Effect of cooling rate on thermal aging in β treated zircaloy-4

Abstract

To obtain various data for the accident condition and the optimization of advanced nuclear fuel cladding material, the effects of the heat treatment on mechanical properties and microstructures of Zircaloy-4 material have been studied. Due to phase difference, hardness and tensile strength values increased with annealing temperatures(α, α+β and β region) for 15 minutes before water quenching. The microstructural development with cooling rate after β-annealing was as follows: coarse plate-like ⇒ fine parallel plate ⇒ basketweave ⇒ martensite+Widmanstatten ⇒ martensite. Hardness and yield stress of the alloy increased with increasing cooling rate and reached to 245 VHN and 64 kg/㎟, respectively, for the highest cooling rate. Hardness and tensile strength values increased dramatically at cooling rate above 1600℃/sec. Upon tempering at different temperatures for 1 hour, recrystallization was completed at 700℃ in as-ice brine quenched and as-water quenched specimens, while as-air cooled specimens did not recrystallize up to 850℃. During aging at 760℃, hardness and tensile strength values in as-ice brine quenched and as-water quenched specimens indicated that initial decrease appeared to be due to matrix recovery, polygonization and recrystallization, while the values in as-air cooled specimens did not. The decrease in hardness and tensile strength values occurred during short-time annealing(20 minutes for as-ice brine quenched and 40 minutes for as-water quenched) at 760℃. Possibly due to precipitation hardening, these values increased to a maximum (207 VHN for as-ice brine quenched and 203 VHN for as-water quenched) at longer aging times. Further aging decreased the strength values because of precipitate coarsening and grain growth.