Suppressed hydride precipitation in the welding zone of a zirconium-based alloy cladding tube

Abstract

The difficulty in characterizing mesoscale hydrides in the highly deformed submicron (< 1 μm) scale grain size of the reactor-grade stress-relief annealed (SRA) zirconium-based alloy tube via electron backscatter diffraction (EBSD) prevented, so far, the field from directly observing the mesoscale characteristics of precipitated hydrides in the welding and tube region. This study presents new high-quality EBSD characterization of precipitated hydrides along the axial length of reactor-grade zirconium-based alloy tubing covering the end cap head, welding zone (WZ), heat affected zone (HAZ), and nominal tube region. The result demonstrates that hydride precipitation is suppressed in the WZ, indicating increased resistance of the hydride embrittlement. Extensive EBSD analyses coupled to thermodynamic modeling were conducted to unveil the mechanism of hydride precipitation suppression at the WZ. Suppressed hydride precipitation in the WZ region is caused primarily by the combined effects of a higher critical nucleation Gibbs free energy of the WZ (ΔGWZ*) due to a greater matrix stiffness, increased misfit strain, and greater grain boundary misorientation angles.