|Fig. 3. Ring hardening machine for large bearings up to 6 meters (tilting design)|
Traditionally, case carburizing has been used to harden very large workpieces such as slewing rings, ring gears and bearing races. The carburizing process is straightforward and has been well understood for many decades.
|Fig. 1. Induction principle – An alternating magnetic field induces eddy current heating below the workpiece surface|
The Case for Induction Hardening
A growing number of manufacturers, however, are augmenting or replacing their carburizing processes with induction hardening, in which the surface material’s metallurgical structure is transformed (hardened) through a precisely controlled sequence of rapid surface heating followed by rapid cooling (quenching). Practical advantages of induction hardening include:
- Minimal workpiece distortion – Induction hardening heats the workpiece surface. Through-heating is avoided. The rapid heating/quenching sequence minimizes distortion, thereby reducing requirements for expensive and time-consuming secondary machining and straightening operations.
- Superior metallurgical properties – Workpiece heating is limited to the surface area only, and the heating cycle is kept short. Unwanted grain growth is avoided.
- Higher throughput – With short cycle times (minutes instead of hours or even days), induction hardening supports higher throughputs and streamlined manufacturing.
- Compact machine footprint – Induction hardening requires less factory floor space. Processing the ring in a vertical or near-vertical plane further reduces the equipment footprint.
- Ability to harden extremely large workpieces – Induction hardening can treat very large workpieces (e.g., 6-meter-diameter rings).
- Production scheduling flexibility – Like the microwave oven in your kitchen, induction heating is instant on/instant off, with no need for lengthy warm-up, cool-down cycles. Production planners have greater freedom when scheduling small lot sizes and one-off parts.
- On-line quality control – Modern induction systems feature energy monitoring systems and hardness checkers for automated on-line QC of each and every workpiece while it is in process.
- Reduced energy consumption – Induction hardening primarily heats the surface instead of the entire workpiece. The heating cycle is necessarily short, and total energy consumption is low.
- Low emissions – Induction hardening produces essentially no CO2 emissions.
|Fig. 2. Skin effect – Higher frequencies are used to concentrate heating near the surface and away from the core.|
Principles of Induction Hardening
Induction hardening is comprised of two process steps: heating followed by rapid cooling (quenching) with a quenching fluid. The heating is caused by alternating current flowing through a coil (sometimes called an inductor) that is sized and shaped according to the workpiece. The alternating coil current creates a corresponding alternating magnetic field that, in turn, induces eddy currents inside the workpiece. These eddy currents produce heat inside the workpiece. The heating depth is inversely proportional to coil current frequency. Higher frequencies produce shallower heating in the workpiece. This very useful phenomenon is known as the “skin effect.” Heat originates inside the workpiece and does not have to be transferred into the workpiece via radiation or convection at the surface. The heating period is therefore short (e.g., a few seconds), and unwanted core heating is avoided (Figs. 1 and 2).
Before hardening, the steel workpiece surface contains a mixture of a-iron (ferrite) and cementite (Fe3 C) at room temperature. The induction coil heats the surface material to a temperature of at least 723°C (the eutectoid temperature), causing the ferrite to transform into g-iron (austenite). The steel is thus “austenitized.” At the same time, carbon from the available cementite dissolves into the austenite because carbon has a much higher solubility in austenite than in ferrite. This dissolved carbon is essential because a steel workpiece must contain at least 0.02% carbon to be hardenable.
In the next process step – quenching – the austenitized steel is rapidly cooled at a controlled rate. This rapid cooling prevents the diffusion of carbon atoms and the reformation of original ferrite and cementite mixture. Dissolved carbon atoms are trapped in the ferrite, causing the ferrite body-centered cubic (BCC) crystal structure to deform into a body-centered tetragonal (BCT) structure called martensite. The temperature difference and the cooling rate, which can be controlled by selecting the right quenching medium (such as oil, or water with polymer additives), determine the level of martensite formation. Faster cooling below the transformation temperature produces more martensite. The fresh martensite is hard and brittle. Tempering (controlled heating to prescribed moderate temperatures for defined time periods) reduces this brittleness and gives the steel the desired combination of hardness, strength and toughness.
|Fig. 4. Illustration of the inductor setup with flux concentrators and spray heads during the starting sequence|
Conventional Induction Hardening Methods for Large Rings and Bearing Races
With single-shot hardening, either the workpiece rotates past one or more stationary inductors or a complete 360-degree ring inductor interfaces with the entire workpiece. The entire bearing surface (race) is heated to the appropriate austenitizing temperature followed by a quench of the entire surface. Quenching may be done by submerging the workpiece in a bath or by using spray nozzles that are integrated into the inductors and tailored for the process requirements. The single-shot process is best suited for workpieces with a diameter less than 2 meters. The induction heating electrical-power requirement grows quadratically with increasing workpiece diameter. For example, single-shot hardening of a 2-meter-diameter ring would require about 1.6 MW of power. Compared to scan hardening, the single-shot process with its high power is very fast, typically measured in seconds.
