Laser Hardening 2h7270

03 I 2012

International Magazine for Industrial Furnaces Heat Treatment & Equipment

ISSN 1611-616X

heat processing

03|2012

Vulkan-Verlag

www.heatprocessing-online.com

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Induction hardening of complex geometry and geared parts by Fabio Biasutti, Christian Krause, Sergio Lupi

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Volume 10

High-efficiency quenching and tempering plants based on comprehensive process expertise

LOI Thermprocess GmbH - Tenova Iron & Steel Division Am Lichtbogen 29 - 45141 Essen / Published in heat processing 3/2012 Phone +49 (0)201 1891.1 - Fax +49 (0)201 1891.321 [email protected] - www.loi-italimpianti.com

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Tenova S.p.A., LOI Italimpianti Via Albareto 31 - 16153 Genoa Sestri Ponente / Italy Vulkan Verlag GmbH Essen () Phone +39 010 6054807 - Fax +39 010 6054741 [email protected] - www.tenovagroup.com

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Induction Technology

Induction hardening of complex geometry and geared parts by Fabio Biasutti, Christian Krause, Sergio Lupi Mechanical gear parts for automotive or aeronautical industry are highly stressed toothed workpieces, which need to be heat treated in order to improve their mechanical properties. Up to now the most common heat treatment technology is carburizing. This method permits to achieve strength and wear resistance but cannot be integrated in a production line. Moreover, the carburizing process takes several hours and the workpiece must be completed heated up to the austenization temperature, resulting in extensive deformation. The induction contour hardening process is an alternative to carburizing. The purpose of this paper is to present the induction contour hardening process with particular reference to geared parts. In the paper are first recalled the basic laws of induction heating and is given an historical overview of the induction hardening process of gears. Induction hardening involves electromagnetic and metallurgical phenomena, which are described in the third and fourth paragraph. Some practical industrial applications of the induction hardening are presented in the fifth paragraph.

BASICS OF INDUCTION HEATING The induction hardening process consists of two steps: first the heating above the austenization temperature of the surface layer that has to be hardened, and second the quenching below a defined temperature (martensite finish or room temperature). The heating is performed by electromagnetic induction, in which are involved different physical phenomena and laws, i.e.: Electromagnetic Induction

Faraday, Neumann, Lenz laws

Electromagnetic field distribution

Maxwell laws

Conversion of electromagnetic energy into heat

Joule law

Temperature field distribution

Fourier equation

With reference to the system of Fig. 1, in which an alternative current I flows in the exciting coil and produces a magnetic flux Ф variable in time, according to the law of the electromagnetic induction an e.m.f. is induced in every closed path inside the work piece. This e.m.f. causes induced currents Ik (eddy or Foucault currents) to flow within the mass of the body which, in turn, generates power according to the Joule’s law, thus heating the body.

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Like the coil current, the induced currents Ik also produce an alternating magnetic field which, in accordance with Lenz’s principle, is in opposition to the exciting field. The two fields are superimposed, and the result is a radial reduction of the magnetic field towards the inside of the work piece and the outside of the inductor. This is known as “skin effect”, because the resulting induced current and power density distributions (the heating sources) are not uniform inside the work piece and concentrate in a surface (or “skin”) layer. This surface layer has radial dimensions comparable with the so called “penetration depth”, given by:

δ=

2ρ [mm] 2π f µ0 µ

(1)

with: ρ - resistivity [Ωm]; μ0 = 4 π ·10 -7 - magnetic permeability of vacuum [H/m]; m - relative magnetic permeability; f – frequency of the inductor and workpiece currents [Hz]. The heating sources cause the heating of the workpiece, and the temperature increase follows the law of thermal conduction (Fourier equation).

HISTORICAL OVERVIEW Since the beginning of the development of induction heating, the induction hardening of gears has been at the attention of researchers and engineers because of the advantages of

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Fig. 1: Sketch of the inductor coilwork piece system: a current I [A] flowing in the coil produces a magnetic flux Ф [Wb] which induces eddy currents Ik [A] in the work piece.

