Identification and Visualization of Fine Precipitates in a Cr-Mo Steel Using SEM Contrasts

A precise characterization of fine precipitates is crucial because these precipitates control mechanical properties of steel materials. Different types of carbide precipitates appear in heat resistant Cr-Mo steels. Their identification requires characteristic X-ray and diffraction analysis because it is usually not possible to differentiate carbides from their size and morphology alone. Latest scanning electron microscopes that are equipped with multiple image detectors can give rich information on microstructures. Thus, in this paper, we study the conditions where secondary electron (SE) and/or backscattered electron image contrast will clarify the types of precipitates in a common 2.25Cr-1Mo steel. A mechanically polished bulk specimen was observed by systematically changing the SEM the acceleration voltages and working distances. We have found that low accelerating voltages with a short working distance enables differentiation of four types of carbides and AlN precipitate in this steel. A small penetration depth of primary electrons and selective acquisition of SE/BSE is the key to extracting material contrast. This technique will allow quantification of size, distribution and area fraction of each precipitate for a large area using a bulk specimen.