ASM Handbook, Volume 4C, Induction Heating and Heat Treatment Copyright # 2014 ASM InternationalW V. Rudnev and G.E. Totten, editors All rights reserved www.asminternational.org Design and Fabrication of Inductors for Induction Heat Treating Rob Goldstein, Fluxtrol William Stuehr, Induction Tooling Micah Black, Tucker Induction Systems FOR INDUCTION MELTING AND MASS based method. Concepts for induction coils were personal computers is capable of making cal- HEATING, the early induction heating coils created by physicists. Calculations of coil para- culations in seconds, minutes, or hours that were manufactured from copper tubing wrapped meters were determined by manually solving would have taken days, weeks, or months to in multiple turns around a mandrel. The first complex equations. Induction coil designs initi- solve manually. In many cases, no analytical induction heat treating coils were developed for ally were made by draftsmen on blueprints. formula exists for complex coils, and the only crankshaft hardening in the 1930s (Fig. 1, 2) Most inductors were manufactured by crafts- way to make calculations prior to tests is with (Ref 1–4). Unlike the melting and mass heating men using a combination of copper tubing, a computer modeling program. Induction coil coils, the heat treating induction coils were manually machined components, and copper designs and process recipes are “virtually” machined. These inductors consisted of two plates brazed together. Coil optimization was tested to determine resulting heat patterns. In parts with a hinge on one side that would open made based on experimental results through many cases today (2013), virtual tests and eva- and shut around the crankshaft journal. Quench the trial-and-error method, because tests and luations can be made more quickly and at a holes were drilled on the inner diameter of the modifications took much less time than the lower cost than physical ones (Ref 10–12). induction coil to deliver quench to the part after calculations did (Ref 1, 5). The process of mechanical induction coil heating. This pioneering development was the While many induction heat treating coils are design also has evolved over the years. While culmination of many years of hard work by a still manufactured in this way, the tools for the initial drawings were made by hand, induction large team and clearly demonstrated the different the design and fabrication of complex induction coils now are drawn and detailed using computer- requirements for induction heat treating as com- heat treating inductors have evolved greatly aided design (CAD) packages. Over the past sev- pared to melting and mass heating. over the years. For many types of applications, eral years, CAD packages have increased their During the infancy of induction heat treating the preferred induction coil style already is interactivity with computer-aided manufacturing coil development, the specific features of the in- known based on years of empirical data. In (CAM) software packages. In many instances, duction coil primarily were determined through many cases, comparisons to known results can induction coil drawings can be transferred from an analytical-, experience-, and experimental- even determine induction coil dimensions that a CAD drawing to a CAM program into the com- will lead to a successful heat treating pattern, puter numerically controlled machine. This cre- greatly limiting experimental testing times and ates the possibility for greater repeatability of shortening development cycles (Ref 1, 5–9). complex machined induction coils (Ref 13, 14). For newer applications or optimization of existing ones, computer modeling tools are being incorporated into the induction coil design Methods of Induction Heat Treating procedure. Sophisticated software running on Today (2014), there are many different types of induction heat treating coils. The induction heat treating coil style depends on the induction heat treating process. All induction heat treating processes can be classified in two categories: single shot and scanning, based on whether the induction coil is moving (excluding rotation) relative to the part during the heating process. A brief description is given subsequently, because more in-depth information on induction heat treating equipment is discussed in other sections in this article. In single-shot (Fig. 3) induction heat treating applications, the position of the induction coil relative to the heated length of the part Fig. 1 Tocco coil. Source: Ref 1 Fig. 2 Vologdin coil does not move. In many single-shot heating 590 / Equipment applications, the part rotates while heating and Besides single shot, the other type of induc- the component will be produced on an existing quenching occur to ensure uniformity of pattern. tion machine is scanning (Fig. 6) (Ref 5, 9). In system or if a new machine must be built. In Within the family of single-shot heat treating scan-hardening applications, the induction coil many cases, the part producer’s desire is to applications, there are several varieties of