Fig. 1. Temperature distribution for 5.75-inch-diameter x 5/8-inch-wall pipe with 10 kHz hardening – 3 kHz tempering
Recent innovations have introduced the use of induction heating technology to provide the heating requirements. Induction can be tailored to case harden the part, providing greater ductility than through-hardened parts. Energy requirements are reduced. Overall conversion efficiency of an induction heating system is superior to that of gas, contributing to a “greener” environment. Additionally, the adaptability of induction heating into “in-line” processes reduces the amount of material handling and streamlines the entire process, reducing overall heat-treat costs.
Heat-treatable materials are a requisite part of the application. Heat treating steel for optimum strength and ductility consists of a hardening process followed by a tempering process. Hardening steel increases the tensile strength and abrasion resistance of the steel, but it also reduces ductility. Hardened steel is relatively brittle due to the residual stresses and the typical characteristics of the hardened microstructure, referred to as martensite.
By sacrificing some strength and wear resistance, ductility can be introduced again by tempering the material. Tempering is accomplished by reheating the already hardened material to a temperature significantly below that used for the hardening process. In the tempering process, residual stresses are relieved and the hardened microstructure is altered to allow greater toughness and ductility while marginally reducing hardness (wear resistance) and overall strength. This processing sequence is often referred to as “quench and temper.”
Large truck suspension-component manufacturing incorporates the quench-and-temper process into the heat treatment of trailer axles. Axles can be processed in as-cut lengths or as much longer tubes and then cut to length following quench and temper. Heating was initially accomplished via gas furnaces. Thermal energy is transferred from the furnace to the surface of the axle. Thermal conduction provides heat from the surface to the interior of the target part.
Heating materials to the required austenitic temperature using gas-fired furnaces can take a substantial amount of time and is dependent on the cross section being heated. Larger cross sections will require more time in the furnace. During this time, scale formation is usually quite heavy. During this heating process, 1–2% of the steel is typically lost to oxidation in the form of scale. Excessive scale formation poses a number of problems for the axle manufacturer. Scale formation can result in metallurgical abnormalities, present added costs to the manufacturer for removal and disposal, and impede gas flow and heat transfer within the furnace. Induction heating technology provides an energy-efficient alternative with significantly lower energy costs, minimizes scale formation and provides overall superior performance.
Fig. 2. Temperature distribution for 5.75-inch-diameter x 5/8-inch-wall pipe with 3 kHz hardening – 1 kHz tempering
Heating Process
Induction heating provides an excellent methodology for quench-and-temper processing. Heating is quick and uniform. Due to the relatively short heating times, scale formation and decarburization of the surface are generally minimal. Parts are usually heat treated one at a time, providing consistent results part after part. Warm-up time is minimal (generally less than one hour for the quench system), and there is no need to keep expensive furnaces running during off-production hours in order to maintain process temperature and avoid lengthy warm-up times. Modern material-handling systems allow multiple heating and quenching processes to be integrated into one cell with proper sequential timing while occupying minimal floor space.Unlike most other heating processes where thermal energy is transferred from the surface through the cross section of a part, induction heating heats from within the part. Induction heating is an electrical process for heating a metal object whereby eddy currents are generated within the metal and that metal’s resistance to the eddy-current flow leads to Joule heating of the metal. Eddy currents are induced by way of electromagnetic radiation.
Fig. 3. Picture of the entrance end of a 40-foot horizontal scanner processing a 5 3/4-inch-diameter tube with a 5/8-inch wall.
Coil/System Design
Using best practices in a modern coil design, one can confidently determine the optimum frequency for a given application. Figure 1 represents a coil/thermal design using 10 kilohertz (kHz) for hardening and 3 kHz for tempering.This calculation was made assuming a 5.75-inch-diameter tube having a 5/8-inch wall thickness. The tubes were approximately 40 feet long and were scanned at a rate of 1.0 inch per second. Each of the hardening coils was 12 inches long. The tempering coil was 24 inches long. The dark-blue line represents the external surface temperature. The pink line represents the internal surface temperature. The orange line represents induction-coil power. This particular calculation required three induction-heating coils (for hardening) with a 12-inch gap between coils. The gaps between the coils were intended to prevent overheating the surface and allow the internal surface to continue heating after surface heat was suspended. The system was designed to heat the entire cross section to approximately 1750°F as it entered the quench station. It should be obvious that the sharp reduction in temperature was due to quenching. Each quench station was 36 inches long. Two were required for hardening while only one was required for post tempering (Fig. 3).
Figure 2 represents the same processing cycle except the hardening frequency was 3 kHz and the tempering was accomplished at 1 kHz. The graph clearly demonstrates improved temperature uniformity through the cross section. The application was supplied with three 3-kHz power sources for hardening and one 1-kHz power source for tempering.
When overall straightness of the axle shaft is critical, the material-handling system was designed with a lance-pusher rather than a typical skewed-roll drive (Fig. 4). A skewed roll drive places undesired force upon the tube while at the elevated temperature. Since steel was relatively soft at typical hardening temperatures, the force exerted by the skewed roll imparts distortion into the tube. In this new design, a series of long rolls were aligned using a laser alignment device.
