Heat Transfer Studies During Quench Hardening of Steels

Abstract

In the present work, heat transfer during quench hardening of steels was investigated incorporating the effect of phase transformation, section thickness, and the quench medium. The work is divided into three sections. In the first part of the work, the effect of section thickness on the steel/quenchant interfacial heat flux is studied. The second part defines a critical heat transfer coefficient as the minimum heat transfer coefficient required to form 50% martensite at the core of the steel cylinder of a particular grade and section thickness. A simulation study was carried out for this purpose. In the last section of the work, an attempt was made to repurpose and reuse the used cooking oil as a blend for oil quench media. The first part of the work investigates the effect of phase transformation on interfacial heat flux during quench hardening treatment of steel. Experimental and modeling approaches comprising the inverse heat conduction problem (IHCP) phase transformation coupling were employed to analyze the thermal behaviour of different steel grades with varying section thicknesses. The study revealed a distinct heat flux pattern with phase transformation, showing a dip followed by a rise. Increasing section thickness increased the surface heat flux for stainless-steel probes (no phase transformation), the heat flux was reduced for plain carbon steel probes due to phase transformation. A was introduced for quantifying the enthalpy change during quenching of steel probes. was found to be consistent for a steel grade and independent of section thickness but varied with cooling rate and quench medium. Incorporating IHCP models with phase transformation simplifies the process of quenching simulation and minimizes data inputs. A database on Q as a function of temperature and cooling rate would greatly facilitate heat transfer modeling during quench hardening of steels. In the second part of the work, a simulation study is performed to obtain the critical heat transfer coefficient for the selection of quenchants for a particular grade of steel and section thickness. The simulation study is conducted by solving phase transformation coupled transient heat conduction equation using the finite element method. The finite element model adopted in this work uses the one-dimensional radially symmetric model with the constant heat transfer coefficient boundary condition at the surface. The variables in the simulation study are the carbon content, the diameter of steel, and the heat transfer coefficient. The effect of the variables on the martensite formation at the core of the steel is studied. The critical heat transfer coefficient significantly varied with the section thickness and the carbon content of the steel. The usefulness of the study in selecting a quenchant for quench hardening plain carbon steels with varying carbon content is illustrated. In the last part of the work, the fatty acid methyl ester (FAME) produced from the used sunflower cooking oil through transesterification was blended with sunflower and mineral oils at various proportions. The cooling characteristics of the FAME/oil blends were assessed using the cooling curve analysis according to ASTM D6200 and ISO9950 standards. The inverse heat conduction method estimated the spatiotemporal metal/quenchant interfacial heat flux. The uniformity of heat flux was analyzed. The results indicated that blending of used cooking oil-derived FAME with sunflower oil up to 60vol% and mineral oil up to 50vol% provided comparable cooling characteristics to pure oils. The estimated heat flux transients showed a marginal decrease in peak heat flux for FAME blends in sunflower oil, whereas an increased peak heat flux with mineral oil. In the subsequent part, the thermal-oxidative stability of the FAME blend is improved through the epoxidation reaction. The stability of epoxidized FAME/mineral blended oil is assessed by thermogravimetric analysis (TGA) and thermal quench cycling. The quench cycles were performed using an ISO 9950 Inconel 600 standard probe. The viscosity and cooling performance of the oil was assessed periodically at the 1 st , 10 th , 50 th, and 100 th quench cycle. The results indicated that the thermal stability of the blend quenchant was improved with the epoxidation of FAME. The study showed that blending the epoxidized form of FAME (EFAME) in oil provided better thermal-oxidative stability than the FAME/oil blend.