Design and Fabrication of Inductors for Induction Heat Treating

FOR INDUCTION MELTING AND MASS HEATING, the early induction heating coils were manufactured from copper tubing wrapped in multiple turns around a mandrel. The first induction heat treating coils were developed for crankshaft hardening in the 1930s (Fig. 1, 2) (Ref 1–4). Unlike the melting and mass heating coils, the heat treating induction coils were machined. These inductors consisted of two parts with a hinge on one side that would open and shut around the crankshaft journal. Quench holes were drilled on the inner diameter of the induction coil to deliver quench to the part after heating. This pioneering development was the culmination of many years of hard work by a large team and clearly demonstrated the different requirements for induction heat treating as compared to melting and mass heating. During the infancy of induction heat treating coil development, the specific features of the induction coil primarily were determined through an analytical-, experience-, and experimentalbased method. Concepts for induction coils were created by physicists. Calculations of coil parameters were determined by manually solving complex equations. Induction coil designs initially were made by draftsmen on blueprints. Most inductors were manufactured by craftsmen using a combination of copper tubing, manually machined components, and copper plates brazed together. Coil optimization was made based on experimental results through the trial-and-error method, because tests and modifications took much less time than the calculations did (Ref 1, 5). While many induction heat treating coils are still manufactured in this way, the tools for the design and fabrication of complex induction heat treating inductors have evolved greatly over the years. For many types of applications, the preferred induction coil style already is known based on years of empirical data. In many cases, comparisons to known results can even determine induction coil dimensions that will lead to a successful heat treating pattern, greatly limiting experimental testing times and shortening development cycles (Ref 1, 5–9). For newer applications or optimization of existing ones, computer modeling tools are being incorporated into the induction coil design procedure. Sophisticated software running on personal computers is capable of making calculations in seconds, minutes, or hours that would have taken days, weeks, or months to solve manually. In many cases, no analytical formula exists for complex coils, and the only way to make calculations prior to tests is with a computer modeling program. Induction coil designs and process recipes are “virtually” tested to determine resulting heat patterns. In many cases today (2013), virtual tests and evaluations can be made more quickly and at a lower cost than physical ones (Ref 10–12). The process of mechanical induction coil design also has evolved over the years. While the initial drawings weremade by hand, induction coils now are drawn and detailed using computeraided design (CAD) packages. Over the past several years, CAD packages have increased their interactivity with computer-aided manufacturing (CAM) software packages. In many instances, induction coil drawings can be transferred from a CAD drawing to a CAM program into the computer numerically controlled machine. This creates the possibility for greater repeatability of complex machined induction coils (Ref 13, 14).