An experimental characterization of grain boundaries using field-emission Auger electron spectroscopy has been carried out for Ni₂M-stabilized alloy (where M mainly corresponds to Cr) and Alloy 690, which have duplex and non-duplex grain sizes, respectively, in order to determine the relationship between grain-boundary precipitates and the grain structure. Thermodynamic calculations based on the Scheil-Gulliver model with and without back diffusion of the C solute in the solid phase were also performed in order to investigate the solidification process in both alloys. Chromium carbide precipitates, with a predicted composition of M₂₃C₆, were observed at grain boundaries in both the Ni₂M-stabilized alloy and Alloy 690. The M₂₃C₆ precipitates in the Ni₂M-stabilized alloy wer considerably coarser than those in the Alloy 690. A small number of coarse titanium carbonitride precipitates were also observed on the fracture surfaces of both alloys at intergranular and intragranular positions. The simulations predicted that the M₂₃C₆ precipitates are likely to be formed during the final stages of solidification, and it is thought that this occurs more readily in the Ni₂M-stabilized alloy. The results indicate that the duplex grain structure observed in the Ni₂M-stabilized alloy is most likely due to the presence of undissolved coarse M₂₃C₆ grain-boundary precipitates.

The influence of hydrogen in steel on the high temperature water oxidation behavior of low carbon austenitic stainless steel Type F316L was investigated in order to clarify the mechanism of SCC initiation in the BWR environment, which is closely associated with the localized oxidation and its acceleration. The steel was charged with hydrogen by means of cathodic electrolysis. And then, the solution-treated (non-charged) and hydrogen-charged steels were subjected to the oxidation test in simulated BWR environment. Experimental results revealed that the size of outer oxide particle increased with increasing hydrogen content, resulting in the hydrogen accelerated oxidation (HAO). Additionally, the oxide of the hydrogen-charged steel was mainly NiFe₂O₄, whereas Fe₃O₄was predominantly formed on the non-charged one. From the result of the small punch test in the BWR environment, it was also indicated that the effect of hydrogen on the oxidation might be almost equivalent to that of applied stress.

Quantifying the low-cycle fatigue damage accumulated in nuclear power plant components is one of the important issues for aged plants. In this study, detailed observations of crack initiation and growth were made using scanning electron microscopy in order to correlate the crack size and the magnitude of the fatigue damage. Type 316 stainless steel specimens were subjected to the strain-controlled axial fatigue test (strain range: 1.2%) in air at room temperature. The test was interrupted several times in order to observe the specimen surface. The spatial distribution of inhomogeneously accumulated damage by cyclic loading was identified by crystal orientation measurements using the electron backscatter diffraction technique. Cracks were initiated at grain boundaries and slip steps, where relatively large damage accumulated. The changes in the number of cracks and their length were quantified. The crack growth rates were well correlated with the strain intensity factor. The change in crack size during the fatigue test was predicted using the obtained growth rate and assumed initial crack size. The fatigue lives estimated by the crack growth prediction agreed well with those obtained experimentally. It was concluded that the fatigue damage could be estimated from the crack size measured in plant components.