High-temperature creep rupture behavior of dissimilar welded joints in martensitic heat resistant steels

The creep rupture behavior and microstructure changes of dissimilar welded joints of two types of martensitic heat resistant steel F92 and Co3W2, under a creep testing of 115-200 MPa and 873-893 K, were investigated. Fracture position and mode change from transgranular ductile fracture in parent F92 to intergranular brittle fracture in the fine grain heat affected zone (type IV cracking) near F92, with the drop of applied stress. Microstructural degradation resulted from creep with respect to dislocation density decrease, precipitates coarsening, dislocation cells, microvoids, macroscopic secondary cracks, and fracture surface morphology are researched based on detailed experimental results. The creep rupture data are analyzed employing Norton's power law, Monkman-Grant relation, creep damage tolerance, and Larson-Miller parameter. Main creep damage micromechanisms are analyzed with the aid of calculated and experimental data. The extrapolating creep rupture stress for a creep life of 100,000 h are obtained and the influence of stress change and temperature change on creep life are compared in terms of calculated normalized creep life growth rate.