Two‐Step Nitriding Behavior of Pure Iron with a Nanostructured Surface Layer

A two‐step gas nitriding treatment (280 °C for 2 h plus 550 °C for 6 h) is designed for pure iron with a nanostructured surface layer induced by surface mechanical attrition treatment (SMAT). The microstructure and surface properties of the SMAT sample after two‐step nitriding treatment are investigated in comparison with those of the SMAT sample after single‐step nitriding treatment (550 °C for 6 h). Experimental results show that a much thicker compound layer (up to 25 μm in thickness) is generated on the surface of the SMAT sample after two‐step nitriding treatment, in which the mean grain size of the Fe_2‐3N phase is below 25 nm. The enhanced nitriding kinetic may be attributed to the formation of a large number of Fe(N) solid solutions and a small amount of dispersed Fe_2‐3N particles on the SMAT sample after first low‐temperature nitriding, which enhances the thermal stability of nanostructured iron grains. Due to the enhanced nitriding kinetic and refined nitride structure, the SMAT sample after two‐step nitriding treatment displays higher surface hardness and improved wear resistance relative to its single‐step nitrided counterpart.