Abstract Scope |
In ERW, rolled steel strips are resistively heated using high-frequency currents and then squeezed together to form a weld bondline. The notched impact toughness near or at the bondline is typically lower than in the pipe body. To help understand the origin of reduced toughness, this study aims to quantify and then model the compositional and microstructural characteristics of the bondline. In the as-welded condition, the bondline depicts a solute-depleted allotriomorphic ferrite phase and elongated, alloy-enriched features. The elemental partitioning during welding is believed to be responsible for the formation of these microstructural constituents. The microsegregation behavior is predicted by assuming diffusion as a one-dimensional planar problem based on Thermo-Calc’s diffusion module (Dictra). The thermodynamic and kinetic calculations suggested that during the squeeze, both liquid and δ-ferrite phases were present. On cooling, δ-ferrite began to solidify and progressed into the liquid. The alloying elements were partitioned into the liquid phase, and therefore, the solute content was lower in δ-ferrite. On subsequent phase transformations, this region will transform into a solute-depleted allotriomorphic ferrite phase. On the other hand, the composition of the solute enriched liquid obtained at the end of the squeeze force suggests that the formation of martensite is possible under the high cooling rates involved during the welding. The simulation results were verified by the electron probe micro (EPMA) analysis. This current investigation illustrates the potential of computational thermodynamics and kinetics simulation tools to understand the phase transformations at the bondline of pipeline grade steels during electric resistance welding. |