Pero primero es necesario que padezca muchas cosas, y que sea desechado por esta generación

It is not always possible to employ externally located coils, and this can be a distinct disadvantage in terms of the overall heating efficiency of an induction brazing process.

6.1.3 Heating Parameters

In the opening paragraphs of this chapter reference is made to the effect that the electrical resistance of the work has upon the efficiency of heating by

FIGURE 6.3

Typical inductor designs used when induction is the heat source for: (a) external heating, (b) internal heating and flat surfaces. (From Roberts, P.M., Brazing, Engineering Design Guides, The Design Council, 1975. With permission.)

FIGURE 6.4

The efficiency of heating in the vicinity of an inductor.

Single turn

Multi-turn round

Multi-turn formed

External coil Internal coil

Hairpin

Pancake

Double-turn internal

Skid coil Internal coil

(a) (b)

Multi-turn rectanglar

Multi-turn conical

Zones of moderate heating efficiency Zones of poor heating efficiency (i.e., when the inductor is located

inside the assembly)

Area (a) Zone of maximum heating

efficiency

Inductor Inductor

(b) (c)

(i.e., when the inductor is located parallel to the assembly) 2112_book.fm Page 148 Tuesday, November 4, 2003 1:07 PM

induction. There are five other factors that have to be taken into account when the overall efficiency of heating likely to be experienced by a particular workpiece is examined:

1. The thermal conductivity of the materials to be brazed 2. The thermal capacity of the parts

3. The distance of the work from the inductor

4. The frequency at which the induction generator operates 5. The rate of power input to the work

6.1.3.1 Effect of Thermal Conductivity

As a general rule, a material that has a low resistance to the passage of an electric current (e.g., copper, brass, and aluminum) tends to heat up relatively slowly by induction. These types of material also generally possess excellent heat conduction properties. As a result, the heat that is generated tends to be conducted away relatively rapidly from its generation point and into the body of the material.

The reverse situation applies to materials that have a high resistance to the passage of electric current. When induction is employed as the heat source the materials that can be heated very efficiently (e.g., high-carbon steels) find it quite difficult to dissipate the heat by conduction from the point of generation into the body of the component. In these circumstances there is always the risk that the rate of heat input will be so high that the surface of the component burns and, in extreme conditions, actually begins to melt.

With older models of induction heating equipment these potentially harm-ful effects were often avoided by incorporating a device in the output control circuit of the machine that automatically and rapidly switched the current being fed to the inductor on and off during the heating operation. This had the effect of providing time for the heat that was generated during the on periods to be conducted into the surrounding material during the off periods.

This ensured that the heating rate was smoothed, and surface burning was avoided. Figure 6.5 illustrates this concept.

This procedure is universally known in the lexicon of brazing related to heating processes employing electrical energy as “pulsing the current.” It is tending to fall into disuse in induction heating, but it is still used fairly extensively in resistance brazing procedures.

With modern solid-state induction machines temperature control is very often achieved by using an optical pyrometer to feed temperature informa-tion to the power source of the generator. It is, however, important to under-stand that the pyrometer is not actually reading the temperature, it is taking note of, and responding to, changes in the emissivity of the surface at which it is pointing.

The emissivity of a surface can change for a number of reasons; one reason is the change in its temperature. Consider the case where a white flux paste is coating the surface of a part. The pyrometer will see white, and a white surface has a high emissivity. When the flux melts to form a clear liquid, the emissivity of the surface is reduced. This is because the color seen by the pyrometer changes from white to parent material’s color, and if it is steel that is being heated this might be dull red. The fact that the surface at which the pyrometer is looking is no longer white, and so now has a lower emissivity, can lead to problems of temperature control. This is because at the very time when the pyrometer ought to be telling the generator to reduce power because the temperature of the work is approaching brazing temperature, it thinks it is looking at a cooler object because the emissivity, to which it is reacting, has fallen. The system of which the pyrometer is a part compensates for this by telling the generator to provide more heat (i.e.. more power). This can lead to quite serious problems of overheating occurring in the work.

Quite often the pyrometer is sited so that it looks at the joint area and is calibrated so that its output signal represents a certain predetermined tem-perature value. The set temtem-perature is generally a few degrees higher than the working temperature of the brazing alloy that is to be used to make the joint. The power circuit of the induction generator is controlled automatically by the electrical output from the pyrometer. This action results in control of the amount of power being fed to the inductor, which is progressively reduced as the temperature of the work approaches the set temperature. The overall effect is that very accurate temperature control of the joint area becomes a practical reality. Indeed, it is not uncommon to find that a control range of ± 3ºC is typical of the accuracy that can routinely be achieved.

