Important processing steps after hardening a steel part include tempering, stress relieving, and subcritical annealing. The transformation to martensite through quenching creates a very hard structure with low ductility. Untempered martensite retains a high level of residual stresses, and typically is too brittle for many commercial applications, except components that primarily require wear resistance. Reheating as-quenched steels and cast irons for tempering produces a tempered martensite microstructure. Tempering temperatures are below the lower transformation temperature A1 (usually in the range of 120 to 650°C, or 250 to 1200°F) comprising four overlapping tempering stages.
Localized tempering of threads by induction heating allows focusing thermal energy in the areas where greater ductility is desired. Developing such a process presents several challenges:
• The thread area of the shaft must be heated just below the A1 critical temperature to properly soften it, while making sure that the temperature of neighboring areas (including shoulders and diameter changes) are not reaustenized.
• Thermal conditions and process recipe should be selected to avoid or reduce the possibility of such undesirable metallurgical phenomena as tempered martensite embrittlement and temper embrittlement from occurring.
• A specific length of a “tempered-untempered” transition zone should not be exceeded.
• The process must be reliable and repeatable.
Numerical computer modeling helps to determine not only an optimal coil design and process recipe, but also to evaluate the robustness of a particular tempering process by estimating the impact of real-life process deviations, such as dimensional tolerances, fixture integrity, and part-to-inductor positioning.
To illustrate, the figures below show the dynamics of electromagnetic field distribution (top) and temperature variation during induction tempering of a thread area using an optimized two-turn inductor insert (inductor with electromagnetically coupled secondary). This type of inductor was selected because of the load tuning specifics of the available inverter used by the customer.
• V. Rudnev, D. Loveless, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.