Análise da microestrutura óptica segmentada e da topografia local em ferros fundidos alto cromo: comparativo entre ferros fundidos com diferentes graus de eutecticidade

Abstract

High-chromium white cast irons are widely used as wear-resistant linings due to the high hardness of the M7C3 carbides embedded in their microstructure and the controlled toughness of their metallic matrices. The present study analyses, in combination, the segmented optical microstructure and the local topography of four high-chromium white cast irons with increasing carbon contents, ranging between 2.57% and 3.36%. The analysed surfaces correspond to the worn tracks from rubber wheel abrasion testing according to ASTM G65 and were observed by confocal microscopy on an Olympus DSX1000. The images from this acquisition provided both the basis for carbide segmentation and the height maps used in the topographic analysis. The separation of matrix and carbides was carried out by automatic thresholding of the luminance of the images, producing binary masks from which morphological descriptors (area, equivalent diameter, circularity, aspect ratio and solidity) were extracted. The height maps were levelled using robust Huber regression, with automatic selection between planar and quadratic trend models through spatial cross-validation, thereby isolating the local variation of the surface. The segmented microstructure was then overlaid on the levelled height map. The qualitative analysis of relief and local depth revealed distinct patterns among the four compositions. In the hypoeutectic compositions, the residual relief is organised at a scale larger than the individual carbide, following the alternation between eutectic colonies and proeutectic matrix corridors. This arrangement indicates that the hard phase and the matrix respond as a single set, rather than in isolation, in the analysed field. The near-eutectic composition showed the deepest and most continuous grooves of the set. The hypereutectic composition showed large primary carbides coexisting with regions of lower relief, with evidence of micro-spalling of these particles. These behaviours were interpreted in agreement with the classical literature on microstructure-wear relationships in high-chromium white cast irons.