DSC “peak temperature” versus “maximum slope temperature” in determining TSSD temperature

Abstract

One of the concerns of the nuclear industry is the deleterious effect of hydrogen on the structural integrity of the reactor core components due to delayed hydride cracking (DHC). The DHC process occurs when hydrogen concentration exceeds the terminal solid solubility (TSS) in the component. Thus, the accurate knowledge of TSS is necessary to predict the lifetime of the components. Differential scanning calorimetry (DSC) is normally used to measure the hydrogen TSS in zirconium alloys. There is a measurable change in the amount of heat absorbed by the specimen when the hydrides dissolve. The hydride dissolution process does not exhibit a well-defined “sharp” change in the heat-flow signal at the transition temperature. A typical DSC heat-flow curve for hydride dissolution has three definite features; “peak temperature” (PT), “maximum slope temperature” (MST) and “completion temperature”. The present investigation aims to identify the part of the heat-flow signal that closely corresponds to the TSS temperature for hydride dissolution ( T TSSD). Coupons were cut from a Zr–2.5Nb specimen, which had been previously hydrided using an electrolytic cell to create a surface hydride layer of ∼20 μm thick on all sides of the specimen. The coupons were then annealed isothermally at various temperatures to establish the T TSSD under equilibrium conditions. Subsequently the hydride layer was removed and the coupons were analyzed for TSSD temperature using DSC. The PT and MST for each DSC run were determined and compared to the annealing temperature of the coupon. The results show that the annealing temperature (the equilibrium T TSSD) is much closer to the DSC PT than any other feature of the heat-flow curve.