Oxidation behaviour of some modern heat-resistant cast steels

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Abstract
The kinetics and morphological development of the oxidation of a selection of modern heat-resistant cast steels have been examined and compared with those of the traditional material, HK40. The materials examined had Cr contents of 24 to 29 weight percent (wt %), Ni contents of 30 to 46 wt %and in several cases minority additions of Nb, W, or both. One steel contained 3.3 wt % Al. Kinetics were measured gravimetrically over periods of 6 to 100 h and found to be parabolic in all cases except for the Al-containing steel, which oxidized in an irregular and irreproducible fashion. All steels formed an external scale of Cr2O3 with a Mn-rich spinel layer at the outer surface. Beneath this scale was a layer of alloy depleted in both Cr and Mn. Within the depleted layer inter-dendritic carbides had been destroyed, leaving either oxide near the external alloy surface or voids deeper within the alloy.

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