Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
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Stanley Zinn

Induction Brazing and Soldering    View as PDF

by STANLEY ZINN, Induction Heating Consultants • June 2010

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Uniform Heating
Power densities for brazing and soldering are considerably lower than those used for heat treating applications. It is desired to have the heat penetrate through the joint and since the materials must be at the same temperature, a low power density is required so that the outer section of the joint does not overheat. For that reason, power densities for brazing and soldering range from 0.5 to 1.5 kW/in2.

A generalized formula for determining approximate power required for brazing is:

P = WTC/0.95t

P = absorbed power in kW
W = pounds of material heated by the induction coil and by thermal conduction away from the Joint
T = temperature rise in degrees Fahrenheit
C = Specific Heat of the material
T = heating time to meet production requirements, in seconds

With respect to the alloy, capillary action will cause the alloy to flow toward the part with the highest temperature. Liquid alloys tend to flow in the direction of the hottest temperature. It is important therefore, that all components come to the brazing temperature at the same time. For this reason the coil should be designed to place greater heat close to the greatest mass. In addition, if there is a great difference in the resistivity of one material versus another, the part with the greatest resistivity will heat at a faster rate due to its I2R loss. Balancing the heat between the components is usually an empirical endeavor. The coil can be moved closer to the greater mass or more coil turns can be located in this area.

An indication of the difference in temperature of the two parts can be seen in the movement of the flux. The flux will melt sooner and tend to flow toward the part with the greater heat.

Once this has become apparent, it is necessary to look for the flow of alloy in the joint. A properly processed assembly will have the alloy flow fully through the joint area. This can be readily seen as the alloy appears at the opposite side of the joint.

Normally, the time to heat at this point is sufficient and will be repeatable thereafter, as long as the applied power remains the same. In some instances however, because of the mass of the larger part and the position of the alloy it is necessary to heat for a longer period, enabling conduction to bring the joint to the brazing temperature. This soak period must be controlled in order to protect the part.

If the heat is put in at the same rate as is used to bring the part to temperature, the large mass will continue to heat beyond the flow temperature of the alloy. This could result in greater oxidation and the forming of scale. The power should be reduced at the flow temperature to allow soaking of the heat to the joint and preventing overheating of the outer component. This can be controlled, once parameters have been determined, either by a programmable temperature controller or with an optical pyrometer sighting on the part and controlling the power output of the power supply via a PID controller.

Brazing and Hardening in a single operation
In some applications, it is beneficial to incorporate the brazing and heat treatment of the material in a single operation.

Heat treatment can precede brazing. The brazing is done at the lowest temperature and the shortest time possible (per the table). Process times must be short to prevent possible problems with the steel.

A preferred technique is that of selecting an alloy whose melting and solidification temperature are above the hardening temperature of the material. Once the alloy is melted and resolidification takes place, the part can then be quenched at any temperature without recrystallizing the brazing alloy. Short heat times in this system can be used to limit the hardness case depth. It should be noted that hardening phase transformations result in contraction and expansion of the joint. Allowance for variation in dimension due to these changes should be made in the fixture design. Hardness can also be affected (see Figure 10).

Where materials cannot be heated directly by the induction field, intermediate materials called Susceptors are often utilized.

Susceptors are materials that can be heated by induction and which pass on their heat by conduction, radiation or convection.

The most common of these materials is carbon graphite which, because of its high resistivity, heats extremely well in the induction field. Typically, where glass lenses must be soldered to a metal frame, the periphery of the glass is coated with a material, which will wet to the solder. Multiple lenses, with the mountings to which they are to be joined, are placed in a graphite fixture, which is then placed in an induction coil. The carbon susceptor heats readily in the induction field raising the assembled to the alloy flow temperature. The additional benefit of the susceptor, in this instance, is the slow cooling that results from the mass of the carbon after it is removed from the induction field. This greatly reduces stresses in the glass lenses reducing cracking and shattering.

A reducing atmosphere must be used to prevent oxidation of the carbon, and this in turn aids as a flux for the joining operation.

Figure 10
Base Metal Condition Before Brazing After Brazing at 635ºC (1175ºF) After Brazing at 705ºC(1300ºF) After Brazing at 760ºC (1400ºF)
Low-carbon steel Annealed 55-70 HRB 55-70 HRB 55-70 HRB 55-70 HRB
Cold-rolled 60-90 HRB 55-80 HRB 55-75 HRB 55-70 HRB
Low-alloy or low-carbon steel (0.4-0.5% C) Annealed 90-100 HRB 90-100 HRB 90-100 HRB May harden slightly
Heat treated to 1030 MPa (150 ksi) 32 HRC 22-32 HRC 18-25 HRC May harden slightly
Carbon and low-alloy tool steel Hardened and tempered 50-65 HRC 28-32 HRC 20-25 HRC May harden
High speed steel Hardened 65 HRC 59-63 HRC 46-50 HRC - -
Affect of Brazing on Hardness of Adjacent Areas

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