Fig. 2: Classification of the different spin hardening methods [14]

this technique, such as short heating times, strict process control, low distortion of the heat treated parts. An historical document is, in this sense, the patent US1687656 of Brown [1] as much as the work of Hogel [2] in 1938 and the work of Vologdin [3] 1939. In the 1940s many researches and publications related to the improvement of the gear hardening were developed in several countries around the world [15]. It was found relatively early that the best service properties could be obtained by a hardness layer more or less uniform along all the surface of the gear, following its contour (“contour hardening”). At the end of the 40 s the basic influence of the main process parameters was known and the main results were available, e.g. in the book of prof. V. Vologdin; examples of the influence of frequency and heating time on the hardening pattern are reported in the 2nd edition of this book [4]. But the industrial application of the process to mass production took place only later, first for the needs of the military production during the II World War and then for the development of the automotive industry, starting from the beginning of the 50 s. The different techniques developed for contour hardening of gears are summarised in the scheme of Fig. 2. The first technique proposed and used for gear contour hardening is the “single frequency” one, i.e. the simultaneous hardening of the whole contour with an inductor encircling the gear, excited with current at a convenient optimal frequency. It corresponds to the process cycle of Fig. 3 left. But since the beginning it was clear that, for obtaining comparable penetration depths in gear regions characterised by very different geometrical dimensions (i.e. the teeth and the part of the gear below the roots, as schematically shown in Fig. 4a, it would be useful to use two different frequency values.

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The optimal single frequency represents therefore a compromise, which gives relatively good results for modules between 4 and 10, if used in connection with high power densities and short heating times, as shown in Fig. 4b which gives the process parameters suggested in [9]. For these modules, frequencies up to 30 kHz can be used in practice. But for gears with module lower than 3 to 4 mm, peak power densities above 5 to 6 kW/cm2, heating times of few hundreds of milliseconds and frequencies in the range of few hundreds kHz are required. Additional flexibility in the selection of the single frequency process parameters may be achieved by a preheating stage at lower power, followed by one or more pulses; this technique, so called also “Pulsing Single Frequency”, allows to expand the possibility to achieve results close to contour hardening for different gears. As shown in Fig. 3 right, the process comprises a preheat stage with a reduced power to approximately 500 to 750 ºC (dependent on the material), a soaking stage, and a short final heating to the hardening temperature with higher specific power, followed by the quenching and a final low-power heating stage for tempering [12]. To overcome the limits of the single frequency technique, dual frequency processes have been proposed since the middle of the 20th century [5,6,8]. In these processes (named “Pulsing dual-frequency processes”) two frequencies are applied separately in sequence to the same work piece for realizing the heating cycle of Fig. 5: pre-heat is accomplished in the first step of the heating cycle by applying a low power density at MF (usually in the range 3 to 10 kHz), while during the final heating stage, which is much shorter than the first one, a high power density at HF is used for contour hardening. The selection of suitable values of the frequency of the final pulse, in the

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range 30 to 450 kHz - depending on the type of gear, its size and material – and the heating time allows to obtain the desired contour hardened depth. The MF and HF frequency converters can be connected to two different coils which are spatially separated, as shown in Fig. 6. The dual frequency process can also be carried out without transferring the gear wheel from one inductor into another. In this case the inductor is made in the form of two half rings connected to a MF transformer and a HF transformer, as shown in Fig. 7. A limit of this technique is the time required for switching the power from one generator to the other. As in the single frequency process, the heating cycle can comprise a pre-quenching and tempering stage of the tooth area before the actual hardening step; this stage

Fig. 3: Single-frequency hardening processes [12]

is used for improving the final metallurgical results also when the prior microstructure of the steel is not particularly suitable for hardening. Though the “Simultaneous Dual Frequency” (SDF) process is a still emerging technique, it has been already successfully introduced in the automotive, airspace and other industries. This technique also uses both medium frequency and high frequency currents, but they flow simultaneously in the same coil. The idea of this process is also very old, since it was patented in 1948 in USA by J. Jordan, General Electric Company [5]. According to this patent, medium and high frequency currents were supplied to the same inductor by two power supplies, such as spark or tube generators for HF and motor-generators for MF. The power supplies were connected in parallel via a system of filters.

Fig. 4a: Schematic of a gear highlighting two different geometrical dimension;

Fig. 5: Pulsing dual-frequency cycle [11]

Fig. 6: Inverters connected to separate coils for pulsing dual- frequency process [11]

Fig. 4b: Process parameters suggested for single shot hardening [average surface specific power; heating time; frequency]

Fig. 7: Dual frequency scheme using one coil and two inverters [11]

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However at that time there were no effective generators producing simultaneously two significantly different frequencies with sufficient power. The situation changed later with the development of solid state generators in the range 20 to 300 kHz, which have made possible to improve the dual-frequency hardening process and, in particular, to avoid the need for switching frequencies, supplying simultaneously a single inductor with medium and high frequency energy. The frequency mixture at the inductor current consists of a high-frequency oscillation superimposed to a fundamental medium frequency, as shown in the example of Fig. 8.

