Fabrication and Characterization of Bulk Nanocrystalline Layer on Steel Components With Different Microstructures by Air Blast Shot Peening

The fabrication of nano-crystalline (NC) layers in pearlitic and martensitic carbon steels viz. AISI 1018, 1045 and 1075 alloys using ABSP has been studied. The cross-sectional microstructure of the samples showed that a gradual reduction of the grain size near the surface until a clearly demarcated NC layer was observed. The NC layers were characterized using optical, scanning electron microscopy (SEM), transmission electron microscopy (TEM), microhardness and nanohardness measurements. 2D surface topography of the peened surface was also carried out to measured surface roughness. The roughnesses of the peened surfaces were dependent on the hardness of the sample. Samples with higher hardness had lower surface roughness value. NC layers varying from 5 to 15 μm were observed using the optical and SEM images in the different types of steels. The SEM images clearly showed the dissolution of cementite phase in the NC layers. The nanohardness measurements of the steels showed that the NC layers had much higher hardness than the bulk material. The microhardness measurements also confirmed the mentioned phenomena. A TEM study was carried out on the NC layer and the grain size was confirmed to be 50 to 80 nm.