Creep Rupture Properties and fracture type of 9Cr-1Mo-V-Nb/18Cr-8Ni steel dissimilar joints

Abstract

By conducting long-term creep rupture tests for dissimilar weld joints (9Cr-1Mo-V-Nb/18Cr-8Ni) made with Gas Tungsten Arc Welding (GTAW) and Friction Joining (FJ), creep rupture properties and microstructures were examined, and the relationships between changes in microstructure and nucleation and propagation of creep cracks that occur during creep deformation in weld joint specimens were studied. Creep rupture tests of GTAW joints were conducted at three temperatures: 550, 600 and 650 °C, under applied stresses of 160–240 MPa, 80–160 MPa and 40–80 MPa, respectively. Creep rupture tests of FJ joints were conducted at 600 °C and 650 °C under stresses ranging from 40 to 160 MPa and 30 to 80 MPa, respectively. The creep-rupture strength of dissimilar weld joint specimens was lower than that of the 9Cr-1Mo-V-Nb base metal specimen at all temperature levels. In addition, the differences in creep strength between the dissimilar weld joint specimen and the base metal specimen tended to be greater at higher temperature levels. The fracture type of dissimilar weld joint was transformed from a Type V fracture and a Type VII fracture mode to a Type IV fracture mode following long-term creep rupture tests. The fracture type of the dissimilar weld joints was predicted as a Type IV fracture mode at 550 °C and 600 °C and with rupture occurring after 100 000 hours.