TECHNOLOGY The hardening treatment of steel parts, gears in particular, is performed in order to improve their mechanical properties. Sometimes it is required to improve only the wear resistance and the fatigue strength, other times the goal can be to increase the compressive residual stress in a sub-surface layer of the workpiece, to the end of increasing the applicable load on the mechanical component, maintaining or reducing its volume and weight (economical savings). The work of Sigwart [7] has shown, from a qualitative point of view, how the compressive residual stress and the hardness pattern are related, and that higher values of the compressive residual stress can be achieved with contour true hardening of the gear. Such contour hardening pattern is achievable by thermo-chemical processes like carburizing, nitro-carburizing, nitriding or pure thermal processes like flame, laser, electron beam or electromagnetic induction heating. In the thermo-chemical processes the whole volume of the work piece has to be heated during several hours in an oven, with consequent waste of energy and high distortion levels of the component. Among the thermal process, induction heating is interesting from the economical and technological point of view, in particular for hardening geared work pieces with modules between 1.5 and 5 [10]. This technology allows on one side to achieve contour hardening of the component in order to improve its mechanical properties, on the other hand to concentrate the heating sources directly into the outer surface layer to be hardened. The possibility of concentrating the heating sources where they are needed, implies a reduction of the heating times and leads to a minimal distortion produced by the heat treatment, thus allowing to reduce/eliminate expensive post-treatment hard machining. A comparison between gas carburizing and induction contour hardening is presented, for automotive industry, in the work of Jenhert [16] and for aeronautical industry by Jones [19].

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Process This paragraph highlights the electromagnetic phenomena relevant for the induction hardening process. The geared steel workpiece to be treated is placed inside or in front the inductor coil where an alternating current flows (Fig. 9). As previously mentioned, the depth of the outer layer, in which the induced currents and practically all the induced power concentrate, has a dimension comparable with the so-called skin depth d (eqn. 1), which as a consequence – due to the short heating time which prevents heat diffusion – can be considered the parameter that defines how deep will be the heated layer. For a cylindrical workpiece the coupling distance between coil and work piece is constant along the circumference and it is possible to control the depth of the heated layer and its hardness only by choosing suitable values of frequency, heating time and power density. The achievement of a uniform hardness pattern is much more complicated in case of mechanical workpieces having an irregular or complex contour such as pinions, gears, toothed racks and screws. In fact, in the case of toothed mechanical parts, the root area does not have the same good coupling with the inductor as the tips; moreover, roots and teeth are characterized by very different surface areas and thermal capacities. Therefore it is difficult to induce the proper amount of energy into the gear roots and teeth, required for obtaining a uniform heated layer, which in turn is strongly affected by the frequency of the exciting current and the geometry of the inductor. Influence of the frequency on the skin depth In the work of Slukhotsky [9] is presented a parameter m is defined as follows, for characterising the induction heating of cylindrical workpieces: D m= (2) 2δ where D is the diameter and δ is the skin depth. This parameter, describing the current and power distribution inside the workpiece, depends on its geometrical dimensions and the applied frequency. It is known that, in order to achieve a good energy transfer from the coil to the workpiece, relatively high values of m are necessary. As published by Di Pieri [29] and showed in Fig. 4a, two different geometrical dimensions R1 and R2 characterise the gear; since R1 is much smaller than R2, in order to achieve a good energy transfer m1 and m2 should be both high enough, thus δ1 should be much smaller than δ2 or f1 much higher than f2. If we consider the two regions independent one from the other, it is easy to understand that two independent frequencies would lead to an optimum energy transfer between the coil and these two regions.

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Fig. 10 left, Fig. 10 right illustrate the heating effect of medium frequency MF (left) and high frequency HF (right) currents in gears with module between 1.5 and 5. As schematically shown in the figures, the current direction in the coil is normal to the teeth axis, while the induced current will flow in the opposite direction within a layer δ that depends on the frequency (eqn. 1).

Fig. 8: Shape of the inductor supply voltage with HF superimposed on MF

Fig. 9: Section of the coil – work piece system

Heating Time The heating time is a fundamental parameter which determines the hardening result. If the heating process is not short enough, the heat will be transmitted from the surface layer to the core of the tooth, resulting in the through hardening of the teeth. The smaller is the gear module the shorter as to be the heating time, so that for gears of module between 1.5 and 5, the heating times should not be longer than 250 ms. Heating a mechanical part to the austenization temperature means to transfer a certain amount of energy. If short heating times are needed, as a consequence high power densities must be applied. Influence of the power density on the hardness pattern The applied power density has a strong influence on the “quality” of the hardness contour, as shown by the experimental results of Fig. 11, obtained on the same gear with 3, 5, and 8 kW/cm2 [20]. The higher is the power density the better is the quality of the contour hardness pattern, which can be evaluated through the “contour parameter” C% [20], defined as: C% =

TH − SHDT + SHDR * 100 TH

(3)

where: TH - height of the teeth, SHDT - hardness depth in the tooth tip and SHDR - hardness depth in the tooth root.

