During an induction hardening process, the electromagnetic field generated by the inductor creates eddy currents that heat a surface layer of the part, followed by spray quenching to convert the austenitized layer to martensite. The critical process parameters include the power and frequency of the inductor, the heating time, the quench delay time, the quench rate, and the quench time, etc. These parameters may significantly affect case depth, hardness, distortion, residual stresses, and cracking possibility. Compared to a traditional hardening process, induction hardening has the advantages of low energy consumption, better process consistency, clean environment, low distortion and formation of beneficial residual stresses. However, the temperature gradient in the part during induction hardening is steep due to the faster heating rate of the surface and the aggressive spray quench rate, which leads to a high phase transformation gradient and high magnitude of internal stresses. Quench cracks and high magnitude of residual stresses are more common in induction hardened parts than those of conventional quench hardening processes. In this study, a scanning induction hardening process of a large part made of AISI 4340 with varying wall thickness is modeled using DANTE. The modeling results have successfully shown the cause of cracking. Based on the modeling results, a preheat method is proposed prior to induction heating to reduce the in-process stresses and eliminate the cracking possibility. This process modification not only reduces the magnitude of the in-process tensile stress, but also converts the surface residual stresses from tension to compression at the critical inner corner of the part, which improves the service life of the part. The modified process has been successfully validated by modeling and implemented in the heat treating plant.
Residual Stresses in Case Hardened Materials
