AEA Technology, Nuclear Science, Harwell, Didcot, Oxfordshire

    Gage, G
    Technical Staff, AEA Technology, Nuclear Science, Harwell, Didcot, Oxfordshire

    Server, WL
    President, ATI Consulting, Dublin, CA

    Rosinski, ST
    Project Manager, EPRI, Charlotte, NC

    Pages: 23    Published: Jan 2001


    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.

    Keywords:

    phosphorus segregation, intergranular embrittlement, grain boundaries, HAZ, thermal anneal heat treatment


    Paper ID: STP10531S

    Committee/Subcommittee: E10.07

    DOI: 10.1520/STP10531S


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