Study on the creep deformation behavior and characterization of 9Cr-1Mo-V-Nb steel at elevated temperatures

Abstract

In this study, creep tests were conducted on a set of specimens made from the same tube of T91 steel at different temperatures (600 °C, 650 °C and 700 °C) with the applied stresses ranging from 30 MPa to 180 MPa and rupture times from 28.7 to 7957 h for understanding the creep mechanism. The stress dependence of minimum creep rates exhibits a power law with different exponents were found to be 14.3, 11 and 4.6 at 600 °C, 650 °C and 700 °C, respectively. Activation energy of creep was determined to be ~573 kJ/mol, which is ~53% higher than that of the lattice self-diffusion in α-iron. Based on the stress exponents, the modified Bird-Murkherjee-Dorn (BMD) equation was used to find the threshold stresses which has shown the strongly dependent on the test temperature. After incorporating the threshold stress, true activation energy of creep was found to be decreased ~271 kJ/mol, which is equivalent to that of lattice-self diffusion in α-iron. This subsequently led to evaluation of true stress exponent ~4. The results of creep data suggest that the rate-controlling mechanism of creep is dislocation climb for T91 steel. Microstructural observation of T91 steel is mainly consisting of lath martensite structure with fine MX carbonitrides and M23C6 carbides were characterized using the scanning and transmission electron microscopes. The stress dependence of creep rupture life has shown that the operating mechanism of creep is similar. Furthermore, creep data was analysed for validity of the Monkman-Grant relationship and creep damage tolerance parameter. The qualitative microstructure of the crept samples show that less number of MX carbonitrides within grains is responsible for creep cavity formation and decreasing the rupture life. Thus, microstructural degradation is attributed to M23C6 carbides coarsened and dislocation sub-structure boundaries are mainly responsible for creep damage. The fracture morphology of the samples has revealed with dimples is a typical transgranular ductile failure.