Contribution a l'etude du durcissement par irradiation d'un acier faiblement allie destine au caisson de reacteurs

Abstract

Tensile specimens of one low alloy steel for a reactor pressure vessel were irradiated in a flux of fast neutrons and the changes in mechanical properties were measured at ambient temperature. The integrated neutron flux was ≅ 2 × 10 19 n/ cm 2 ( E > 1 MeV); the irradiation temperature was below 100° C. Irradiation caused an increase in the elastic limit (≅ 20 kg/ mm 2) and in the fracture stress (≅ 9 kg/ mm 2) together with a reduction of the total and uniform elongations. We examined the relationship between the hardening obtained for two series of specimens prepared in different ways and the initial elastic limit, and found that the relative change in the elastic limit ( Δσ 0,2 σ 0,2 ) × 100 depends on the original value of σ 0,2, and, at the same time, the influence of other metallurgical factors (microstructure, chemical composition etc.) becomes obvious. Some irradiated specimens were annealed at different temperatures above the temperature of irradiation and tensile tests were done at ambient temperature. These samples showed an additional hardening which had a maximum value between 150–200° C. (In the literature, this phenomenon has been termed “thermal hardening”). When the annealing temperature was raised to 450–500° C, the mechanical properties of the irradiated steel gradually returned to the original (unirradiated) values; the recovery process took place. The contribution of these results to the understanding of irradiation-hardening and “thermal hardening” in this type of steel are discussed.