Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask (October 2016)

Question of the Month: We process copper alloy bars and billets in our factory. Gas furnaces have been used to heat billets made of C14500, C44300, C46400, C48500, C64200, and others. Billet/bar diameters are within the 200-240 mm range. The maximum temperature of our furnace is 860^oC, which is sufficient for these jobs. However, we are expanding our business and processing grades such as C71500, C63000, and some other copper alloys that require temperatures of 950^o-1050^oC. Could we use our gas furnace for billet preheating and then use a gantry system to move billets to an induction heater for final heating to take place?

Answer: Yes, this approach will help you reduce capital costs because your new induction system will only require raising billet mean (average) temperature less than 200^oC. Depending on the necessary production rate and required temperature uniformity (surface-to-core and end-to-end), you may use a number of static induction heaters to reach target temperatures. Keep in mind that if the lengths of your billets/bars vary noticeably, you will need to properly control electromagnetic end effects by applying adjustable turns, magnetic flux extenders, and/or flux concentrators. Considering the diameters of your billets, the use of low frequencies (50-120 Hz) will most likely be appropriate. Time of billet transportation after preheating in the gas furnace and final heating in the induction system should be kept to a minimum. Lengthy transportation times can disrupt the heat uniformity obtained in the gas furnace, requiring compensation by the induction heater and complicating system design.

The challenges with induction heating of copper alloys such as C71500, for example, are associated with their extremely low thermal conductivity. In the case of C71500, alloy thermal conductivity is more than 14 times lower than the thermal conductivity of pure copper and more than 12 times lower than the C14500 alloy you are now processing. Due to the lack of thermal conduction, much lower surface-to-core heat transfer will occur, leading to a reduced heat equalization effect and increased thermal gradients. Thus, sufficient time is needed.

In addition, electrical conductivity of C71500 is only 4.6% that of pure copper. This increases current penetration depth and electrical efficiency, but reduces required power consumption: This means the maximum required power might be deceiving for a similar size billet and production rate for a different copper alloy such as C14500. Electrical resistivity of copper alloy C71500 is only about 54% of austenitic stainless steel and its thermal conductivity is about 1.6 times that of nonferrous stainless steel. In other words, both electromagnetically and thermally speaking, copper alloy C71500 responds to induction heating more like a stainless steel than a copper.

The subtleties of new copper alloys must be addressed when designing an induction heater to be positioned after a gas furnace. Numerical computer modeling should be used to determine subtleties of induction heating of workpieces fabricated from different alloys.

Dr. Valery Rudnev, FASM
Director, Science & Technology
Inductoheat Inc
www.inductoheat.com

Beginning in July 2016, Professor Induction started a new article series called “Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask.” The most commonly asked questions related to different aspects of induction heating and heat treating will be reviewed and explained. All are welcome to send questions to Dr. Rudnev at rudnev@inductoheat.com. Selected questions will be answered in this column without identifying the writer unless specific permission is granted.