Ductility Loss in Ni-based Weld Metal under High Temperature Creep Condition

In advanced ultra-super critical (A-USC) power generation system, the steam condition becomes severe for conventional structural materials, i.e., high chromium heat resistant steels, because the steam temperature and pressure are 973 K and 35 MPa, respectively. It means that dissimilar welding between nickel base alloys and the heat resistant steels is inevitable. However, because the dissimilar welding engenders interface fracture at high-temperature creep conditions, understanding a relation between creep behavior and ductility of the weld metal as well as the base metals is important to ascertain safety of the plant. Alloy 82 weld metal decreased ductility with increasing creep lifetime because of Ni3Nb in the matrix and two kinds of GB precipitates: M23C6, with Cr, Ni, and Fe; MX with Ni, Nb, Cr and Mn. On the other hand, high B-9Cr steel showed high ductility even after long term creep test. Therefore, the difference of mechanical property leads to incompatibility of strain at the interface between steel and weld metal, resulting in interfacial crack in dissimilar welding condition.