Fig. 10 left: Heating effect of MF frequency current in the teeth of a gear (A: coil current, B: induced currents path, C: heating zone, D: tooth dimension) [28] Fig. 10 right: Heating effect of HF frequency current in the teeth of a gear (A: coil current, B: heating zone, C: induced currents path) [28]

Fig. 11: Hardness patterns achieved with different applied power density and constant energy; A) 3 kW/cm2, B) 5 kW/cm2, C) 8kW/cm2. The values of the “contour” parameter are respectively A) 47 %, B) 67 % and C) 88.3 %.

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Ring and proximity effects If we consider a straight electrical conductor, like a copper tube, where an alternating current flows, it is well know that the current distribution in the conductor cross-section is generally not homogeneous and it concentrates on the external surface layer due to the skin effect. In case of two adjacent current flowing conductors, the current distribution in their cross-sections will depend also on the distance between the conductors and the currents directions. If the currents have opposite directions, the currents will concentrate towards the adjacent surfaces and vice versa (proximity effect). If we consider finally a bent conductor, e.g. a turn of a circular inductor coil, again the distribution of the current will be non-uniform in the cross section of the conductor. In fact, the magnetic field intensity is considerably higher in the inner

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space of the coil turn, this will give rise to a concentration of the current towards the inner coil cross section, and the current will flow through the lowest impedance path (ring effect). In an induction heating system, where the work piece is encircled by the inductor coil, both ring-effect and proximity effect will take place: in particular, since the induced currents have a direction opposite to the coil current, in the work piece they will be concentrated in the outer layer while in the coil conductor in the internal one . On the contrary, if the surface to be hardened is the internal diameter of the work piece and the inductor coil is internal to it, the current in the coil conductor will distributed both on the external layer of the cross-section (due to proximity effect between coil and work piece) and the internal one (due to the ring effect). In this case the coil efficiency would be drastically reduced because of the lower coupling between the inductor current and load; for this reason the coils for hardening the inside diameter of a workpiece (ID) should always be equipped with field concentrators. This effect depends on the frequency of the applied current; in particular, the lower is the frequency, the more prominent it is. This means that for hardening ID, high frequency currents must be preferred. An analogous phenomenon arise in case of hardening bevel gears or similar workpieces, where the conductor of the inductor coil is bent to a circular form and the inductor is placed in front of the work piece (Fig. 12, left). Here again, due to the simultaneous ring- and proximity-effects, the distribution of the current will be non-uniform in the cross section of the coil conductor. The effect will be more evident in case of medium frequency supply, and the hardening results will be different in the outside diameter (OD) and inside diameter (ID) cases. The choice of a suitable variable distance between coil and work piece along the coil cross section (Fig. 12, right) allows to control the heating pattern in both cases.

End and Edge effect The end effect is a phenomenon present in the hardening inductors, due to their geometry, where, usually, the coil diameter is much larger than its axial length. Inside such coils the lines of the electromagnetic field are non uniform in the axial direction; in particular, at the coil ends the lines are not anymore straight and this leads to an axial variation of the induced power density. This effect can lead to the reduction of the heating of the workpiece near the ends of the coil. At the workpiece edges another effect takes place, which is known as “edge effect”. In these regions the work piece is influenced at the same time by the longitudinal component of the magnetic field along the axial direction and by the radial magnetic field component on the edge surface. This leads to an increase of the induced power density at the workpiece corner. The experience has shown that the above effects

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depend on the choice of the process parameters, i.e. frequency, power density and coil geometry, which in turn strongly influence the final hardening pattern. Uniform heating along the axial length of a gear can be achieved by the use of magnetic flux concentrators, placed at the top and bottom of the work piece. They allow to achieve a quite uniform hardening pattern along the axial direction while, without them, the root edges tend to be overheated. When the use of this solution is not possible, excellent results along the axial length of the gear can be achieved by the use of the SDF® process, through a suitable coil design and applying high power density levels [17, 18, 20].

