The Effect of Water Vapour on the Oxidation of High Speed Steel, Kinetics and Scale Adhesion

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An investigation has been carried out into the effects of steam on the adhesion of oxide scales grown on four different chemical compositions of high speed steels. The oxidation tests were carried out in a thermobalance at 650 °C for up to 14.4 ks. The mass gain of the specimens increased with increasing contents of water vapour, and in no case were parabolic kinetics obeyed. Increasing the alloy chromium content reduced the oxidation rate in dry conditions but increases in the V and W content had no effect. In moist conditions, the alloys containing increased Cr and W had the lowest oxidation rates followed by the alloy with increased V levels, however these effects were small. The “adhesion” of the oxide scale was determined using indentation with a Rockwell C diamond and the pull adhesion test to determine the interfacial toughness and the tensile strength of the oxide metal bond, respectively. Generally, the results of the investigation indicated that oxide adhesion was lowest for the specimens exposed to dry conditions, and that with increasing water vapour content, the scale was more adherent. It is noteworthy that whereas the scale formed in dry conditions spalled at the metal oxide interface, that formed in moist gas failed cohesively within the scale, mostly at the Cr spinel/magnetite interface. Examination of the fractured scale after indentation shows that the outer oxide layer became more porous as the water vapour content of the gas increased. Indentation test results showed that an increase in the V concentration was deleterious to oxide adhesion in both the dry and wet environments, whereas an increase in the Cr-content had little effect on adhesion. The results with the pull test were less clear.