Creep rupture behaviour of Cr-Mo ferritic steels under multiaxial state of stress

摘要

Effect of multiaxial state of stress on creep rupture behaviour of different Cr-Mo ferritic steels viz. 2.25Cr-1Mo, 9Cr-1Mo and modified 9Cr-1Mo steels have been investigated by performing creep tests at 873 K over a stress range of 110-230 MPa on plain and circumferentially U-notched specimens. The notch root radius was varied from 0.25 to 5 mm keeping the notch throat diameter of 5 mm as in plain specimen. The creep rupture life of the steels increased in presence of multiaxial state of stress. The strengthening was found to increase with increase in notch sharpness and tend to saturate at higher notch sharpness. The extent of strengthening was comparable in 2.25Cr-1Mo and 9Cr-1Mo steels and was significantly higher for modified 9Cr-1Mo steel. Fractographic observations revealed plasticity induced intragranular ductile failure with dimple appearance for relatively shallow notches and creep cavitation induced intergranular brittle failure for relatively sharp notches. The 2.25Cr-1Mo steel was found be more prone to creep cavitation under multiaxial state of stress than both the 9Cr-steels. Finite element analysis of distribution of components of stresses across the notch throat plane was carried out on incorporating material's elasto-plastic-creep behaviour to understand the effect of notch sharpness on creep damage and rupture life. Fracture behaviour of the steels has been explained based on the stress distribution in the notched specimens of the steels. Higher notch strengthening in modified 9Cr-1Mo steel has been attributed to the higher reduction in von-Mises stress across the notch throat plane than those in other two steels. Higher principal stress along with higher von-Mises stress at notch root region for relatively sharper notches induced intergranular creep cavitation.