An Integrated Model to Predict Microstructure and Mechanical Properties in the Heat Affected Zone for X80 Linepipe

There is a complex interplay between welding procedures and the steel chemistry which determines the final engineering performance of the heat affected zone in a large diameter girth weld. This work uses a combination of experimentally determined thermal histories from laboratory scale single and dual torch multi-pass gas metal arc welding (GMAW) with phenomenological models to predict microstructure and mechanical properties in the heat affected zone (HAZ) of an X80 steel. The integrated model consists of sub-models for austenite grain growth, dissolution of Nb based precipitates and austenite decomposition. These models have been calibrated with detailed experimental studies using a Gleeble 3500 thermomechanical simulator. The models are fully integrated so that the austenite grain size and the Nb solid solution level are used as inputs into the austenite decomposition model where these two factors strongly affect the final microstructure. The decomposition model includes ferrite and bainite models with suitable criteria for transition from one model to the other and a simple first order empirical relation to predict the final fraction of martensite/retained austenite (MA). The integrated model has been applied to a variety of thermal scenarios which are derived from experimental measurements of thermal histories including dual torch conditions where, for example, the Nb solid solution level has to be tracked through both thermal excursions into austenite. Using the integrated model, microstructure maps of the HAZ can be generated for the different welding scenarios.