In this article we will discuss about:- 1. Introduction to Induction-Hardening 2. Steels for Induction Hardening 3. Types of Induction Coils 4. Methods 5. Metallurgical Control 6. Advantages and Disadvantages.
Contents:
- Introduction to Induction-Hardening
- Steels for Induction-Hardening
- Types of Induction Coils for Induction-Hardening
- Methods of Induction-Hardening
- Metallurgical Control in Induction-Hardening
- Advantages and Disadvantages of Induction-Hardening
1. Introduction to Induction-Hardening of Steels:
When heating of an electrically conductive material for surface-hardening is done by means of induction-heating, the method is known as induction-hardening.
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The principle of induction-heating is:
When an electric current passes through a coil, a magnetic field flows through the coil. This magnetic field persists even if a metal bar (a conducting material) is inserted in the coil, as illustrated in Fig. 8.58 (a). But when high frequency alternating current is passed through this coil, highly concentrated varying magnetic field is setup. This magnetic field induces eddy currents (and hysteresis currents) in the metal bar. The induced-current is not uniform throughout the cross section of the metal bar.
This current passes chiefly through surface layers, i.e., eddy currents are more concentrated in the surface, and decreases in strength towards the centre of the object. This phenomenon of eddy currents travelling closer to the surface of the metal bar is called ‘skin effect’ as illustrated in Fig. 8.58 (b). With the increase of frequency of the current, the eddy current is much more concentrated to the surface.
The resistance of the metal bar to the flow of this eddy current comes out in the form of heat (Joule heat), which also remains concentrated to the skin, or surface layers of the steel as illustrated in Fig. 8.59. (Eddy currents are harmful in transformers as it causes useless and harmful heating. Special measures are taken to reduce the eddy currents to minimum level in transformers). Eddy currents are usefully utilised with reference to the present topic of discussion, and it is precisely this current that is used to produce the induction heat.
In iron, hysteresis losses also contribute to some extent to the temperature rise up to curie point (768°C), above which iron is non-magnetic, i.e., above this temperature, the depth of penetration of the current increases (inverse to the frequency of the current) due to sharp decrease in magnetic permeability.
At the same time, the heating rate is reduced. This aspect should be taken into account in assigning heating conditions. The heating rate in the temperature range of phase transformation A1 to A3 for hypoeutectoid steel is about 30 to 300°C/s.
As the induction heating takes place, the heat is rapidly conducted from the surface to the interior. The overall depth of heating is larger.
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The additional penetration due to this heat conduction is given by:
where,
dx = depth of heating in mm (due to conduction)
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t = time in seconds.
The depth of heating is controlled by the duration of heating, the power density of the coil and the frequency of the current. The normal power input is 0.1 – 2 kW/cm2 of the heating surface.
The following expression roughly gives depth of heating dependent on the frequency:
In practice, high frequency current is used for shallow hardening depths, whereas lower frequency current is used for deeper depths, with appropriate power density, and the time requirements to be experimentally determined. Table 8.12 can be used for selecting the frequency of the current, based on hardening- depth needed, taking also into consideration equation 8.57.
The main consideration in selecting proper operating conditions, i.e., the power, time and frequency for a given required depth of hardening is that the surface should not get overheated by the time the austenite is obtained in the required depth, or that the heating is not too slow such that the material is heated to a depth larger than specified in the time the proper austenitising temperature is reached.
As the frequency controls the depth of hardening, and the depth of hardening generally increases with the size of the part (see Fig. 8.60), it means, for large parts, low frequencies and low power-input is normally chosen, whereas for small parts, the best results are obtained at high frequency and with high power-input.
Table 8.13 gives the various common power sources used for induction-hardening:
Motor-generate power source is used for frequencies up to 10000 c/s. Higher frequencies are obtained with valve-generators.
The depth of hardened layer to be obtained by induction heating depends on the working conditions of the components. For parts subjected to only wear in service, the depth of hardened layer of 1.5 to 2 mm is normally sufficient (also for small components). This normally needs valve-generator set for high frequency current. A depth of 4 to 8 mm is normally sufficient to withstand crushing or squirting.
This is normally obtained from medium frequency current. Automobile and machine tool industries, also commonly require such range of depth for parts. For rolls used in cold- rolling, depths of 10 mm, or more are suitable and is obtainable from low frequency current (2500-150 c/s) with optimum frequency of 500 c/s and power input of 0.1 kW/cm2.
