Microstructural changes of Al Hot-Dipped P91 steel during high-temperature oxidation

Abstract

The 9Cr-1Mo steel (ASTM P91) was hot-dip aluminized, and its microstructural changes during oxidation were studied. Before oxidation, the coating consisted of (Al-rich topcoat containing a small amount of Al 5 Fe 2 and Al 13 Fe 4 )/(Al 13 Fe 4 -rich, Al 13 Fe 4 -containing alloy layer)/(Al 5 Fe 2 alloy layer containing a small amount of Al 9 Cr 4 precipitates), from the surface. During oxidation at 700-900 °C for 20-100 h, Al diffused inward and the substrate elements migrated outward to broaden and soften the coating, and also to transform (high Al)-Fe intermetallics to (low Al)-Fe intermetallics. The phases in the coating progressively transformed during oxidation as follows; (Al-rich topcoat)/(Al 5 Fe 2 -rich, Al 13 Fe 4 -containing alloy layer)/(Al 5 Fe 2 alloy layer)→(α-Al 2 O 3 scale)/(Al 13 Fe 4 -rich, Al 5 Fe 2 -containing layer)/(Al 5 Fe 2 layer)/(AlFe interlayer)→(α-Al 2 O 3 scale)/(AlFe-rich, Al 5 Fe 2 -containing layer)/(AlFe layer)/(AlFe 3 layer))→((α-Al 2 O 3 , Fe 2 O 3 )-mixed scale)/(AlFe 3 layer)/(Fe(Al) layer) from the surface. As the oxidation progressed, the scale changed from α-Al 2 O 3 to the (α-Al 2 O 3 , Fe 2 O 3 )-mixture, which provided the necessary oxidation resistance.