Sigma-Phase Formation in Heat-Resistant Cast Stainless Steels
Viherkoski, Mimmi (2012)
2012
Master's Degree Programme in Materials Science
Automaatio-, kone- ja materiaalitekniikan tiedekunta - Faculty of Automation, Mechanical and Materials Engineering
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Hyväksymispäivämäärä
2012-05-09
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201205211141
https://urn.fi/URN:NBN:fi:tty-201205211141
Tiivistelmä
Heat-resistant cast stainless steels are widely favoured materials for their resistance to corrosion even at elevated temperatures. These steels maintain their good mechanical properties, for example high strength, even when the material is heated above 650 °C. However, when they are cooled to room temperature, embrittlement often takes place. This loss in ductility is usually due to the formation of the sigma-phase, which is hard and extremely brittle.
This thesis focuses on the formation of the sigma-phase. The factors that affect the formation are presented and studied individually. The theoretical part includes also background information of cast stainless steels and heat-resistant steel grades. The embrittlement phenomenon is discussed in more detail. Probable causes for embrittlement are briefly discussed one by one. The emphasis is kept on the sigma-phase. The question of how the sigma-phase influences the mechanical properties of the steel is covered as well.
The experimental part consisted of the studies on the influences of the sigma-phase on the mechanical properties. The test series was started with long-term exposure (aging) at elevated temperature. Impact toughness tests were performed in order to find out the possible connection between the aging time and the eventual changes in ductility. In addition, hardness values were measured. The experimental part included also microstructural characterisation. Optical microscopy was used in the microstructural characterisation of all test samples. Part of the test samples was characterised using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).
The studied samples showed a major decrease in the impact toughness with increasing time of aging, whereas the hardness values showed only a slight increase. Micro-structural characterisation revealed significant changes in the microstructures of the samples between different steel grades and different times of aging.
This thesis focuses on the formation of the sigma-phase. The factors that affect the formation are presented and studied individually. The theoretical part includes also background information of cast stainless steels and heat-resistant steel grades. The embrittlement phenomenon is discussed in more detail. Probable causes for embrittlement are briefly discussed one by one. The emphasis is kept on the sigma-phase. The question of how the sigma-phase influences the mechanical properties of the steel is covered as well.
The experimental part consisted of the studies on the influences of the sigma-phase on the mechanical properties. The test series was started with long-term exposure (aging) at elevated temperature. Impact toughness tests were performed in order to find out the possible connection between the aging time and the eventual changes in ductility. In addition, hardness values were measured. The experimental part included also microstructural characterisation. Optical microscopy was used in the microstructural characterisation of all test samples. Part of the test samples was characterised using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).
The studied samples showed a major decrease in the impact toughness with increasing time of aging, whereas the hardness values showed only a slight increase. Micro-structural characterisation revealed significant changes in the microstructures of the samples between different steel grades and different times of aging.