Questions about AMS 2750 are common for readers of this journal. We review the impact of this important pyrometry specification to help you deal with the complexities and unknowns in the high-temperature environment of sintering.
Operating and maintaining industrial heat-treating furnaces is complex and involved. Some factors that are vital to your process and furnace operations include:
1. Position and orientation of the thermal sensors
2. Type of thermal sensors used
a. Type K is popular.
b. Type T, R, N and even B are becoming more attractive for certain applications.
c. Type C is becoming more popular as the sintering temperatures rise, and it still provides excellent accuracy over a much wider temperature range.
3. Range of processing temperatures and atmospheres at temperatures
4. Load density of the furnaces and resultant layering of parts
5. Pressure, vacuum and gas types used
The parts and materials undergoing heat treatment in these furnaces are very valuable. This value often exceeds tens or hundreds of thousands of dollars, which leaves no room for error in the operation of these furnaces.
The calibration of these furnaces for processing aviation-related parts is governed by AMS 2750E. AMS 2750E is very prescriptive and establishes requirements for the testing, use, number of cycles at temperature and reuse of the devices along with the specialized calibration of these complex furnaces.
Meeting the strict requirements of AMS 2750E starts long before the furnace is turned on for the first time. The process begins at the start of the furnace design, which needs to meet the thermal-cycle range necessary for its type of payload. This may require the ability to ramp the temperature up at a specified rate, provide a controlled hold at a given temperature (e.g., debinding) and maintain a uniform temperature – all of this being performed with specified accuracy. The factors that come into play include:
- Size, location and type of heaters
- Atmosphere circulation and type of gas(s) and flow rates (if not a vacuum furnace)
- Insulation to prevent heat loss and non-uniform heating
- Accurate temperature measurements to characterize the heating process
- Placement of thermocouples to ensure temperature uniformity
- Materials of construction for the furnace
- Types and accuracy of the controllers
- Any algorithms used by the controller in reading the thermocouple temperatures or controlling the furnace
The accurate measurement of temperature is often assumed to be easy and taken for granted. In these high-temperature furnaces, though, the selection, location and operation of the thermocouples is of critical importance.
In a vacuum furnace, for example, radiative heat transfer is responsible for the heating of the temperature probe and the reported temperature. If a probe is placed too near a heating element, it will respond quickly to the heat applied to it. In the other extreme, if the thermocouple is placed behind a metallic support structure, the temperature it reports may lag the actual temperatures of the rest of the system. At steady-state operating conditions during a temperature hold, these issues are not much of a problem as long as there is uniform heating throughout the furnace. The temperature uniformity in the furnace is often controlled by fans and some type of circulation system.
In furnaces where there are recirculating flows, the placement of thermocouples needs to be considered as well. An example is the thermal response of a thermocouple placed in a region where gas flow will be more rapid than that of one hidden behind a support structure. Differences will also occur when an exposed-tip versus an ungrounded-sheath thermocouple is used. As before, once steady state has been reached and the thermal load in the furnace is held at a constant temperature, the variation in readings based on position can be less – as long as there is uniform heating and no regions of excessive heat loss from the furnace.
Under AMS 2750E, there are a number of applications and classifications listed in Table 1 as follows:
- Reference standard includes type R or S noble-metal thermocouples calibrated every five years.
- Primary standard also requires type R or S thermocouples calibrated every three years (accuracy +/-0.6°C or +/-0.1%).
- Secondary standard base-metal thermocouples calibrated every year (accuracy +/-1.1°C or +/-0.4%); type R or S calibrated before first use and recalibrated every two years (accuracy +/-1.0°C or +/-0.25%); type B calibrated every two years (accuracy +/-0.6°C or +/-0.5%).
- Temperature uniformity survey (TUS) accuracy for noble metal is +/-1.0°C or +/-0.25% for type R and D, and +/-0.5% for type B; system accuracy test (SAT) base-metal thermocouples calibrated every three months (+/-2.2°C or +/-0.75%); type B, R and S calibrated before first use then recalibrated every six months thereafter (accuracy provided here); type E and K are NOT permitted to be used in these tests.
- Control, recording and monitoring system-accuracy tests (SAT) base-metal (accuracy class 1 and 2 +/-1.1°C or +/-0.4%, class 3 to 6 +/-2.2°C or +/-0.75%) or types B, R and S, calibrated before first use.
