Study On Microstructure And Mechanical Properties Of Pure Iron Subjected To Surface Mechanical Attrition Treatment And Nitriding

The surface mechanical attrition treatment (SMAT) technique can be used to process a nanostructured surface layer on metallic materials without changing their structural and chemical composition. Surface property, such as hardness and friction and wear properties, can be effectively improved by the surface nanocrystallization technique. Additionally, mechanical property can be improved for the SMATed material due to its gradient structure. Besides, large amount of grain boundaries in the surface layer of the SMAT materials can act as fast nitrigon diffusion channels, leading to greatly enhanced atomic diffusivities and accelerated nitriding procedure and thicker nitrided layer.In this work, a pure iron was treated to achieve a nanostructured surface layer via surface mechanical attrition treatment (SMAT) technique. Nitriding was performed for the iron with nanostructured surface layer and its coarse-grained counterpart. Structure and thermal stability of the SMATed Fe were characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM). The tensile properties and the effect of rolling on SMAT Fe were examined. Additionally, microstructure and mechanical property of the SMATed iron after nitriding at500℃were investigated. The main results are as follows:A plastic deformed layer of about150μm thick is formed in the top surface layer of pure iron after SMAT treatment, with an average grain size of about10nm and random crystallographic orientation. Surface hardness of the SMATed Fe is much larger than that of the coarse-grained sample, and it decreases with depth. Strength of the SMAT sample is much larger than that of the coarse-grained sample, whereas the elongation of the SMAT sample is lower than that of the coarse-grained sample. These may be attributed to work hardening and hinder of dislocation by grain boundary as well as the lack of movable dislocations. Fracture of surface layer of the SMAT sample behaves in a brittle mode, whereas the substrate tends to fracture in a ductile mode.There is no significant grain growth for the SMATed pure iron sample after annealing at500℃for2h, indicating satisfactory thermo stability. After cold rolling of the SMATed sample, the surface roughness is decreased and the strengthening layer is redreased. Additionally, strength is increased, whereas plasticity is decreased. The satisfactory stability of the nanocrystallization layer is maintained on the SMATed and cold rolled sample after annealing at550℃, whereas the change to recrystallization grains are observed in the substrate.Gas nitriding of the SMATed pure iron at500℃resulted in the formation of a compound layer which is thicker than that in the coarse-grained iron nitrided under the same condition. The nitrided layer in the SMATed sample is mainly composed of ε-Fe3-2N compound. This indicates that the SMAT procedure can significantly promotes the diffusion kinetic of nitrogen. Strength and surface hardness of the nitrided SMAT sample is higher than that of the coarse-grained sample, whereas elongation of the nitrided SMAT sample is lower than that of the coarse-grained sample. Fracture mode of the nitrided layer in the nitrided SMAT sample tends to be brittle fracture whereas the substrate tends to be ductile fracture.