Effect of temper embrittlement and specimen size on Charpy impact testing of a Cr-Mo-V rotor steel

Full and sub-size Charpy V notch specimens from several locations of a high pressure-intermediate pressure Cr-Mo-V turbine rotor were tested. A compar ison between full and sub-size impact energy data showed that the smaller s pecimens exhibited qualitatively similar behaviour, with a systematic reduc tion in fracture appearance transition temperatures (FATTs). The full and s ub-size impact energy data were normalised against the specimen area and vo lume. The latter normalisation produced the closest match and the offset be tween the two data sets was described by a simple linear equation. The sens itivity of impact energy and FATT to specimen size was examined in samples possessing different degrees of temper embrittlement. It was found that the difference in FATT between full and sub-size specimens for embrittled samp les was at least double that of de-embrittled samples. It is proposed that the observed specimen size/impact energy/FATT variations with degree of emb rittlement arise from sensitivity of intergranular fracture to lineal speci men thickness, since fracture occurs predominantly through a two-dimensiona l network of grain boundaries.