ISOTHERMAL OXIDATION BEHAVIOUR OF Fe-20Ni-14Cr-6Al-1,8Ti-3,2Mo-0,02Y-0,2C ALUMINA-FORMING AUSTENITIC STAINLESS STEEL AT TEMPERATURES OF 800, 900, AND 1000oC

• Main Author: Ester Veronica Sirait, Clara
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/41969
• Institution: Institut Teknologi Bandung

The total demand for electricity in Indonesia has increased significantly due to population growth, economic growth, urbanization and industrialization. To meet this need, Indonesia produces electricity through various types of plants, of which 40% are electric steam power plants. With fuel consumption in the form of coal which is a non-renewable energy source, fuel savings are necessary with one condition: same electrical energy output. This means increasing process efficiency at the electric steam power plants. According to the principle of thermodynamics, the efficiency of boilers in power plant facilities can be increased by increasing the temperature and vapor pressure. Therefore, materials that can meet the operating conditions are needed. There are many recent developments in metal alloys that can be used as options for this application, ranging from oxide-dispersion strengthened (ODS) alloys, intermetallic alloys, and nickel-based superalloys. However, all of these alloys are either prohibitively expensive or have yet to exhibit the desired combination of mechanical properties, oxidation resistance, and manufacturability necessary to make them viable for widespread use as heat-resistant components in energy-conversion systems. Therefore, an alloy of the type of Alumina-Forming Austenitic Stainless Steel (AFA-SS) was developed. In this study, the chemical composition of the AFA-SS alloy studied was Fe-20Ni-14Cr-6Al-1,8Ti-3,2Mo-0,02Y-0,2C wt% (AFA-m) at temperatures of 800°C, 900°C and 1000°C with isothermal testing time for 2 hours, 20 hours, 50 hours and 100 hours. A series of experiments have been conducted to study the resistance of isothermal oxidation of the AFA-m alloy. Tests are carried out in a horizontal tube furnace under free air conditions. Weighing samples is done before and after testing to study the severe changes that occur. Data on the weight changes to the surface area of each sample are then processed to analyze the oxidation kinetics. Samples that were tested at each temperature with time variations of 2 and 100 hours were then characterized using XRD (X-ray Diffraction), and SEM-EDS (Scanning Electron Microscope-Energy Dispersive X-Ray Spectroscopy). Vickers hardness tests were carried out on as-homogenized samples and samples with 100-hour testing duration. The results showed that the AFA-m alloy had an Fe-? matrix and three types of precipitates called B2-NiAl, TiC carbide, and Laves phase. The AFA-m hardness of as-homogenized sample is 376.25 HV and continues to decrease with increasing operating temperature. The smallest hardness value of the tested samples is at the temperature of 1000°C, which is 355.5 HV. The oxides formed on the surface of the sample are Al2O3, Cr2O3, TiO2, FeO, Fe2O3, Fe3O4, NiO, Cr2NiO4, and some metastable oxide phases. The main protective oxide in this alloy is Al2O3 and the most protective condition is at the temperature of 800°C. The oxidation kinetic rate of the AFA-m alloy is following the logarithmic rate law.