Well-controlled laboratory heat treatments were performed in the range from 460 to 610°C (733 to 883 K) and from 1 to 8 h at temperature on Zircaloy-4 cladding tubes with three different degrees of initial cold work (40%, 64%, and 76%). Within this range the influence of annealing temperature T and time t and of cold work on the yield strength R_p0.2 at 400°C (673 K) and on the degree R of recrystallization was experimentally determined.

This data base was used to verify a semi-empirical approach to describe analytically the dependence of yield strength and recrystallization on the aforementioned technological parameters T and t for the annealing and φ = ln 1/l_o as a measure for the applied cold work.

Using a “growing function” y=lnRp0.2max−Rp0.2Rp0.2−Rp0.2min to correlate the normalized annealing temperature A = t·exp(-Q/RT) with the change of the yield strength R_p0.2 between the values Rp0.2max and Rp0.2min, the correlation Rp0.2=Rp0.2min+ΔRp0.2·11+k·A can be obtained, where ΔRp0.2=Rp0.2min.

It was found as a good expirical approach that ΔRp0.2=c·ω4

and the yield strength can be described as Rp0.2=Rp0.2min+c·φ41+k1·φ2·A

The influence of the same mentioned technological parameters on the recrystallization can be described as R1−R=k2·ω2·A

This leads to the correlation Rp0.2=RP0.2min+c·φ41+k1k2·R1−R

between yield strength and recrystallization.