Modeling of grain boundary stresses in Alloy 600

Corrosive environments combined with high stress levels and susceptible microstructures can cause intergranular stress corrosion cracking (IGSCC) of Alloy 600 components on both primary and secondary sides of pressurized water reactors. One factor affecting the IGSCC is intergranular carbide precipitation controlled by heat treatment of Alloy 600. This study is concerned with analysis of elastic stress fields in vicinity of M{sub 7}C{sub 3} and M{sub 23}C{sub 6} carbides precipitated in the matrix and at a grain boundary triple point. The local stress concentration which can lead to IGSCC initiation was studied using a two-dimensional finite element model. The intergranular precipitates are more effective stress raisers than the intragranular precipitates. The combination of the elastic property mismatch and the precipitate shape can result in a local stress field substantially different than the macroscopic stress. The maximum local stresses in the vicinity of the intergranular precipitate were almost twice as high as the applied stress

Status and Future of Corrosion in PWR Steam Generators

This review considers the broad set of corrosive degradations that can occur in PWR steam generators. The general status is reviewed, and potential future problems are identified. Recommendations for future work in each category are given. Main categories in this review include thick sections, tubing, and water chemistry. Tubing materials included are Alloys 600MA, 600TT, 690TT, 800NG, and Type 321 stainless steels. It appears that the Alloys 690TTand 800 give the best performance in vertical SGs and that the Type 321 gives adequate lifetimes for the horizontal steam generators. For tubing, the most likely location for degradation, mainly by stress corrosion cracking (SCC), is at the top of the tubesheet (ITS) of vertical steam generators, especially from the secondary side owing to both the superheated crevices and in some cases denting. These locations are the most likely to concentrate impurities, especially lead and possibly reduced sulfate species, which are inimical to both Alloys 690TT and 800. Available data from operating plants and from laboratory studies of these two alloys suggest that low potential stress corrosion cracking (LPSCC) is not a major concern at present although the presence of cold work, due to surface abuse or bulk deformation, may produce such SCC at longer exposure times. The line contact tube supports are expected, in principle, to be improvements over ones with drilled holes. However, in the long term this degree of improvement is not so clear since the interstices in some plants can fill with deposits; the possible concentration of impurities here is not clear but should be expected. Special attention should be paid to Alloys 690TT and 800 when exposed to lead since, in addition to their possible (neither has cracked in service so far) susceptibility to SCC, based on laboratory data, these alloys produce relatively thick scales when exposed to nominal concentrations of lead. There appears to be little question that degradation could occur in the future in modern steam generators. Only the mode of such degradations may differ from those in the past. There is substantially insufficient information upon which to predict future performance; however, the directions and types of work required are quite clear

Modeling of stress distributions on the microstructural level in Alloy 600

Stress distribution in a random polycrystalline material (Alloy 600) was studied using a topologically correct microstructural model. Distributions of von Mises and hydrostatic stresses at the grain vertices, which could be important in intergranular stress corrosion cracking, were analyzed as functions of microstructure, grain orientations and loading conditions. Grain size, shape, and orientation had a more pronounced effect on stress distribution than loading conditions. At grain vertices the stress concentration factor was higher for hydrostatic stress (1.7) than for von Mises stress (1.5). The stress/strain distribution in the volume (grain interiors) is a normal distribution and does not depend on the location of the studied material volume i.e., surface vs/bulk. The analysis of stress distribution in the volume showed the von Mises stress concentration of 1.75 and stress concentration of 2.2 for the hydrostatic pressure. The observed stress concentration is high enough to cause localized plastic microdeformation, even when the polycrystalline aggregate is in the macroscopic elastic regime. Modeling of stresses and strains in polycrystalline materials can identify the microstructures (grain size distributions, texture) intrinsically susceptible to stress/strain concentrations and justify the correctness of applied stress state during the stress corrosion cracking tests. Also, it supplies the information necessary to formulate the local failure criteria and interpret of nondestructive stress measurements