in- moves relative to the part. Similar to single-shot develop new tooling for an existing machine duction coil configurations, depending on the inductors, scanning inductors can be either MIQ with spare capacity. This reduces the degree quench-delivery mechanism. or quenching in place. They also may be sepa- of freedom and can make the induction coil Machined integral quench (MIQ) installations rate quenching, but this is less common. design procedure more complicated, because have quench integrated right into the induction a less-than-optimal frequency or coil style will coil itself. The most common method is for the be necessitated to fit the existing machine quench to be delivered through the coil copper Considerations for Inductor Design (Ref 16). itself. These inductors typically have separate To determine the ability to use existing pockets for water cooling and quench delivery. Induction heat treating coils are available in equipment, it is necessary to make an analysis In some other cases, the quench may be deliv- many shapes and sizes and must perform a vari- of the part to be heat treated. Part material, ered through a magnetic flux concentrator. ety of tasks in a given induction heat treating prior processing, geometry, production rate, Another type of single-shot heat treating instal- application. Depending on the application, the and heat treatment specifications all play roles. lation is quenching in place (Fig. 4). Quenching in induction coil design requirements include: The part material and prior processing deter- place is similar to MIQ, because the quench ring is Meet heat treatment specifications in desired mine what the minimum heat treatment temper- a part of the induction coil assembly. What differ- production rates ature should be, along with how much time is entiates quenching in place from MIQ installa- Be robust enough to tolerate manufacturing allowed for cooling. The part geometry and tions is that the quench is not designed to be an variations heat treatment specifications indicate how active component of the induction circuit. Mount into the induction machine much energy is required, what the preferred The final single-shot induction heat treating Have electrical parameters that match the frequency ranges are, and what type of induc- coil style is separate quenching (Fig. 5). In induction power supply tion method (i.e., single shot, scanning) is best these installations, the quench is not part of Deliver quench suited for the application. Finally, the produc- the induction coil assembly. In separate-quench Have a satisfactory lifetime tion rate determines how much power and/or installations, the quenching often is delivered in Have satisfactory efficiency how many spindles or stations are required. a different station or position in the machine. Be repeatable from inductor to inductor More details on this topic are given later in These systems are common when a delay this and other articles in this Volume. The dis- between heating and quenching for heat soak- In developing a new induction heat treating cussion in this article is limited to the relation- ing is desirable. coil and process, the first question is whether ship between these factors and the induction coil design. Current Flow in the Part Eddy currents are the primary source of power dissipation in most induction heat treat- ing applications. Eddy currents, just like all other electrical currents, must form a closed contour. In most cases, the current flow in the workpiece follows the shape of the induction heating coil, due to the proximity effect. The power density in a given section of the work- piece depends on the current density. The current density can be influenced by electro- Fig. 5 Separate quenching. Source: Ref 15 magnetic effects (end effect, edge effect, etc.), the presence of magnetic flux concentrators, the width of the copper, the geometry of the Fig. 3 Single-shot induction heat treating application part, and the distance between the coil and the workpiece (coupling gap). The next step in the coil design process is determining how the current will flow in the part. This is critical, especially in cases where the geometry changes. Some common geometry changes encountered in induction heat treating are fillets, undercuts, corners, shoulders, cham- fers, splines, keyways, and oil holes. The heat- ing of these critical areas relative to adjacent ones is strongly dependent on the induction coil style and frequency of heating. The first primary choices for current flow, either in the plane of the geometry change or perpendicular to the geometry change, are dis- cussed here. When current flow is perpendicu- lar to the direction of a geometry change, the Fig. 6 Machined integral quench scanning inductor natural tendency is for the current to concen- Fig. 4 Quench-in-place inductor for hardening of a spindle with magnetic flux concentrator trate on the part surface that is closer to the Design and Fabrication of Inductors for Induction Heat Treating / 591 induction coil, due to the proximity effect.