Enhancement of Hot Corrosion Resistance of Modified 9Cr-1Mo Steel Through Surface Nanostructuring and Pre-oxidation

Nanosize grains of similar to 65nm were developed with large misorientation angles of grain boundaries in surface region of normalized and tempered modified 9Cr-1Mo steel by ultrasonic shot peening (USSP) with hard steel balls of 3mm diameter. The surface roughness and the depth of nanostructured surface region were found to increase with the duration of USSP from 0.5 to 5min. Hot corrosion tests were performed for the Un-USSP and the samples USSPed for the durations of 0.25 and 1.5min and also for the samples subjected to USSP and subsequently pre-oxidation at 600 degrees C for 4h. The hot corrosion resistance at 600 degrees C under 100% NaCl coating was significantly improved by USSP for 1.5min and subsequent pre-oxidation at 600 degrees C for 4h. The results are discussed in terms of the formation of more uniform and effective protective iron-chromium oxide layer on the USSPed surface from the subsequent pre-oxidation treatment, prior to hot corrosion of the specimens under 100% NaCl coating at 600 degrees C.