The effects of variations in the beta-quench parameters during the tube reduction process on the final microstructure and microchemistry of Zircaloy-2 and Zircaloy-4 cladding were studied by TEM/STEM and correlated with corrosion rates in 400 and 500°C steam autoclaves.
The Process A Zircaloy-2 specimens (recrystallization annealed) revealed primarily two types of ternary intermetallic compounds: tetragonal Zr2(Fe,Ni) and hexagonal Zr(Fe,Cr)2. The solutelean Zr2(Fe,Ni) particles were much coarser than the solute-rich Zr(Fe,Cr)2 particles. The Process A Zircaloy-4 specimens revealed only one type of ternary particles, Zr(Fe,Cr)2, but with much higher Fe/Cr ratios and mean particle size than those of the corresponding particles in Zircaloy-2. Also, both the cubic and hexagonal forms of Zr(Fe,Cr)2 were noted in this sample.
More aggressive beta quenching reduced the mean particle size and volume fraction of the intermetallic particles and increased their number density in both alloys; however, the changes were greater in Zircaloy-2. In both alloys, the degree of these changes was most pronounced when beta quenching was performed close to the final tube reduction step. In Zircaloy-2, the Zr-Fe-Cr type particles showed higher refinement than the Zr-Fe-Ni type particles.
The variations in Zircaloy microstructure with the timing and aggressiveness of beta quenching in the tube reduction process were attributed to differences in quench severity and thermal exposure after the quench. A critical discussion of the influence of alloy chemistry was also presented to reconcile the differences in beta-quenching response between Zircaloys 2 and 4. The 500°C steam autoclave corrosion data revealed significant improvement when beta quenching was performed close to the final tube dimensions.
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Exxon Research & Engineering Company, Florham Park, NJ
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