Oxidation Start Detection on Heating Parts of Thermogravimetry Curves for High Temperature Alloys Based on Nickel, Cobalt or Iron

The heating parts of thermogravimetry curves performed for cast alloys in air at high temperatures were analysed, after correction from the effects of air buoyancy variation, to characterize the transient oxidation before reaching the targeted temperature for the isothermal stage usually applied thereafter. This was done here for three binary M-30Cr alloys, six ternary M-30Cr-0.4 and 0.8%C alloys, and three M-30Cr-0.4C-6Ta quaternary alloys, heated in synthetic air at 20 K min-1 until 1000, 1100 or 1200°C. The cobalt alloys begin to be oxidized with a sufficient mass gain to be detected by thermogravimetry, earlier than the nickel or iron alloys (i.e. at lower temperatures). The temperature of oxidation start is lowered by the presence of tantalum for the three families of alloys. When the carbon content (or the carbides density) increases in the alloy, the temperature of oxidation start decreases for the cobalt alloys and remains almost constant for the nickel or iron alloys, while the total mass gain due to oxidation during heating increases for the nickel and cobalt alloys and decreases for the iron alloys.