Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.