Scan Hardening with a Remaining Soft Zone
Scan hardening with a remaining soft zone has been a standard process for hardening single raceway and multi-raceway large bearings. The inductor/spray head assembly is stationary while the ring rotates with a low, constant tangential velocity past the inductor. A small soft zone (unhardened area) necessarily remains at the end of the scan path. With 100 kW of power, a 3-meter-diameter bearing race can be hardened in less than one hour (Fig. 3).
|Fig. 5. The polished section and hardening depth of a ring segment after the starting sequence shows uniform hardening with no soft zone.|
New Techniques for Scan Hardening with No Soft Zone
With its residual soft zone, traditional scan hardening is inadequate for mission-critical components with stringent requirements for smoothness and heavy loads such as:
- Ultra-low vibration systems (e.g., magnetic-resonance imaging, MRI, technology)
- High mechanical-stress systems (e.g., tunnel-drilling machines)
- Systems where continuous rotation is required or where challenging environmental conditions mandate maintenance-free systems with high lifetime (e.g., wind turbines, tidal plants and oil platforms)
Carburizing is limited as a process alternative by furnace size and long process times. Single-shot induction hardening is impractical due to its high power requirement. A better hardening method is needed to efficiently and reliably harden very large, high-value rings. In response to this need, a process to scan harden arbitrarily large rings with no remaining soft zone has been developed and patented. This process differs from conventional scan hardening primarily at the start and end location of the scan, where steps are taken to avoid unwanted tempering and changes in the hardened-zone microstructure.
Comparing this process to single-shot induction hardening, a 6-meter-diameter ring can be processed with a 200-kW scanning system, which is 1/8th the power the single-shot process would require. In contrast to carburizing, which would require several hundred hours in the furnace, the time required for the induction scan-hardening process (less than two hours) is negligible. Moreover, the cost-intensive and time-consuming straightening operation to clean up the distortion caused by carburizing can be avoided altogether.
Scan hardening and tooth hardening on both the inside and outside diameters of the workpiece can be done on the same induction machine with minimal setup times. Manufacturers of large ring bearings with small production runs appreciate this flexibility and the freedom to process diverse workpieces while holding equipment costs at a minimum.
|Fig. 6. Inductor movement and spray setup during the end sequence of induction hardening|
Characteristics of Induction Scan Hardening with no Soft Zone
Scan hardening with no soft zone requires two inductor assembles, each comprised of an inductor and a spray head. The inductors are narrow to create a compact hardening zone. Flux concentrators focus the magnetic field for one-sided hardening.
The inductor assemblies are brought together in a back-to-back orientation and are then energized with independent power supplies. Both assemblies travel side-by-side in the same direction for a short distance. One of the assemblies reverses directions, so the inductor assemblies travel in opposite direction. This technique avoids the formation of a soft zone at the start location (Fig. 4).
Figure 5 depicts the hardening result with the etched case and with two hardening passes along the workpiece axis at 0.5 mm and 5 mm depths after tempering. The start location is still recognizable, and one can see that the entire area has been hardened with a relatively constant case depth.
Like the start sequence, the end sequence relies on precise control of the quenching sprays and the tight motion control of the inductors and sprays to achieve a uniform case depth with no soft zone at the end location (Figs. 6 and 7). The resulting bearing surface meets stringent requirements for high loads and smooth operation.
|Fig. 7. The result of the end sequence of induction hardening is uniform hardening with no soft zone.|
Induction hardening has proven itself in the manufacturing of high-value, large ring bearings. Wind power certainly owes its success in part to surface-hardened bearing races and gear teeth. Construction and mining equipment would wear out quickly, and military and aerospace endeavors would entail unacceptable risks without the benefits of induction hardening. Alternative hardening methods for large rings and bearing races struggle with distortion, soft spots, grain growth, furnace dimensions, energy efficiency, emissions and extraordinarily long process times. Induction scan hardening with no remaining soft zone bridges the gap between conventional hardening methods and the growing demand for very large bearings with high load ratings, low noise and longer service lives. IH
For more information: Contact George Burnet, general manager of SMS Elotherm North America at tel: 724-553-3471; e-mail: firstname.lastname@example.org. Otto Carsen is sales manager, hardening, for SMS Elotherm GmbH and Dirk M. Schibisch is vice president of sales.