The key to minimizing distortion is to maximize the support of the tube while at the elevated hardening temperatures. No more than 18 inches of tube should be unsupported in this application. The lance sealed the end of the tube while pushing it through the system. The rolls provided rotation and support. Distortion was reduced to less than 0.030 inches over 3 feet of length, well within the customer’s specification of 0.045 inches maximum.
Fig. 4. Picture of the pusher-lance for precise heat treating of tubular products.
Quenching
Quenching is as critical to the hardening process as heating. In order to transform the heated material into martensite, the cooling process must be fast. Slower cooling rates allow other undesirable microstructures, such as bainite and pearlite, to form. Unfortunately, there is a possibility of cooling some steel grades too quickly, resulting in quench cracks. Cracking can be eliminated by controlling the quench-fluid temperature and/or adding a material to the quench that is intended to retard and control the quenching rate. Close control and monitoring of the quench concentration levels are vital to a successful process. Excessively high concentration levels will render low hardness, while excessively low concentration levels will promote cracking.Uniform application of the quench fluid around the perimeter is equally important to ensure proper hardness and minimal distortion (Fig. 5). Quench rings should be designed with internal baffles to ensure uniform pressure around the perimeter. The baffle also eliminates the potential of high portal pressures adjacent to the inlet connections. Several quench rings are generally required to prevent excessive distances between support rolls.
Fig. 5. Quench barrel for small-diameter, heavy-wall tubular products.
Tempering
As stated earlier, tempering is accomplished by reheating hardened material to a temperature well below the temperature required for hardening. This re-applied heat relieves the residual stresses in the part created by the hardening process and toughens the hardened microstructure (martensite) to increase ductility.Typically, the reheating is done in an oven set at a temperature between 275 and 450°F. Parts are held at temperature for an hour prior to slow cooling. Comparable results can be ascertained by heating very quickly with the induction heating process, however, the temperature must be several hundred degrees above the temperature normally considered appropriate for tempering in an oven. The material used in the subject application would normally be tempered in an oven at 450°F. Using an induction heating process, the surface was heated to 975°F within 24 seconds. This provided enough tempering to relieve the residual stresses and provide the desired ductility.
To determine if the quench-and-tempered part was successfully processed, the material must undergo complete mechanical tests for tensile, yield and elongation. Although hardness can be an indication that the material was properly processed, it should not be the determining criteria. Other factors (such as improper microstructure) can yield a similar hardness reading that could result in lower-strength material. Rather, hardness should be used as a spot check on a verified process to determine the consistency of the process. Periodic mechanical tests should be performed to assure the process integrity (Fig. 6).
Fig. 6. Surface hardness as a function of tempering temperature for furnace and induction heating of SAE 1050 steel quenched in brine from 1575°F.
System Features
This induction quench-and-temper system has several unique features that allow it to adapt to a number of processing variables. The induction coils are automatically positioned to adapt to the various tube diameters, as required. The tubes are processed on a fixed set of rollers. The centerline of the tube, relative to the fixed rollers, varies as the tube diameters change. The system automatically positions the coils such that the coil’s centerline and the tube’s centerline are co-linear. The end of the lance rotates with the tube as it rotates and is able to automatically adapt to the varying centerline as well. This adaptability is seamless and requires no direct operator intervention.The system provides dual-input racks with one rack located on each side of the input stage of the scanner. The operator can be loading the next series of tubes to be processed onto one rack while the other rack feeds current production tubes to the scanner. This significantly reduces the setup time when part changeovers occur.
The unique control system allows processing of any length tube as required, on the fly. It identifies the end of the tube and precisely starts and stops the heat-treat process correctly, regardless of tube length. Lengths can be random within the series. Additionally, each tube is automatically monitored for correct processing parameters. An incorrectly processed part is identified and controlled by placing those tubes on a second output rack, isolating the properly processed tubes from the improperly processed ones.
Conclusion
Unlike traditional gas-fired furnace operations, induction heating systems lend themselves well to the in-line processes used in automotive production. As clearly demonstrated in the previously described systems, axles are continually fed through the system, providing sequential heating (for hardening) followed by quenching, followed by another heating cycle (tempering), followed again by quenching. The need for batch heat processing is completely eliminated. Tubes exiting the system are generally cool enough to be safely handled. Induction heating does not require extended warm-up times. The system can be in the “ready” mode within seconds of start-up. Equipment can be completely shut down during idle periods such as overnight, weekends, vacations and plant shutdowns. Extended warm-up periods are not required. Each tube is processed individually, allowing traditional SPC data to be collected and saved, which identifies the exact processing characteristics of each axle assembly.IHFor more information:Contact Donald Wiseman, VP advanced applications & product development, ABP Induction, LLC, 21905 Gateway Rd., Brookfield, WI 53045; tel: 800-558-7733; fax: 262-317-5394; e-mail: heating@abpinduction.com; web: www.abpinduction.com