Heat transfer and stress generation during forced convective quenching of steel bars
Creators: name: Hernández-Morales, José Bernardo
Publisher: name: University of British Columbia
Year: 2009

description: An industrial heat treatment based on forced convective quenching of alloyed eutectoid steel bars has been studied within the framework of microstructural engineering. The proprietary process is used to produce grinding rods with improved abrasion resistance and toughness. Mathematical models of heat transfer, microstructural evolution and stress generation have been applied to predict the final microstructure and residual stress distribution in 38.1 mm-dia. bars, quenched in a laboratory facility. The models are based on the finite-element method (FEM) and assume axisymmetric geometry. The principle of additivity was invoked to predict the evolution of the microstructure under continuous cooling conditions. The kinetics of diffusional and martensitic phase transformations were characterized using the JMAK and Koistinen-Marburger equations, respectively. The kinetic parameters were determined through continuous cooling and isothermal tests using the Gleeble 1500 thermomechanical simulator. The existing elasto-plastic stress model incorporates thermal- and transformation-related strains. The mechanical properties were obtained from the literature. The effect of stresses on transformation kinetics was not considered; thus, the thermal-microstructural and mechanical models were effectively uncoupled. The heat-transfer boundary condition has been characterized by acquiring the thermal response in instrumented interstitial-free (IF) steel bars quenched from 1000 °C in flowing water. Three water velocities and water temperatures were investigated. Tests with clean and oxidized bars were also conducted. The surface heat flux, as a function of surface temperature, was estimated by solving the inverse heat conduction problem (IHCP). The solution algorithm was based on the sequential function specification technique. The water temperature was found to have a significant effect on the boiling curves. For the highest water temperature, 75 °C, a film boiling stage was identified, while only transition, nucleate and convective boiling were observed when using water at 50 and 25 °C. No significant differences in heat extraction were observed between clean and oxidized quenched bars, within the parameter range investigated. The residual stress distribution in selected forced convective quenched IF, 1045 carbon and alloyed steel bars has been determined by means of neutron diffraction. The axial and circumferential residual stresses in the IF and 1045 carbon steel quenched bars were compressive at the surface and tensile at the centre, while the radial component was always tensile. In contrast, the alloyed eutectoid steel bar showed compressive axial, circumferential and radial residual stresses at the centre. The final microstructural and hardness distributions have also been determined in the three bars. The IF steel bar transformed completely to ferrite, while the alloyed eutectoid steel specimen showed an essentially martensitic structure. On the other hand, quenching the 1045 carbon steel bar resulted in an outer ring of martensite and a mixture of diffusional and martenistic products in the core. Comparisons between measured and model-predicted thermal response, final microstructure and residual stress distribution have been made. Fair agreement between measured and predicted values was observed. It was found that the position of the 'nose' of the continuous cooling diagram, when a mixture of diffusional and martensitic products was produced, has a significant influence on the predicted final microstructure and, therefore, on the predicted residual stress distribution. The difference in the measured residual stress distributions obtained in the alloyed eutectoid steel specimen, when compared with those found in the IF and 1045 carbon steel quenched bars, has been explained based on the sequence of transformations that took place during the quench. The quench of a bar under industrial conditions was also simulated.