E-Thesis 409 views 39 downloads
Surface Oxides on Advanced High Strength Steels (AHSS): Effect on Applications and Appearance / EDWARD SIMBANEGAVI
Swansea University Author: EDWARD SIMBANEGAVI
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Copyright: The author, Edward T. Simbanegavi, 2022. Released under the terms of a Creative Commons Attribution-Only (CC-BY) License. Third party content is excluded for use under the license terms.
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Abstract
During the annealing of advanced high strength steels (AHSS) in a continuous annealing and processing line (CAPL), alloying elements with a high Oxygen affinity diffuse to the surface causing external oxidation. This results in a surface discolouration that is a departure from the typical finish obs...
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Swansea
2022
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| Institution: | Swansea University |
| Degree level: | Master of Research |
| Degree name: | MSc by Research |
| Supervisor: | Abdallah, Zak |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa61650 |
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| Abstract: |
During the annealing of advanced high strength steels (AHSS) in a continuous annealing and processing line (CAPL), alloying elements with a high Oxygen affinity diffuse to the surface causing external oxidation. This results in a surface discolouration that is a departure from the typical finish observed in dual phase (DP) AHSS steels, which are a type of first generation (1G) AHSS. This can result in decreased confidence in the product’s performance from the customer’s perspective. Therefore, it is important to characterise the surface, and investigate any potential impact on performance. Previous work carried out with regards to applications i.e., contact resistance, highlighted that the darker samples exhibited higher contact resistance values, which would in turn negatively impact their welding performance. Therefore, it is important that surface chemistry be analysed i.e., oxides’ phases be identified; the elements responsible to establish their effect on e.g., wetting performance. It is also important that this work needs to be accessible to both technical and non-technical users. The following research focused on analysis of both ‘best and worst case’ scenarios with regards to surface discolouration for DP800 and DP1000 grade AHSS. The characterisation of the oxide layer has been carried out using optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). SEM and EDS results showed that whilst there were alloying elements present within the oxide layer for both grades of metal, the cross-sectional width of this layer was very small. XPS was also carried out to complement/verify the EDS results. XPS analysis showed that alloying elements such as Mn and Cr diffused to the surface to form oxides (Mn2p, Cr2p) these oxides and Carbonates (Ca2p) are responsible for the surface discolouration observed. This was shown by the concentration of O1s with the typical binding energy for the metal oxides coupled by the peaks for the following spectra: Mn2p1/2 and Mn2p3/2 and Cr2p3/2. The quantity was measured as %atomic concentration of the peak fitted models, this was shown to vary for all the samples depending on the severity of the discolouration i.e., the more discoloured the samples, a higher % atomic concentration was observed at the surface. TalySurf surface roughness average values showed that for the DP1000 (C9321101), DP800 (C7025701) and (6491201) showed very little deviation for majority of the values despite the difference in surface appearance. This is likely due to all samples had to be within the release criteria for roughness for optimal performance in applications such as coating, therefore it can be concluded that surface appearance has a negligible effect on average roughness (Ra), maximum height above (Rp) and below (Rv) the mean line, Rsk was negative for all samples showing the presence of holes on the surface. DP800 had high root mean squared roughness values (Rsk) showing a high number of peaks and valley for the sample. Rv>Rp meant all samples regardless of grade has deeper valleys compared to peaks. |
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| Keywords: |
Materials Engineering, Advance High Strength Steel, Surface Oxidation |
| College: |
Faculty of Science and Engineering |