A GENERALIZED MICROSTRUCTURE MODEL FOR MICROALLOYED HIGH STRENGTH PIPELINE WELDMENTS

Prediction of strength and toughness of weld metal (WM) and heat-affected zones (HAZ) are very important for the design of high strength pipeline steel welding. Although, there exists a wealth of semi-empirical, phenomenological and physics based models, these models are not generalized enough to predict both WM and HAZ microstructures. This paper pertains to a comprehensive project focusing on developing a generic microstructure model that can predict the microstructural constituents in the WM and HAZ regions with minimum experimental calibration. In this work, sensitivities of published austenite grain growth and simultaneous transformation kinetic models on final microstructure were calculated. The evaluations were performed for typical steel compositions corresponding to X70 and X100 line pipe steels and controlled thermal cycles with different peak temperatures. The austenite grain growth models indicated a complex interplay between the activation energy for grain growth and precipitate coarsening. The transformation kinetic models indicated a competition between allotriomorphic and Widmanstätten ferrite can be modified by promoting intragranular nucleation of idiomorphic ferrite. Possible use of these models for improving the HAZ properties of high-strength steel pipelines is proposed.