The term heat treating is associated with a large family of processes. Though other materials can be heat treated using electromagnetic induction, components made of steels and cast irons represent the majority of metallic materials that routinely undergo induction heat treatment.
Hardening One of the most common applications of induction heat treatment is the hardening of steels, cast irons, and powder metallurgy materials. Among other workpieces, includes components (e.g., camshafts, crankshafts, gears, constant-velocity front wheel drive components, transmission shafts, ball studs, pins, working surfaces of tools, and others) that are commonly hardened using electromagnetic induction developing an attractive blend of properties.
A typical hardening procedure for steels and cast irons involves heating the alloy to the austenitizing temperature range, holding it (if necessary) at a temperature for a period long enough for the completion of the formation of a fully or predominantly austenitic structure and then rapidly cooling/quenching it below the Ms critical temperature when martensite starts to form. Rapid cooling allows replacement of the diffusion-dependent transformation of austenite by diffusion-less shear-type transformation, producing a much harder constituent called martensite.
Besides carbon steels, martensitic reaction is observed in many materials and its causes might be quite different. For example, martensite can be thermally induced or formed owing to the presence of mechanical stress (for example, work hardening of austenitic stainless steels in spring wire manufacturing). In cases when martensite reaction is thermally driven (e.g., attributed to intense cooling), the temperature ranges where martensitic reaction occurs and characteristics of obtained martensitic structures (including hardness, strength, ductility, and toughness to name a few) can be substantially different fordifferent materials. In this article, a discussion regarding martensitic reaction and obtained martensitic structures will be limited to carbon steels, cast irons, martensitic stainless steels, and some powder metallurgy materials. As-quenched martensitic structures are commonly associated with being hard, strong but having a lack of ductility and toughness and exhibiting a significant amount of residual stresses.
It should be also mentioned at this point that there are much less frequent cases of hardening when instead of forming martensitic structures, it might be desirable to form predominately bainitic or even fine pearlitic structures. For example, in contrast to the great majority of induction hardening applications, when hardening of high carbon steel rails for railways, owing to the specifics of the process requirements and safety concerns, formation of any martensite in the as-hardened structure is not permitted. Nevertheless, it is more the exception than the rule, and for the great majority of induction hardening applications, the goal is developing fully or predominately martensitic structures. Hardening may be done for purposes of obtaining certain properties or combination of properties such as strength and wear resistance as well as the formation of a desirable distribution and magnitude of residual stresses. In some cases, it is required to harden an entire cross section of the workpiece (so-called through hardening); however, in other applications, only certain selected areas are needed to be hardened (e.g., surface hardening or hardening of a portion of the workpiece). For example, it may be desirable to obtain a certain combination of hardness, wear resistance, and contact fatigue strength at the surface or near-the-surface areas without affecting the core microstructure (e.g., hardening of gears and gear-like parts). Other applications might require an increase of hardness and strength of the entire cross section of the part, and induction through hardening can help achieve the desirable industrial characteristics.
There are four primary methods of induction hardening :
• Scan hardening: The coil and workpiece move relative to each other. The cylinder workpiece generally rotates inside the inductor to even out the induction hardened pattern around the circumference.
• Continuous or progressive hardening of elongated workpieces (e.g., bars, tubes, rods, wires, plates, etc.): Parts progressively pass through a number of coils positioned in-line or side by side. Each coil can have different power/frequency settings and mechanical designs.
• Single-shot hardening: Neither the part nor the inductor axially moves relative to each other, but the part is typically rotated so that the entire region to be hardened is effectively heated all at once.
• Static hardening: This is similar to single-shot hardening, except the part being hardened typically has an irregular geometry preventing its rotation.
Both vertical and horizontal induction hardening designs have been used by different manufacturers. Depending on the process requirements and geometry of the component, induction hardening equipment can be designed as a relatively simple apparatus with manual loading/unloading or can involve sophisticated, fully automated high-production machinery. As an example, Photo below shows a two-station CrankPro™ fully automated system for high-production heat treatment (hardening and tempering) of crankshafts. CrankPro utilizes patented SHarP-C technology, which eliminates the need to rotate the crankshaft and any movement of the inductor during heating and quenching cycles.
This stationary heat-treating method provides several practical and technological benefits that include but are not limited to the following:
• Unique inductor design allows fully encircling main or pin journals of a crankshaft required to be heat treated resulting in short heat time for austenization (typically less that 4 s). Precise, localized heating removes the need for “heat–cool” cycling, which is inevitably associated with rotational systems. This results in dramatically minimized crankshaft distortion (typically 45 μm being the maximum).
• High production rate—up to 120 parts per hour.
• Modular common base allows switching pallets for different crankshaft topology and configuration.
• Dramatically reduced maintenance cost. Coil life is at least doubled (more commonly tripled) compared to technology that requires crankshaft rotation.