Control, Repeatability are Name of the Game with Induction Heat Treating
As manufacturers strive to compete in our constantly evolving, globalized market, they must consider every opportunity to increase efficiency and gain an advantage. When a truly revolutionary process comes along – one that can offer more precise control and more repeatable results than traditional heat treating – producers stop and take notice.
It is for these reasons that manufacturers around the globe are relying more and more on induction heat treating. When they make the switch, they enjoy the following benefits:
• Increased energy efficiency: Induction heating is often far more efficient than other traditional methods.
• Increased precision: Very precise control is maintained over how much energy and where it is put into the part.
• Increased process control: Very repeatable heat-treat patterns can be achieved on parts because of the precise energy input control that induction heating provides.
• Increased speed: Parts can be heat treated in a few seconds to several minutes compared to hours in a furnace.
• Conversion from batch to lean manufacturing: Induction heating lends itself to JIT (just-in-time) and single-piece-flow processing. Because of its speed and the fact that there is no furnace or refractory heat-up and cool-down time associated with induction heat treating, single parts can be processed as needed.
Induction heating is different from other processes used for heat treating parts in the way that the heat is delivered. Traditional industrial heating processes (e.g., furnaces, torches, salt baths) heat the surface of the part by use of a thermal fluid (e.g., hot gas or molten salt) around the part or by thermal radiation (i.e. vacuum furnaces). Heating rates are therefore limited by the thermal-energy transfer rate at the surface of the part.
Induction heating uses a changing electromagnetic field to put energy into the part. This causes heating below the surface of the part due to I2R power losses: where I is the current induced in the part by the electromagnetic induction field and R is the electrical resistivity of the part. If a part is not electrically conductive, it cannot be directly heated with induction.
In some instances, induction is used to heat parts that are not electrically conductive through the use of a susceptor (a material that absorbs electromagnetic energy and converts it into thermal energy, which is then transferred to the electrically nonconductive part through conduction).
What Makes Induction Different?
The differences between how induction and thermal heating processes heat parts necessitate that the processes be controlled in a different ways. Processes that use thermal fluids for heating are controlled by varying the heating fluid temperature and heating time. Radiant heating processes are controlled by varying the temperature of the radiant heating elements (furnace temperature) and heating time.
Induction heating equipment transfers energy into the heated part by means of a changing electromagnetic field. It is varying of the electromagnetic field intensity, frequency and time that controls part heating rate and temperature. Because the technology exists to precisely control electrical devices like an induction generator used in induction heating, very precisely controlled amounts of energy can be put into every heated part. It is this ability to be precisely controlled that makes induction heat treating so attractive to heat treaters.
Induction heat treating is particularly well suited for creating precise, localized heat-treat patterns that are difficult to achieve with other methods. Figure 1 shows photos of heat-treat patterns that are achievable with induction heating. The light areas in these photos are where the steel has been transformed to untempered martensite (hardened). Moving around Fig. 1 in a clockwise manner, the upper left-hand corner is the head of a pinion where the customer only wanted a uniform hardened case on the outside curvature of the pinion ball. In the upper right-hand corner, two ball studs are seen where the ball and shank were case hardened, but the flat on top was to remain soft. The center right-hand-side image shows a thin-wall (~0.04 inch) cylindrical part where the top edge and step areas were to be heat treated to control wear and deformation, but the heat-treat pattern could not extend through the entire wall of the part. The lower right-hand corner is a shift drum where the corner needed to be heat treated to control wear and deformation. Finally, the lower left-hand corner of Fig. 1 shows a 10-mm sheet-metal screw where the first few threads are hardened and a soft core is maintained.
Doing Induction Right
When trying to achieve precise heat-treat patterns, people often forget that a controlled, rapid quench is just as important as the heating portion of the process. Not only does the quench transform austenite into martensite, allowing heat treaters to achieve desired surface hardness, but the quench also helps control heat flow within the part and minimizes the heat-affected zone. With induction, the most commonly used quenching method is spray quench. This may be used in combination with dunk quenching, depending on the size and mass of the parts being heat treated. With quenching (particularly spray quenching), some of the process variables that heat treaters need to control are:
• Quench chemistry – controls the heat-transfer characteristics of the quenchant
• Quench temperature
• Quench flow rate
• Quench pressure
• Timing between heat and quench
• Quench cleanliness – goes hand-in-hand with quench chemistry since the presence of rust, trap oil and iron flakes affect quenchant cooling rates
When designing induction heating processes for precise heat-treat patterns, process developers should consider the following additional factors.
