Oxide reduction and oxygen removal in water-atomized iron powder: a kinetic study

Johan Wendel,Eduard Hryha,Lars Nyborg,Swathi K Manchili

doi:10.1007/s10973-020-09724-6

Johan Wendel, Eduard Hryha + Show 2 more

Open Access

https://doi.org/10.1007/s10973-020-09724-6

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Abstract

Reduction of oxides during sintering is a prerequisite for the manufacturing of powder metallurgy steels. Inadequate control of the sintering atmosphere may impede sinter neck formation and cause entrapment and growth of oxides in sinter necks, ultimately deteriorating the mechanical properties of sintered components. In this study, the oxide reduction and oxygen removal in water-atomized iron powder was investigated by means of thermogravimetric analysis in pure hydrogen. Two principal mass loss events were recorded, corresponding to the removal of the surface oxide layer at around 400 °C and reduction of internal and stable oxides in the range 600–1350 °C. The apparent activation energies of these mass loss processes were determined by means of kinetic analyses, giving values around 100 kJ mol−1 and 200–400 kJ mol−1, respectively. The validity of the results was asserted using hematite reference samples which displayed good correlation with the reduction of the surface oxide layer, thereby showing that the powder surfaces are covered by an Fe2O3 oxide. The high-temperature mass loss, with no analogy in the reference samples, is believed to originate from a combination of oxygen removal from internal oxides and stable oxide particulates on the surface. Analysis of the oxide reduction in iron powder compacts show a slightly lower activation energy for the oxide layer reduction, indicating an influence of the compaction step on the initial state of the powder and oxide layer. At the same time, the high-temperature mass loss event was shifted to higher temperatures, which is believed to be caused by the increasingly restricted mass transport of reduction products along the pores in the sintered compact.

Highlights

The reduction of iron oxides is widely studied due to the industrial importance of iron and steel [1, 2]
Iron oxide reference samples were used to establish a baseline for the subsequent thermogravimetric experiments and kinetic analyses of the water-atomized powder and its compacted state
It should be noted that the actual recorded mass losses will be a function of the heating rate of the experiment as the reduction steps show some degree of overlap and become difficult to separate as a consequence of the different activation energies of the reaction steps

Summary

Introduction

The reduction of iron oxides is widely studied due to the industrial importance of iron and steel [1, 2]. Iron oxide reference samples were used to establish a baseline for the subsequent thermogravimetric experiments and kinetic analyses of the water-atomized powder and its compacted state. The compacted sample shows a similar reduction behavior as the powder, but with three main differences; (1) the onset of the surface iron oxide reduction is at lower temperatures, whereas the completion is at higher temperatures, (2) the mass loss in the ferrite region is lower, and (3) the high-temperature mass loss is shifted to higher temperature by around 50 °C.

Results

Conclusion