An interesting example of how this technology can be employed in brazing is found in those situations where relatively small pieces of polycrystalline diamond- (PCD) tipped tungsten carbide have to be brazed to the shanks of lathe tools. To avoid thermal damage to the PCD it is essential that the

FIGURE 6.5

Concept (and result) of pulsing the current. Note how length of on and off periods change with time.

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brazing temperature be limited to about 730ºC. Placing the prepared and fluxed assembly (tool shank, brazing alloy perform, and the PCD-faced insert) onto the surface of a carbon cylinder that is being heated by induction, where the power fed to the inductor is controlled by an optical pyrometer, satisfies this requirement. As can be seen in Figure 6.6, no matter how long the assembly remains on the upper surface of the carbon cylinder overheat-ing of the work cannot occur.

6.1.3.2 Effect of Thermal Capacity

The thermal capacity of a component is inextricably linked to its physical size as well as the material from which it is made.

If the materials that are to be heated are moderately large and have rela-tively good thermal conductivity properties, it is clear that heating times are likely to be quite extended. In such a case one might come to a situation where the majority of the component could be at a temperature of perhaps 500ºC when the joint attained its brazing temperature of, say, 670ºC. From the technical standpoint such a situation would be nonsense. It would clearly indicate that a choice of heating method other than induction would be much more appropriate.

It is therefore necessary to accept that a joint on a component of large thermal capacity and moderately good thermal conductivity properties will almost certainly be difficult to heat smoothly to brazing temperature with induction unless a fairly high rate of heat input can be achieved. While a generator of sufficient output power could achieve this goal, its use would inevitably lead to an increase in the difficulty of controlling the brazing process. In such conditions an alternative heating method (e.g., furnace heating) might prove to be a more attractive proposition. On the other hand, we have seen in Section 6.1.3.1 that materials that have a low thermal

FIGURE 6.6

A cylinder of carbon (about 30 mm diameter) heated by induction with temperature regulation provided via an optical pyrometer.

Optical pyrometer Surface temperature about 730º±±±±3333ºC

Inductor

Temperature

@@@@ 850ºC

conductivity can sustain surface damage if heated too intensely. As a result, there needs to be compromise with these two conflicting situations. This is specifically the case where one of the components is made from steel and the other from copper.

Another case that has to be considered is where the component and the thermal capacity are very small. Here the difficulty would almost certainly be related to the potential to experience serious overheating, and perhaps even melting of the components. These specific problems can best be over-come by employing one or more of the following:

1. Paying close attention to the coupling factor (see Section 6.1.3.3) 2. Manually controlling the amount of power being fed to the work 3. Using a low-power induction generator

4. Using the output from an optical pyrometer as an essential compo-nent of the control system of the induction generator that is to heat the work

These are matters where experts in the application of induction heating for brazing processes should be consulted before a final decision is made on what route should be followed in a specific case.

6.1.3.3 Effect of the Distance of the Coil from the Work

The distance of the coil from the work is known as the coupling factor and is very important in terms of the efficiency of induction heating.

It can be readily demonstrated that as the distance between the inductor and the workpiece increases the rate of heating of the work decreases. This interrelationship is expressed mathematically as:

where

H = heat generated in the workpiece a = directly proportional to

d = distance between the workpiece and the inductor

It is clear from the above simple formula that an inverse square law governs the heating effect related to the distance of the inductor from the work. This means that when a coil that is x mm from the work is moved so that it becomes 2x mm from the work, the heating rate achieved from the new position will only be one quarter of that which was prevailing at the initial position. The reverse is also true. If the distance between the inductor and the work is halved, the result will be to generate four times the amount of heat at the new position compared with the amount generated at the old position. This characteristic of induction heating technology underlines the

Ha 1 d²

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fundamental need to ensure that close attention to detail is mandatory when determining the ideal positioning of the inductor with respect to the work.

For example, if the inductor is too close to the work, there is a possibility that during heating the brazing flux will bubble up and come into contact with it. Contamination of the inductor in this way is not helpful to the overall efficiency of the process. Figure 6.7 illustrates the concept of both loose and close coupling. In broad terms, loose coupling is preferred for brazing. By adopting this approach, the chance of overheating the components is reduced, providing conditions where smooth heating of the parts to brazing temperature will be achieved. Even where loose coupling is employed, the inductor needs to be close enough to the work to ensure that there is an acceptable rate of heating.

