The preferred manufacturing method for reliably creating hard, wear-resistant component surfaces is induction hardening. The surface material’s metallurgical structure is transformed (hardened) through a well-controlled sequence of induction heating and rapid cooling (quenching). This process is used, for example, to harden ring gears, slewing rings and bearing races. Different hardening-system designs are available for optimum and reproducible results. The most flexible hardening system features a workpiece tilt table to optimize quenching fluid flow to the bearing raceways and/or gear teeth. Additional patented SMS Elotherm technologies such as workpiece net power monitoring and automatic inductor position control combine to make induction hardening a robust and precise manufacturing process that is easily integrated into existing production lines.

Significance of Wind Energy

Today, wind energy supplies the electrical power requirements of almost 8 million households. U.S. Department of Energy projections call for wind power to produce 20% of domestic electrical energy by 2030. About half of this power would come from installations located far offshore. Winds on the open seas are generally stronger and more continuous, offering up to 40% higher power output compared to winds on land. However, the repair and maintenance on the high seas is more involved and expensive. Induction hardened components increase system wear resistance and minimize service costs for offshore wind turbines.

Where is Induction Technology Used in Wind Turbines?

Wind turbine bearings must have a maintenance-free service life of at least 20 years. Particularly harsh demands are placed on the large roller bearings for the rotation of the nacelle azimuth (yaw), the main rotor and the blade pitch adjustment. These critical bearings and associated gearing can be hardened for improved strength and wear resistance. Induction technology provides a fast, energy-saving hardening process with excellent and reproducible results.

Characteristics of Various Surface-Hardening Techniques

The precise process control offered by modern induction systems produces uniform and highly reproducible surface hardening that improves the properties of large roller-bearing rings. The high degree of induction process automation yields consistently superior quality day after day, month and month, year after year.

Induction Principle

Faraday’s Law of Induction governs the inductive surface-hardening process. The changing magnetic field induces an electrical voltage. This voltage causes an electrical current flow in an electrically conductive material, thereby heating the material. By the “skin effect,” the heat input decreases with increasing depth into the workpiece. Adjusting the frequency of the magnetic field used to drive the induction process can influence the effective heating depth. The material to be hardened is heated to the austenitizing temperature and then rapidly cooled (i.e. quenched), causing the formation of a hard surface case, including a very hard material called martensite.

Gear-Tooth Hardening

Gear teeth on the inner or outer diameter of large roller-bearing rings are also hardened with induction. Typically, the tooth base and flank are hardened, and a “soft zone” (unhardened region) is maintained at the tooth tip. An induction frequency range between 4 kHz and 30 kHz is selected to achieve the desired case depth. High-throughput production systems process multiple teeth simultaneously (Fig. 1).

Bearing Raceways

Three process techniques are used to inductively harden roller-bearing raceways:
1. Scan hardening with a soft zone
2. Scan hardening with no soft zone
3. Single-shot hardening with no soft zone

Scan Hardening with a Soft Zone
Scan hardening with a soft zone is the standard process for raceway hardening. The inductor assembly includes a quenching showerhead and is mounted on a linear axis to accommodate a range of different ring diameters. The inductor assembly remains stationary during the hardening process, except for small movements required to maintain the correct standoff distance between the inductor and the ring. The ring rotates with a low, constant tangential velocity past the inductor. For thicker (e.g., 1/4 inch or more) case depths, the workpiece is inductively preheated before final heating to the austenitizing temperature. A small soft zone (unhardened area) remains at the end of the scan path.

The flexible automatic-tilt machine shown in Figure 2 performs horizontal, vertical and tilted scan hardening. The tilt table orients the workpiece for optimal quench control for different hardening applications. For gear-tooth hardening, a horizontal workpiece is preferred. For raceway hardening, a tilted or vertical ring works well. Industrial hardening machines for workpiece diameters up to 20 feet and weights up to 20 tons are reliably working in high-throughput production settings.

Scan Hardening with no Soft Zone
The SMS Elotherm patented process for inductive scan hardening with no soft zone (Fig. 3) begins with inductive heating of a start position by two scanning heads. Each scanning head operates independently with its own inductor coil, quenching spray and servo-motion control. After initial heating at a common start position, both heads travel along the workpiece surface in opposite directions, heating and then quenching the surface as they move. At the opposite side of the workpiece, the two hot zones come together and are quenched with a stationary spray fixture, producing an inductively hardened workpiece with no soft zone. This technique of using dual, independent scanning heads provides a cost-effective solution for hardening even the largest workpieces with minimal peak input power requirements.

Single-Shot Hardening with no Soft Zone
In the single-shot process, the workpiece rotates at high speed past one or more stationary inductor(s). After multiple rotations, the workpiece surface reaches the required temperature and the entire surface is quenched. The single-shot hardening technique is well suited for smaller-diameter workpieces. The costs for the system, factory space, input power and cooling grow quickly as the workpiece diameter increases.

Workpiece Net Power Monitoring

In addition to normal machining tolerances and positioning error, thermal distortion during raceway hardening contributes to workpiece dimensional variations. Continuous online measurement of process parameters is therefore essential to monitor the hardening process in real time.

The patented SMS Elotherm workpiece net power monitor provides robust feedback for online quality control. Previous power measuring methods relied on phase-corrected multiplication of converter current and voltage. The validity of the resulting power calculation was compromised by the inclusion of irrelevant waste power that did not contribute to workpiece heating. The SMS Elotherm workpiece net power monitor, however, filters out this irrelevant wasted power, providing accurate measurement of the real net power to heat the workpiece during the hardening process.

Figure 4 illustrates how the two different power signals (converter output power and workpiece net power) vary in response to changes in the inductor standoff distance. Note how the converter output power signal has poor sensitivity to changes in the inductor position. The workpiece net power monitor, however, provides easily recognizable signal changes in response to small inductor position variations. The workpiece net power monitor works like a magnifying glass, revealing net power variations that would otherwise be masked by losses in the resonant induction circuit. Using this “magnifying glass effect,” even the smallest irregularities and inductor position errors can be recognized and located on the workpiece surface.

Noncontact Automatic Inductor Position Control

The patented SMS Elotherm automatic inductor position control senses and automatically corrects inductor position errors in real-time during the hardening process. Simultaneously, the workpiece net power monitor provides continuous online quality control. The result is consistent and reproducible surface hardness and depth, leading to higher load capacities and longer service lives for large roller and ball bearings.

Summary and Outlook

The hardening process is critical to assure the component properties and wear resistance needed in modern wind turbine rotary joints. Induction hardening technology is particularly useful in these applications. Induction process tools are operationally straightforward for reproducible, high-quality results. Moreover, induction provides flexible, cost-effective and energy-efficient hardening of simple and complex parts – large and small.

Induction hardening already plays a critical role in the successful build-out of renewable wind power, particularly offshore wind power. Improved induction hardening capabilities are already emerging to meet new challenges. Additional productivity improvements will be realized through scan optimization and the reduction of non-process time. The induction technology roadmap also looks forward to inductive hardening of ever more complex and compact geometries incorporating targeted soft zones for higher component fatigue strength. IH

For more information: Contact George Burnet, general manager of SMS Elotherm North America at tel: 724.553.3471; e-mail: g.burnet@sms-elotherm.com. Otto Carsen is sales manager, hardening, for SMS Elotherm GmbH; Dr. Stefan Dappen is principal engineer, process engineering; and Dirk M. Schibisch is vice president of sales.