Two methods are often used to through harden and low alloy steels. The well recognized conventional hardening method involves heating to an appropriate austenitizing temperature for a specific length of time in a furnace with or without a protective atmosphere. This is followed by quenching in oil or water, and then tempering at a specific sub-critical temperature in order to achieve desired mechanical properties and hardness. A second method involves rapid heating to the austenitizing temperature in an induction coil for a length of time sufficient to obtain a uniform temperature followed by rapid cooling. The mechanical properties and hardness are usually dictated by the cooling conditions following austenitizing, since sub-critical tempering is not generally used. While comparable mechanical properties and hardness can often be achieved with both methods, microstructures can vary significantly.
Of interest is a comparison of the fatigue properties that can be obtained with either method. The AISI Bar Steel Fatigue Database contains data for SAE 4140 low alloy steel which was through hardened using both conventional and through-induction hardening. While the data was developed for bar stock as opposed to fabricated parts, it does provide a means of comparing the fatigue properties for the two heat treating methods.
The table below summarizes the mechanical properties and hardness values obtained for the two processes.
Somewhat higher values of yield strength and tensile strength were achieved with the induction hardened process. The microstructure obtained after conventional quenching and tempering was tempered martensite, whereas the microstructure observed in the induction hardened condition contained a large amount of bainite and a lesser amount of ferrite.
Figure 1 shows the strain-life fatigue curves obtained for both heat treating conditions. The strain-live curve for conventional quenching and tempering is given by Iteration No. 68, and the strain-life curve for induction hardening is given by Iteration No. 93.
It can be seen that comparable fatigue properties are obtained for both hardening processes. The fitted strain-life curves show a slight advantage for through-induction hardening at fatigue lives from 103-105 reversals and somewhat better performance for conventional hardening at lives greater than 106 reversals.
The decision as to which process to use can be based on manufacturing considerations and other property concerns, e.g. notch toughness, rather than on fatigue requirements. It should be noted that, while parts of almost any configuration can be hardened using the conventional process, the through-induction hardening process is constrained by part geometry. Induction coils cannot be designed for hardening many complex parts.