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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 6 6 ly to ensure a constant amount of heat is produced. Modern induction technology allows monitoring energy levels with high precision. However, it might be more challenging to ac- curately control the quench severitywith the same precision, in particular when hardening complex geometry compo- nents. Quench time, flow rate, temperature, concentration, and quenchant cleanliness are some of the quench-related factors that must be monitored and held within close toler- ances to ensure a consistent thermal condition after quench- ing. It should be noted that the variations in quench severity are not only affected by actual conditions of the quenchant but they are also affected by surface condition of the work- piece, including surface roughness, presence of foreign res- idue from the previous operation (e.g., machining oil), and other issues. Though factors that are responsible for poten- tial deviations in cooling intensity due to theworkpiece’s sur- face conditions are undesirable and should be minimized, they might not make as dramatic an impact during the first two stages of spray quenching (vapor blanket and nucleate boiling stages). However, they might produce a greater im- pact during the third stage (convective cooling), leading to measurable variations of residual heat. Inevitably, this will negatively affect the repeatability of self-tempering. In some cases, an infrared pyrometer may be used to monitor the self-tempering temperature of the workpiece surface. If applicable, self-tempering can be used in static heat- ing, single-shot heating, and, to a lesser degree, horizontal scan hardening or continuous/progressive hardening ap- plications. It should not be used in vertical scan hardening, because of unequal cooling conditions and variations of the accumulated residual heat in the top and bottom regions of the vertically scan-hardened workpiece. It is easier to use self-tempering when dealing with simple geometries (such as straight shafts, for example). Geometrical irregularities may produce localized variations in both the heating and quenching intensities, particularly when dealing with complex-geometry components. These variations could be sufficient to create too large a deviation in the residual heat and deviations of the thermal conduc- tion heat flow in self-tempering. Some steels and cast irons have relatively low M s tem- peratures and upon completion of the formation of the needed amount of martensite, theremight be an insufficient amount of retained heat accumulated within the workpiece for sufficient self-tempering. It is more challenging to control residual heat when hardening components of small size (e.g., wires, thin-walled tubing, small diameter rods and pins). Thismakes it easier to apply self-tempering in cases where there is sufficient mass. However, large size workpieces with an extremely large ratio of diameter-to-thickness of the austenitized layer might also not bewell suited for self-tempering. This is because the cold core may provide such an overwhelming cold-sink effect, that it eliminates the rise in temperature of the hardened surface layer needed for self-tempering. Self-tempering should be avoided when profiled hard- ening is used (for example, contour hardening of gears and gear-like components, stepped shafts, and shafts that com- prise a combination of thin-wall sections and solid regions, just to name a few). Variations in the neighboring masses may produce a nonuniform tempering effect. The amount of heat stored, as well as the heat sink of neighboring regions, must be the same or sufficiently similar; otherwise, the tem- peratures obtained during self-tempering will be substan- tially different, resulting in unspecified temper structures, hardnesses, and engineering properties. The challenges discussed here prevent the wide use of self-tempering in industry making furnace/oven tempering and induction tempering more popular choices. At the same time, there is a group of applications where self-tempering has been successfully applied alone exhibiting significant process benefits, or in conjunctionwith induction tempering combining the benefits of both processes [1] . For example, a combination of self-tempering and multi-pulse induction tempering is successfully used in a non-rotational crankshaft hardening (SHarP-C technology). In this case, the journals of a crankshaft are stationary heat treated. For most automo- tive crankshafts, it takes approximately 3 to 4 sec to austen- itize a journal surface layer for hardening using frequencies in the range of 10 to 30 kHz (depending on the specifics of the automotive crankshaft and the required case depth). After completion of austenitization, the quenching is applied for only 4 to 5 sec, followed by 3 to 5 sec of the first soaking that accomplishes the first stage of self-tem- pering. Then, low-power induction tempering is applied for approximately 3 to 5 sec, followed by the second soaking and the second induction tempering. The process may be repeated to achieve desirable tempering conditions, provid- ing a multi-pulse induction tempering effect combined with self-tempering and allowing optimization of the tempered structure. SHarP-C technology as well as the subtleties of induction tempering are thoroughly discussed in Ref. 1. ~HTPro For more information: All are welcome to send questions to Dr. Rudnev at rudnev@inductoheat.com. Selected ques- tions will be answered in his column without identifying the writer unless specific permission is granted. Reference 1. V. Rudnev, D. Loveless, and R. Cook, Handbook of Induction Heating, 2nd Edition, CRC Press, 2017. 14