In some applications, such as the forming of tie-rod heads, only the end regions of the workpiece need to be heated for subsequent deformation. Induction end-heating systems of all coil types are more sensitive to electromagnetic and thermal non-uniformities that are innate to the induction heating process. Minimizing the non-uniformities requires computer simulation and careful consideration of workpiece geometry, material properties and the intelligent selection of geometric and electrical parameters of the induction coil.

Induction end-heating systems are used in a variety of forging applications in which only select end regions of the workpiece are required to be heated (e.g., upsetting of axle shafts, forming tie-rod heads). Multiple types of induction end-heating coils exist. These include conventional solenoidal coils, where individual workpieces are heated statically, and oval coils (Figure 1) and channel coils (Figure 2), where multiple workpieces are heated progressively. The selection of coil type is based largely on the nature of the customer’s forging process because each offers unique advantages and disadvantages.

Inherent Challenges

Compared to systems in which entire workpieces are heated progressively along the axis of a series of induction coils, induction end-heating systems of all coil types are dramatically more sensitive to electromagnetic and thermal non-uniformities that are innate to the induction heating process. The subsequent analysis of induction end-heating systems will focus on conventional solenoidal coils. However, the challenges and solutions presented are generally applicable to all types of end-heating equipment.

Electromagnetic Non-Uniformity

Non-uniform electromagnetic conditions exist along the length of any induction coil due to the distortion of the electromagnetic field at the coil and workpiece ends. Described generally by Gauss’ Law for magnetic fields, the distortion of the electromagnetic field, and the associated distribution of current in the coil and workpiece, is commonly referred to as the electromagnetic end effect. In end-heating systems, different regions of the workpiece are subject to substantially different electromagnetic conditions during the heating process. This is notably different than the aforementioned horizontal progressive heating system, in which the full workpiece evenly “sees” the different electromagnetic conditions along the coil as it moves along its length during heating. Consequently, the non-uniform induced power density in the workpiece, an inherent product of the locally non-uniform electromagnetic field, can result in poor temperature uniformity along the heated length of the workpiece.

    Minimizing the non-uniformity of the induced power density requires careful consideration of workpiece geometry and material properties, and the intelligent selection of geometric and electrical parameters of the induction coil. Critical variables in the design of a standard solenoidal end-heating system include: workpiece geometry (diameter, length and heated length); workpiece magnetic properties, which vary non-linearly with temperature and magnetic field intensity; workpiece electrical and thermal properties, which vary nonlinearly with temperature; coil geometry (diameter and overhang); and electrical frequency.  Rudimentary selection of design variables with respect to the workpiece to be heated can result in excessive or inadequate heating at the “extreme end” of the workpiece (Fig. 3), depending primarily on whether the coil overhang is too large or too small with respect to the selected electrical frequency. A poor combination of design variables can also adversely affect temperature distribution within the “transition zone.”

Thermal Non-Uniformity

If a truly uniform induced power density could be attained along the length of an end-heated workpiece, a non-uniform temperature distribution would still exist along the heated length due to natural heat-transfer phenomena. Because of the inherently large temperature gradient that exists near the transition zone (due to the cold mass at the unheated end of the workpiece), a non-uniform rate of axial heat conduction exists along the heated length of the workpiece. This effect becomes more pronounced when the length of the transition zone is small. Additionally, relatively higher rates of convection and radiation are typical at the extreme end of the heated length due to locally higher radiation view factors and lower ambient temperatures.

    The adverse effects of non-uniform conduction, convection and radiation losses along the heated length can be minimized by refining the design variables previously mentioned. The reality that these variables affect both electromagnetic and thermal non-uniformities contributes to the intricacy of engineering induction end-heating systems.

Optimizing End-Heating Systems

The persistent pursuit of process improvement as a means of reducing cost and increasing customer satisfaction is the cornerstone of a viable business model in modern manufacturing industries. The forging industry is no exception. Recently, forgers have specifically pushed for quality improvements in the form of reducing scrap and minimizing secondary operations (e.g., grinding excess flash) and productivity improvements in the form of reduced changeover time.  Though a precursor to the actual forging operation, the induction heating process can have substantial effects on the quality and productivity of the overall forging process. The demand for improvement in the induction heating process is a natural extension of forgers’ pursuit of business advancement through process improvement. Because of the number and complexity of variables involved in the design of end-heating systems, utilizing modern computer simulation software (such as our proprietary ADVANCE – Bar End Heater) is a necessity for optimizing designs and continuing to exceed customers’ induction heating needs.