Fig. 12: Typical heating pattern: left – with constant gap between inductor and workpiece; right - with variable coupling distance between work piece and coil [27]

Fig. 13: Iron-Carbon-Diagram [26]

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METALLURGICAL ASPECTS Induction hardening is based on the martensitic transformation of steels and cast irons. The possibility of the martensitic hardening of steel is due to the allotropy of iron. At room temperature iron has a cubic body centred lattice, called ferrite. At a temperature of 911 °C a solid state transformation starts and the lattice transforms to a cubic face centred structure, austenite. In the cubic face centred lattice 100 times more carbon is soluble compared to the cubic body centred lattice. Fig. 13 presents the iron-carbon diagram [26]. For the martensitic transformation it is necessary to heat the alloy up to the austenising temperature, in order to solute the carbon in the cubic face centred lattice, holding it at a this temperature for a time long enough for completetion of the formation of austenite. In case that a fast cooling follows, the solid state transformation into the cubic body centred lattice takes place without diffusion of carbon and the hard martensitic structure with a tetragonal distorted cubic body centred lattice is created. There is one principle necessary to understand to increase hardness of metals. A higher hardness always occurs because of a restraint or limitation of the movement of dislocations during a mechanical load. By the heat treatment we can change the microstructure in a way that limits the movement of dislocations. Thus there are different mechanisms in steels that can lead to an increase of hardness. The mechanism that leads to limit the dislocation movements are: ■ F ormation of a tetragonal distorted cubic body centred lattice (martensite) ■ Grain refinement ■ P recipitations (depends on dimension and distribution) ■ Increase of density of dislocations. The data of hardenability of induction hardenable materials in most cases are published for a full martensitic structure, where during heating nearly all the carbon is solved in the austenite. In some cases, when the austenitization times are short, the cooling rates are high and the carbon is not completely solved in the austenite, it is possible to reach an increase of hardness (up to 2 or 3 HRC). This increase occurs, in addition to the contribution of martensite, if a refinement of grains takes place (shorter heating times leads to a fine grained austenite and with a fast cooling rate to a fine grained martensite) and, additionally, if some carbides are retained. This phenomenon is sometimes described as “super hardening effect” [31]. The two main requirements to realize a martensitic hardening process are the austenitic transformation and the right carbon content. For induction surface hardening processes normally carbon contents between 0.3 and 0.6 weight percent are used.

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The solid state transformations as well as the solution of the carbon in the austenite are processes controlled by diffusion. We can activate these processes by the heating to a specific temperature and, like all diffusion processes, they need a defined holding time, which also depends on temperature. The temperature acts like a mainspring. Within specific limits we can use higher temperature to shorten the time necessary to complete the transformation into austenite and to solve the carbon in the cubic face centred lattice. Formation of austenite is a function of the starting microstructure. Austenite starts to form ferrite/ cementite interfaces. To continue the austenite formation, cementite dissolves and supplies the carbon (via diffusion) to the austenite/ferrite interface in order to activate the transformation from low carbon ferrite to austenite [25]. Heating times for induction gear hardening are between 100 ms and 3s; in some cases more, by the use of preheating. Short heating times can in the worst case scenario lead to incomplete martensitic transformation. This depends on: ■ Chemical composition / Bond of carbon at room temperature ■ Starting microstructure / Dispersion of carbon ■ Austenitization temperature ■ Heating rate / time. Counteractive measures are [22]: austenising temperature ■ Focus the sources of heat into the critical area ■ Preheating ■ Use of different material and/or initial microstructure. ■ Higher

A benefit of induction hardening is that the heat sources are located inside the part itself. It is not necessary to bring in the heat from outside. Moreover, the use of different frequencies gives the possibility to focus the heat sources into the critical area. Skin effect and the dependence of the induced heat sources from the relative coil-workpiece position are thereby crucial factors of influence. If the carbides are fine distributed in the material, the distance to cover the diffusion processes is shorter. Thus fine grained microstructures with fine distributed carbides are the most suitable materials for short austenitisation times. Also the stability of the bond of carbon plays a major role. The higher the stability the higher energies and temperatures are necessary to dissolve the carbides [21]. Depending on the geometry of the tooth of the gear, temperature differences occur from the flanks to the tooth axis and from the tip to the root [20]. As a consequence, this leads to different microstructures in the heat treated gear. In many cases the reached austenitisation temperature in the tip is lower then in the root; in these cases, a short preheating time with high frequency power leads to a higher temperature, specially in the tooth tip, and

