Recent Developments In Induction Hardening Technology For Diesel Engine Components.
Initially, induction processing was used primarily to harden selected surfaces, typically bearing journals, to improve their wear resistance.
Today however, more manufacturers are recognizing induction processing as a viable technology for improving the fatigue resistance of highly-stressed components such as camshafts and crankshafts. This trend is particularly strong in the diesel engine industry where engine components are normally subjected to much higher operating stresses than are typically found in gasoline engines. Induction hardening and induction tempering are rapidly becoming the processes of choice for processing camshafts, crankshafts, and even cylinder blocks for diesel engines of all sizes.
Diesel engine builders have been induction hardening crankshaft bearing journals since the 1940s, and the process became nearly universal in the '60s with the demand for more power from existing engine designs. By processing the fillets at the same time the journals were hardened, manufacturers obtained improved fatigue strength in the crankshaft. This permitted the use of low-carbon steels for crankshafts with no sacrifice in durability or performance.
In the early years, controlling post-hardening deformation was a major-process challenge because hardened crankshafts are extremely difficult to straighten without introducing micro-cracks that shorten engine life. Research has shown that such deformation is the product of internal stresses introduced into the crankshaft during manufacturing. By controlling the raw material and the manufacturing process, it is now possible to induction harden crankshafts while keeping total deformation well within the stock removal allowance of the finish grinding operation.
Today, a range of induction hardening systems are available to handle both small batch runs and high volume production requirements. Low-volume, semi-automatic systems typically use semi-open inductors to harden one journal at a time with quenching from below using a turbulent bath produced by a quench head.
Fully automatic systems with throughput of 100 cranks per hour or more are the choice for high-volume applications. These systems typically include sophisticated process monitors and control systems to optimize process uniformity within very narrow control limits. Semi-open inductors and separate quench heads are typically used on these systems.
Traditionally, induction hardened crankshafts have been tempered in resistance heated ovens to increase their fatigue strength in the critical fillet area. Several manufactures, however, are now using the residual heat from the hardening process to harden and temper the fillet in a single operation.
Two different approaches are currently in use. One process works by controlling the post-hardening cooling of the fillet area during the quenching operation. The quench is stopped before the area in question reaches ambient temperature, allowing the residual heat to produce the desired tempering.
In the other approach, the fillet is reheated using the same inductors. This method is only applicable to crankshafts where the bearing and filletare hardened at the same time, as heat will flow away from the fillet area too quickly if they are not.
In some engine designs, two wrist pins are placed side by side in the same throw to reduce the overall length of the crankshaft. Such split-pin journals traditionally have been hardened by nitriding only because it has not been possible to induction harden both bearings without losing fatigue strength.
Today however, equipment is available to induction process split-pin journals as efficiently as single-pin journals. Systems are in operation for crankshafts ranging in size from small-displacement automotive engines to large, over-the-road truck engines. Crankshaft designs using a collar between the journals are processed as efficiently as traditional collarless configurations.
The major challenge faced in induction hardening diesel camshafts is the close spacing of the inlet, outlet and injector lobes on these cams. Without precise heating control, it is quite easy to temper a just-hardened lobe while hardening an adjacent one. There are three different processes available to overcome this difficulty.
Single-Hardening -- For low-volume applications, diesel cams are best hardened one lobe at a time in a vertical scanning-type machine. The camshaft is clamped between centers and rotated as it is lowered through the inductor. As each lobe is processed, it is spray quenched and then lowered into a quenching tank below the inductor. A protective shield is also used between lobes to minimize reheating. These systems are typically manually loaded.
Batch-Hardening -- At higher volumes it becomes practical to harden multiple lobes simultaneously. This is normally done either using multiple inductors, or single wider inductors that heat adjacent lobes. In either case, the hardened lobes are quenched using quench heads which are often integrated with the inductors.
Whole-Shaft Hardening -- An adaptation of a process developed for driveshafts and axles in the '60s, this is the fastest and most efficient way of hardening a camshaft. A single inductor is used to process the entire camshaft in one heating, which is then quenched by a quench head. This process lets the camshaft be processed either horizontally or vertically, and simplifies part handling and fixturing requirements. On the other hand, it produces a camshaft in which all features have been hardened, which can complicate subsequent processing.
Cylinder Blocks & Liners
Diesel liners have been hardened by induction processing for many years to improve their wear resistance. Depending on volume, this is typically done on either manually-loaded or fully-automated vertical scanning machines operating at or above 50 kHz frequency. Using today's transistorized converter technology, the same system can be used for tempering by simply switching to a lower frequency typically 10 kHz or less.
Cylinder Block Ring Hardening
Top dead center is a critical area on diesel cylinders because these engines tend to form deposits of highly abrasive carbon at that point. As the deposits build, the piston and/or rings break off pieces which then fall into the bore and cause galling of the piston.
To help alleviate this problem, a technique has been developed to induction harden a ring of cylinder wall material extending through the top dead center zone. In addition to hardening the area, the process reduces the cylinder diameter slightly so that the deposits are scraped off by the piston with each stroke to prevent the damaging buildup from forming.
Cylinder Block Spot Hardening
This process is used to selectively harden selective longitudinal sections of cylinder wall material in the power stroke zone to improve wear resistance. The first fully-automatic system of this type is now in production.
Induction hardening is a process with many applications for diesel engine builders, some of which are long-proven and in widespread use and others of which are truly leading-edge. The ability to combine hardening and tempering in a single operation offers many opportunities for improvements in both process efficiency and part quality. The full benefit of this newly emerging capability remains to be realized in much of the industry.
Robert Madeira is Technical Coordinator, Induction Heating at Robotron Corp., Sourhfield, Mich., and Hans-Rudolf Schwarz is Product Manager, Heat Treatment, at Elotherm GmbH, Remscheid, Germany.
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|Comment:||Recent Developments In Induction Hardening Technology For Diesel Engine Components.|
|Author:||Madeira, Robert; Schwarz, Hans-Rudolf|
|Publication:||Diesel Progress North American Edition|
|Date:||Sep 1, 2000|
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