The Canary in the Furnace: Ceramic Disks Give Early Alerts of Temperature Changes

The question in many heat treaters’ minds is, “Why would I want more documentation on my furnaces?” TempTABs can act as an early warning sign that further temperature monitoring is necessary.

This Technical Tuesday article was written by Thomas McInnerney and Garrick Ackart of The Edward Orton Jr. Ceramic Foundation, for Heat Treat Today's November 2023 Vacuum Heat Treating print edition.

The Need for User-Friendly Documentation

Increased regulations called for in AMS2750G and CQI-9 were, for the most part, driven by the client purchasing the items. With a business climate that can generate a product-liability lawsuit quicker than a rapid quench, clients are trying to protect themselves.

Consequently, most heat treating facilities will perform the necessary and required temperature uniformity surveys (TUSs) as well as thermocouple calibrations. Once the formal TUS is complete, other than the information generated from the control thermocouples, the challenge still exists for the furnace operators to ascertain what happens throughout the furnace between surveys. By passing the last survey but failing the next one, how do you detect that something changed two days after the good survey or two days before the failed survey?

It is true you can run a temperature data logger with an array of thermocouples attached through the furnace to get a complete picture of the furnace performance, but that process includes production interruption, an expenditure of precious manpower, and significant expense in maintaining the data logger. Essentially, we have just defined the need for a cost-effective, user-friendly device to monitor the day-to-day repeatability of the performance of the furnace.

Metals Industry Demands

Driven by the question raised above, The Edward Orton Jr. Ceramic Foundation initiated a development project to provide such a product for the metals industry.

Demands of the metals industry are quite different from those of the ceramic industry. The detection device would have to be able to withstand rapid heat-up schedules, rapid quench, a wide variety of furnace atmospheres (air, nitrogen, hydrogen), and no atmosphere (vacuum), and do all this without introducing contaminants to the products being heat treated — no small challenge for an engineered ceramic product. Following a great deal of consultation and experimentation, Orton developed a product, the TempTAB, that can be used to benchmark and monitor furnace performance in most heat treating applications.

Measuring Dimension: How a TempTAB Works

How does the device work, and how is it made and controlled? The device depends on a constant slope curve of shrinkage versus temperature. When the device is exposed to more temperature and for longer periods of time at peak temperature, the amount of shrinkage increases.

TempTABs are small disks made from exact blends of select ceramic materials prepared in an environment where the processing variables are tightly controlled. The ceramic material is selected based on its predictable shrinkage, which is affected more by temperature than time; even so, holding at or near the peak temperature will have an impact on the final dimension.

Once the TempTAB is out of the furnace, its diameter is measured with a micrometer. The dimension, in millimeters, is entered into an Excel workbook that automatically looks up the equivalent temperature inside the furnace based on the furnace cycle time.

Temperature conversion charts are available with each batch of TempTABs for converting the diameter measurement to temperature. The charts have several columns of data which allow the user to find the data that is best associated with their final furnace cycle hold times (temperatures available for 10-, 30-, 60-, 120-, and 240-minute hold times). The charts are built into the software to allow you to monitor up to nine different locations inside the furnace for up to 360 runs (Figure 2). The software is available free from Orton’s website.

The resulting temperature data generated by the software is graphically displayed in both table and numerical format for easy interpretation. The data can also be copied into other Excel spreadsheets and SPC (Statistical Process Control) programs to be incorporated into existing quality programs.

Primary Uses: Early Warning Device & Quality Control

Heat treat companies use these disks as an early warning device and to document that their processes are under control. First, they benchmark their thermal process by running several TempTABs through the heat treat furnace.

After establishing a benchmark with upper and lower control limits, the company will run the disks on a regular schedule, placing them in the same location alongside the parts being treated in the furnace (see process temperatures graphed with TempTABs in Figure 3).

At a glance, the furnace operator, the quality manager, or the general manager can see if the process is under control. The size of each disk indicates if the thermal process is, or is not, within the established control limits.

The case studies that follow demonstrate these primary uses in real-world heat treat.

Case Study #1: Furnace Documentation When You Need It

A manufacturer with in-house heat treating ran TempTABs alongside the thermocouples in one of its required nine-point uniformity surveys with a data logger. After the formal survey, they continued to run disks in each load, monitoring shrinkage of the disks. The heat treating operations wanted to document the thermal treatment of the product in every load. If something did change inside their furnace before the next required survey, the TempTABs would act as an early warning system alerting them that a formal survey may be necessary.

Case Study #2: Developing Backup Facilities/Preparing for Increased Demand

A company specializing in powder metal sintering wanted to duplicate a sintering process of one of their products, currently only being done at a single manufacturing site, at a second location. Initially, they duplicated all the settings in the new location (temperature settings and belt speed) and found that the resultant parts differed from those of the original site.

The company began to consider TempTABs. They liked the idea of having a device that could provide them with furnace temperature readings since they knew that it was an important variable to the quality of their parts. For one year, TempTABs were used daily for process control of the furnace. This use proved that the furnace was consistent and stable.

Since they had developed a benchmark of the disk dimensions yielding good parts, they were able to adjust the new facility settings so their process could be duplicated in the second facility. Within weeks, the powder metal sintering experts could produce products in the new facility consistent with the original facility.

Case Study #3: High-Value Heat Treating

A heat treating facility serving the aerospace industry historically ran nine thermocouples in every load of their batch furnace for bright annealing stainless steels to document furnace performance. The method required using many type S thermocouples and a data collection unit. Labor costs included setting up the thermocouple array and replacing the certified thermocouples. It was expensive and disruptive; they wanted an alternative.

The time needed to replace the TempTABs was minutes and only required one person’s time to place and gather them. After doing a correlation study of at least five runs over a week, the heat treat facility replaced the thermocouples with TempTAB disks. Now, a single operator places TempTABs inside every load so they can gather information at a lower cost. If they see any change in the amount of TempTAB shrinkage, they will run the thermocouple array to see precisely how the temperature profile has changed.

About the authors:

Thomas McInnerney is the engineering manager of Pyrometric Products at The Edward Orton Jr. Ceramic Foundation. He received his BS in Ceramic Engineering at The Ohio State University and has been a key leader in the development and application of TempTABs for 22 years.

Garrick Ackart is the Marketing and Business Development manager at The Edward Orton Jr. Ceramic Foundation. He received a Bachelor of Science degree from Alfred University in Ceramic Engineering, an MBA from The Ohio State University, and has more than 25 years of experience in the ceramic and glass industry.

For more information:

Contact Thomas McInnerney at mcinnerney@ortonceramic.com or Garrick Ackart at ackart@ortonceramic.com.

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