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 austenite matrix. The increase in SP-DBTT(SP-ΔDBTT) is almost similar independent of the neutron fluence, which is approximately 14 K, suggesting that the irradiation embrittlement appears to be saturated. The increase in SP-ΔDBTT with irradiation was obviously induced by irradiation hardening combined with the preferential failure of δ-ferrite at low temperatures. However, the increase in SP-ΔDBTT was more strongly affected by the initial microstructural factors, such as the amount of brittle σ phase since it caused a crack nucleation site in an early stage of deformation. The fracture appearance of SP specimen was changed from circumferential to radial cracking as a function of test temperature, content of σ phase and neutron irradiation. The dominant cracking process below the transition region was intergranular type. High susceptibility of δ- ferrite to embrittlement was explained by the facts that the higher effective stress and the lower equivalent strain was partitioned to δ-ferrite during plastic deformation, and those were accelerated by irradiation. The change in cracking appearance as test temperature became low was interpreted as a reduction in critical radial distance($r_c$) to reach the fracture stress of ferritic phase. As the $r_c$ was reduced by decreasing test temperature, the governing stress state for crack initiation was also changed from radial to circumferential stress. For that reason, the radial cracking was able to be formed as a dominant feature of cracking at low test temperatures. The fracture stress of σ phase at the disk bottom surface was attained at a relatively small deflection irrespective of test temperature. Hence, the significant embrittlement of K002J cladding accompanied by the radial cracking was inevitable at the entire test temperature regions because it contained a larger amount of σ phase (7.9 vol. percent). The $r_c$ of irradiated SP specimen was also reduced by irradiation hardening showing the transition of cracking appearance from circumferential to radial.
