Effect of ageing on properties of pressure vessel steels

The migration of phosphorus in ferritic iron alloys under irradiation

“…Although theoretical calculations of these jump frequencies are few and rudimentary, they do indicate a weak phosphorus-vacancy binding (Byrne 1996). Data for the thermal diffusion of iron and phosphorus atoms in a-Fe are also compatible with a small phosphorus-vacancy binding (Druce et al 1986, Mehrer 1990.…”

Engineered materials characterization report, volume 3 - corrosion data and modeling update for viability assessments

“…Corrosion pits can be developed at welds and base metals of waste package containers after long exposure to the environment These act as stress risers to initiate crack fissures at the bases of the pits SCC can be initiated at these pits when the applied stress intensity factors are equal to or larger than I&c Using the expression for K given as Eq (75) and the K,,, data from the previous section, the critical flaw size for initiation can be defined (K = Krscc) These critical flaw sizes have been calculated and are given in Table 2 8-28 as a function of aspect ratio (a/2c) at various stress levels Note that the critical flaw sizes for XC initiation is always larger than the thickness of the respective barriers, except at extremely high aspect ratio (u/2c-5) At this very high aspect ratio (u/2c-51, the critical flaw sizes for SCC initiation in unannealed welds of Alloys 625 and C-22 are 12 cm and 14 cm, respectively Because these values are somewhat less than the wall thickness (2 cm), SCC may be possible in the unannealed closure weld Because stresses in the base metals are expected to be much lower than that in the corresponding, unannealed welds, it is concluded that SCC should not occur in the base metals Even though the weld residual stress can be very high, it has been observed by Henshall and Roy that the aspect ratio of corrosion pits in Alloy 825 seldom exceeds one (a/2c<l) (Henshall, 1996a) Such pits are shown in Figure 2 8-16 The results in Table 2 8-28 suggest that SCC will not occur at corrosion pits at welds, even if the welds are not stress relieved The current analysis is based on linear elastic fracture mechanics When the stress applied on a crack or corrosion pit is close to or beyond yield stress, there is a possibility that the linear elastic fracture overestimates the critical flaw size for initiation of SCC Under this situation, elastic-plastic fracture mechanics based on the J-integral approach should be used To use this elastic-plastic fracture mechanics approach, accurate stress-strain curves for each material are needed to characterize its strain-hardening behavior We will pursue such data and conduct elastic-plastic fracture mechanics analysis in the future In the interim stage, we recommend that the maximum stress on the welds be relieved to less than 75% of the yield strength of the material 2.8 Corrosion Model Development Estimates of the extent of TE rely on both thermodynamics, kinetics, and transport phenomena McLean developed a theory of grain-boundary segregation using statistical thermodynamics (McLean, 1957) His expression [Eq (80)] is used to calculate the segregation of P at grain boundaries after thermodynamic equilibrium is reached (Eq 2 8-80) where X, is the equilibrium fraction of grain boundary being covered with a monolayer of the impurity of concern, X, is the solubility of the impurity in the matrix, and 6G is the Gibbs free energy of segregation For phosporous segregation in steel, Bruce and his coworkers have derived 6G as a function of temperature T based on experimental data (Druce, Gage, and Jordan, 1986) This is represented by GG(J/mol) = -63000 + 210 x T(K) (Eq 2 S-81)…”

Segregation behavior of phosphorus in the heat-affected zone of an A533B/A182 dissimilar weld joint before and after simulated thermal aging

“…Owing to its potential of causing temper embrittlement, creep cavitation and intergranular stress corrosion cracking, grain boundary segregation of phosphorus induced by long term aging has been suspected as a major cause of degradation of RPV steels, especially at locations such as the core shroud support where the irradiation rate is negligibly small. Equilibrium segregation occurs during aging [1][2][3][4][5][6], i.e., the saturated grain boundary concentration increases with increasing bulk P concentration and decreasing aging temperature, while the kinetics of segregation decrease with the increase of aging temperature.…”

“…The concentration of P at grain boundaries was found to be linearly related to the increase in ductile-to-brittle transition temperature (DBTT) [1,2,7] and the percentage of intergranular failure [8,9]. Attention was also paid to the microstructure dependence of phosphorus segregation, especially in the heat-affected zone (HAZ) of weldments.…”