Initiation and Arrest of Delayed Hydride Cracking in Zr–2.5Nb Tubes

Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr–2.5Nb tubes that had previously been investigated with the previous DHC models. At the test temperatures above $180°C$⁠, DHC initiation temperatures fell near the terminal solid solubility for precipitation temperatures, requiring some undercooling or $ΔT$ from the terminal solid solubility for dissolution (TSSD) temperatures, and increased toward TSSD with the number of thermal cycles. At the test temperatures below $180°C$⁠, DHC initiation occurred at temperatures near TSSD with little $ΔT$⁠. DHC arrest occurred on heating toward TSSD where the hydrogen concentration difference between the bulk region and a crack tip $ΔC$ decreased to a minimum $ΔCmin$⁠, under which nucleation of the hydrides was restrained. $ΔCmin$ after the first thermal cycle increased with increasing temperature, demonstrating that nucleation of the hydrides becomes more difficult with increasing temperatures. Different DHC initiation and arrest temperatures with the test temperatures or hydrogen concentrations are discussed in view of a supersaturation of hydrogen $(ΔC)$ for nucleation of hydrides in the zirconium matrix.