Improving Temperature Uniformity

Metal formability is a function of temperature. Consequently, temperature uniformity along the heated length of the workpiece is essential to forging quality. Poor temperature uniformity in the heated workpiece is a common cause of excessive flash and/or underfill conditions during forging and can ultimately result in unwanted increases in scrap production and secondary operations (e.g., grinding of excessive flash, non-standard machining operations). Customers’ increasingly stringent radial and axial temperature-uniformity requirements are not surprising due to the potential reductions in the cost of quality associated with improved temperature uniformity.

    Radial temperature uniformity, relative to axial temperature uniformity, is affected to a much lesser extent by the previously discussed electromagnetic and thermal non-uniformities innate to the induction end-heating process. Improving radial uniformity along the heated length is relatively straightforward, as it is predominantly a function of workpiece diameter and material, electrical frequency and process recipe. Improving axial temperature uniformity, on the other hand, is a more challenging task because of its sensitivity to the aforementioned electromagnetic and thermal non-uniformities inherent to the process.

    In order to address customers’ desire to reduce temperature disparity along the heated length, standard coil designs can be modified to redistribute the electromagnetic field along the workpiece. Variable-pitch and variable-diameter coils, in which magnetic field intensity is locally increased or decreased to subsequently increase or decrease local induced power density, can prove effective in improving a deficient temperature distribution. Additionally, copper end plates and electromagnetic flux concentrators can be mounted at coil ends to change local electromagnetic conditions and further modify the induced temperature distribution. While these additional design features can offer notable improvement in axial temperature uniformity along the heated length of the workpiece, they also introduce new variables into the engineering process and further the necessity of leveraging computer simulation for design development.

    The case study presented in Figure 4 illustrates the benefits of utilizing variable-pitch coils in end-heating systems. In this common end-heating application, a 6 inch length at the end of a 12-inch-long, 2.75-inch-diameter steel bar (AISI 1035) is to be heated to a target forging temperature of 2230?F (1221?C). The maximum allowable radial and axial temperature differentials are ±50?F and ±75?F respectively. Despite identical process parameters (frequency, supply current, heat time, etc.), the variable-pitch coil offers substantially improved axial temperature uniformity due primarily to a 100?F (38?C) relative increase in temperature at the cold end of the heated length (Fig. 5). The variable-pitch coil also offers a shorter transition zone and reduced heating at the cold end of the workpiece. While a standard-pitch coil could be used to achieve similar temperature uniformity along the heated length, the need for additional turns and coil overhang near the transition zone would likely result in decreases in electrical and thermal efficiency and a less desirable axial temperature distribution. 

Reducing Changeovers

Reducing changeover time can considerably increase the net productivity of a forging line. Now more than ever, customers expect induction end-heating systems to be capable of heating a variety of different workpieces – with varying diameters, heated lengths and overall lengths – while requiring minimal downtime for changeover activities.  Meeting this expectation is possible, but the selection of critical design variables becomes significantly more complicated. Introducing multiple workpiece configurations into the design “formula” markedly increases the scope of an already complex task.

    This increasingly common need requires pragmatic consideration of the many possible geometric configurations of the workpiece and computer simulation of the associated heating processes. Because physical shortening of the induction coil is not feasible, coil “taps” – a series of electrical contactors at different positions along the coil – allow the effective electromagnetic length of a coil to be changed without having to conduct a coil change. Similarly, “jumpers” – moveable electrical connections that allow supply current to bypass select turns along the coil –
can be used to redistribute the electromagnetic field in local regions of the coil. While both of these features offer flexibility in response to different workpiece geometry, each has unique advantages and disadvantages. Therefore, their use varies depending on the specific application.

Conclusion

In addition to the inherent electromagnetic and thermal non-uniformities associated with the induction end-heating process, the increasing demand for quality and productivity advancements in end-heating systems presents complex design challenges. While a variety of potential solutions exist, addressing customers’ specific needs with optimal solutions requires experience and expertise in induction end-heating and the employment of modern computer simulation software in the design process. 

Author Collin A. Russell is a software modeling design engineer with Inductoheat Inc. (an Inductotherm Group Company), Madison Heights, Mich. He may be reached at 248-629 5024 or at crussell@inductoheat.com.