Section 3.16 of CQI-9 Special Process: Heat Treat System Assessment calls for the following data monitoring from an induction heating process:

“The heat treater shall control the energy or power for each part.
• A signature monitor for each machine is preferred. A signature monitor gives the energy unit (voltage, kilowatt, etc.) vs. time or distance (for scanning systems).
• An energy monitor or equivalent is acceptable if approved by the authorized customer representative.”

This article will explore current available technology to fulfill this requirement including monitoring basic control parameters of voltage, frequency, power, scan time and related quench parameters.

Introduction

Induction hardening is a form of heat treatment in which a metal part is heated above austenitizing temperature by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing hardness and brittleness of the part.

As a non-contact heating process based on electromagnetic induction to produce heat inside the surface layer of a workpiece, induction hardening is used to selectively harden areas of a part without affecting the properties of the part as a whole.

Section 3.16 of CQI-9 Special Process: Heat Treat System Assessment calls for the following data monitoring:

“The heat treater shall control the energy or power for each part.
•  A signature monitor for each machine is preferred. A signature monitor gives the energy unit (voltage, kilowatt, etc.) vs. time or distance (for scanning systems).
•  An energy monitor or equivalent is acceptable if approved by the authorized customer representative”

Induction coil signature monitoring has been around for over 20 years. With the advancement in sensor/transducer technology and the improved speed and robustness of industrial computers, it has become much more practical and affordable to apply this new technology on existing machines.

With the large variety of induction equipment manufacturers and various ages of installed equipment, it can be cost-prohibitive to replace power supplies or complete control systems. There is now technology available that can be retrofitted on existing equipment without the need for complete system replacement.

Monitored Parameters

In the process, the part is fully or partially exposed to a strong alternating magnetic field inducing an electrical current (eddy current) flow, creating heat due to I²R losses and hysteresis losses as long as temperature is still below Curie Point while heating. The depth of the heated layer is given by the frequency of the alternating field, the power density, the permeability of the treated material, heating time and the part’s diameter or thickness. Transformation into martensite requires rapid cooling often achieved by water spray or oil/polymer quenching.

Monitoring current, voltage (power) and frequency to the coil in real time enables one to determine whether or not uniform heating, proper flux density and depth are being achieved.

Because power transferred to the part, time, quench time, and time between heat and quench affect the final metallurgy of the part, the parameters shown in Figure 1 must be monitored and alarmed. The software allows monitoring up to 12 parameters in real time.

Fluctuation in frequency and power can lead to “under” or “over” casing and incomplete phase transformation. Variability in quench flow and temperature can result in unacceptable variations in part hardness (Fig. 2).

Since heating rates are primarily controlled by coil voltage and coil current is roughly proportional to voltage, monitoring either of these parameters will provide a good indication of variances in the heating rate. Variations in scan time can lead to improper metallurgy, and variation measurement is a good indication of forthcoming mechanical problems with the equipment.

Proper data acquisition and alarming may reduce the frequency of destructive testing such as cutting parts.

Acquiring the Data

Sample rate:It is imperative that sampling rates be less than 10 milliseconds. If one considers the duration of a heat cycle that can be only a few seconds, it would be impossible to develop a statistically meaningful sample with a slower sample rate.

Interface to existing electronics: Many older machines and equipment are still equipped with analog panel meters. These meters require annual calibration. Current sensor technology eliminates the calibration requirement. One can typically interface directly to the existing electronics with appropriate signal conversion.

Parameters such as quench temperature and flow are easily monitored with the addition of appropriate sensors.

Digital inputs to the system are required to automatically record the signature for each part run. In the case of a continuous operation (e.g., fastener hardening using a channel coil), a signal such as scale dump may be used to signal a new set of data.

System Reactions

Digital outputs for alarms, part deflection, power-supply shutdown, etc. are common. Once the system recognizes an error, output signals can be used to stop the machine or automatically eject the part and continue with treating the next part.

Setup

This process typically requires running test parts and “teaching” the system what the ideal signature would look like for that part. Once established, alarm bands can be assigned around the various monitored parameters. Figure 3
demonstrates the alarm bands that become the recipe for that part (Fig. 4).

The system has to be intelligent enough to only alarm when the process is within initially established control limits. It is also possible to allow some deviations once the cycle is in process. For example, the system is heating with 100 kW and has set a tolerance of +/-10 kWon this parameter. The system can be set to accept two out-of-band deviations during the cycle without rejecting the part.

Reporting for Compliance

Ideally, we should look for a signature per part. This may not be possible in some continuous operations.

Key process parameters must be charted as well as any alarms on the system during a run. This will allow production of reports to distribute to customers, auditors and for compliance with regulations and internal ISO. Another advantage of collecting the data is the value it has to maintenance personnel. If a trend is spotted on one of the monitored parameters, maintenance can deal proactively with repair and adjustments to the overall system. Data should be stored on the company server or another secure server to prevent the heat treat from having to manage the data.

Conclusions

Induction signature monitoring has become more important in light of ever-evolving heat-treat specifications such as CQI-9. Compliance to these specifications forces heat-treat shops to improve manufacturing tolerances. An extra benefit from monitoring is that it helps maintenance departments proactively solve problems. With systems becoming more affordable as a retrofit to existing equipment, it has become easier to comply with specifications and possibly reduce the number of destructive tests required.

References

1. John Davies and Peter Simpson, Induction Heating Handbook, McGraw-Hill, ISBN 0-07-084515-8, 1979

2. ASM Handbook, Volume 4: Heat Treating, ASM Handbook Committee, p. 164-202

3. Peter A. Hassell, Hassell Associates, and Nicholas V. Ross, Ajax TOCCO Magnethermic Corporation, “Induction Heat Treating of Steel”

4. Proceedings of the 27th ASM Heat Treating Society Conference, Sept. 16–18, 2013, Indianapolis, Ind.

5. Fred R. Specht, “Control Systems for Induction Heat Treating, the Obsolete & Newest Designs”

6. Richard E. Haimbaugh, “Practical Induction Heat Treating,” p. 183-214

7. 21st ASM Heat Treating Society Conference Proceedings, Nov. 5-8, 2001, Indianapolis, Ind.

8.  D.J. Williams, “Basics of Induction,” Welduction Corporation

9.  Proceedings of the 22nd Heat Treating Society Conference and the 2nd International Surface Engineering Congress, Sept. 15-17, 2003, Indianapolis, Ind.

10.  T. Kennamer, Ajax TOCCO, and D. Collins, Borg Warner Automotive, “Process Monitoring to Reduce/Eliminate Destructive Testing in Induction Heat Treating”