Wear and fatigue properties of isothermally treated high-Si steels

Detta är en avhandling från Luleå tekniska universitet

Sammanfattning: In recent years steels containing Si contents of 1,5% or more have been isothermally transformed in order to obtain a microstructure consisting of bainitic ferrite laths and retained austenite. The refined microstructure and presence of retained austenite has allowed the manufacture of steels with high tensile strength and considerable elongation. The work in this thesis is motivated by the need for more knowledge, especially regarding wear and fatigue properties before they can be used in engineering applications. In this work the rolling-sliding wear resistance and fatigue strength of ausferritic (carbide free bainitic) steels have been investigated. The dry rolling-sliding tribological behavior of 60SiCr7 steel, with 1.65% of Si was investigated in austempered conditions. The obtained ausferritic microstructure contained laths in the sub-micron range. It was found that the retained austenite content decreased and the hardness increased with lower austempering temperatures, and these changes resulted in decreasing the wear rate. The wear behavior of nano-structured ausferritic steels was investigated using the same rolling-sliding conditions as the previous study. If steels of the same hardness are compared, the wear rate was reduced by half in nano-structured steels in relation with the results obtained previously for the 60SiCr7 steel. Initial hardness was an important property in reducing rolling-sliding wear. Surface hardness after wear was inversely proportional to the wear rate. It was found that the increased plasticity obtained from the transformation of retained austenite into martensite (TRIP effect), present in these steels can be beneficial for the wear resistance. The effect of austempering 55Si7 spring steel on its fatigue strength was also investigated. Three heat treatments were done, isothermal transformation at 300 and 350°C respectively and quenching and tempering at 460°C. The samples were tested in rotating-bending fatigue. It was found that by austempering at 300°C the endurance limit was improved by approximately 25% in comparison with the other two heat treatments. The improved fatigue life was mainly due to the carbide-free microstructure. The transformation of austenite into martensite improves fatigue strength, but this depends on the stability of the retained austenite. With the current work some insight has been gained on the behavior of carbide-free bainitic steels in wear and fatigue. The future work will include high resolution techniques in order to further investigate in-depth both the wear and fatigue mechanisms. In addition, other wear modes will be investigated, particularly pure sliding.

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