Effect of phase difference on growth kinetic of alloy layer in aluminized and diffusion-treated 12% Cr heating resistant steels

12% Cr heating resistant steels with single (martensite) and dual (martensite and ferrite) phases were prepared to investigate not only the effect of the phase difference in substrate on the alloy formation but also the growth kinetic of the alloy layer during aluminizing and diffusion treating. Scanning electron microscopic observation and X-ray diffraction analysis revealed that the alloy layer was identified as Fe₄Al₁₃ (FeAl₃) during aluminizing and annealing treatment and the thickness of the layer in both specimens increased as the temperature and time increased. For comparison of the parabolic coefficient and activation energy of two specimens, the specimen with the single phase presented a higher coefficient and lower activation energy than that with dual phases. This may be due to structural differences relating to the ease in which the Al atom migrates into the substrate and the fact that martensite contains a larger fraction of defects. Additionally, the results for the annealing treatments for diffusion after aluminizing clarified that both specimens showed lower activation energies than during aluminizing treatment because the formation of voids and their aggregation during annealing treatment after aluminizing contributed to lower activation energies for diffusion.