Kinetics of oxidation and decarburization in Al-Si transformation induced plasticity steel

This research elucidates the evolution of internal oxidation, external oxidation, and decarburization and their interdependence under different gas atmospheres for a transformation induced plasticity (TRIP) steel sample. A rolled steel sample chemistry of 0.14 wt pct C, 1.8 wt pct Mn, 0.6 wt pct Si, and 1 wt pct Al is used as a case study. Thermogravimetric techniques are used to examine the net mass change in a sample during isothermal heat treatment under five different atmospheres: P_H2O/P_H2 = 0.01, 0.06, 0.13, 0.20, and 0.32. The results show an initial mass decrease and later an increase. Scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), optical microscopy, and glancing-angle X-ray are used to quantify the microstructure and show that the samples have four distinct zones: (1) the external scale, often rich in fayalite; (2) an internally oxidized region with Al, Si, and Mn oxides in an Fe matrix; (3) a decarburized steel region, characterized by large ferrite grains and low carbon content; and (4) a relatively unaffected steel core with a structure similar to the starting material. The results show that the oxidation is predominantly internal and proceeds independently of decarburization, following a parabolic oxidation law where the parabolic rate constant is higher, as would be expected, at higher water vapor contents. Decarburization rates are slightly lower than the predicted rates based on carbon diffusion only, suggesting that the external scale might have an effect in blocking CO escape from the surface.