Assessment of Rupture Behaviour of 9Cr-Steels Under Multiaxial State of Stress

Abstract

In this investigation, detailed finite element analysis has been carried out to assess the creep behaviour of 9Cr-1Mo and modified 9Cr-1Mo steels under multiaxial state of stress. The multiaxial state of stress was introduced by incorporating the circumferential notches of different notch radii in smooth cylindrical specimens. The creep experiments were carried out on smooth and notched specimens of the steels at 873 K and in the stress range of 110–230 MPa. The modified 9Cr-1Mo steel possessed significantly higher creep strength than the 9Cr-1Mo steel. The rupture life was found to increase in presence of notch for both the steels. However, the extent of strengthening was found to be significantly higher for modified 9Cr-1Mo steel compared to 9Cr-1Mo steel. The fracture appearance was found to be dependent on the applied stress and the notch geometry. Typical transgranular ductile fracture was observed for shallow notches and mixed mode fracture consisting of ductile and creep cavitation for relatively sharper notches, which was more prevalent at lower applied stress. Finite element analysis was carried out to understand the distribution of stresses across the notch throat plane. The stresses were found to relax in the notch throat plane. The stress relaxation at the notch root was found to be more for modified 9Cr-1Mo steel than that of 9Cr-1Mo steel, resulting in higher extent of strengthening in modified 9Cr-1Mo steel.