1. Choosing the right inductor/coil design for the application:
• The inductor/coil must effectively and efficiently heat the part only in the areas desired. In the past, most inductor/coils were designed based on past experience and trial-and-error. With the development of FEA software and powerful high-speed computers, suppliers of induction heating equipment and inductors/coils – such as Radyne – are capable of virtually simulating induction heating processes. This software models the field intensity and penetration depth along with the material properties to predict coil performance prior to creating a final inductor (coil) design for an application.
• The coil inductance must be within the inductance range capabilities of the induction generator being used for the application, and the induction generator tuning will need to be adjusted to achieve maximum peak power delivery.
2. Position of the part relative to the inductor/coil: Parts must be consistently placed in the same position relative to the inductor/coil to achieve consistent power and heat patterns since electromagnetic field intensity is an inverse-squared function of the distance from the inductor according to Biot-Savart’s law.
3. Controlled split-second timing of power and quench: As heating and cooling rates go up and processing time goes down, small deviations in timing can have a big impact on the amount of variation that occurs with very defined case-hardening patterns. Figure 2 is an example of what happens when the process timing is not controlled well enough. Process Monitoring Systems or Quality Assurance Systems (QAS) can help identify problems like this and monitor the process to prevent their reoccurrence.
4. High power densities: Power density defines how sharp the transition is between heated and non-heated steel. While you are trying to inductively heat the steel, the surrounding steel is taking heat away by conduction. High power densities for short periods of time result in crisp transition zones, whereas low power densities for long heat times result in fuzzy or diffuse transition zones between hardened and unhardened areas.
5. Quench is important: This was discussed previously.
6. Correct induction frequency: Frequency is the primary driver of the energy penetration depth in an induction heating process. As a rule of thumb, the higher the frequency, the thinner the achievable case depth.
7. Process Monitoring Systems or Quality Assurance Systems (QAS): These systems can be an invaluable tool in the development and maintenance of an induction process by measuring process variables, recording them for traceability and detecting variations in induction-system response that exceed process-control limits. Validating process parameters in real-time helps ensure consistent, repeatable results.
QASs for induction heat treating are independent systems used for monitoring system response and the process variables used to control an induction heat-treating process. Depending on the specific process, the process variables that a good QAS should be capable of measuring are:
• Generator power (kW)
• Quench temperature
• Quench pressure
• Quench flow
• Spindle rotation (assuming rotation is involved)
Note: If an induction heat-treating system has more than one quench circuit, both circuits should be monitored.
Figure 3 shows a typical display of a QAS screen. The screen shows the values of nine different variables being monitored. The blue line on the various screens represents the actual value of the variable being monitored. The red lines on the screens represent acceptable limits that each variable needs to stay within for the process to yield an acceptable product.
Process limits are initially generated by monitoring a process that yields an acceptable part, saving the process-variable traces as a template and recalling the saved template as a standard to compare future processes against. By default, process limits are automatically generated at plus/minus some default value, such as 10%. But because some variables have a greater impact on the end results than others, process developers and others with supervisory clearance are able to reset process limits. Figure 4 shows the screens used by supervisors for setting the limits on multiple and individual process variables.
Capabilities that process developers and quality professionals should look for in a good induction heat-treating QAS are:
• Multi-channel monitoring system with extra channels for expansion
• Displays multiple channels at once
• Real-time process monitoring
• Records faults
• Logs process parameters
• Records process parameters from part cycles for traceability
With induction technology evolving every day and the development of the latest QASs and solid-state induction generators, now is a good time to look at induction heating technology for precise process control and repeatable end results. IH
For more information: Contact Scott R. Larrabee, process development lab manager; Radyne (An Inductotherm Group Company), 211 W. Boden St., Milwaukee, WI 53207; tel: 414-481-8360 x134; fax: 414-481-8901; e-mail: firstname.lastname@example.org; web: www.radyne.com