Characterisation of Non-metallic Inclusions in Special Steels using Electrolytic Extraction
Abstract: When producing high strength special steels strict requirements are in place to assure the quality of the final product, both in terms of harmful elements and the content of non-metallic inclusions (NMI). Achieving cleanliness is a gradual process where all the steps of the production process are inter- connected, meaning that it is crucial to know the influence of each step in order to assure the contents of the final product. The purpose of this thesis was to investigate the presence of non-metallic inclusions in wear plate steel, produced through two different process routes, one with an added round of Ca treatment and vacuum treatment. The investigations were performed using electrolytic extraction of in- clusions followed by analysis with a scanning electron microscope (SEM). Backscattered electrons (BSE) were used for imaging and morphology anal- ysis and composition analysis was performed using energy dispersive spec- troscopy (EDS). Four different types of inclusions were found in the samples, whereof only one type is present in liquid steel. It was found that the ini- tial Ca treatment stabilised the composition for the vacuum treated samples, while the sizes of the inclusions found in these samples were scattered. The reverse was observed in the reference samples, where compositions were scat- tered but a mode value could be found for inclusion size at around 2.5 µm. Inclusions in the reference samples were found to have an average composi- tion lying outside the desired liquid region, not achieving full modification of harmful inclusions, while only modified inclusions could be found in vacuum treated samples. Additionally, around 70% of the analysed inclusions in the vacuum treated samples could be found in the liquid region. It was also found that the use of an electrolytic extraction method was appropriate for the investigated steel grade, while highlighting the need for some parameter adjustments to be made for future investigations in order to extract a sufficient number of inclusions.
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