The effect of thermomechanical processing on the creep behavior of Alloy 690

The effect of thermomechanical processing on the microstructure and elevated-temperature creep behavior of Alloy 690 was investigated. Commercially available sheet was subjected to four cycles of cold rolling to 25% deformation followed by annealing at 1000 °C for 1 h. Both the resultant microstructure and the original microstructure were characterized using electron backscattered diffraction. The thermomechanically processed microstructure exhibited a slightly lower fraction of twins and a smaller average grain size than the original microstructure. Tensile-creep experiments were performed in an open-air environment at temperatures between 650 and 690 °C and stresses between 75 and 172 MPa. The measured creep stress exponents (4-5) activation energies (320-368 kJ/mol) suggested that dislocation creep with lattice self-diffusion was dominant. The thermomechanically processed microstructure exhibited significantly worse creep resistance than the original as-processed microstructure. Thus, cyclic strain and annealing processing, which has been shown to improve the ductility-dip cracking susceptibility of Alloy 690, is not recommended for enhancing the creep resistance.