THE LONG-TERM, CYCLIC-OXIDATION BEHAVIOR OF SELECTED CHROMIA-FORMING ALLOYS

Long-term, cyclic-oxidation testing in still air for about 2 years (72 0 days) at 982 degrees C and 1 year (360 days) at 1093 degrees, 1149 d egrees, and 1204 degrees C has been conducted on the commercial, high- temperature chromia-forming HR-120(R), HR-160(R), and 230(R) alloys (a ll trademarks of Haynes International, Inc.). Each thermal cycle consi sted of 30 days at temperature followed by about 4 hr at ambient. The results demonstrated the significant effects of alloy composition on l ong-term, cyclic-oxidation resistance. Each of the alloys exhibited sc ale spallation; however, the manner by which spallation occurred varie d between the alloys. The 230 alloy, which contains 0.02 wt. % La, exh ibited partial scale spallation, thus allowing for the easier formatio n of a protective or semiprotective Cr2O3-rich scale during subsequent oxidation. The HR-160 alloy exhibited complete spallation owing large ly to its relatively high silicon content (2.75 wt.%). However, the si licon was also beneficial in promoting protective or semiprotective sc ale 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-pro tective scale. All of the alloys underwent internal attack in the form of internal oxidation and void formation. In most cases, the extent o f internal attack was significantly greater than that of metal loss.