جواد بهزادي فر

Abstract: The acceptable wear behavior of nano-structured bainitic steels has increased their industrialization potential, and reducing the density of these steels can expand their applications in the automotive industry. In this research, the wear behavior of high-strength nano-structured bainitic steel containing 3.5 weight percent aluminum (cost-effective and low-density with a chemical composition of Fe-0.8C-1.5Mn-0.6Cr-0.35Mo-3.5Al) was investigated, eva‎luating the relationship between microstructure (austenitization temperature and time) and the applied load on the sliding wear resistance and dominant sliding wear mechanism in this steel. Accordingly, by qualitatively and quantitatively examining the microstructure using XRD and FE-SEM, the existing phases were identified, and the volume fraction of phases, residual austenite carbon content, and average thickness of bainitic ferrite plates under various isothermal treatment conditions were calculated. Then, considering a combination of the highest volume fraction of bainitic ferrite with residual carbon-rich austenite and the maximum hardness in the shortest possible time, the optimal time at each of the isothermal treatment temperatures was selected. Tensile tests were performed on the optimal samples, and dry sliding wear tests under applied loads of 50N, 100N, and 150N were conducted on samples that fell within the mechanical properties range of nano-structured bainitic steels. The results indicated that in this steel, the volume fraction of residual austenite and its carbon content (the stabilizing factor of residual austenite) is not the only important factor in determining sliding wear behavior; rather, the thickness of bainitic ferrite plates (the strengthening factor) is the most significant factor affecting its sliding wear performance. With the increase in the microstructural parameter V_(α_b )/t_(α_b ) due to a decrease in the austenitization temperature, the specific wear rate decreases, and the sliding wear resistance increases. Moreover, as the wearing conditions change from mild (low load 50N) to severe (high load 150N), the dominant sliding wear mechanism shifts from oxidative wear to adhesive wear, and the thickness of the tribological transformation zone (TTZ) increases. Additionally, the tribological behavior of this steel is controlled by the formation of various tribological layers on the sliding surfaces, and the microstructure before the sliding wear test does not significantly affect the average coefficient of friction in its steady state. This study demonstrated that for a carbon-rich steel containing 3.5 weight percent aluminum, by performing an austempering at 250 °C for 24 hours, a microstructure consisting of bainitic ferrite plates with an average thickness of 53 nm surrounded by carbon-rich residual austenite can be achieved, exhibiting mechanical properties such as a hardness of 561HV30, yield strength of 1614MPa, ultimate tensile strength of 1981MPa, along with a total elongation of 9.12% with minimal weight loss, the lowest specific wear rate, and the highest dry sliding wear resistance under applied loads of 50N, 100N, and 150N after a distance of 1000m.

سايش چسبان , سايش اكسيدان , مقاومت به سايش لغزشي خشك , محتواي كربن آستنيت باقيمانده , پارامتر ريزساختاري V_(α_b )/t_(α_b ) , دما و زمان آستمپرينگ , 3/5درصد وزني آلومينيوم , نانوساختار بينايتي

Keywords: nanostructured bainite, austempering temperature and time, microstructure parameter V_(α_b )/t_(α_b ), retained austenite carbon content, dry sliding wear resistance, wear mechanism of oxidative and adhesive , nanostructured bainite , austempering temperature and timee , microstructure parameter V_(α_b )/t_(α_b ) , retained austenite carbon content , dry sliding wear resistance , oxidative and adhesive wear

Javad behzadifar

Dr. Seyed Mohammad Ali Boutorabi & Dr. Hassan Saghafian Larijani