OXIDATION OF HIGH-TEMPERATURE ALLOYS (SUPERALLOYS) AT ELEVATED-TEMPERATURES IN AIR .2.

In continuation of previous work, the oxidation behavior of three more alloys, namely UHBA 25L, Sanicro 28, and Inconel 690, was studied at high temperatures (600 to 1200 degrees C). The oxidation kinetics of UHBA 25L and Sanicro 28 followed the parabolic-rare law at 800 and 1000 degrees C. At 600 degrees C oxidation rates were very low, while at 1200 degrees C the parabolic-rate law was initially observed, followed by ''breakaway.'' Both alloys suffered extensive spalling at all temperatures, despite their high Cr contents of about 25% and 27%, respectively This is attributed to significant amounts of Mn present. Sanicro 28, with higher Cr, suffered an early breakaway at 1200 degrees C because of the presence of Mo, which forms a low-melting oxide. Inconel 690 showed mixed behavior at 600 and 800 degrees C, parabolic at 1000 degrees C and cubic at 1200 degrees C. Inconel 690 and Incoloy 800H gave the best overall performance for the range of temperature and exposure time under study. This may be ascribed to the absence of Mo, the presence of small amounts of Ti and Al, and relatively small amount of Mn in these alloys.