Spalling mechanism of oxide scale formed on Cr-Mo steels in steam

Various kinds of Cr-Mo steel tubes are used in superheaters and reheaters of boilers. The troubles related to the spalling of the oxide scale formed on Cr-Mo steels in steam had been reported in the U.S.A. and Europe. The present study is aimed at identifying the allowable operating conditions for boilers using Cr-Mo steels. The effect of various factors on the spalling behavior of the oxide scale formed on Cr-Mo steels in steam was examined and analyzed. Among all the factors considered, the thermal strain generated by the thermal shock showed the strongest effect on the spalling behavior. When the temperature difference of the innner and outer surface of the oxide scale was above 150 K, the spalling of the oxide scale on 2.1/4Cr-1Mo steel tube was observed in the thermal shock tests. The effects of other factors were found to be insignificant for the spalling of the oxide scale.