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therewith to a more homogeneous austenite and finally to a better transformation into martensite. An important fact is that, during a correct induction surface hardening, the mechanical properties of the core will not be affected. This means that the required core properties of the mechanical parts (i.e. hardness and core strength) must be already reached before the part are heat treated. In Fig. 14 is shown a typical hardness profile, from the surface to the core, after a successful induction surface hardening process. Some important factors can influence the shape of the hardness profile. These factors and the characteristics on which, in turn, they have influence are: Austenitization time and temperature: ■ surface hardness in of grain size, carbon distribution and carbon bond ■ width (b) of the transition zone ■ width (c) of over tempered zone ■ hardness collapse (d) in over tempered zone Starting microstructure: hardness ■ hardness collapse (d) in over tempered zone ■ core

Carbon content: of surface- and core hardness

■ level

Fraction and content of alloys: ■ hardenability

Cooling rate: (b) of the transition zone ■ level of surface hardness ■ width (c) of over tempered zone ■ hardness collapse (d) in over tempered zone ■ width

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Therefore the hardness profile depends on material starting microstructure as well as heating and cooling parameters. This means that the design of the heat treatment process has a big influence on the final hardness pattern. All the above described factors/parameters determine also the residual stress after heat treatment. In most cases a compressive residual stress on the surface is desired after the heat treatment of geared parts [7]. A small hardness penetration depth and a low cooling rate give benefits for reaching high compressive residual stresses. Therefore it is important that the cooling rate is as high as possible to form the necessary amount of martensite. However, it is difficult to foresee the amount of residual stress after hardening, since this amount depends also on the workpiece geometry and the residual stresses before heat treatment. Thus the whole process chain, for one specific workpiece, leads to a specific pattern of final residual stress. In most cases a tempering process is done after hardening to give more ductility to the material. Tempering of steel is a diffusion process. In the martensite the carbon is trapped in the cubic body centred lattice. If we heat up the martensite the diffusion of carbon is activated. The tempering process is composed of five stages, described in the reference [23]. In each stage definite transformation processes are activated as a function of temperature. In the first tempering stage, from 100 to 150 °C, the precipitation of ε-carbides Fe2C begins for carbon content over 0.2 %. According to [23], the second stage (150 to 300 °C) is characterised by transformation from retained austenite in bainite or martensite. The third tempering stage (325 to 400 °C) is denoted by the precipitation of cementite (Fe3C) and the disappearance of carbide Fe2C. The fourth stage, above 400 °C, is characterised by the recovery and recrystallisation of the martensitic microstructure, where defects such as vacancies and dislocations are removed.

Fig. 15: Worm gear true contour surface hardened with total power 566 kW and heating time 0.35 s

Fig. 16: Bevel gear true contour surface hardened with total power 580kW and heating time 0.2 s [30]

Fig. 14: Hardening profile after induction surface hardening

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In the fifth tempering stage special carbides develop in high-alloy steels above 450 °C. The temperature at which the Ostwald ripening occurs is reported at approx. 500 °C. All these stages can be found, during tempering of the steels used in the induction hardening of gears. They occur at different temperature ranges for each material, in dependence of its chemical composition. An example of the tempering process on microscopic scale, for the 42CrMo4 steel, is described in [24]. Tempering is a diffusion process, which is controlled by temperature and time. That means that it is possible, within specific limits, to compensate temperature by time and time by temperature in order to reach the desired material requirements. For low alloyed carbon steels the temperature is the mainspring [25], time plays a secondary role. The more alloyed is the material, the more the time has influence on the tempering result. Thus tempering with short heating times can lead to the desired results also by using induction heating. The authors of [25] suggest the use of heating rates not exceeding 50 K/s.

APPLICATIONS In this paragraph are presented some practical applications of induction contour hardening of geared parts. As explained in the historical overview, the idea to contour harden geared parts by mean of induction is not new, since from the beginning of induction industrial application hardening of gears was a main topic. The range of application of induction hardening has being increasing during the last 15 years due to developments in power electronic components. In particular, the development of solid state generators, which can deliver a higher power level and two different frequencies to the same coil, allows today to harden by induction parts, which were carburized before. Typical power densities for these processes are in the range 5 to 15 kW/cm². In this paragraph some examples are discussed were the heat treatment has been switched from carburizing to induction or were this change is an actual topic. Why changing the kind of heat treatment? Advantages of induction surface hardening are: ■ one piece control, i.e. the possibility to follow and monitor the heat treatment of each single workpiece, ■ intelligent use of the energy, since the heating sources are only placed were they are need, ■ low distortion, ■ r epeatability, since the process parameters are the same for each work piece both during the heating (i.e. not depending on the position in the carburizing oven) and the quenching, ■ increase of productivity, due to heating times shorter than one second, with consequent reduction of the total cycle time.