The common practice of doing induction-hardening is:
First select the frequency of the current based on hardened depth required and table 8.12 and Fig. 8.60 could be used. Time is then fixed for an arbitrary period (usually 5-10 seconds) based on previous experience. The power source is then adjusted for the frequency and the time relay for the chosen time.
The test trial hardening is then performed. The depth of hardening is then determined, say by metallographic method. Now, based on this actual result of depth of hardening, and how close this is to the desired depth, the induction-hardening factors are adjusted to obtain the exact depth.
2. Steels for Induction-Hardening:
In induction-hardening, two factors are to be considered for selecting the carbon content of steels. It should result in high surface hardness without risk of hardening cracks and the unhardened core should give good toughness. Thus, normally the carbon content is kept in range 0.3 to 0.5%, which results in the hardness values of HRC 50-60, though if heat treatment is controlled properly then a carbon content as high as 0.8% (and 1.8% Cr with 0.25% Mo) is used as for rolls.
Table 8.14 illustrates some induction and flame hardenable steels. Induction hardening is applied mostly to hardenable steels, although some slowly cooled carburised parts are also induction hardened.
Some typical induction-hardened steels are:
1. Medium carbon steels (Table 8.14) used for automobile drive shaft, gears, etc.
2. High carbon steels (1070) used for hand tools, drill and rock- bits.
3. Alloy steels used for automotive valves, bearings etc.
3. Types of Induction Coils for Induction-Hardening of Steels:
The coils, also called inductors are made of copper tubes and are cooled during operation by internal water flow as these too get heated up by electric current as well as by radiation from the heated steel. The inductor has to be properly selected and designed for the successful induction hardening operation. For power output normally required, around 50 kW, copper tubing of internal diameter of around 5 mm is used.
To quicken the process of heating, inductors are designed to have maximum flow of current in the inductor, and the closest coupling (distance between the coil and the component) is normally between 2-5 mm. This is normally also the distance between turns. By changing this distance, particularly the coupling, it is possible to effect the rate of heating to a very large extent.
The tighter (gap) the coupling between the component and the inductor, more strong magnetic field it contacts, more rapid is the rate of heating. That is why, while designing a coil for an irregularly shaped component, care should be taken that portions closest to the coil will usually be heated at a very fast rate. Fig. 8.61 illustrates some coil designs and the heating patterns obtained.
The coupling for stationary component is about 2-3 mm, for rotating component is 2-4 mm, and for a progressive shape is 2-5 mm. Simultaneously, inadequate spacing may cause a contact with coil, or puncture the air gap between them, and more important, may overheat the external layer. Large coupling may be chosen for deeper depth of hardening.
4. Methods of Induction-Hardening of Steels:
1. Induction-Hardening with Static Coils, or Single-Shot Hardening:
This method is used for small parts having small area to be hardened so that the power output can heat it in one step such as head of a bolt. After heating, the quenching can be done in one of the following methods also illustrated in Fig. 8.66.
i. The heated component can be dropped from the fixture in the quenching liquid, or mechanically lifted to be immersed in the quenching liquid. (Fig. 8.66 a).
ii. In the hardening fixture, the hot steel component is fed automatically into a quenching spray (Fig. 8.66 b), where compressed water-sprays quench it.
iii. The induction coil has provisions of quenching-sprays, which start to operate when the high frequency current is switched off.
If the component has rotational symmetry, i.e., is circular like a gear, the component may also rotate during heating and, if possible during cooling in the quenching bath to avoid soft spots. When the component is rotated within the inductor, the width of the inductor is equal to the breadth of the surface area to be hardened. This rotation gives greatest possible degree of uniformity of healing in depth and width.
This also takes care of irregularities in the coil. As soon as the part has been heated by the required induction time, the part descends by means of a hydraulic device into the quenching bath to be immersed completely.
2. Progressive Hardening:
When whole area of a component cannot be heated in a single-shot hardening, or if particularly shallow depth of hardening is needed on a part, then progressive hardening may be used. Normally, it is used for long components with almost uniform cross sectional area. The component is normally held in position in a rotating chuck.
The component not only rotates within the inductor, but moves forward at a certain velocity through the coil, so that it gets heated by required depth, and then moves in front of quenching sprays to be quenched progressively during the motion. The quenching sprays may be as separate unit (Fig. 8.67 a). Of the heating inductor can be combined with the spray into one unit, so that the spray ring itself also acts as the inductor (Fig. 8.67 b).
