Computer modeling reveals that the region of the acute corner is noticeably underheated because the path of eddy currents in the transverse cross section does not match the contour of the rhomboid, and most of the induced currents close their loops earlier, without reaching the acute corner. The situation is quite different when heating the obtuse corners.
Various proprietary coil designs and process recipes were developed to ensure uniformly heated products and to accommodate geometrical features of the workpieces that are different compared to heating components of traditional shape.
Numerical computer modeling helps determine optimal recipe and process details that could be costly, time-consuming, and, in some cases, extremely difficult, if not impossible, to determine experimentally.

Further reading:
V. Rudnev, D. Loveless, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.

The edges of RCS billets can be nonuniformly heated depending on heating process parameters. The transverse electromagnetic edge effect and thermal edge effect are primarily responsible for temperature nonuniformity within the transverse cross section of the RSC billets, including edge/corner regions. Experience gained on previous jobs and the ability to model induction processes provide a comfort zone when designing new induction bar and billet heating systems. The combination of advanced modeling software and a sophisticated engineering background enables manufacturers of induction heating machinery to quickly determine process details that could be costly, time-consuming, and in some cases, extremely difficult, if not impossible, to determine experimentally.

To illustrate, the figure shows two-dimensional temperature profiles representing the dynamics of induction heating the top-right cross section (1/4 of the part) of a 4-in. (0.1-m) RCS carbon steel bar using a frequency of 500 Hz. Appreciable temperature gradients may occur within the bar cross section. It is important to have a clear understanding of the magnitude of these gradients during the initial and intermediate heating stages (particularly the initial stage) to prevent cracking.
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References:
• V. Rudnev, D. Loveless, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.
• V. Rudnev, Successful induction heating of RCS billets, Forge, July, p15-18, 2008.

It is important to remember that immediately after completion of the heating stage, the inductor must be quickly retracted from the heating position allowing the two piston’s halves to be rapidly pressed together forming sound weld. Even though the time of retraction of inductor is a fraction of a second, it might be sufficient to noticeably distort temperatures of pre-heated weld faces noticeably affecting thermal condition of the welding process.

An ability to simulate soaking / cooling stage is as important as an ability to computer model the heating stage. An analysis show that only 1 sec of an inductor retraction time can result in temperature reduction of weld faces that exceeds 250deg.C/450deg.F. Therefore, it is important to have clear understanding regarding an effect of time delay between end of heating and beginning of welding.

As an example, Figure below shows dramatic variation of temperature distribution in the proximity to weld faces during 1 second of soaking / cooling on air helping determining the most appropriate maximum time of inductor retraction.

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References:
1. V.Rudnev, D.Loveless, C.Ribeiro, J.Boomis, Unleashing a superior induction heating design with computer modeling, Industrial Heating, August, 2009, p.43-47.
2. V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003, 800p.
3. V. Rudnev, Simulation of Induction Heating Prior to Hot Working and Coating, ASM Handbook, Vol. 22B: Metals Process Simulation, editors D.U. Furrer and S.L. Semiatin, ASM International, 2010, p.475-500.

Induction preheating is a critical part of this technology because it provides over 95% of the weld energy input. Induction provides the required heat input quickly and efficiently. The ability to achieve the required temperature uniformity is imperative for the process to work successfully. Immediately after the part is heated, the inductor is retracted within the fraction of second, and the two piston halves are rapidly pressed together.

The figure above shows a finite element mesh (right) and the variation of temperature profiles (left) during induction preheating of the top and bottom halves of the piston using 30 kHz. Total heating time is 6 seconds. A novel inductor concept made possible a retraction time of less than 1 second.

The figure below shows a magnified view of the temperature variation within the right half of modeled induction preheating system.

References
1. V.Rudnev, D.Loveless, C.Ribeiro, J.Boomis, Unleashing a superior induction heating design with computer modeling, Industrial Heating, August, 2009, p.43-47.
2. V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003, 800p.
3. V. Rudnev, Simulation of Induction Heating Prior to Hot Working and Coating, ASM Handbook, Vol. 22B: Metals Process Simulation, editors D.U. Furrer and S.L. Semiatin, ASM International, 2010, p.475-500.

Inductoheat, Inc. is committed to finding efficient, effective and economic solutions for your induction heating needs. We continue to embrace break-through technologies and customer service in all that we do, and look forward to serving the metals and materials industry for 50 more. www.Inductoheat.com

• The presence of a thermal refractory and the necessity of considering radiation factors
• The heated workpiece can simultaneously move, rotate, and oscillate with respect to induction coil
• The presence of nonuniform initial temperature distributions, such as reheating after continuous casting and billet holding stage
• The presence of end plates, guides, fixtures, liners, etc.

Results of computer simulation of the sequential dynamics of end heating of carbon steel bars utilizing an oval coil (top half is shown only). Bar OD=2”; required heated length = 5¼”; @ 112 bars per hour; 5 bars were progressively heated in oval inductor using 3kHz. Temperature variation “degree C” of four critical points is also shown there.

