Abstract:Creep testing was conducted on two types of welded joints made of martensitic heat-resistant steel (F92/Co3W2 and Co3W2/Co3W2) at temperatures ranging from 873 K to 948 K and stress levels ranging from 105 MPa to 250 MPa. The Larson-Miller parameter method was employed to extrapolate the allowable creep fracture stresses for a service life of 100,000 hours at various temperatures. Additionally, the cavity damage mechanisms under different creep conditions were investigated. As the applied stress decreases, the location, mode, and mechanism of fracture in both types of welds transition from transgranular plastic fracture in the base metal to intergranular brittle fracture in the fine grain heat-affected zone (termed type IV fracture). It was observed that cavities undergo a sudden change when the fracture mode shifts from plastic to brittle fracture. Specifically, cavities associated with plastic rupture exhibit larger size but smaller number density, while cavities associated with brittle rupture show smaller size but greater number density. When the fracture mode remains the same, the cavities exhibit an increase in size, quantity, and area fraction as the stress decreases. Notably, a coalescence phenomenon was observed for cavities near the type IV fracture site, with the coalescence direction being perpendicular to the tensile direction.
