Effects of Minor Elements on the Cyclic-Oxidation Behavior of Commercial Fe-Base 800-Series Alloys

Abstract

The cyclic-oxidation behavior of commercial iron-base 800-series chromia-scale-forming alloys was studied at 1000°C in still air. Each thermal cycle consisted of one day at temperature followed by air cooling to room temperature. Two different types of 800-series alloys were studied (800 and 800HT), with each type being supplied by two different producers. Although the alloys were of similar chemical composition, their variations in minor-constituent contents were apparently sufficiently different to cause marked differences in oxidation behavior. Compared to the 800 alloys, the higher-strength 800HT alloys generally performed the worst, undergoing rapid weight-loss kinetics after an initial period of relatively protective oxidation. The time required for the commencement of rapid weight-loss kinetics depended significantly on the producer of the 800HT alloy. In general, however, the 800HT alloys developed protrusion of Ti- and Cr-rich oxides into the alloy along the alloy/scale interface, which promoted spallation and poorer cyclic-oxidation resistance. The formation of Si-rich oxide particles in the vicinity of the alloy/scale interface could improve cyclic-oxidation resistance; however, higher Si contents would cause internal Si-rich oxide formation and promote the formation of Ti- and Cr-rich oxide protrusions at the alloy/scale interface. Good cyclic-oxidation resistance requires optimization of the minor-element constituents, Si, Ti, and Al in particular.