It is important to be aware that certain critical features of induction heating the workpiece prior to forging could be limiting factors for all-purpose software, dramatically affecting accuracy. This was the reason why some induction heating manufacturers developed several proprietary application-oriented programs, which allow taking into consideration process specifics and important subtleties.

Results of computer modeling of the dynamics of end heating a 2-in. (50-mm) diameter low-carbon steel bar using a solenoid multiturn coil are shown in the figure. The required heated length is 5in. (127 mm). Because the bar is symmetrical, only the top half was modeled using Inductoheat’s proprietary FEA software. Temperature variations at four selected areas of the bar are shown. A 0.5-in. thick thermal refractory is placed between the heated bar and a multiturn inductor (inductor is not shown). Prior to heating, the thermal refractory has certain nonuniform initial temperature distribution that remains from the previous heating cycle.

During the heating cycle, the temperature of both bar and refractory changes. Bar temperature increases due to Joule heat generated by induced eddy currents. The thermal condition of the refractory is a complex function of the heat radiated from the bar surface (due to thermal radiation and convection) and its cooling due to surroundings and water-cooled induction coil.

Computer modeling using appropriate software makes it possible to address all of the important process features, reducing the learning curve in the development of the most appropriate process recipe.

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References

1. V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.
2. V. Rudnev, Simulation of Induction Heating Prior to Hot Working and Coating, ASM Handbook, Vol. 22B: Metals Process Simulation, editors D.U. Furrer and S.L. Semiatin, ASM International, p 475-500, 2010.

Click here to read more about Induction Contour Hardening.

Induction heating the internal surfaces of a workpiece can be used in applications such as hardening, tempering, annealing, shrink fitting, stress relieving, coating, drying, and brazing. Application specifics requires the use of a variety of different inductor styles to heat internal surfaces including solenoid-type, cylindrical single- and multi-turn coils, hairpin inductors, C-core inductors, and others.

Single- and multi-turn solenoids are the most popular inductors for heating internal surfaces. Such inductors are called ID coils or internal coils. Internal coils are typically made of copper tubing that is spiral wrapped the same way a solenoid is wrapped. In some cases, the head of the internal inductor is machined from a solid copper bar.

Installation of a magnetic flux concentrator inside the internal inductor is frequently necessary to increase coil efficiency and reduce coil current, particularly for heating internal surfaces of small to moderate diameters. The use of magnetic flux concentrators on internal coils provides a noticeable reduction in required coil current and power, reduces coil water-cooling requirements, and often simplifies load matching of the induction coil and inverter.

The figure shows the results of computer modeling of the dynamics of induction scan hardening of inside diameter of carbon steel pipe using two-turn solenoid inductor with a U-shape magnetic flux concentrator. The pipe ID and OD are 11.8 and 7.8 in. (300 and 200 mm), respectively. Spray quenching follows induction heating. Because of the pipe symmetry, only the top-half of the system was modeled using Inductoheat’s proprietary FEA software. Temperature variations at four selected areas of the pipe are shown at different locations.

Computer modeling shortens the inductor development time and helps reveal many important process subtleties, which would be costly and time-consuming to determine experimentally.

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Further reading:

V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.

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Certain design features make a part prone to overheating during induction heating. Typical examples are parts containing longitudinal and/or transverse holes, keyways, grooves, shoulders, flanges, diameter changes, undercuts, hollow areas, splines, and sharp corners. However, such features are not unique, as they are commonly found on many transmission and engine components.

The presence of these features distorts the magnetic field generated by an inductor during scan hardening, potentially causing the appearance of hot or cold spots and leading to an excessive shape distortion, undesirable microstructures, and cracking. The eddy current flow and temperature fields should be carefully evaluated to determine the optimal process parameters and coil design to prevent cracking and ensure the quality of heat-treated component.

The figure above shows the results of computer modeling of the dynamics of induction scan hardening of a hollow shaft that has diameter changes, a chamfer, and a groove. Because the shaft is symmetrical, only the top half was modeled using Inductoheat’s proprietary FEA software. Temperature variations at four selected areas of the shaft are shown at different inductor positions. The scan rate and coil power were varied during scanning to accommodate geometry changes of the shaft.

In some cases, it is required that the hardened case should be prevented at an undercut. In other cases, hardening is required in these areas, such as hardening snap-ring grooves. This necessitates developing precise process control algorithms that provide appropriate coil power variations when needed and terminating power when approaching groove or undercut regions. For example, at the end of scanning, the inductor power is off but spray-quenching is still applied.

Computer modeling reveals numerous important process subtleties that were discussed in Ref.1.

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Further reading

1. V. Rudnev, Computer Modeling Helps Prevent Failures of Heat Treated Components, Advanced Materials & Processes, ASM International, October, 2011, p 6-11.
2. V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, N.Y., 2003.