Economic Fabrication of Bulk Nanostructured Surface on Steel Components with Different Microstructures by Severe Plastic Deformation

摘要

The nanostructured metallic surface may enhance mechanical properties and functional performance. Current techniques such as inert gas condensation, electrodeposition, powder-consolidation, severe plastic deformation (SPD) including equal-channel angular pressing (ECAP) are often used to refine grains. However, these grain refinement techniques are time consuming, expensive, and often ineffective to fabricate bulk nanostructured metallic materials. In this study, a top-down Air Blast Shot Peening (ABSP) technique has been developed to economically fabricate bulk nanostructured surface on low carbon steel AISI 1018 and high carbon steel AISI 1075 of pearlitic and martensitic microstructures. A design-of-experiment based experiment was performed to isolate the effect of peening process parameters on the nanocrystalline (NC) layer formation. The NC surface layers were characterized by optical, SEM and TEM microscopy, microhardness and nanohardness measurements, and surface profiles. The experimental results have shown that the cross-sectional microstructure of the samples has a gradual reduction of the grain size near the surface until a clearly demarcated NC layer was observed. The roughnesses of the peened surfaces depends on the hardness of the sample. Samples with higher hardness have lower surface roughness values. NC layers varying from 5 to 15 μm can be observed using the optical and SEM images in the different types of steels. The SEM images clearly shows the dissolution of cementite phase in the NC layers. The nanohardness and microhardness measurements of the steels show that the NC layers have much higher hardness than the bulk material. A TEM study was carried out on the NC layer and the grain size was confirmed to be 50 to 80 nm.