Relationship between oxide inclusion and changing rule of low experimental temperature impact toughness of Ni-Cr-Mo-V steel weld metal

The Charpy impact tests of Ni-Cr-Mo-V steel used as reactor pressure vessel material weld metal were performed, and the result of the impact experiments shows that at low experimental temperature, the impact toughness of weld metal is worse gradually at the same experimental temperature along the direction from cosmetic welding to weld root. The spherical oxide inclusions with a diameter of 0.3-2.0 μm, located at the crack initiation site, dimples and cleavage facet initiation, were observed at the low temperature experimental fracture. In the process, the oxide inclusions and matrix separate to form microcracks, and develop into crack initiation site, dimples and cleavage facet initiation. It is concluded that the oxide inclusions are the main reason for low temperature failure of the weld. At the same time, the amount of oxide inclusions along the cosmetic welding to the weld root increases gradually, which causes the increase of the microcracks nucleation rate, and then the low temperature impact toughness of the weld is worse gradually along the direction from cosmetic welding to weld root. (authors)