Neutron irradiation effect on the hardening and embrittlement in reactor pressure vessel cladding material

Abstract

The embrittlement of mechanical properties in nuclear pressure reactor vessel cladding induced by the presence of σ-phase as well as neutron irradiation was investigated. Three kinds of overlay-weld cladding were fabricated on SA508 cl.3 pressure vessel steel plates with ER309L welding consumable strip by differing in heat input rates. The cladded plates were then subjected to postweld heat treatment at 893 ~ 901 K for 41 hours. The microstructural characteristics and their effects to the mechanical properties were evaluated by using a TEM, SEM, micro-Vickers hardness and small punch (SP) tests. The stress analysis of deformed SP specimen was carried out by finite element method to understand the cracking appearances of SP specimen. The JMTR and HANARO was utilized for neutron irradiation and irradiation fluences at 563 K were ranged from $5.1＼times10^{18}$ to $5.79＼times10^{19}n/cm 2$ (E>1MeV). The microstructure of the cladding was composed of a main portion of fcc austenite and a few percent of bcc δ-ferrite, and the transformation of δ-ferrite during postweld heat treatment resulted in the formation of chromium rich $M_{23}C_{6}$ type carbides at the initial δ/γ grain boundaries and brittle bct σ phase. Volume fraction of σ phase was ranging approximately 1.9 ~ 7.9 percent depending on welding conditions. After irradiation, the cladding exhibited an increase in yield stress and showed a shift of SP-DBTT. Although both the δ-ferrite and γ austenite were hardened by irradiations, any visible defect clusters were not observed in those phases, suggesting that the defect clusters contributing the irradiation hardening are too small to be observed by TEM, namely smaller than 1 nm. On the other hand, many small dots like structures were observed in the σ phase. The general hardening may come from the irradiation-induced fine size defect clusters (<1~2 nm) in the γ austenite matrix, even though the δ-ferrite showed about 1.5 times higher hardening ability than the ...