It has already been mentioned that the maximum intensity of the magnetic field is generated inside an inductor. Therefore, the part that is to be heated has to be placed centrally within the coil to achieve even heating of the assembly. If the component is off-center within the coil, that part of it that is closest to the coil will be heated preferentially. This will lead to the gen-eration of an uneven heat pattern and, in extreme cases, localized serious overheating of the component in the area that is being preferentially heated (see Figure 6.8).

FIGURE 6.7

The meaning of “loose” coupling and “close” coupling.

FIGURE 6.8

Even heating demands that the components are placed centrally within the inductor.

Inductor

Closely wound inductor Loosely wound inductor

"Close" Coupling "Loose" Coupling

Inductor Inductor

A.Poor heat pattern B. Good heat pattern C. Poor heat pattern

6.1.3.4 Effect of Frequency

It is important to understand that the density of the induced current is greatest at the surface of the workpiece, and that the current density decreases as the distance from the surface toward the center of the compo-nent increases. This phenomenon is known as the skin effect. The value of the depth of penetration is an important consideration for a production engineer who is thinking of using induction as a heat source for brazing. He needs to know how powerful a machine, and what output frequency it needs to have, to heat the parts effectively.

The value of the depth of penetration, d, can be calculated by using the following formula:

where

r = electrical resistivity of the material in ohm meters mp = relative permeability of the material that is to be heated

f = frequency of the applied inductive power in hertz (for modern solid state generators the maximum frequency obtainable is 400 kHz)

For low temperature brazing applications where steels have to be joined and where the Curie point for steels will not be attained (see Figure 6.2), typical values for mp will be in the range 20 to 40. In cases where heating above the Curie point will occur, the value for mp will be 1.

The use of this formula will show that as the frequency of the alternating current increases, the depth of penetration of the induced current flowing in the work decreases. This factor can be decisive when it is necessary to select an induction generator for a particular application, such as the brazing of cruciform rock drills. These often have a large diameter, perhaps 100 mm or more, and to make the brazed joint it is invariably necessary to heat them through to their center. Employing machines that have very low frequencies, typically in the range 0.5 to 70.0 kHz, normally satisfies this requirement.

Tubular components with thin walls are best heated with machines that have a frequency in the range 70 to 500 kHz.

In cases where perhaps only skin-effect heating is required, such as where surface hardening of the material is needed, the frequency selected is likely to lie in the range 0.8 to 1.5 MHz. From this it follows that machines that have frequencies in this range, even if they are obtainable, are unsuitable for brazing applications.

The practical effect of these different frequency ranges is illustrated in Figure 6.9.

d r

m_r

=500 ¥

f

2112_book.fm Page 154 Tuesday, November 4, 2003 1:07 PM

6.1.3.5 Rate of Power Input Applied to the Work

The higher the power of the induction generator, the greater will be the intensity of the magnetic field that will be produced in the vicinity of the inductor. This will have a marked effect on the rate of heating experienced by the components (assuming that the inductor is maintained at unit distance from all parts of the work). In these conditions the temperature of the part will rise in direct proportion to the increase in the power being fed to the inductor.

We have already seen that the choice of frequency has a marked effect upon the depth of penetration of the inductive power into the work. In consequence, it is clear that in the case of a large-diameter rock drill (see Chapter 10, Section 10.6.2.6), a machine capable of delivering high power at low frequency is needed to achieve the required depth of penetration of the heating effect. The relationship between the frequency and the depth of penetration of the inductive power when nonmagnetic steel sections are to be heated is shown in Figure 6.10. These values are also applicable to other high resistivity materials such as stainless steel. In practice, the ideal fre-quency derived from Figure 6.10 may not be the most cost-effective choice.

Other factors such as the range of cross sections likely to be heated and the proportion of the total production that each size represents will influence the final selection. From the point of view of economics, it should always be remembered that a machine that has an output frequency that is lower than that indicated as ideal may allow a generator of lower cost to be purchased.

There are no hard and fast rules and Figure 6.10 is provided simply as a pointer to the ideal situation.

Earlier in this chapter we saw that a very high rate of heating, particularly if the components are made from materials that have poor thermal conduc-tion properties, can lead to quite severe localized overheating at the surface and perhaps even localized melting. One must never lose sight of the fact that what is required is smooth and even heating of the joint (see Section 6.1.3.1 and Figure 6.5). It is only by adhering closely to this fundamental rule that one can be certain that best-practice brazing will result. Further detailed information related to this very important aspect of brazing technology will be found in Chapter 1, Section 1.4.7.

FIGURE 6.9

Variation in the depth of heating with various applied frequencies.

Skin effect Through heating

Applied frequency: 0.8 –1.5 MHz 70- 500 kHz 0.5 – 70kHz

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