Efficient reconstruction of prior austenite grains in steel from etched light optical micrographs using deep learning and annotations from correlative microscopy
The high-temperature austenite phase is the initial state of practically all
technologically relevant hot forming and heat treatment operations in steel
processing. The phenomena occurring in austenite, such as recrystallization or
grain growth, can have a decisive influence on the subsequent properties of the
material. After the hot forming or heat treatment process, however, the
austenite transforms into other microstructural constituents and information
on the prior austenite morphology are no longer directly accessible. There are
established methods available for reconstructing former austenite grain
boundaries via metallographic etching or electron backscatter diffraction
(EBSD) which both exhibit shortcomings. While etching is often difficult to
reproduce and strongly depend on the investigated steel’s alloying concept,
EBSD acquisition and reconstruction is rather time-consuming. But in fact,
though, light optical micrographs of steels contrasted with conventional Nital
etchant also contain information about the former austenite grains. However,
relevant features are not directly apparent or accessible with conventional
segmentation approaches. This work presents a deep learning (DL)
segmentation of prior austenite grains (PAG) from Nital etched light optical
micrographs. The basis for successful segmentation is a correlative
characterization from EBSD, light and scanning electron microscopy to
specify the ground truth required for supervised learning. The DL model
shows good and robust segmentation results. While the intersection over
union of 70% does not fully reflect the model performance due to the
inherent uncertainty in PAG estimation, a mean error of 6.1% in mean grain
size derived from the segmentation clearly shows the high quality of the result