Effect of heat-treatment on delayed hydride cracking in Zr-2.5wt%Nb pressure tube material

Abstract

The mechanical properties, microstructure, and texture of Zr-2.5wt% Nb pressure tube material were changed by annealing, α+β, or β heat-treatment. After the specimens were hydrided to 100ppm, the morphology of hydrides was observed with optical microscope. The crack extension during delayed hydride cracking (DHC) test was measured with direct current potential drop method. After DHC test, fracture surfaces were observed with optical and scanning electron microscope. The hydrides in annealed specimens had similar morphology which was aligned to circumferential direction due to residual compressive stress introduced during manufacturing. However, α+β, or β heat-treated specimens had the random hydrides because residual compressive stress was eliminated by the heat-treatment. Specially for β-quenched specimen, thin and short hydrides were precipitated in martensite structure. Because of such a difference in hydride morphology, secondary crackings appeared in annealed specimens but not in the β-quenched specimen. Striation spacing was about 30~40㎛ and wider in the region where crack velocity depended on $K_I$. The striation spacing was tend to decrease with $K_I$ increase. The activation energy of DHC was 34kJ which was comparable to Coleman``s result. As yield stress decreased, $K_{IH}$ increased while crack velocity decreased. Semi-empirical DHC model was proposed to describe the relationship between $K_I$ and crack velocity. The calculated crack velocity was proved to be comparable to measured one. Thus crack velocity can be predicted by the proposed DHC model if the material has similar texture to the current pressure tube material.