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.

Automated summary

The research focuses on the creep behavior and fracture characteristics of welded joints of heat-resistant steel 12Cr10Co3W2MoNIVNbNB under different creep conditions. Ductile rupture in the parent material and intergranular type IV failure in the heat-affected zone were observed. The industry requirements for extrapolating creep rupture stresses for 100,000 hours were met. The influence of mediate stress range on creep life was found to be greater compared to high or low stresses. The growth of creep cavities and formation of cracks were analyzed, with precipitates serving as preferential cavity nucleation sites. Nanoindentation results showed no decreasing trend in local physical-mechanical properties with increasing creep time.