Characterization of heterogeneous creep deformation in vanadium-modified 2.25Cr1Mo steel weldments by digital image correlation

Abstract

Low alloy ferritic steels are widely used to manufacture critical components that operating at elevated temperatures in the process industry, but a better understanding of the creep responses of their weldments are still needed to provide guidance for the design of high-temperature welded structures. In the present paper, the heterogeneous creep behavior of vanadium-modified 2.25Cr1Mo ferritic steel weldments was investigated by using the digital image correlation (DIC) technique. Three-dimensional plot of creep strain across the weld joint was constructed to intuitively represent the spatio-temporal evolution of creep strain for the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). Combining the analysis of strain contour evolution during creep with the microstructural examination, it is found that creep strain was primarily concentrated in the HAZ, resulting in the creep cavitation at grain boundaries of the coarse-grain bainitic HAZ. In addition, BM exhibited a faster deformation rate than WM during creep. In order to quantitatively describe the heterogeneous creep deformation, a sub-region extensometer method based on the construction of virtual extensometers was proposed to obtain the representative creep strain data of individual regions of weldments. Furthermore, a parameter identification procedure combining a global optimization method (i.e. genetic algorithm) with an initial value determination scheme, was developed to determine the model parameters of BM, WM and HAZ for the hyperbolic-sine creep constitutive model. The results show that the heterogenous creep behavior of the V-mod 2.25Cr1Mo steel weldment can be well characterized by combining the sub-region extensometer method, the hyperbolic-sine creep constitutive model and its genetic algorithm-based parameter identification method.