When designing a tempering process, the type and condition of tempering equipment must be considered. In particular, airflow and temperature uniformity play a crucial role. Tight temperature uniformity is required throughout the load, typically ± 10°F (± 5.5°C), preferably ± 5°F (± 2.75°C), especially for high-speed and precipitation-hardening steels. The ability to have a fast heating rate will shorten the overall cycle time.
Various forms of self-tempering and accelerated tempering by induction or ultra-high air convection ovens and furnaces have shown promise in many applications [2,3]. While soaking time and temperature are usually determined by steel chemistry, processing time can be significantly reduced by speeding up the heating time. This can be achieved by designing more efficient heat transfer between the heated atmosphere and the load using high-speed convection, and turbulent flow modes.
For example, it was found that the effective tempering of an axle after induction hardening depends on the actual heat transfer rate and component temperature, rather than time at a specific temperature. This study achieved the following results and achieved a cycle time savings of over 80% with a comparable post-temper hardness of the bearing journal surfaces. Equivalent hardness and residual stress distribution within the shaft, equivalent yield strength and fracture strength, and equivalent reverse torsional and rolling bending fatigue life.