S. Krishna Srivastava

Long-Term Oxidation Behavior of Selected High Temperature Alloys

Long-term cyclic oxidation resistance is often needed for high temperature alloys used in gas turbine engines for extending their operating lives. This study evaluates the cyclic-oxidation behavior of commercially available Ni- (HAYNES ® 230 ® , 214 ® , HASTELLOY ® X, and HR-160 ® alloys) and Fe-based alloys (HR-120 ® and 556 ® alloys) in flowing air at 982 o C, 1093 o C, and 1149 o C for a total exposure of one year. Test samples were thermally cycled every 30 days at temperature followed by air cooling to room temperature. Alloy performances were assessed by analyzing the weight-change behavior and extent of attack, as measured by metal loss and average internal penetration. The results clearly demonstrated the effects of alloy composition and temperature on long-term cyclic oxidation resistance. The 214 alloy exhibited superior oxidation performance owing to its ability to form and maintain protective alumina scale. Amongst the chromia-formers, 230 alloy performed the best at all temperatures; while Fe-based alloys exhibited rather poor oxidation resistance due to poor scale adhesion. By contrast, the HR-160 alloy showed the lowest weight-loss at 1149 o C of the chromia-forming alloys; however, this alloy underwent extensive internal attack. This study also compared cyclic oxidation resistance of 230, HR-120, and HR-160 alloys in flowing and still air. It was found that alloy composition has a profound effect on the extent of oxidation in flowing air compared to that in still air. For instance, the Fe-based HR-120 alloy exhibited improved performance in flowing air while Ni-based alloys (230 and HR-160 alloys) performed better in still air. The factors that may have influenced the oxidation behavior of alloys in flowing and still air will be discussed.

HAYNES NS-163 Alloy: A Novel Nitride Dispersion-Strengthened Co-Base Alloy

HAYNES® NS-163® alloy was developed by Haynes International Inc. for high-temperature structural applications with a dual manufacturing approach in mind: the fabrication of components in the readily weldable and formable mill-annealed condition, and their subsequent strengthening by means of a gas nitriding process. The alloy was designed to be conducive to dispersion strengthening by virtue of the formation of internal Ti- and Nb-rich nitrides. Since this process is diffusion-controlled, component section thicknesses are presently limited to approximately 2.0 mm (0.080″). The paper presents key parameters of the devised commercial nitridation process and the resulting microstructures and mechanical properties of sheet samples, which originated from a production-scale heat. Oxidation resistance and the need for coatings at temperatures exceeding 980°C (1800°F) are addressed as well. It is shown that this new alloy has the potential to fill the gap between today’s strongest solid-solution strengthened alloys and the oxide-dispersion strengthened mechanically alloyed materials.Copyright

Long-Term Oxidation Behavior of Various Chromia-Forming Alloys: Effect of Thermal Cycle Length

Thermal cycle length can severely affect long-term oxidation behavior of high-temperature alloys. The present study is focused on the effect of thermal cycle length (twelve 30-day cycles versus six 60-day cycles) on the long-term (360 days) oxidation behavior of various chromia-forming alloys at 982°C, 1092°C, and 1149°C (1800°F, 2000°F, and 2100°F). The alloys included in this study are HAYNES® 230® alloy, 617 alloy, HR-160® alloy, HR-120® alloy, and 800HT® alloy. Alloy performances were assessed by analyzing the weight-change behavior and extent of attack as measured by metal loss and average internal penetration. The 230 and 617 alloys exhibited excellent oxidation resistance under both cycling conditions, presumably due to their ability to form and maintain adherent oxide scale. In particular, alloys with high Fe contents underwent accelerated oxidation attack. There was a significant increase in the extent of attack with increase in number of cycles (i.e. shorter cycle length). Moreover, the effect of cycle length was most pronounced at the highest test temperature (1149°C), and a strong correlation was found between oxidation kinetics and alloy composition as well as oxidation kinetics and the cycle length. HAYNES, 230, HR-160, HR-120 are registered trademarks of Haynes International, Inc. 800HT is a registered trademark of Special Metals Corporation.Copyright