Annealing behavior of cold-rolled reactor pressure vessel steel

Abstract

An isochroanl and isothermal annealing study was performed with use of the cold rolled RPV steel(SA 508 Cl. 3 steel). Vickers microhardness and positron annihilation technique were used to understand the behavior of recovery and recrystallization. The cold-rolled specimens have represented work hardening, of which major mechanism was thought as a role of density and distribution of generated dislocations by cold rolling. Isochronal annealing experiment indicated that at least two recovery stages were observed during the annealing of the cold-rolled specimens. The first recovery process appeared, in the lower temperature, below $250^\circ C$. It is due to annealing out of vacacies, which was indicated with a drop of S parameter to the measured value of underformed specimen. The second drop in the Doppler Line-shape parameters (Stage II) occured in the temperature range of $400^\circ C$ $\sim$ $550^\circ C$. Because there was slight hardening (6.5\% increase of Micro-hardness) in this temperature range, it can be thought that there were formation and sharpening of dislocation cell, and/or interaction of alloy elements with dislocations. Furthermore, only above $550^\circ C$, the cold-rolled (60\%) RPV steel initiated the recrystallization. The behavior of recovery and recrystallization in the cold-rolled RPV steel, is very complex, and therefore must be systematically studied.