Prior austenite grain measurement: A direct comparison of EBSD reconstruction, thermal etching and chemical etching

It is well known that the prior austenite grain (PAG) microstructure of steels has a significant impact on their microstructure evolution and mechanical properties. Many PAG measuring techniques have been developed over many decades, with the effectiveness of each varying alloy-to-alloy. Some of the more common techniques, specifically for bainitic and martensitic grades, are thermal etching and picric acid etching, which both involve the preferential etching of PAG boundaries in order to reveal the austenitic microstructure. More recently, parent grain reconstruction techniques using EBSD (electron backscatter diffraction) mapping have shown good promise in recreating PAG microstructures from BCC-FCC orientation relationships, and can now be deployed at speed with the advent of rapid detectors (1000s of pixels per second). This study aims to compare the accuracy and relative advantages/disadvantages of picric acid etching, thermal etching and EBSD reconstruction methods. A TESCAN and NewTec In-Situ Testing (TANIST) capability was used to directly observe and measure the true high-temperature PAG structure during the austenitisation of SA-540 B24 low alloy steel. These high-temperature PAG measurements were then compared to measurements from the same area obtained using thermal etching, picric acid etching and EBSD parent grain reconstructions after quenching to room temperature. Reconstructing the parent austenite grains from EBSD data resulted in the closest measurement of PAG size. PAG boundaries were delineated well by thermal etching but surface effects (such as surface relief and ghost traces) created complexities when identifying the exact position of boundaries. Picric acid etching, which produced the least accurate measurement, was found to reveal PAG boundaries well, however it was limited by its ability to sufficiently etch annealing twin boundaries and its susceptibility to microsegregational effects. EBSD reconstruction and thermal etching were more consistent at reconstructing/revealing these boundaries, although inaccuracies with the techniques were still observed.