Effect of hold time and force ratio on creep fatigue crack growth (CFCG) behaviour of P91 weldments at 600 °C

Abstract

Modified 9Cr-1Mo steel is the chosen material for the construction of thermal and mechanical stress-bearing structures in fossil fuel-fired power plants, aiming for greater operational efficiency, extended life, and a considerable reduction in greenhouse gas emissions. The detailed study of welded joints and their properties, along with the underlying reasons and mechanisms for failure propagation, is crucial for quality control. Hence, there is a need to study the creep fatigue crack growth behaviour (CFCG) of P91 weldments and to evaluate the consequences and the underlying mechanisms behind crack propagation. This article intends to provide a preliminary comparison of the behaviour of welded P91 compact tension C(T) samples with those of the base metal. CFCG of P91 welded specimens have been performed at 600 °C for a force ratio of 0.1, 0.5 and 0.8 with hold times of 10, 60 and 600 s. The test results were compared with corresponding tests for base metal samples. The variation in cyclic crack growth rate has been evaluated with respect to stress intensity factor range (ΔK) and (C_t)_avg parameter and correlated with the variation in hardness corresponding to the change in microstructural features across the welded joint. The phenomenon of crack tunnelling has been evaluated along with detailed fractography of the tested samples. The angular distortion of the terminal crack front is subdued for the welded samples. The welded samples can thus be said to exhibit a higher degree of resistance to lateral deformation due to the advent of triaxial stresses.