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Another very important factor, that increases the utility of induction technology, is the possibility to have a full automated process. The process parameters, like transferred energy, input current and power, can be monitored and/or regulated by the generator controller. The evaluation of the advantages is specific for every application. It has to be determined if it is possible to achieve a cost reduction in the heat treatment process and/or in the whole process chain, maintaining or improving the quality standards of the produced workpieces. A typical workpiece that can be conveniently heat treated by induction is worm gear (Fig. 15). By the simultaneous dual frequency process it is possible to reach a contour true austenitisation and therewith a contour true hardening pattern. These parts can be hardened in single shot operation under rotation inside a coil that induces currents in the workpiece flowing at 90 degrees to the teeth direction. Another interesting example of application is the contour induction hardening of the bevel gear shown in Fig. 16. By using of `single shot` simultaneous dual frequency induction hardening minimal distortion of the workpiece was achieved. This bevel gear processed by SDF was compared with a case hardened bevel gear in of distortion. Induction hardening allows much closer control of surface distortion (less distortion) than conventional carburizing, as explained in [19], this implies a significant reduction of production costs. Another new field of application of induction hardening is the treatment of ring gears and pinions used in the automotive industry. Fig. 17 shows an induction hardened ring gear. Major savings in the process chain are due to the energy costs and reduced distortion. The lower distortion leads to a significant shorter lapping process of the induction hardened workpiece. The economical and heat treatment quality comparison between the induction hardening and the carburizing processes has been presented in paper [16]. Up to now, steering parts - in particular steering pinions and steering racks - are induction hardened by a scanning process using low power density. This process is well known but has some drawbacks since it gives rise to the through hardening of the teeth, resulting in high mechanical distortion. An actual topic is the hardening of these parts by a `single shot` process, using simultaneous dual frequency and a high power density (5 to 15 kW/ cm²). Fig. 18 presents a hardening example heated by the SDF process, with 500 kW power and 200 ms heating time. The switching to the `single shot` process leads to the reduction of the mechanical distortion and the consequent lowering of the heat treatment and whole process chain costs.

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Fig. 17: Ring gear contour true surface hardened with total power 1.5 kW and heating time 0.4 s [16]

Fig. 18: Steering pinion contour true surface hardened with total power 500 kW and heating time 0.25 s

CONCLUSION

[13] Wrona E.: Numerische Simulation des Erwärmungsprozesses für das induktive Randschithärten komplexer Geometrien, Ed. Cuvillier- Verlag, Göttingen (), 2005

This paper describes state of the art of induction hardening of complex geometry and geared parts. The electromagnetic and metallurgical aspects which are involved in the induction hardening process have been highlighted. In particular has been pointed out the advantages of using the simultaneous dual frequency process combined with high power densities applied to the workpiece.

[14] Stiele H., Marquis F.:”An Overview of inductive gear spin hardening”, Conf. Proc. of HES-07 – Heating by Electromagnetic Sources, Padua (Italy), June 19-22, 2007, 183-190, ISBN 88-86281-92-7 [15] Mühlbauer A.: History of induction heating and melting, Ed. Vulkan-Verlag GmbH, Essen (), 2008, pp.202, ISBN 978-3-8027-2946-1

LITERATURE

[16] Jenhert H., Peter H.-J.: Einsatzhärten vs. Induktionshärten, HTM J. Heat Treatm. Mat., 64, 2 (2009), pp 72 – 79

[2] Hogel L.: Induction hardening cuts distortion in worm gears, Steel 103, 57, 1938

[17] Ulferts A., Nacke.: B Improvements in induction hardening by adapted field guiding. 54th international scientific colloquium IWK 2009, Ilmenau, September 7-11, pp.

