Identification and visualization of fine precipitates in a Cr-Mo steel based on SEM contrast

Precise characterization of fine precipitates in steels is crucial owing to the influence of these precipitates on the mechanical properties. Various types of carbide precipitates occur in heat-resistant Cr-Mo steels. Their identification typically requires characteristic X-ray and diffraction analysis because it is usually not possible to differentiate carbides based solely on their size and morphology. Modern scanning electron microscopes equipped with multiple image detectors can provide abundant microstructural information. Thus, we investigated the use of secondary electron (SE) and backscattered electron (BSE) image contrast to determine the types of precipitates in the commonly used 2.25Cr-1Mo steel. A mechanically polished bulk specimen was examined and the observation parameters were optimized by systematically varying the SEM accelerating voltage and working distance. We found that the use of a low accelerating voltage and short working distance enabled the differentiation of four types of carbides and AlN precipitate in the steel specimen. A small penetration depth of the primary electrons and selective acquisition of SE/BSE were the key to extracting the material contrast. This technique is expected to allow quantification of the size, distribution, and area fraction of precipitates over large areas of bulk specimens. (author)