Temper Embrittlement, Irradiation Induced Phosphorus Segregation and Implications for Post-Irradiation Annealing of Reactor Pressure Vessels

Abstract

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.

Grain Boundary Phosphorus Segregation in Thermally Aged Low Alloy Steels

“…Various values of ÁG P and D for A533B have been reported. 10,11,15,18) The experimental results well agreed with the estimated value by the model with adopting the follow-ing values, D 0 ¼7:12Â10 À3 m 2 /s, Q¼253 kJ/mol, ÁH P ¼ À63 kJ/mol and ÁS P ¼À26:0 J/mol, as shown in Fig. 8.…”

“…4,5) Therefore, several studies on the effects of phosphorus segregation on intergranular fracture 6,7) and on the segregation modeling [8][9][10] have been carried out. Furthermore, there have been some studies implying phosphorus segregation to grain boundaries during annealing of the irradiated reactor pressure vessels of US and European LWRs, 11) showing that the intergranular fracture increases with increasing grain boundary phosphorus segregation, the higher phosphorus segregation was observed in the steel with the higher phosphorus content, and there was a linear relationship between the grain boundary phosphorous concentration and the ductile to brittle transition temperature (DBTT).…”

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

“…(2) Neutron irradiation significantly enhances P segregation at GBs. This was confirmed by a sample of data in irradiated commercial steels [113,[116][117][118][119][120][121] and model alloys [122,123]. The enhancement is thought to be caused by accelerating thermal segregation due to radiation-enhanced diffusion (vacancy dragging) and also radiationinduced diffusion (mobile P-interstitial dumbbells) [114,[124][125][126].…”

“…(4) It is well established that high-level P segregation at GBs enhances intergranular fracture and transition temperature shift in thermally aged steels. In such cases, the transition temperature shift is proportional to the P segregation level [112,116]. Recent studies carefully examined the correlation between transition temperature shift, radiation-induced hardening and P segregation in A533B-type steels containing a wide range of P contents (0.008-0.57%P), and found that P segregation affects the transition temperature shift only when the P concentration at GBs exceeds approximately 0.3 monolayer coverage [120,121].…”

Effect of the bainitic and martensitic microstructures on the hardening and embrittlement under neutron irradiation of a reactor pressure vessel steel

“…Recently, the sensitivity of martensite bearing microstructures to thermal ageing of A508 type steels without prior coarsening of the austenite grain has been demonstrated [7], [10]. Neutron irradiation, in general, increases only slightly the P segregation at grain boundaries and nonhardening embrittlement of RPV base metals is low [11][12][13]. However, some cases of irradiation enhancement of brittle intergranular fracture (BIF) are reported in the literature for the formally called "western" RPV base metals (A302, A508 and A533):…”