Temper Embrittlement, Irradiation Induced Phosphorus Segregation and Implications for Post-Irradiation Annealing of Reactor Pressure Vessels
Authors: McElroy RJ , English CA , Foreman AJ , Gage G , Hyde JM , Ray PHN , Vatter IA 
Published: March 2009

Three steels designated JPB, JPC and JPG from the IAEA Phase 3 Programme containing two copper and phosphorus levels were pre- and post-irradiation Charpy and hardness tested in the as-received (AR), 1200°C/0.5h heat treated (HT) and heat treated and 450°C/2000h aged (HTA) conditions. The HT condition, was designed to simulate coarse grained heat-affected zones (HAZ's) and showed a marked sensitivity to thermal ageing in all three alloys. Embrittlement after thermal ageing was greater in the higher phosphorus alloys JPB and JPG. Charpy shifts due to thermal ageing of between 118 and 209°C were observed and accompanied by pronounced intergranular fracture, due to phosphorus segregation. The irradiation embrittlement response was complex. The low copper alloys, JPC and JPB, in the HT and HTA condition exhibited significant irradiation induced Charpy shift but very low or even negative hardness changes indicating non-hardening embrittlement. The higher copper alloy, JPG, also exhibited irradiation hardening in line with its copper content. Fractographic and microchemical studies indicated irradiation induced phosphorus segregation and a transition from cleavage to intergranular failure at grain boundary phosphorus concentrations above a critical level. The enhanced grain boundary phosphorus level increased with dose in agreement with a kinetic segregation model developed at Harwell.

The relevance of the thermal ageing studies to RPV Annealing for Plant-Life Extension was identified early in the programme. It is of concern that annealing of RPV's has been performed, or is proposed, at temperatures in the range 425–475°C for periods of about 1 week (168h). Much attention has been given to the use of in-situ hardness measurements and machining miniature Charpy and tensile specimens from belt-line plate and weld materials. However, HAZ's, often containing higher phosphorus levels than the present materials, have largely been ignored. A post-irradiation annealing (PIA) experiment was performed to provide an indication of the strength of the above concerns. The highest annealing temperature (475°C) and longest time (1 week) used in actual RPV anneals were chosen. In addition to Charpy tests on reconstituted specimens, hardness was used to determine hardening recovery, since this is common practice in RPV annealing. The hardness tests indicated that complete recovery of irradiation hardening was achieved by the anneal. Charpy tests, however, indicated a further increase in transition temperature, such that the shift due to irradiation, 69°C, was more than doubled to 155°C after PIA. The latter embrittlement was consistent with model predictions. The significant increase in shift after PIA is cause for concern in RPV annealing if coarse grained HAZ's are present. This is particularly so since hardness measurements, which gave a contrary indication, have traditionally been used to monitor the progress of annealing. The model calculations suggest that, in a sensitive material, annealing temperatures should probably not exceed about 425°C, if a net deterioration of toughness is to be avoided.