L52347 Microstructure Model for Welding Simulations

Welding of micro-alloyed, high strength steels such as X100 poses a number of challenges due to, in part, the sensitivity of weld mechanical properties to the welding parameters such as heat input, preheat temperature, etc. For design purpose, it is often required that the mechanical properties of the welds overmatch those of the pipe materials in terms of yield strength, ductility, and toughness. In general, high strength pipe steels exhibit lower strain hardening capability, lower ductility, and increased anisotropy than the traditional lower grade steels. For these steels, when exposed to welding process, the heat-affected zone (HAZ) in the base metal can be softened and strain localization can take place. For weld metal, it is even more difficult to maintain a balance of ductility and fracture toughness at high strength because the weld metal performance is highly sensitive to welding parameters in comparison with lower strength steels.

Gas metal arc welding (GMAW) processes have become the popular choices for pipeline construction. In recent years, a number of high-productivity variants of GMAW such as tandem wire and dual torch have been employed for field welding in order to increase welding productivity. These new GMAW variants introduce more welding variables and further complicate the relationship between weld mechanical properties and welding conditions.

To understand the relationship between the mechanical properties of the weldment and the welding conditions, an effective approach is to simulate the thermal and microstructure processes in GMAW processes. By predicting the thermal cycles and the phase transformations, the microstructure of both weld metal and its HAZ can be determined numerically. With the assistance of experimental measurements, it is possible to identify the welding essential variables and evaluate their influences on the final weld performance. For this reason, an integrated thermal-microstructure model was developed in the project. The in-depth discussion of the thermal model was presented in a separate technical report[2]. This report will cover the microstructure model only.