Characterization of the transient oxidation at heating in the case of simple chromium-containing alloys reinforced by carbides

摘要

During heating up to high temperatures, in laboratory experiment as well as in service in industrial apparatus, refractory alloys or superalloys undergo an early oxidation before reaching a stationary temperature. This transient oxidation, which becomes faster and faster when temperature increases, induces a mass gain which finishes being significant. This first mass gain achieved when the targeted temperature is reached, must be known, for example in order to better specify the following parabolic rate. Indeed, the quantity of oxides formed during the heating is related to the initial thickness of the external oxide scale become continuous, the growth rate of which is then governed by ions diffusion through it.This work concerns six simple alloys: three low carbon ones (M-30%Cr-0.2%C) and three high carbon ones (M-30%Cr-0.8%C) where M=Fe, Co or Ni, and the contents of which are expressed in weight percents. The purpose is to consider the mass measurements recorded during the heating parts of thermogravimetry tests performed with these alloys, the targeted dwell temperatures of which were 1000, 1100 and 1200°C. It was looked at which temperature mass change of the sample began to be significant, and the total mass gain obtained during the whole heating was estimated. Preliminarily, the heating parts of the curves were corrected from the apparent variation of mass due to the decrease in Archimede's thrust because of heating (almost linear when temperature has become higher than about 500°C). The temperature at which a "real" (i.e. due to oxidation) mass gain occurred was determined. Secondly, the total mass gain between the temperature of oxidation start (with detectable mass gain) and the targeted temperature (at dwell beginning) has been assessed for each alloy and each temperature. The first results show that the temperature of oxidation start is slightly higher for a high fraction of carbides than for a low fraction of carbides (i.e. oxidation becomes significant earlier for the low carbon alloys than for the high carbon alloys), and in general for cobalt alloys than for the iron-based and nickel-based alloys. In the case of the nickel alloys and of the cobalt alloys, a higher carbides fraction leads to a higher mass gain at the end of heating to a same temperature. However, it is the contrary to what is observed in the case of the iron alloys.