Low Temperature Boronizing of Surface Nanostructured Ni-Cr-Mo Steel Using SMAT

Abstract

Boronizing is a surface thermochemical treatment in which boron atoms are made to diffuse into a metallic surface at high temperatures. A nano-crystalline surface with larger defect density assists in enhancing the diffusion rate even at low temperatures. In the present work Ni-Cr-Mo steel is subjected to a surface mechanical attrition treatment (SMAT) to activate the surface with nanocrystalline structures and crystal defects. Subsequently the samples were boronized at low temperature regime (400°C - 600°C) for 5 hours using a pack boronizing technique. The microstructure, chemical analysis and hardness of borided layers were investigated using optical microscope, SEM – EDX and Microvicker’s Hardness Tester. The SMAT treated samples showed severe plastic deformation of the surface, nano-structured grains (10-30 nm) and larger defect density illustrating mechanically activated surface for diffusion. The boronizing had clearly demonstrated the diffusion of boron even at 400°C. The thickness of diffused layer was found to be about 20 µm at 400°C and 50 µm at 600°C for SMAT samples while the untreated samples showed practically no diffusion at 400°C and 12 µm at 600°C. The SEM-EDX results had confirmed the presence of boron at the diffused layer; however the hardness was found to be low. A maximum of 650 HV0.3was achieved by low temperature boronizing of SMAT treated samples.