Thermal analysis of ball-milled Fe–14Cr–3W–0.4Ti–0.25Y2O3 ferritic steel powder

Abstract

The contamination from the air (the identification of the impurity phases and their thermal evolution) of Fe–14Cr–3W–0.4Ti–0.25Y2O3 oxide dispersion-strengthened ferritic steel powders ball-milled for different times was evidenced by thermal analysis in correlation with X-ray diffraction and scanning electron microscopy. The powders loaded under an argon atmosphere and milled up to 170 h with and without interruption of the milling process were compared. A steady state of Fe–Cr alloying was reached within the first 12 h. The reflections corresponding to a new phase with an fcc-CrN structure were found in XRD patterns of powders milled for a long time with interruption. The differential thermal analysis of powders milled over 12 h with interruption shows an exothermic peak at 625.5–653 °C, ascribed to the coarsening of the fcc-CrN, and an endothermic reaction above 1000 °C accompanied by a mass loss in thermogravimetric analysis. (Cr,Fe)2O3 and retained austenite were also found upon the heating of as-milled (with interruption) powders to different temperatures. The endothermic feature was associated with the decomposition of CrN accompanied by the degassing of N2. The observed phenomenon was explained to be the manifestation of contamination with nitrogen and oxygen from the air during the milling; the rate of contamination with nitrogen was estimated to be 0.011 mass% h−1. The contamination level depends on the degree of alloying at the moment of the interruption of milling process. The contamination of the powders milled without interruption of milling process was insignificant. The experimental conditions described in this work can be further developed for the removal of impurities (e.g. nitrogen, nitrides, oxides) from alloyed ferritic steel powders.