The role of austenite twins on variant selection during decomposition in low carbon steels

TL;DR

This study investigates how austenite twins influence variant selection during the decomposition process in low carbon steels, using advanced 3D microscopy and reconstruction techniques to analyze microstructural evolution.

Contribution

The paper introduces a novel application of 3D pFIB-SEM and PAG reconstruction to link austenite twin boundaries with microstructural variant selection in low carbon steels.

Findings

High-temperature twin boundaries influence variant selection.

Large volume 3D analysis enables comprehensive grain analysis.

Twin boundaries govern grain growth and microstructure evolution.

Abstract

Thermomechanical Controlled Processing (TMCP) is widely used to control the microstructure and properties of linepipe or high strength low alloy steels (HSLA). These steels are often joined by welding and used in demanding environments such as the Arctic. In these materials, the thermal path the steel experiences is critical for understanding microstructural evolution during processing. A key step is the solid-state phase transformation during cooling from the high-temperature austenite to the room-temperature microstructure which significantly influences the final mechanical properties. Specifically, the population of different variants and grain shapes that form affect the types and morphology of the grains, and grain boundary network which influence strength and toughness of the final component. In this paper, we apply 3D microscopy using a Xe-plasma focussed ion beam scanning electron microscope (pFIB-SEM) which is equipped with electron backscatter diffraction (EBSD) in a static configuration to characterize the room temperature microstructure of a steel sample, and then use a '2.5D' prior austenite grain (PAG) reconstruction code to explore the relationship between the austenite phase and the room temperature microstructure. A significant result for the present work is the collection, and analysis, of data from a large volume (150 x 150 x 100 {\mu}m3, with a (200 nm)3 voxel size) which enables analysis of a complete prior austenite grain. Analysis of variants within this grain demonstrates that high-temperature twin boundaries likely govern variant selection and grain growth in the child microstructure. This suggests opportunities to engineer novel microstructures by controlling the high-temperature grain boundary character.