SURFACE MODIFICATION OF AISI 4340 STEEL BY IN-SITU GROWN TITANIUM CARBIDE THROUGH TUNGSTEN INERT GAS (TIG) ARCING

Abstract

This work comprises of surface modification of AISI 4340 steel by changing the chemistry of the surface by employing TIG arcing process, where detailed analysis was carried out for the in situ formed titanium carbide precipitates in the bainitic matrix of substrate. The critical cooling rate required for bainitic transformation in substrate is obtained by Continuous cooling diagram developed using JMatpro software. The TIG arc process parameters needs to be optimized in such way so that it can give a thermal profile having same desired cooling rate. Utilizing the COMSOL Multiphysics software, a 3D finite element analysis is being carried out to determine the thermal profile at each point along the heat source travel path during TIG arcing process. Heat source is modelled using Goldak’s double ellipsoidal equation. By varying the parameters like current and arc scanning speed, various thermal profiles are generated and average cooling rate over transformation temperature range is calculated. Cooling rate resembling with the one obtained from thermodynamic calculation is then selected and corresponding TIG arcing parameters are then used experimentally to develop the required bainitic microstructure in the matrix. Various microstructural characterization techniques such as optical microscopy, X Ray diffraction, field emission scanning electron microscope, High resolution transmission electron microscope are utilised to confirm the formation of predicted phases and to configure the precipitate morphology also its compatibility with the matrix. Mechanical properties evaluation using microhardness tester and pin on disc tribometer for sliding wear testing is done to show enhancement in hardness and wear resistance as a result of modification.