[1] Brown W.J.: Patent USA n. 1 687 656, filed July 1926

[3] Volgdin V.P.: Method of surface gear hardening by means of induction, Patent USA n. 61 405, filed Sept. 1939 [4] Vologdin V.P.: Surface induction hardening, 1st ed.: Metallurgizdat, Moskow-Leningrad, 236 pp., 1939; 2nd ed.: Oborongiz, Moskow, 291 pp., 1947, (in Russian) [5] Jordan J.: Patent USA n. 2 444 259, June 29, 1948 [6] Knolwlton H.B., Kincaid H.F.: Induction-hardening gears, Soc. Autom. Eng. Quart. Trans, 4, n.1, 1950 [7] Sigwart H.: Direkthärtung von Zahnrädern., HTM J. Heat Treatm. Mat., 12 (1958), 9-22 [8] Reboux J.: Die Zhanradoberflaechenhaertung, eine typische Anwendung der Induktions erwaermung, Deut. Elektrotechnik, 11, 375, 1957 [9] Slukhotsky A.E., Ryskin S.E.: Inductors for induction heating, Ed. Energhia, Leningrad, pp.263, 1974, (in Russian) [10] Schwenk W.: Simultaneous Dual-Frequency Induction Hardening, Heat treating Progress, April/May 2003,35 - 38 [11] Rudnev V., Loveless D., Cook R., Black M.:”Handbook of Induction Heating”, Ed. Marcel Dekker, Inc., New York, 2003, 777 pp., ISBN: 0-8247-0848-2 [12] Rudnev V., Loveless D., Cook R., Black M.: ”Induction Hardening of Gears: a Review”, Heat Treatment of Metals, Wolfson Heat Treatment Centre, Aston University, UK, Part 1: 97-103 (2003); Part 2: 11-15 (2004)

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[18] Schwenk W., Nacke B., Ulferts A., Häußler A., Biasutti F.: Härteeinrichtung, Patent DE102008021306A, (2009). [19] Jones K.T., Newsome M.R., Carter M.D.: Gas carburizing vs. contour induction hardening in bevel gears, Gear Solutions, 1 (2010), 213 - 233 [20] Biasutti F., Krause C.: Experimental investigation of process parameters influence on contour induction hardening of gears, Conf. Proc. of HES-10 Heating by Electromagnetic Sources, May 18-21, 2010, Padua (Italy), 189-199, ISBN 978-8889884-13-3 [21] W.C. Roberts-Austen, Fourth Report to Alloys Research Committee, Proceedings, Istitution of Mechanical Engineers, 1897 [22] Ch. Krause, R. Springer, F. Biasutti, G. Gershteyn, Fr.-W. Bach: Mikrostrukturelle Untersuchungen an randschichthärtbarem Stahl Cf53 nach induktiven Hoschgeschwindigkeitsaustenitisierung mit anschließendem Abschrecken, HTM J. Heat Treatm. Mat. 65 (2010) 2, S. 96 - 100 [23] Hrsg. Dahl, W.,. Eigenschaften und Anwendung der Stähle, Verlag der Augustinus Buchhandlung, 5. Auflage, Aachen 1998 [24] Christian Krause 1 | Ronald Springer 1 | Gregory Gershteyn 1 | Wlodzimierz Dudzinski 2 | Friedrich-Wilhelm Bach 1In-situ high temperature microstructural analysis during tempering of 42CrMo4 using transmission electron microscopy; Appeared in International Journal of Materials Research 2009/07, Page 991-1000.

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[25] Crafts, W.; Lamont, J.L.: Härtbarkeit und Auswahl von Stählen. Springer Verlag, 1954 [26] Krauss,G.: Steels: Processing, Structure, and Performance von George Krauss von A S M Intl (Oh) (Gebundene Ausgabe -30. August 2005 [27] Benkowski, Induktionserwärmung.5. Auflage Verlag Technik GmbH Berlin [28] W. Schwenk, H.-J. Peter. Application for surface induction hardening using Simultaneous Dual Frequency Induction Heat Treating. Elektorwärme international Heft 1/2002-März. Page 14 [29] Di Pieri, C. Moderni impianti di riscaldamento ad induzione. Vol.LXXV – N.12 – Dicembre 1988 – L´elettrotecnica. Page 1186 [30] Schwenk, M. Substitution/replacement of case hardened gearbox components in the automotive as well as airplane industry through contour true induction hardening, Conf. Proc. of HES-07 Heating by Electromagnetic Sources, June 19-22, 2007, Padua (Italy), 175-182, ISBN 88-89884-07-X [31] L.C.F. Canale, C.R. Brooks, T.R. Watkins, and V. Rudnev. Effect of prior microstructure on the hardness and residual stress distribution in induction hardened steel, Proc. SAE Intl. OffHighway Cong.,Las Vegas, Nev. March 19-21, 2002

AUTHORS Dipl.-Ing. Fabio Biasutti eldec Schwenk Induction GmbH Dorstetten, Tel.: +49 (0)7443/ 9649 20 [email protected]

Dr.-Ing. Christian Krause eldec Schwenk Induction GmbH Dorstetten, Tel.: +49 (0)7443/ 9649 73 [email protected]

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