Abstract
The secondary-phase particles in metals, particularly those composed of non-metallic materials, are often detrimental to the mechanical properties of metals; thus, it is crucial to control inclusion formation and growth. One of the challenges is determining the three-dimensional morphology and inner structures of such inclusions. In this study, a non-aqueous solution electrolytic method and a room-temperature organic technique were developed based on the principle of electrochemistry to determine the three-dimensional morphologies and inner structures of non-metallic inclusions in Al-killed steel, Si-killed steel, and ductile cast iron. The inclusions were first extracted without any damage to the inclusions, and then the collected inclusions were wrapped and cut through Cu ion deposition. The results revealed that the inclusions in Al-killed steel had an irregular morphology, that those in the Si-killed steel were mainly spherical, and that almost all the spheroidal graphite in the ductile cast iron featured a uniform globular morphology. In addition, nucleation was not observed in the inner structures of the inclusions in the Al-killed steel, while some dendritic or rod-like MnS phase precipitates appeared on the silicate inclusion surfaces, and some silicate-rich phases were detected in their inner matrix. For spheroidal graphite, rare-earth oxides (one particle or more) were observed as nuclei in the center of almost every graphite particle. The formation and evolution of inclusions in these types of metals can be better understood by means of the two developed methods.
Highlights
As secondary-phase particles, non-metallic inclusions are detrimental to the mechanical properties, fatigue life, and strip surface quality of steel
A non-aqueous solution electrolytic method was developed to extract the inclusions from the steel matrices to observe their three-dimensional morphology and surface characteristics
The inclusion diameters range from a few micrometers to more than 100 μm, most of them have irregular morphologies
Summary
As secondary-phase particles, non-metallic inclusions are detrimental to the mechanical properties, fatigue life, and strip surface quality of steel. Many studies have explored inclusion morphology and inner structures with regard to non-metallic inclusion formation and growth mechanisms. Certain measures, such as limiting the precipitation or phase modification of the liquid phase, can be used to reduce the negative effects of such secondary particles, or even to render them beneficial to the overall steel product. Metallographic methods are typically used to detect non-metallic inclusion properties in steels. By this method, one can only observe
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