Grain size effects on the austenitization process in a nanostructured ferritic steel

A surface layer with a depth-dependent microstructure was produced on a ferritic steel (P92) plate by means of surface mechanical attrition treatment (SMAT). The austenitization processes of ferrite and carbides in the surface layers with different average grain sizes were investigated by means of in situ X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Experimental results showed that the onset temperature of the austenitization process decreases gradually with decreasing sizes of ferrite grains and carbide particles, being ∼120 K lower in the top SMAT surface layer compared with that in the original sample. In addition, the two-step austenitization process in the surface layers becomes a one-step one when the mean size of carbide particles is smaller than 20 nm. The effects of microstructure refinement on the accelerated austenitization processes were discussed in terms of thermodynamic and kinetic.