Effect of quenching and post heat-treatment process to the microstructure and physical properties of electro-carb
Limited studies were performed pertaining to the post-heat treatment on low carbon steel that is carburised by electro-carburisation process. In this study, the electro-carburisation (EC) was carried out using the electrolyte mixture of sodium carbonate (Na2CO3) and sodium chloride (NaCl) with molar...
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| Format: | Thesis |
| Language: | English English |
| Published: |
2023
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| Subjects: | |
| Online Access: | https://eprints.ums.edu.my/id/eprint/40854/1/24%20PAGES.pdf https://eprints.ums.edu.my/id/eprint/40854/2/FULLTEXT.pdf |
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| Summary: | Limited studies were performed pertaining to the post-heat treatment on low carbon steel that is carburised by electro-carburisation process. In this study, the electro-carburisation (EC) was carried out using the electrolyte mixture of sodium carbonate (Na2CO3) and sodium chloride (NaCl) with molar ratio of 4:1. The process was done under the temperature of 800 ℃ with constant voltage of 4.0 V for three hours in the carbon dioxide, CO2 gas environment. After electro-carburised, the sample was quenched in either water or oil. Post-heat treatment such as tempering, or annealing was carried out to relieve the residual stresses. After the heat treatment processes, Vickers hardness machine was used to measure the hardness profile from surface (0 μm) towards the core. It was found that the peak hardness of all samples was located at the 100 μm from the surface. The peak hardness of the EC/water quenched sample (WQ, 702 ± 5 HV) was higher than the EC/oil quenched sample (OQ, 571 ± 3 HV). This is due to the high cooling rate of water compared to oil. The high cooling rate of water caused the carbon had inadequate time to escape or react with oxygen, but the carbon was trapped to form the martensite. Apart from that, the peak hardness of the EC/post-heat-treated sample which is tempered (WQ-T, 623 ± 9 HV; OQ-T, 495 ± 4 HV) or annealed samples (WQ-A, 571 ± 3 HV; OQ-A, 367 ± 6 HV) was lower than the EC/quenched samples (WQ and OQ) due to the internal stress was relieved and the rearrangement of structure that lowered the distortion of the steel. Changes in the microstructure such as martensite, tempered martensite and ferrite were observed by optical microscope which the formation of martensite increased the hardness of the sample. While the metal phase was examined by using X-ray Diffraction (XRD). XRD analysis showed that EC/post-heat-treated sample consisted of iron oxide which are the magnetite and hematite. Full width at half maximum (FWHM) values at the XRD peak (between 44.4° to 44.7°) showed that EC/post-heat-treated samples had lower FWHM value than EC/quenched sample, which indicated that the stress was relieved in both EC/tempered and EC/annealed sample. Elemental composition of the samples was determined by Energy Dispersive X-ray (EDX). The EDX analysis showed the carbon content on the outer surface of as-received sample, AR (2.79 wt. %); WQ (5.24 wt. %), WQ-T (5.16 wt. %), WQ-A (2.94 wt. %); OQ (4.73 wt. %), OQ-T (2.78 wt. %) and OQ-A (2.63 wt. %), respectively. Relatively, the carbon content of WQ was higher than OQ sample, whereas the EC/tempered samples had high carbon content compared to the annealed samples, but lower than the EC/quenched samples. The formation of martensite depends on the carbon content of the steel as more martensite was formed with high carbon content, this matched with high carbon content had higher hardness. The oxygen content on EC/post-heat-treated samples was more concentrated compared to other samples which provided the formation of iron oxide on its surface due to occurrence of oxidation and decarburisation. Oxidation and decarburisation happened during cooling process of tempering and annealing. High temperature during the cooling process caused the reaction between oxygen and ferrite. In short, electro-carburisation process increased the carbon content and improved the hardness of the samples. In quenching process, the water quenching provides better hardness than the oil quenching, while tempering or annealing process relieved the internal stresses that led to hardness reduction. |
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