In-situ study on tensile deformation and microvoid formation in a nuclear pressure vessel steel

Tensile deformation and microvoid formation of quenched and tempered SA508 Gr.3 steel are studied by in-situ digital image correlation (DIC) technique and in-situ EBSD measurement. The quenched steel with a mixture of bainite and M-A islands exhibits a high ultimate tensile strength (UTS~795 MPa) and an elongation of about 25%. After tempering, long rod carbides and accumulated carbide particles are formed at the interface of bainite-ferrite subunits and prior austenite grain boundary (PAGB), respectively. UTS of the tempered steel decreases to about 607 MPa, while the total elongation increases to about 33% with a local strain of about 191% at the necked area. In-situ EBSD results show that strain localization in the bainite-ferrite produces lattice rotation and dislocation pileup, thus leading to stress concentration at the discontinuities (such as M-A island and carbides). As a result, the decohesion of PAGB dotted with M-A islands is the dominate microvoid initiation mechanism in the quenched steel, while the microvoids initiate by fracture of long rod carbides and decohesion of PAGB with carbides aggregation in the tempered steel. The fracture surfaces both for quenched and tempered specimens are featured by dimples, indicating the ductile failure mechanism caused by microvoid coalescence.