13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Beremin model to predict the variation of fracture toughness with temperature. One heat of 18 MND5 grade steel (type A533B) was used for this purpose. The material (a 100mm thick hot-rolled plate) was tested in the as-received condition and after aging at 450°C for 5000 hours. This aging heat treatment was introduced to reinforce the segregation of phosphorus at grain boundaries and to decrease the grain boundary fracture resistance. The McLean [6]–Guttmann [7] theories for impurity intergranular segregation was used to calculate the grain boundary enrichment in phosphorus and in carbon under isothermal conditions. These theoretical values were compared to experimental values determined by Auger spectrometry. The interaction between C and P atoms was adjusted from this comparison. Therefore the approach developed in the present study contributes to an improvement in the prediction of the shift in DBTT with thermal aging. 2. Material and experimental procedures The composition of the investigated steel is (in weight %): C = 0.192, Mn = 1.44, Ni = 0.683, Mo = 0.473, Cr = 0.194, Si = 0.249, S = 0.0053, P = 0.010. It was received as a thick plate (100 mm) produced for simulating welding operations. This material was used in the past for the manufacturing of steam generator secondary pressure vessel, and is very close to plates used in the past for pressurizer vessels. The material was given the conventional heat-treatments applied to PWR components, including the final stress-relieving heat-treatment (615°C-Air cooling at ~ 30°C /hour). The prior austenite grain size was between 20 and 30 µm. The yield strength determined at RT was 490 MPa. The microstructure of the material is shown in Fig.1. Segregated zones (SZ) aligned along the rolling direction (L) and distant of about 0.2 mm in the short-transverse (ST) directions are observed in Fig.1a. This is the consequence of micro-segregations of alloying elements during solidification, deformed by the rolling process. The dark zones are much harder than the matrix (~ 300 HV instead of ~ 210 HV) (Fig.1b). They are enriched in all alloying elements (C, Mn, Mo, Si) and in P as revealed by microprobe analysis (Fig.1b). The surface fraction covered by these segregated zones is about 10%. SEM and Auger spectrometry were used to examine the fracture surfaces and to determine the amount of phosphorus segregated along the grain boundaries. Figure 1. a) Optical micrograph showing two segregated zones aligned along the rolling direction; b) Microprobe analysis across one segregated zone showing the bulk concentration CB in carbon, manganese, molybdenum, silicon and phosphorus. The variations of micro-hardness (100g) are also shown. a) b)
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