2004 Volume 45 Issue 4 Pages 1299-1305
Steam oxidation resistance of 50Ni-50Cr coatings sprayed by a high velocity oxyfuel process on modified 9Cr-1Mo steel substrate was evaluated in the temperature range of 873-1023 K. The present study focuses on the microstructural changes during the oxidation tests. The coated samples showed the formation of chromium carbide at the coating/substrate interface during the steam oxidation. The magnitude of the chromium carbide formation was increased with increase in the temperature and the duration. Microstructural examination of the etched cross sections showed that beneath the chromium carbide layer the substrate microstructure transformed from tempered martensite to higher grain-width ferrite microstructure. The carbon depletion in this region resulted into longer ferrite grain formation. Micro hardness studies showed that at the coating/substrate interface the micro hardness values increased with increasing the test temperature and the duration. The substrate layer just beneath the coating showed the decreased hardness values compared to the core substrate region.