Image-processing-based tool for carbide dissolution analysis in bearing steels

Carbide volume fraction and size distribution in 100Cr6 steel have been studied using experimental and computational methods. The industrial production cycle of bearing steels includes spheroidization and hardening steps, which result in coarse globular carbides in the martensitic matrix. It is well known that coarse precipitates adversely affect the mechanical properties. Therefore, microstructural evaluation is necessary to determine the carbide size and distribution. Scanning electron microscopy (SEM) studies have been carried out after austenitization with different annealing parameters. SEM micrographs have been evaluated according to ASTM E 562-11 (manual point count method), as well as by using the ImageJ software and the proposed image-processing-based tool. The proposed tool includes (a) preprocessing steps, such as converting to grayscale, smoothing, thresholding and morphological operations, to improve the results of later processing steps; (b) finding contours and convex hull representation; and (c) calculation of different features, such as volume and size. The experimental results have shown that the proposed tool can be used for analysis of carbide volume fraction and size distribution.