Characterisation of Select Nanofluid and Vegetable Oil Quenchants and Assessment of Heat Transfer during Quench Hardening of Steels

Abstract

The present investigation involved the characterization of nanofluids and study of heat transfer characteristics of vegetable oil quench media for heat treatment of steels. Nanoquenchants were formulated by the two-step method. The wetting kinetics and kinematics of quench media were studied by measuring the contact angle and online video imaging during quenching. CuO, MWCNT and graphene based nanofluids showed better wetting and spreading ability compared to distilled water. In nanofluids, the stabilization of the vapour phase stage resulted in low severity of quenching. Spatiotemporal heat flux transients were estimated by using a 2-D IHCP model during of quenching of ISO 9950 inconel probe. The study showed increased heat extraction capabilities of graphene and MWCNT nanofluids compared to distilled water under agitated quenching conditions. Heat extraction rates were found to be lower for CuO nanofluids. The use of edible and non-edible vegetable oils for quench hardening was investigated by comparing their heat transfer characteristics with a mineral oil. The study showed the excellent potential of non-edible vegetable oils for quench hardening of steels. Karanja oil was found to be superior compared to neem and sunflower oils. To simulate the industrial quench heat treatment, reference probes made of medium and high carbon steels were quenched and heat flux transients were estimated by taking into account the phase transformation. The cooling curves obtained with reference probes made from G 10450 and G 10900 steels showed kinks indicating enthalpy change accompanied with phase transformations during continuous cooling. This was reflected in the estimated heat flux curves. The effect of viscosity, density and surface tension of quench media on the mean peak heat flux was quantified using a power fit model. The section thickness effect on heat flux transients was examined by using probes of diameters 25 mm and 50 mm. The cooling rates measured at various locations along the cross section of reference probes of both thicknesses were related to the hardness using the Quench Factor technique. The heat transfer characteristics of the quench media, the evolved microstructure and the resulting hardness were in complete agreement.