Application of Integrated Computational Materials Engineering (ICME) Tools for Improving Fracture Toughness on ERW Line Pipe

This study addressed an integrated computational materials engineering (ICME) approach to understanding the relationships between in-line annealing conditions and low-temperature fracture toughness of electric resistance welded (ERW) line pipe. Work was based on a pipe mill located at Axis Pipe and Tube in Bryan, Texas. The major assumption leading into this work is that the root cause for low-temperature fracture toughness in ERW welds is the presence of a decarburized band along the bond line. This decarburized band is inherent to the ERW welding process, providing a region of highly constrained undermatched material. In the presence of a flaw, a highly constrained fracture along this band can result. For Charpy testing at the bond line, dramatic reductions in fracture energy can be observed. In-line annealing offers the potential for back diffusion of carbon into this bond line, alleviating the fracture toughness concern. To characterize relationships between weld geometry, annealing processing conditions, and the resulting recovery of carbon in the bond line, ICME tools were employed. The approach here has been to use a series of ICME tools to accomplish this effort. This has included creating analytical constructs for pairing the coil with the pipe as well as for subsequent heat transfer in the workpiece itself. Microstructural modeling to address temperature-dependent carbon diffusivity and final phase distributions was done by a combination of design of experiment (DOE) techniques and commercial software. Essentially, commercial software was used to generate microstructural data as a function of processing and steel compositions that could be mapped using the DOE tools. Finally, carbon back diffusion was modeled using a one-dimensional approach with linearized compositional gradients. Tools were integrated into a stand-alone computer application. This application estimated thermal profiles, final microstructures, and degrees of carbon recovery based on inputs of steel geometry, chemistry, and annealer processing conditions. Results were compared with experimental work conducting Charpy impact testing at − 20 °C, using a GLEEBLE thermo-mechanical simulator to replicate various annealing conditions. The results showed a direct correlation between the predicted degree of carbon recovery and the subsequent low-temperature Charpy impact energies.