Progressive hardening can also be done on components which are not axially symmetrical, i.e. without the rotation of the part as illustrated in Fig. 8.68. In (a) part of the figure either the component is moved continuously close to inductor, or the inductor is moved on the flat surface of component. The spray quench at incidence angle of 40° to 50° should be used so that it sprays the surface with an even film of water or oil producing an even depth or hardness and eliminates local over hardened spots.
Fig. 8.68 (b) illustrates an example where progressive hardening is done but the mass of the component being large compared to volume of the induction hardened part, so that air cooling, or remaining part acts as quenching medium. Fig. 8.69 illustrates some loop type and Zig-Zag type inductors to be used for surface hardening of flat components.
Induction Hardening of Gears:
Induction hardening is probably the best method of hardening gears. Depending on the requirements, the gear teeth and roots can be selectively hardened. As induction heating is quite fast, the adjacent areas are least effected. For example, the central portions of the gears should not get heated up beyond 200°C, particularly of the gears with finish-machined internal splines, which otherwise may be deformed during heating. This can be checked from the tempering colours obtained there.
The depth of hardening below the base of the spline should at least be equal to the height of the spline ribs. Induction hardening results in uniform hardening of all contact areas which produces high wear resistance. Even costlier alloy steels could be substituted by cheaper steels like AISI 1045 or 1335.
Gears can be induction hardened by any of the methods described in Fig. 8.66, but preferably the gear should rotate during heating.
Some of the common methods used in induction hardening of gears are:
1. Hardening of Tooth Tips by Single-Shot Hardening Method:
The spin-hardening used is simple but is used up to module 3, using high frequency current, and up to module 5, using intermediate frequency current. As only the tips are hardened, the wear resistance of teeth is increased but the strength remains unaffected (as the remaining body is unaffected.
2. Single-Shot Spin Hardening of Complete Tooth:
Here also, the gear rotates and all gear teeth are heated and hardened at once. Normally the inductor goes entirely around the gear, and a quench ring concentric to gear is used. This method thus improves the wear resistance as well as bending strength of the tooth. The method is used for gears with modules up to 5.
3. Tooth-by-Tooth Hardening Technique:
Each tooth is individually induction heated and quenched. This is used for modules ≥ 2 when high frequency current is used and for modules ≥ 5 when intermediate frequency is used. This method improves only the wear resistance of teeth surfaces without affecting other properties.
4. Tooth-Gap Hardening (Progressive Hardening):
This method leads to improvement in wear resistance, bending and fatigue strength. It is an ideal method for gears.
Through Hardening and Tempering:
Through hardening can also be obtained by induction heating. As the whole part is to be uniformly heated, low frequency currents and power inputs are essential. The whole depth can be penetrated by using low frequencies, and the low power input does not permit over heating of the surface layers. This method can be economically used for through hardening.
Induction tempering can be used to reduce the hardness of the components, in particular the ones which were earlier induction hardened. Symmetrically shaped components are preferred. By this method, it is possible to obtain differential hardness in a component.
Tempering of induction-hardened parts in which maximum wear resistance, or fatigue properties are desired, is not done after induction-hardening. Other parts may be tempered to required hardness values. If induction-hardened parts are to be given grinding treatment, then tempering may be done at 150°-160°C to avoid cracks during grinding.
5. Metallurgical Control in Induction-Hardening of Steels:
In induction-hardening as well as in conventional hardening, martensitic hardening is aimed and performed, but in induction heating, the austenitising time is inherently very small (few seconds compared to 1/2-2 hours), but because of very rapid heating rates during continuous heating (in induction heating), the Ac1 and Ac3 temperatures are raised and austenite is seen to form in a fraction of second at these raised temperatures.
But the starting microstructure and the composition of the steel have a far greater influence to obtain fine and homogeneous austenite during induction heating, and thus effects the choice of different induction hardening factors.
Ac, temperature is the temperature at which austenite formation is complete but it is raised with increasing rate of heating, and this increase in critical temperature depends on initial microstructure.
This is because the diffusion distance to redistribute carbon (in austenite) is shorter in finer than in coarser microstructure in which carbides are thicker and far-spaced. For example (Fig. 8.62), the quenched and tempered state of the steel has finely dispersed carbides, and thus is easily austenitised (i.e., Ac3 temperature is less), than in normalised, or annealed state.
