Creep-fatigue interaction behavior of simulated microstructures and the actual weldment of P91 steel

The paper presents first of its kind study on the creep-fatigue interaction (CFI) behavior of simulated coarse grain (CG), fine grain (FG) and inter-critical (IC) heat-affected zones (HAZs) of P91 steel weldment. Total axial straincontrolled fatigue tests were conducted at 550 C using trapezoidal waveform and introducing 30-min hold at peak tensile strain of +/- 0.6%. Irrespective of the initial microstructure, the alloy exhibited continuous cyclic softening throughout the deformation under CFI condition. Strain localization together with interlinkage of subsurface creep cavities caused enhanced crack propagation that resulted in the lowest fatigue life in the case of ICHAZ. The actual weldment failed in the interface between FGHAZ and ICHAZ. The mode of failure was found to be largely governed by Type IV cracking.