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

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. •The creep behavior on ruptured welds of 10Cr-3Co-2W with two different failure modes of six crept specimens under 898–948 K and 105–250 MPa is analyzed.•The failure mode shifts from ductile rupture in parent alloy to brittle type Ⅳ rupture in fine grain heat affected zone.•The decline of λDP values at 898 K, 2.87 for 250 MPa and 2.43 for 195 MPa, is in conformity with the reduced creep ductility and intergranular brittle creep in heat affected zone under 195 MPa.•The extrapolating creep stress based on LMP method for a service time lasting 100,000 h is 82.8 MPa under 898 K, 42.5 MPa under 923 K, 2.26 MPa under 948 K, which is satisfactory to the industry requirements.•Nanoindentation hardness values for three regions (base metal, heat affected zone and weld fusion zone) decrease after 74 h' creep, and then start to increase slightly for type Ⅳ fractured sample.