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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 | M A Y / J U N E 2 0 1 9 4 3 *Member of ASM International INDUCTION HEATING: EVERYTHING YOU WANTED TO KNOW, BUT WERE AFRAID TO ASK As a regular contributor to the HTPro eNewsletter, Professor Induction answers a wide variety of questions regarding induction heating and heat treating. Valery Rudnev, FASM,* IFHTSE Fellow Inductoheat Inc., Madison Heights, Michigan I nduction heating is a multifaceted phenomenon com- posed of complex interactions involving electromagnet- ics, heat transfer, materials science, metallurgy, and cir- cuit analysis—with applications across multiple industries. Figure 1 shows a small portion of a virtually endless variety of workpieces where electromagnetic induction heating is used to develop an attractive blend of microstructures and properties at a competitive cost. Question: In some of our facilities, the quenchant tempera- ture for older induction hardeners varied between 80° and 100°F. We currently try to hold the quenchant temperature for our induction hardening machines around 95°F ±5°F. However, it is difficult to hold this temperature range during the summer months for some of the older machines. If the quenchant is allowed to reach a temperature as high as 105°F, what impact could this have on induction hardening results? Answer: The primary objective of quenching is to provide the necessary rate of heat removal within specific areas of the workpiece to arrive at the desired microstructure, hard- ness level, and pattern while optimizing the residual stress distribution. Quenching systems for induction hardening may use a variety of different quench techniques, quench- ing modes, and media, beginning with something as simple as water or brine and progressing through various aqueous solutions including water-based polymer solutions, petro- leum oil-based quenchants, and vegetable oils [1] . A number of quench modes can be applied in induction hardening us- ing liquidmedia (Fig. 2): • Conventional immersion (dunk) quenching • Open spray quenching • Flood quenching • Submerged quench or submerged spray quench Open spray quenching using an aqueous polymer solu- tion is the most popular quenching technique, used in the vastmajorityof inductionhardeningapplications. Your ques- tion leadsme to believe that you are applying this technique. A variety of complex thermo-hydro-dynamic processes are involved in spray quenching, with the cooling intensity being a multifaceted function of several factors including the surface temperature of the workpiece, type and purity of quenchant, pressure/flow, design specifics of the quench apparatus, number and distribution of quench holes, size of orifices, impingement angle, and more. Quenchant temperature is one of the most critical factors af- fecting the outcome of induction hardening. Because the tempera- ture of the quench will increase as a result of cooling the hot part, it is necessary to have some type of heat exchanger tomaintain the tempera- ture of the quenchant within an ac- ceptable range. A range of quenchant tem- peratures between 80° and 100°F is far too wide and should be avoided formost hardening applications. As- suming the quenchant temperature specified by the OEM for the partic- ular application is 95°F ±5°F, signifi- cant temperature variation outside 10 Fig. 1 — Induction heating applications across a variety of industries. 11