Microstructure-Fracture Toughness Studies in Friction Stir Processed API X80 Pipeline Steel

摘要

The evolution of microstructure associated with friction stir welding/processing is very complex. This makes microstructure-property-parameter studies very challenging. Although, it has been studied extensively in friction stir welded aluminum, there has been limited work in steels. The complexity of such studies increases when allotropic phase transformations are present such as in steel and titanium. In this paper, Electron Backscattered Diffraction is used as a means to reconstruct prior austenite grain structure and orientation in order to investigate the effect of microstructure on fracture toughness of different welds as a function of heat input. Additionally, room temperature textures were generated to study the nature of deformation. Results show that post-weld microstructures change from polygonal ferrite to lath ferrite with increasing heat input, and fracture toughness decreases with increasing heat input. Ferrite exhibits well-defined shear deformation texture components. However, texture intensity and fraction of the components do not vary significantly with heat input. Prior austenite reconstruction results suggest that variant selection along austenite S3 boundaries may be the cause for low fracture toughness observed in samples with high heat input. Variant selection in austenite S3 boundaries leads to elongated parallel ferrite laths which have low misorientations across austenite grains.