Bauer, V. (2015). LA-ICP-MS analysis of boron additives in PM-steel samples [Diploma Thesis, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2015.23750
E164 - Institut für Chemische Technologien und Analytik
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Date (published):
2015
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Number of Pages:
76
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Keywords:
LA-ICP-MS; PM-steels; Boron
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Abstract:
The activated sintering of steels by addition of boron is an attractive method to attain high densities. But still there is the danger of an enrichment of brittle phases on grain boundaries, as the solubility of boron is extremely low which on the other hand is a key factor for the sintering with activating liquid phase. Chromium as an alloying element changes the solubility a little, and therefore the sensitivity against the formation of these brittle phases. Also the sintering atmosphere plays a major role, as boron tends to react with hydrogen and nitrogen. When using argon as an inert sintering gas, the formation of volatile boride compounds is prevented but porosity of the bulk material occurs. The quantitative analysis of boron is known to be very tricky, as the usual methods such as XRF and SEM-EDS, are not very sensitive because of the low atomic number of boron. In this work, Laser Ablation -Inductively Coupled Plasma Mass Spectrometry analysis, LA-ICP-MS, is demonstrated on boron containing PM-steels. It is a very sensitive and accurate detection technique which enables measurement of most of the elements and provides high sample throughput. Moreover, for boron there aren-t any noticeable interferences with other elements which could disturb the analysis. However, LA-ICP-MS measurement of solid samples requires matrix matched calibration standards since the ablation process is strongly matrix dependent which is also the downside of that instrumental analysis. The goal was to allow valid and representative quantification of boron in steel samples. The Development of measuring methods or rather the optimization of measurement conditions was a major key point. Also, evaluation of physical and mechanical properties as well as a metallographic examination were carried out to establish a correlation between analytical results and material characteristics. In the course of this work, the successful set up of a calibration function for the quantification of boron in solid steel-samples could be accomplished by utilizing stock solution for liquid measurements and self-made, vacuum-sintered pellets as standards for solid-sample measurements. Based on that, the total concentration of boron in hydrogen- and argon sintered sample bodies could be quantified. Investigations show that boron enhances densification in PM-steel samples, forming a liquid eutectic phase which increases diffusion of material during sintering. Low solubility of boron leads to the formation of a brittle boron-rich phase at grain boundaries, resulting in poor mechanical properties in PM-steels. The imaging of sample cross sections reveals those boron-rich phases and emphasizes a much less uniform distribution of boron in argon sintered samples. Results for LA-ICP-MS analysis of boron depth-profile show lower normalized boron signals at surface adjacent areas compared to those in the bulk material for hydrogen-sintered samples, suggesting a concentration gradient due to the formation of volatile borides during sintering. Additionally, hydrogen-sintered samples exhibit lower quantities of boron compared to their concentration equivalents which were sintered in argon, when measuring total boron concentration with the LA-ICP-MS. The optimization of this specific method is far from done, whereby methods enabling the quantification of bulk concentration of boron have successfully been established. Improvements in imaging (lateral quantification) and analysis of depth profile of boron in terms of spatial resolution are primarily limited to the inhomogeneous distribution of boron in investigated solid samples. Therefore, there is a need for enhancements for the fabrication of solid standards in order to improve homogeneity, otherwise purchasing certified standard material should be taken into consideration. Finally, replicates of investigated samples need to be analyzed in order to validate the results and achieve statistical security.
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