- Load base or types B, S and R (load-sensing LS) calibrated before first use; recalibrated every six months for types B, R and S (accuracy is +/-2.2°C or +/-0.75%); other base metal not permitted.
Also see the various classifications for maximum-permissible error in Table 1 (ASM 2750) with error ranges provided.
What makes this even more complicated is the potential incorrect use of type K or similar TCs because these are subject (as stated in the AMS standard) to degradation, which depends on the maximum temperature-cycle exposures and can result in drift. Importantly, the AMS standard states that calibration above 1000°C (1832°F) reduces life just as use does.
Because of changing sintering profiles, the automatic acceptance of type K as the best TC is then questioned based on metal or metal matrixes used. The use of type K thermocouples in the correct location is acceptable if it does not function in a manner that is precluded by AMS 2750E and acts as the types of TCs used in Table 1 outlines. AMS 2750E restrictions on type K thermocouples should be carefully managed by the furnace user and designer.
The operating temperature range then becomes the key factor in selecting and using the correct thermocouple. The type K thermocouple has a usable range (per ASTM E230) of -270°C to +1370°C. It is a known fact, however, that performance of type K thermocouples are adversely impacted when regularly used above 1000°C. The thermocouple alloys change with temperatures above 1000°C. Therefore, the readings will also change.
As we have already seen, the furnace design can impact the measured temperatures. Therefore, beyond the heat transfer and physics of the furnace itself, the temperatures need to be measured accurately. The accurate measurement of temperature ensures that the furnace payload receives the proper heat treatment.
The uncertainty of a temperature measurement is much more than just the calibration of a thermocouple probe or the wire itself. The total uncertainty of the measurement will depend on the uncertainty of the controller, the type of lead wire used between the furnace and controller, and potentially even the connectors used. Small errors can be introduced if there is a temperature gradient across the connector. These additional uncertainties can be very small, but when AMS 2750E requires TUS testing to have maximum errors of +/-4°F (+/-2.2°C), small uncertainties can add up quickly. Attention to detail is required.
In a recent study conducted jointly with a large vacuum furnace manufacturer, a series of thermocouples (type C) were mounted to a frame that would hold the parts that were being sintered up to 1650°C (3000°F). A summary of the observed outcomes is as follows:
- A variation of 1/8 to 1/4 inch in location of the thermocouple tip relative to surrounding structures had an impact.
- The type of ceramic insulator had an impact on the cross-furnace result (a measurement of more than eight thermocouple locations fixed to the frame).
- The assembled thermocouple-junction distance from the inside tip of the sheath had a measurable impact.
- As the gas atmospheres changed, the temperatures shown changed up to 4°C between the various locations and in some cases more than 4°C for the control and sensor TCs.
- Pressure and vacuum conditions also had an impact on the temperatures measured.
In this case, the manufacturer was working with a customer who required very accurate sintering of turbine blades used in aircraft turbines. While earlier than the recent Southwest incident, it was prescient in working to better control the sintering temperatures.
It was determined and shown that it was possible to improve the properties of these complex parts by addressing and improving furnace operations through design and the furnace measurement and control algorithms. This was accomplished by providing the furnace manufacturer-calibrated type C thermocouples that were accurate to within +/-0.4°C of indicated temperature. Through this calibration, they were found to actually be better than the previously used type B and also had a wider operating range than the type B (800-1600°C). The type C range was 300-2200°C (572-3992˚F), allowing good measurements at the debinding range as well as the higher 1650°C range.
To successfully meet the requirements of AMS 2750E requires a well-designed furnace and a thorough understanding of thermocouples and how they interact with the system. In particular, how can the operation of a thermocouple be impacted by its environment and change over time? The take-away from this article is to understand:
- The goal of AMS 2750E is to provide well-defined and characterized furnace operating conditions, which lead to the production of high-quality parts.
- The uncertainty of the temperature measurements in these furnaces depends on the wire; the probe design and connectors; lead wire if used; the accuracy of the controller; and any internal algorithms used by the controller.
- The location, selection and calibration of furnace thermocouples are critical to the successful operation of these furnaces.
- The physics of the heating that occurs within the furnace – vacuum-controlled or dependent on internal gas flow.
For more information: Contact Herb Dwyer, COO/CTO, Nanmac Corp., P.O. Box 6640, 1657 Washington St., Bldg. 3, Holliston, MA 01746; tel: 508-872-4811 x250; fax: 508-879-5450; e-mail: firstname.lastname@example.org; web: www.nanmac.com.