A STUDY OF RESIDUAL-STRESSES IN THE SURFACE HARDENING OF A BLADE MOLDBY HIGH-FREQUENCY INDUCTION-HEATING

High frequency induction heating is a method of surface treatment whic h restricts the hardening area using the skin effect. Since the penetr ation depth of the magnetic field in the workpiece is dependent on the frequency, the required size of hardening area can be obtained by sel ecting an appropriate frequency. High frequency induction heating is a ble to harden a large area at once, in which the shapes of the coil an d workpiece and the distance between them are important factors for th e hardening area shape and the distribution of residual stresses. In t his study, the transient heat flow and thermal stress were analysed fo r the high frequency induction surface hardening of a blade mould by u sing the modified two-dimensional finite element method. Besides the v olume change in the phase transformation, the effect of transformation plasticity was also considered as an additional strain in the numeric al analysis of the high frequency induction hardening process. The har dening area was fairly uniform in the mould surface except around the corner where the distance between the coil and workpiece was slightly larger than on the other parts. The thermal stress was induced mainly by the temperature gradient and martensitic phase transformation, whil e the latter was found to have a greater influence on the residual str ess than the former. Simulation results revealed that compressive resi dual stresses occur in the hardening area, while the maximum tensile r esidual stress occurs near the boundary of the hardened zone.