The effect of hydrostatic pressure on the uniaxial creep life of a 2 ¼ % Cr 1% Mo steel

Abstract

The effect of a superimposed hydrostatic pressure on the ductility, the creep life and the failure mechanism of a 2 ¼ % Cr 1 % Mo steel, with an over-aged upper bainite microstructure, subject to different uniaxial stresses is described. Creep tests have been made at 923 K with uniaxial stresses in the range 55-80 MPa and superimposed hydrostatic pressures up to 35MPa. Optical and electron optical microscopy have been used to assess the accumulation of grain boundary damage arising from creep deformation. When failure is controlled by intergranular cavitation, increasing the hydrostatic pressure causes an increase in the creep ductility and a decrease in cavitation, and thus an increase in time to failure. In addition, increasing pressure effects a change in failure mode from one controlled by the nucleation and growth of intergranular cavities to one controlled by plastic flow. The results for the creep of this 2¼ % Cr 1 % Mo steel are discussed in terms of a diffusional cavity growth model which includes continuous nucleation. Moreover, these results are compared with data previously obtained for single phase materials tested with a superimposed hydrostatic pressure. The relative contributions of the principal and equivalent stresses to the creep fracture of this low alloy steel are also examined. The estimation of realistic long-term creep life from the results of short-term creep tests is also discussed.