Wüstite (FeO)

Name: Wüstite (FeO)
Diagram No.: 978
Type of diagram: TTT
Chemical composition in weight %: Steel sample: 0.0021% C, 0.04% Si, 0.12% Mn, 0.009% P, 0.006% S, balance is Fe
Group: Oxides
Note: Oxide scale on the steel samples.
The time-temperature-transformation (TTT) diagrams for each initial scale structure are obtained based on the scale microstructure observed by SEM. Figures 2 show the diagram and typical examples of SEM images for 550°C pretreatment, i.e. for bilayer initial scale structures, respectively. When the scale is kept at 500°C, magnetite precipitations appear from the interface of magnetite and wüstite, and grow in the direction of thickness. At temperatures of between 400 and 450°C, magnetite precipitations from the magnetite/wüstite interface are initially observed, followed by a small magnetite layer at the wüstite/steel interface. Finally the eutectoid magnetite/ferrite structure appears for transformation times exceeding 30 min. At temperatures below 400°C, the granular magnetite precipitations appear from the inside of wüstite.
Next, the diagram and corresponding SEM images for 700°C pretreatment, i.e. the monolayer scale of wüstite alone, are shown in Figs. 4, respectively. When the scale is kept at 500°C, no magnetite precipitations appear, but they do emerge from the wüstite/steel interface at temperatures of between 400 and 450°C, which suggests that the monolayer scale can easily generate a magnetite seam at the wüstite/steel interface compared with the bilayer scale. Finally, the eutectoid structure appears, but it takes about 240 min, which is longer than the case of the bilayer structure. At temperatures below 400°C, the granular magnetite precipitations appear from inside the wüstite layer, which is same as for 550°C pretreatment.
Reference: Not shown in this demo version.

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