Annealing with Induction Heating

Annealing is a heat treatment in which a material such as copper is exposed to an elevated temperature for an extended time and then slowly cooled. Annealing heat treatments are largely characterized by induced microstructural changes which are ultimately responsible for altering the material's mechanical properties. The ultimate goal of this process is to reduce the hardness of the metal and improve its ductility.

Annealing specifically refers to the process of bringing the material to its softest possible point. A tempering process softens the metal but not to the full extent possible.

Modern induction heating provides many advantages over other heating methods and is commonly used for annealing applications such as copper pipe bending. Heating through induction provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time without the use of flame or torch. Solid state systems are capable of heating very small areas within precise production tolerances, without disturbing individual metallurgical characteristics.

Induction can be used for either surface or through heating; case annealing is possible depending on time, temperature and the material's characteristics.

The degree of temper depends on the material, the maximum temperature reached and the length of the cool down time. Process or Stress Relief Annealing is used to negate the effects of cold work; that is, to soften and increase the ductility of a previously strain-hardened metal. Internal stresses may develop as a result of plastic deformation processes such as machining or grinding, non-uniform cooling in a welding or casting process, or a phase transformation. Distortion and warping may occur if the internal stresses are not removed. Annealing will eliminate these stresses when the part is heated to the recommended temperature, held there long enough, and slowly cooled to room temperature.

Closed loop control, through the use of an optical pyrometer or other temperature sensing device, can provide constant heat with a tolerance as low as 3°C at 700°C. Induction heating also ideal for in-line production processes because of its ability to produce repeatable, rapid and accurate heating cycles.

Induction systems typically used for induction annealing range from 1 to 20kW, depending on the parts and application requirements.

Any annealing process consists of three stages. First, the metal part is placed inside an induction coil and power is supplied until the part reaches the correct temperature. The temperature can be checked with an optical pyrometer, temperature sensing paint or some other temperature-sensing device. The second stage is holding or soaking at the correct temperature, which can be accomplished with a closed loop temperature control system. The annealed part must then be allowed enough time to cool down to room temperature.

Metal to be annealed:
Temperature Sensing Device: The temperature of the metal must be checked with an optical pyrometer, temperature sensing paint or other sensing mechanism.
Closed Loop Temperature Control System: The control system monitors and constantly adjusts the output of the power supply so as to maintain the correct surface temperature on the part. This allows the core of the part to reach a uniform temperature. A typical system would include the power supply, heat station, coil and an optical pyrometer.
Heat source: Fast, precise heating works best.

Flux: The functions of flux are to dissolve the oxides formed during the heating process, shield the alloy and joint from oxidation, provide clean surfaces to promote even spreading of the alloy, and to promote alloy flow by capillary action. There are many different types of fluxes available for use at different temperature ranges. Black flux is used for high temperatures (up to 1800°F) and is good for steel brazing. White flux is most often used for lower temperature (1100°F to 1500°F) applications. Ideally, the flux should have a lower melting point than the base metal, and should be entirely liquid before the braze alloy melts.

Heat source: Fast, precise heating works best.

The proper tempering temperature is determined by the type of material and the amount of temper required; you may need to experiment a bit to find the optimum level. Once the correct parameters have been established, the process can be repeated with accuracy.
During both heating and cooling, temperature gradients exist between the inside and outside sections of the part. If the rate of temperature change is too great, these gradients and internal stresses may lead to warping or even cracking.

Time is of the utmost importance. The actual annealing time must be long enough to allow any necessary transformation to take place.
Surface oxidation or scaling may be prevented or minimized by annealing at a relatively low temperature or in a non-oxidizing atmosphere.