The Long-Term, Cyclic-Oxidation Behavior of Selected Chromia-Forming Alloys

Long-term, cyclic-oxidation testing in still airfor about 2 years (720 days) at 982°C and 1 year(360 days) at 1093, 1149, and 1204°C has beenconducted on the commercial, high-temperaturechromia-forming HR-120®, HR160®, and 230® alloys(all trademarks of Haynes International, Inc.). Eachthermal cycle consisted of 30 days at temperaturefollowed by about 4 hr at ambient. The resultsdemonstrated the significant effects of alloy composition on long-term,cyclic-oxidation resistance. Each of the alloysexhibited scale spallation; however, the manner by whichspallation occurred varied between the alloys. The 230 alloy, which contains 0.02 wt.% La, exhibitedpartial scale spallation, thus allowing for the easierformation of a protective or semiprotectiveCr2O3-rich scale during subsequentoxidation. The HR-160 alloy exhibited complete spallation owinglargely to its relatively high silicon content (2.75wt.%). However, the silicon was also beneficial inpromoting protective or semiprotective scale formationwhen the exposed alloy was subsequently oxidized.The HR-120 alloy showed the poorest cyclic-oxidationresistance, due in part to poor scale adhesion and thetendency of the iron in this alloy (33 wt.%) toeventually oxidize and result in the formation of aless-protective scale. All of the alloys underwentinternal attack in the form of internal oxidation andvoid formation. In most cases, the extent of internalattack was significantly greater than that of metalloss.