Also, the normalised state is easily austenitised than the annealed state. The spheroidised pearlitic structure where carbides are present in relatively large sized spherical particles, is most difficult to austenitise in the short (induction heating) time as these large carbide particles will not dissolve easily. Ac3 temperature is also raised with the increase of heating rate, and is higher for coarse initial microstructure. Fig. 8.63 (a) illustrates the effect of initial microstructure during induction-hardening a AISI 1070 steel.
Quenched and tempered state attains maximum hardened case depth on induction hardening under similar conditions. Normalised state attains greater case depth than annealed state. Fig. 8.63 (b) illustrates the temperature gradient during the process, and the Ac3 temperatures for different initial micro- structure. It is apparent that higher the Ac3 temperature, lower the case-depth obtained.
The Fig. 8.63 (a) also illustrates that softening occurs in the region next to the hardened region particularly in steel with initial structure of hardened and tempered state. This region, next to the austenitised region, when attains high temperature, the coarsening of carbide occurs which results in slight softening there.
Normalised state also shows this but to a lesser extent. This region is called ‘heat affected zone’. Thus, for induction hardening of steels, initial sorbitic structure is normally recommended. Thus, while fixing the hardening temperature and other induction hardening parameters, the initial structure, i.e., the thermal history of the component should also be taken into account.
The austenitising temperature for induction hardening is always higher than used for conventional hardening. This is because of the short austenitising induction times, and also that rapid heating rate increases its Ac3 temperature. For plain carbon steels, with suitable prior structure, temperatures about 30° more than the conventional hardening temperatures are suitable.
Table 8.15 compares these temperatures for some steels. In alloy steels, the response to induction hardening treatment is further effected by the presence Of carbide forming elements, like Cr, Mo, V, W, Nb, etc. if present. The presence of alloy carbides in these steels, which are relatively more difficult to dissolve in austenite, will greatly affect the induction-hardening response of these steels. Fig. 8.64 illustrates the induction-hardening response of some steels under similar conditions.
The figure also illustrates the individual hardenabilities of the steels in terms of ideal critical diameter, Dl. The depth of hardening-obtained is increased with increasing Dl, except for SAE 52100 steel. Though this steel has high hardenability, but it responds poorly to induction-hardening treatment, because it contains high chromium, which gives it a high Dl value.
But, during induction-hardening, the chromium carbides are not dissolved and hence, the steel responds poorly to induction-hardening treatment. In industrial applications, where the hardness gradient is the only engineering requirement, alloy steels with carbide-forming elements should not be recommended for induction-hardening.
However, if the alloying elements are added in a steel to derive some other property, but the steel is to be induction-hardened, then it is advisable to use a relatively higher austenitising temperature in order to obtain a better hardening response.
An austenitising temperature 50 to 100°C more than the temperature used for conventional hardening is normally used if the steel has carbide-forming elements. It shall help in improving the response if the initial microstructure of such steels is very tine scorbutic.
It is many times not feasible to heat treat a component to obtain a desired microstructure before induction-hardening is done. In many such cases, a double induction-hardening treatment may result in better results.
Surface hardness as well as case-depth increase. In double hardening, a variation could be made in which the first austenitising temperature could be at higher temperature to obtain better carbon (and some alloying elements) diffusion and more uniform distribution. The second treatment could be from the normal austenitising temperature.
6. Advantages and Disadvantages of Induction-Hardening of Steels:
a. It helps to obtain selective localised hardening without effecting the core or other sections of the part, and the properties.
b. The rapid heating drastically reduces the heating time to increase productivity of the heat treatment section.
c. Cheaper steels could be used as better properties could be developed.
d. Fully-automatic method for similar components is always preferred.
e. No surface decarburisation and oxidation occur.
f. Only slight deformation occurs.
g. Because of finer martensite, higher hardness can be obtained.
h. Higher fatigue strength is obtained.
i. Some straightening can be done in unhardened or even hardened state.
j. The process can be incorporated in production line.
k. Operating cost per part is less.
l. Case depths can be easily controlled.
Disadvantages of Induction-Hardening:
a. High capital investment is needed which requires justification to utilise it by having large number of parts to be induction-hardened.
b. Only a limited type of steels could be induction-hardened.
c. Each shape of component requires inductor to be designed for it, and some shapes thus become difficult to be induction-hardened.