The long-term, cyclic-oxidation behavior of selected chromia-forming alloys

Long-term, cyclic-oxidation testing in still air for about 2 years (720 days) at 982 degrees C and 1 year (360 days) at 1093 degrees, 1149 degrees, and 1204 degrees C has been conducted on the commercial, high-temperature chromia-forming HR-120(R), HR-160(R), and 230(R) alloys (all trademarks of Haynes International, Inc.). Each thermal cycle consisted of 30 days at temperature followed by about 4 hr at ambient. The results demonstrated the significant effects of alloy composition on long-term, cyclic-oxidation resistance. Each of the alloys exhibited scale spallation; however, the manner by which spallation occurred varied between the alloys. The 230 alloy, which contains 0.02 wt. % La, exhibited partial scale spallation, thus allowing for the easier formation of a protective or semiprotective Cr2O3-rich scale during subsequent oxidation. The HR-160 alloy exhibited complete spallation owing largely to its relatively high silicon content (2.75 wt.%). However, the silicon was also beneficial in promoting protective or semiprotective scale formation when the exposed alloy was subsequently oxidized. The HR-120 alloy showed the poorest cyclic-oxidation resistance, due in part to poor scale adhesion and the tendency of the iron in this alloy (33 wt.%) to eventually oxidize and result in the formation of a less-protective scale. All of the alloys underwent internal attack in the form of internal oxidation and void formation. In most cases, the extent of internal attack was significantly greater than that of metal loss.