Enhanced strength and plasticity of gas nitrided iron by surface mechanical attrition pretreatment

Nitriding treatment at 520 °C for 6 h was performed on a pure iron plate (1.5 mm thick) with nanostructured surface layer, produced by surface mechanical attrition treatment (SMAT). Microstructure, surface hardness and tensile behavior of SMAT nitrided sample were investigated compared to those of coarse-grained nitrided sample. Experimental results showed that a much thicker compound layer and a unique transition zone were developed on the SMAT nitrided sample, leading to an obvious enhancement in surface hardness. Tensile tests revealed that the SMAT nitrided sample exhibits an ultimate tensile strength of ~ 450 MPa with a total elongation of ~ 44%, which is much higher than that of the coarse-grained nitrided counterpart (390 MPa, 18%). The higher tensile strength of the SMAT nitrided sample may originate from the contribution of a much thicker compound layer and a unique transition zone. The extraordinary tensile plasticity may benefit from that the precipitation of needle-like nitrides layer was hindered on the SMAT nitrided sample. This enhanced processing method demonstrates the technological significance of nanomaterials in improving traditional processing techniques and provides a new approach to obtain ferrous materials with excellent surface properties and outstanding global tensile properties simultaneously.