Experimental Assessment of High Heating Rates in Induction Heating with Temperature-Sensitive Lacquers

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JMEPEG (2018) 27:3831–3843 https://doi.org/10.1007/s11665-018-3499-z

Experimental Assessment of High Heating Rates in Induction Heating with Temperature-Sensitive Lacquers N. Vanderesse, B. Larregain, F. Bridier, and P. Bocher (Submitted November 21, 2017; in revised form April 23, 2018; published online July 5, 2018) The fast evolving temperature fields at the surface of induction-heated parts were measured by a contactless, optical method based on temperature-sensitive lacquers recorded with a high-speed imaging system. The positions of specific isotherms over time were precisely measured by image analysis. A 4340 steel cylinder and two spur gears of the same material were investigated. One spur gear was treated at high frequency, and the other one at simultaneous dual frequency. In both cases, the temperature started to rise at the root of the teeth and then expanded along the flanks. The heating rates were higher at the tips than at the roots for both treatments, with higher values for the dual-frequency treatment. The layer of transformed material was similar for both treatments, but the transformation temperature varied greatly. It was found to be 50-100 °C higher than the equilibrium values, depending on the local heating rate. Keywords

heat treatment, image analysis, induction heating, steel

1. Introduction Heat exposure is a critical element in many manufacturing processes of structural parts. It may locally modify their strength, wear, and corrosion characteristics, but also optimize their formability and machinability. While several heat treating processes are now well mastered, simulated, and understood in many aspects, processes involving high thermal gradients and heating rates still challenge experimental investigations (Ref 14). The lack of experimental data, in turn, limits the validity of simulations whose implementation is further complicated by the nonlinear and strongly coupled dependency of thermophysical properties to temperature and heating rate. This is especially true whenever phase transformations are involved, as is the case in induction hardening. The principle of induction treatment of steel is to heat the surface of a workpiece in order to locally austenitize the material, and then quench it so as to obtain a martensitic microstructure. Surface heating is produced by the application of a strong, alternating, magnetic field that induces eddy currents in the part. The zone where the material has been transformed is referred to as the transformed, or hardened, layer. Its extension depends on the positions at which the partial and complete austenitization temperatures at heating, Ac1 and Ac3, were reached at the end of the treatment (Ref 5). Additionally, compressive residual stresses, beneficent for fatigue life, can be induced (Ref 6).

N. Vanderesse, B. Larregain, F. Bridier, and P. Bocher, LOPFA, Department of Mechanical Engineering, E´cole de technologie supe´rieure, 1100, rue Notre-Dame O, Montreal, QC H3C 1K3, Canada. Contact e-mail: [email protected].

Journal of Mater