Abstract
A hard Nano crystalline surface was produced by simultaneous surface mechanical attrition treatment (SMAT) and mechanical alloying (MA) with activated carbon, followed by low temperature annealing below 500°C. The thermal stability of surface Nano crystals produced by SMAT and low temperature diffusion of carbon into Nano crystals were also investigated by annealing 200°C to500°C for 1h. Ni-Cr-Mo casehardening steel was used in the present study. The formation of Nano crystals and surface alloying with activated carbon by SMAT, microstructural changes during annealing, thermal stability of the Nano crystals and the surface hardness were been investigated by using Optical microscope, SEM, XRD and microvicker’s hardness tester. Experimental results had showed 20-50 nm crystals by SMAT operation by enhancing 0.19C to 0.4C. Enhanced diffusion kinetics of Nano crystals observed during the annealing process even at 200°C. The simultaneous SMAT and MA with carbon increased the surface hardness from 1.5 to 3.0 GPa in 20 minutes and subsequent annealing further increased the hardness steadily up to 450°C. A maximum hardness of 4.5 GPa was achieved by annealing treatment. The hardness decreased considerably at 500°C indicating grain growth at thistemperature. The results had clearly demonstrated that the surface Nano crystals formed by SMAT were found stable up to 450°C.
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