Investigation of non-metallic inclusions in steel using hot stage Scanning electron microscope

Abstract

Non metallic inclusions in steel are very small non metallic particles trapped inside steel matrix that can have either positive or negative effects on properties of steel. Thus identifying and tracking the behaviour of these inclusions is vital for development of new manufacturing methods and refinement of old ones. Steel is the most used metal in the world and often used as a structural element in buildings, vehicles and tools etc. Therefore improving its properties, such as decreasing weight and improving strength, can have notable positive impact on performance and environmental effects in aforementioned use cases. In this thesis well known non-metallic inclusions were studied by using a hot stage scanning electron microscope and the respective results are discussed. The objective of this experiment was to see if there are any noticeable changes in either size or shape of multicomponent inclusions and if there is any diffusion between NMIs and the steel matrix. At the same time it was evaluated if the hot stage SEM even is suitable for analysing inlcusions in steel.

Hot stage scanning electron microscope was found to be a suitable method for carrying out relatively quick and generalized analysis of non metallic inclusions in steel. Poor resolution and accuracy of EDS, lack of proper depth information and the fact that inclusions have to be exposed limit it's capabilities in more thorough local analysis of inclusions or parts of them. In terms of inclusions analysed in this thesis, it was found that CaS has a tendency to dissolve in to the surrounding steel matrix in multicomponent inclusions at temperatures around 900 °C. Some dissolving was also visible in couple of single component CaS and MnS inclusions. Phase changes in surrounding steel matrix were noted to partially prevent any interaction between inclusions and surrounding steel.