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.
It has been shown earlier that the crystallographic texture of a Zircaloy tube is determined by the ratio of wall reduction to diameter reduction during tube fabrication. The use of this ratio as applied mathematically to the starting tube size and the final tube size does not take into account the complex nature of the tapers in the tube reducing tooling. Instantaneous values of the reduction ratio along the tube transition are shown to vary widely in tools of conventional design. A tooling design scheme that holds the reduction or strain ratio constant is developed by mathematical integration of the basic premise. Data showing the results of various reductions made holding the reduction ratio constant are shown. These tests indicate that the texture rotation is not a linear function of the reduction ratio, and there is a strong indication that the strain field is not homogeneous in thick-walled tubes.
Description The Fourth International Conference on Zirconium in the Nuclear Industry was held 26-29 June 1978, in Stratford-upon-Avon, England. This conference was sponsored by the American Society for Testing and Materials (ASTM) Committee B10 on Refractory Metals and Alloys in cooperation with the American Nuclear Society, the British Nuclear Energy Society, and The Metals Society (U.K).. The program was planned by representatives of ASTM Subcommittee B10.02 on Zirconium and Hafnium and K. G. Sumner of British Nuclear Fuels, Ltd. J. H. Schemel, Sandvik Special Metals Corp., and K. G. Sumner, BNFL, served as the conference co-chairmen, T. P. Papazoglou, The Babcock & Wilcox Company, as editorial chairman, and H. M. Cobb, ASTM Staff, as conference coordinator.
Zircaloy-2 fuel tubing with different compositions has been manufactured by a variety of thermomechanical processes to evaluate manufacturing variables that could influence nodular corrosion. The primary variables studied include: (1) alloy composition. (2) thermal annealing history, (3) tube texture, and (4) recrystallized versus stress relieved tubing. A total of 48 tubing variants were fabricated and tested. Results from 793 K steam autoclave tests revealed that all variables examined influence the corrosion behavior with complex interactions between the variables. Alloy composition and thermal history exhibited the strongest influence. Improved corrosion resistance was observed for the material containing a lower Sn concentration and annealed at lower temperatures. The effect of thermal history was not consistent with the accumulated annealing parameter formulation. A single 1005 K anneal late in the manufacturing process produced extremely high weight gains compared to lower values when the same anneal occurred early in the process or when annealing for longer times at lower temperatures to obtain the same accumulated annealing parameter. Small improvements in corrosion resistance for the more sensitive materials were obtained from tubing produced with a more radial texture. Stress relieved tubing was generally less sensitive to nodular corrosion than comparable recrystallized tubing. Microstructural examinations did not identify the cause for the observed variations in corrosion behavior. Particle size distributions were independent of the material or processing histories. This lack of any correlation indicates that the particles themselves do not control the corrosion behavior, but a specific microstructural feature was not found that would explain the corrosion behavior.
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