Creep behavior analysis on ruptured welds of heat-resistant steels with two different failure modes

Abstract

The crept welded joints of heat-resistant steel 12Cr10Co3W2MoNIVNbNB under different creep conditions, 898–948 K and 105–250 MPa, were researched, and the fracture surface reveals two modes, ductile rupture in parent material for high level stress and intergranular type Ⅳ failure in fine grain heat affected zone for low stresses. Creep data are analyzed with Monkman-Grant relation and Larson-Miller parameter method. The extrapolating creep rupture stresses for 100,000 h satisfy the industry requirements. The calculated normalized creep life growth rate values indicate that mediate stress range (150–195 MPa) yields greater influence on creep life compared with high or low stresses. Micro-voids along with precipitates near the fracture surface are observed and analyzed, and creep cavities in fine grain heat affected zone grow and coalesce together and form cracks during creep with precipitates serving as the preferential cavity nucleation sites, leading into the eventual type Ⅳ fracture. Nanoindentation results in different welding zones demonstrate that local physical-mechanical properties do not maintain a decrease trend when creep time increases.