Creep constitutive equations for as-received 9Cr1Mo steel

Abstract

9Cr1Mo has a wide application in petrochemical and power generation plants, prediction of its creep behavior is a major concern in these industries. In this paper, the creep behavior and creep lifetime of test samples machined from 15 years of service exposed boiler tubes in an oil refinery plant, are studied using uni-axial creep tests. Tests are carried out according to ASTM E139 at different stress and temperature levels (from 80 MPa to 190 MPa and at 550 °C, 600 °C and 650 °C). Based on the experimental data, the creep constitutive coefficients for three types of creep laws: Norton, Plrandtl and Garofalo are obtained together with Monkman-Grant and Larson Miller parameters. It has been shown that the constitutive equations in the form of Garofalo law predict the creep behavior of the material more accurately. The results show that the creep life relationships obtained based on both Garofalo and Norton power law give acceptable estimation. Also, the results are implemented in reference stress method to produce a set of loci for the optimal geometry of the tubes under combined loadings with maximum creep lifetime.