The utilization of electromagnetic induction for heat-treatment purposes is no longer a novelty. However, the current excitement associated with induction heat treatment may suggest otherwise.

A combination of factors including efficiency and environmental initiatives in automotive and aerospace manufacturing, innovations in induction power supplies and advancements in the simulation of induction heat-treatment processes suggests a “golden age” of induction heat treatment is on the horizon.

 

Efficiency and Environmental Advantages

In an era of environmental consciousness and cost reduction, the fundamental physics of electromagnetic induction make it a highly attractive heat-treatment (and general thermal-processing) method. Induction heating is a direct heating method in which thermal energy is generated within the heated component – as opposed to transferred to it from the environment. Because induction heating results in both surface and subsurface heat generation, rapid heating and high thermal efficiency are generally achievable.

Induction heat-treatment processes also typically deliver high electromagnetic efficiency. For hardening ferromagnetic materials such as carbon steels and martensitic stainless steels, this efficiency is often on the order of 70-80% (for tempering such materials, this figure can approach 90%). Induction hardening also does not involve the diffusion of chemicals into components. For this reason, induction is often considered a “cleaner” hardening method relative to thermo-chemical alternatives such as carburizing and nitriding.

 

Induction Power-Supply Innovation

The development of HF transistor power supplies in the 1950s and 1960s dramatically and positively altered the future of induction heat treatment. The introduction of simultaneous dual-frequency power supplies in the late 1990s and early 2000s provided a significant improvement in induction hardening capability, particularly with respect to hardening small-to-medium gears. Very recently, after nearly two decades of relatively minor advancements in induction power supplies, a revolutionary technology – an inverter capable of practically instantaneous, in-operation frequency modulation – was introduced.

In an induction heating process, the frequency of the applied electromagnetic field (i.e., the frequency of alternating current passing through the induction coil) influences the depth in which thermal energy is generated in the heated component. The depth in which the majority (approximately 86%) of induced heat generation occurs in a body conducting alternating current is often called the skin depth. Skin depth (d) is a function of the body’s electrical resistivity (r) and magnetic permeability (µ) and the frequency (F) of the applied magnetic field. It can be approximated by:

 

 

 

Practically speaking, this means that the selection and manipulation of frequency is the only means of controlling the depth in which heat generation occurs in an induction heated component. Accordingly, the ability to measurably and deliberately change frequency during a heat-treatment process, a characteristic unique to Inductoheat’s IFP™ (Independent Frequency & Power) inverter, represents a massive heat-treatment opportunity.

 

Case Study: Scan Hardening

Scan hardening is a proven application of variable-frequency power supplies. The ability to change frequency is the ideal solution for accommodating variations in geometry and hardening requirements along the length of scan-hardened components.

The process outlined in Figure 1, a scanning process for hardening a medium-carbon steel (SAE 4140) shaft, provides a convenient case study. This shaft, representative of many modern automotive components, is hollow, features a flanged end and includes an appreciable change in diameter. The diameters above and below the transition are approximately 45 mm and 50 mm respectively.

This 5-mm-diameter change, which is large relative to the required case depth, creates inherent electromagnetic and thermal challenges. In the internal corner of the diameter transition, it is inherently difficult to induce sufficient heat generation to overcome the conduction of heat into the relatively large surrounding mass. The presence of a 0.5-mm undercut, which effectively increases the localized coupling between the coil and component, provides an additional obstacle. Meanwhile, the external corner can easily be overheated because it protrudes outward into the path of the magnetic flux lines looping around the single-turn coil.

If a single frequency were selected for hardening this component, 30 kHz would be a likely choice given the effective case-depth target of approximately 2 mm. As shown in Figure 2, this process provides good results along the majority of the hardened length of the component, but there are problems in the diameter transition area.

Due to insufficient austenitization (i.e., heating), very little martensite formation is projected in the undercut. Increasing coil power and/or effective heating time in this region may seem like logical corrective actions. However, this would further increase the peak temperature in the adjacent external corner. Given that this temperature is already on the order of 1060°C (1940°F), further temperature increase could produce undesirable (and potentially unacceptable) localized grain coarsening.

Increasing the temperature in the internal corner without doing so in the external corner is a seemingly paradoxical task given that these features are separated by just 3 mm. As shown in Figure 3, however, this is achievable by altering frequency when heating the diameter transition area (i.e., when the coil is directly adjacent to this feature).

A reduction in the inverter output frequency from 30 kHz to 10 kHz, which increases the skin depth in the component by a magnitude of about 1.7, simultaneously alleviates the electromagnetic proximity challenge in the internal corner and reduces the risk of overheating the external corner. This variable-frequency process provides a significant increase in case depth in the undercut while reducing the peak temperature in the adjacent shoulder by almost 40°C.

This case study, while relatively simple, illustrates one of the quality advantages afforded by variable-frequency scan-hardening systems. If this component featured substantially different case-depth requirements along its length, the ability to modulate frequency would have additional benefits. Furthermore, while outside the scope of this article, IFP technology also offers quality and flexibility advantages in various other applications including horizontal continuous hardening, spin hardening (of gears and sprockets) and tempering/stress relieving.

 

Integration of Simulation into Equipment, Process Design

Incorporating computer simulation into the design of induction heat-treatment systems can provide improved product quality, reduced engineering time and manufacturing costs, and faster process development. These advantages, however, can be easily negated by the amount of time required to create a representative model and compute numerical results.

In certain applications, particularly those that require 3-D simulation, the amount of time required to obtain useful information from simulations is simply impractical. Fortunately, the increasing capabilities of simulation software and decreasing cost of computer hardware are diminishing this barrier.

 

Case Study: Single-Shot Hardening

In a single-shot hardening process, the entire region of the component that is to be hardened is heated using an induction coil that induces both circumferential and longitudinal current flow (Fig. 4). Rotation of the part during heating and quenching promotes circumferentially uniform hardening results. The ability to simulate single-shot hardening processes reliably and in a reasonable amount of time offers manufacturers and users of induction heat-treatment equipment substantial value because:

• Single-shot hardening is a very common induction heat-treatment process.
• The design of single-shot coils is much less intuitive than that of most other induction hardening coils.
• Hardening results are predominantly determined by the geometry of the coil, as opposed to process parameters (unlike scan hardening).
• Coil fabrication costs can be considerable, especially taking into account the iterative modifications often associated with trial-and-error design.
• Single-shot coils are often subjected to very high power densities and can therefore be prone to premature failure.

Unfortunately, the physical characteristics of single-shot hardening processes necessitate 3-D electromagnetic-thermal simulation, and the amount of time required to set up 3-D finite-element models and compute accurate solutions has historically been quite prohibitive. For this reason, the simulation of single-shot hardening processes is still quite rare in industry.

As illustrated in Figure 5, however, this reality is changing. Continuous improvements in software functionality and the increasing affordability of computational resources are reshaping the practical feasibility of conducting these complex, resource-intensive simulations. Both manufacturers and users of induction heat-treatment equipment are reaping the rewards.

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For more information: Contact Collin A. Russell, software modeling design engineer, Inductoheat Inc. – An Inductotherm Group Company, 32251 N. Avis Dr., Madison Heights, MI 48071; tel: 248-629-5024; e-mail: crussell@inductoheat.com; web: www.inductoheat.com