Thermal transformation of bementite in air has been studied minutely in the temperature range up to 1200°C, using the differential thermal, thermogravimetric, chemical, and X-ray analyses.
A correlation of thermal and chemical data with phase determinations shows that the chemical reactions caused thermally are dehydration, oxidation of a part of Mn
2+, to Mn
4+, and reduction of Mn
4+ formed at the former stage to Mn
2+, in order of increasing temperature, and bementite transforms, in general, into braunite at the low-temperature oxidation stage and into two phases, i. e. rhodonite and hausmannite, at the high-temperature reduction stage.
The braunite formed at the low-temperature oxidation stage contains a greater amount of SiO
2 in its structure than naturally occuring braunite.
At the high-temperature reduction stage, quartz, if it is present, combines with hausmannite released from braunite and forms rhodonite. This reaction promotes the decomposition of braunite.
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