Lower-temperature aluminizing behaviors of a ferritic-martensitic steel processed by means of surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), a gradient nanostructured surface layer was fabricated on a ferritic-martensitic (F-M) steel plate. Its aluminizing behaviors were investigated during a packed aluminization process and a subsequent diffusion annealing treatment at lower temperatures. In comparison with the initial sample, a much thicker Al5Fe2 layer was formed on the SMAT sample after the packed aluminization, with a growth constant of ~ 3 times higher at 600 °C. The transformation kinetics from Al5Fe2 phase into AlFe phase and α-(Fe,Al) solid solution are also enhanced in the subsequent annealing treatment at ~ 700 °C. The enhanced aluminizing kinetics originates from the increased atomic diffusivities by numerous grain boundaries and a higher concentration of vacancies in the nanostructured surface layer. With the determined growth kinetics, a duplex aluminizing process was demonstrated for achieving a gradient surface layer with lower-Al containing aluminides below the tempering temperature of F-M steel.