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It has been demonstrated that thermal cycling of a previously quenched and tempered reactor pressure vessel (RPV) steel induces a hardness increase equivalent to an increase in flow stress of about 70 MPa. Single cycle heat treatments produce no such changes in hardness, flow stress, ductility or upper shelf energy (USE). Equivalent cycles will occur during reactor operation and may thus contribute to the observed changes during service of the properties of RPV steels. Preliminary microstructural examination offered no clear explanation for these effects, although it was observed that thermal cycling induced decomposition of retained austenite. It is demonstrated that, should the defects involved be shearable defects similar to those introduced by irradiation, they would promote ductile fracture by enhancing the growth and linking of microvoids and thus contribute to a drop in USE.
An analysis was performed which demonstrates that, of the several defects of the shearable type produced during irradiation of steels, vacancy clusters (or voids) are unstable with respect to both individual vacancies and collapsed vacancy loops. Thus, the latter defect and depleted zones would be the predominant vacancy hardening defects. Since irradiation produces both shearable (Cu/Ni/Mn rich precipitates, depleted zones, and loops) and non-shearable (Mo2C) hardening defects, it is important to resolve whether the shearable or non-shearable contribution is greater in order to be able to adequately model the effect on ductile fracture. For this purpose, a study of light water reactor (LWR) surveillance tensile data trends was performed and the results show negligible copper dependence of the yield strength change on fluence for fluences less than the boiling water reactor (BWR) end-of-license (EOL) fluence. These trends also show that the change in strain hardening behavior for U.S. steels is also negligible which indicates that shearable particles are the primary defect responsible for hardening. In addition to the Charpy curve shift, the shearable obstacles also cause a shelf drop during the strain controlled ductile fracture process. Rather than diffusing strain by slip plane hardening, as with non-deformable particles, shearable obstacles cause localization of strain in the unhardened slip plane. Strain localization in ligaments between voids leads to premature ligament failure in the irradiated material by the flow localization mechanism of void linkage.
ductile fracture, upper shelf energy, pressure vessel embrittlement, microstructural evolution under neutron irradiation
Consultant, Metallurgical Consultants, Boalsburg, PA
President, MPM Research & Consulting, Lemont, PA
Professor, Research Centre Rossendorf, Inc., Dresden,
Student, Pennsylvania State University, University Park, PA