Pyrometry per AMS 2750D has struck fear in the hearts of many a stalwart heat treater. This article provides clarity to what can be a challenging subject. Additional training is also a consideration for those who feel they need a greater understanding.
Webster’s Third New International Dictionary of the English Language Unabridged, copyright 2002, has the following definition for pyrometry: “The techniques and methods of measuring high temperatures: esp: the art of using a pyrometer.”
Well, if your business is involved in heat treatment or the use of heat-treated metals, you might want to read this article.
First, the above definition falls considerably short of a definition that would be applicable to the heat-treatment industry. The devices and instruments used to measure temperature in AMS 2750D are highly calibrated thermocouples (sensors) and other instruments that convert electromotive force (EMF) into recognizable temperature readings. Additionally, these EMF readings can be used to measure, record, monitor and control temperature input into a heat-treating furnace. Pyrometers, while useful, are not included for temperature measurement in AMS 2750D.
Second, sensors used in the heat treatment of metals are used to measure temperatures from -450°F (-263°C) to over 3000°F (1700°C). So high-temperature measurement is not the only application.
Fig. 1. A sealed-quench furnace line
The temperature tolerance band for the heat treatment of metals varies from +/-5°F (3°C) for certain aluminum alloys to +/-25°F (14°C) for many other alloy groups like steels, titanium, cobalt and nickel. Metals heat treated or processed outside of the specified tolerance range are subject to rejection or, in the worst case, being scrapped. If your processed parts happen to cost several tens of thousands of dollars, this error could be extremely expensive.
Risk Avoidance and AMS 2750
How do we control heat-treating equipment to avoid such risks? One way is to apply the requirements of AMS 2750D. But, you say, AMS 2750D is so very difficult to understand and is subject to various interpretations. There are calibration requirements for not only sensors, but also instruments.
In addition, we need to conduct system accuracy tests (SATs) and temperature uniformity surveys (TUSs). How do we assure that we are doing those tests and surveys correctly and with the required frequency?
Understanding AMS 2750
One of the best ways to understand and comprehend AMS 2750D is to read through it completely – at least once – and get an idea of what you understand and what you don’t. Highlight the paragraphs that seem the most incomprehensible. Then, take the specification with you down to the shop floor and pick out a particular heat-treating furnace (Fig. 1). Look at the outside of the furnace and try to find the ports where the sensors enter the furnace. If the furnace is cold, open the door and try to determine where the sensors are located on the inside.
If the furnace is operating, wait until the load comes out and then, if possible, look into the furnace (Fig. 2). There should be at least two sensors inside, one for the over-temperature device and one for the furnace controller. If there are only two, this would be Type D instrumentation (Paragraph 188.8.131.52 of AMS 2750). If there are more than two, the furnace may be multi-zoned or it may be equipped with Type A, B or C instrumentation (Paragraphs 184.108.40.206 through 220.127.116.11).
Once you have determined the instrumentation “Type,” try to determine the furnace “Class” by looking at the SAT and TUS records. Once you have determined the class and instrumentation type, Tables 1 through 9 will start to have more meaning. Table 10 is particularly important because it tells you how long a calibration period or test interval lasts.
Fig. 2. An open front-loading vacuum furnace
It is extremely important to actually witness an SAT and a TUS. This will give you a better understanding of the size of furnace work zones, field test instruments, sensor types, correction factors, furnace instrumentation and how we verify that the furnace is actually operating within the requirements of AMS 2750D.
You will probably still have numerous questions about pyrometry, but I can almost guarantee that you will understand AMS 2750D better than you ever have before.
Frequently Asked Questions – AMS 2750D
Q: There are varying interpretations of the meaning of “maximum number of uses or maximum elapsed usage time, whichever occurs first” (para 18.104.22.168) when referring to the life of nonexpendable base-metal load thermocouples. What is the definition of maximum elapsed usage time?
A: “Maximum usage time” begins with the first use. The maximum elapsed usage time (where specified) is the number of calendar days from the first use, regardless of the number of temperature cycles.
Q: Paragraph 22.214.171.124 notes that “any limitations or restrictions of the calibration shall be indicated on the sticker.” What are some examples that might indicate the intent of this requirement?
A: Examples could include a limited temperature range or limited channels of a data logger or multi-point recorder because of calibration issues. If space on the sticker does not permit listing of the limitations, a notation such as “See Report” on the sticker should be acceptable.
Q: Paragraph 126.96.36.199 states that SAT frequency may be reduced one step if “two sensors in each control zone are Type N, R or S.” If the over-temperature sensor is N, R or S, is an SAT required for qualification of the over-temperature sensor as the additional N, R or S sensor for this purpose?
A: Yes. If the sensors are to be used for the basis of frequency reduction, then an SAT will be required on both sensors at the specified frequency.
FAQs provided by Nadcap, the global aerospace-industry special process and product audit and accreditation program.
John Gourley is a principal engineer in the Materials and Process Engineering Department at Honeywell Aerospace in Phoenix, Ariz. John served as the chairperson of the Nadcap Heat-Treat Task Group from January 2003-July 2005. He currently works as an eQualLearn instructor teaching pyrometry to heat-treatment, engineering and quality professionals.