EFFECT OF BETA-HEAT TREATMENT ON MICROSTRUCTURE AND NODULAR CORROSION OF ZIRCALOY-4

The effect of beta-heat treatment on microstructure and nodular corrosion of Zircaloy-4 were investigated for the specimens which were beta-heat treated and then cooled in various cooling process, such as, iced brine, water, oil, air, and furnace cooling. It was observed that the martensitic structure with substructures of dislocations as well as twins is formed at the cooling rate of 2,100-degrees-C/s, whereas at the cooling rate of 1,200-degrees-C/s the classical Widmanstatten structure appears with dense dislocations in a lath and precipitates along the lath boundary. The transformation in the water quenched specimens was suggested to take place by both diffusion- and diffusionless transformation process. The nodular corrosion behavior was investigated at steam of 500-degrees-C and 10.3 MPa. The corrosion resistance of the beta quenched specimens decreased with the decrease in the cooling rate so that the furnace cooled specimens exhibit the worst corrosion resistance, resulting in nodular corrosion. The corrosion of the furnace cooled specimen progressed mostly in the interior of the grain where alloying elements of Fe and Cr were largely depleted during the slow cooling process. However, the grain boundary has revealed relatively high corrosion resistance due to higher concentration of the alloying elements. The microstructural characterization leads to conclusion that the nucleation of nodules can be related with depletion of alloying elements of Fe and Cr.