Simplifi ed Temperature Uniformity Surveys Utilizing Ceramic Technology for the Metals IndustryThomas McInnerney – The Edward Orton Jr. Ceramic Foundation, Westerville, OhioDaniel H. Herring – The Herring Group, Elmhurst, Ill.

Heat treaters have been searching for ways to simplify furnace temperature uni-formity surveys. Using proven technology developed in the ceramics industry, this is now a reality. With emphasis today on the importance of meeting the require-ments of AMS 2750D, CQI-9 and Nadcap, the heat-treating, brazing and sintering industries now have a tool to help them focus on the uniform and effi cient delivery of heat to their products to ensure consistent and repeatable quality.

or many years the ceramics industry has known that pre-dictable shrinkage can be ob-tained by various nonmetal-

lic, inorganic materials when exposed to a known amount of thermal energy. This technology offers a simple, economical option for the measurement and record-ing of temperature (Fig. 1). Various types of ceramic temperature-measurement de-vices are in common usage throughout the ceramics industry.[1]

Product Development for Heat Treating, Brazing and SinteringStarting from product mixes known to work in the ceramics industry, various

blends of inorganic materials were devel-oped and tested to determine the best fi t for the rapid heating and cooling cycles utilized in the metals industry. Also nec-essary was the investigation of the various furnace atmospheres used in the heat-treating, brazing and sintering industries, including air, various hydrogen/nitrogen mixtures, endothermic and exothermic gas and even gases used for vacuum partial-pressure and high-gas-pressure quenching (up to 20 bar). Material selection also took into account the need to prevent contam-ination of the work or the furnace from an atmosphere with unwanted contaminates released from inorganic compounds. As a result, a number of blends of materials

were investigated and an optimum mix se-lected to give the best performance over the wide range of temperatures necessary to meet the requirements of the metals industry. The ceramic-disc product devel-oped is called a TempTab®.[2]

Once the blends were established, the size and shape of the measurement device was carefully chosen to provide ease of use and to maximize its ability to withstand the thermal shock of rapid heat-up and quench rates (Fig. 2). Dry pressing was the forming method of choice due to the ability to control size and density of the fi nished piece. Once pressed, the product is further dried to re-move processing water and then calcined

or many years the ceramicindustry has known that predictable shrinkage can be obtained by various nonmeta

lic, inorganic materials when exposed ta known amount of thermal energy. Thtechnology offers a simple, economicaoption for the measurement and record

FF

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Ceramics & Refractories/Insulation

Fig. 2. Final TempTab® shape seen cooling in airFig. 1. Typical relationship between temperature and fi nal dimen-sion for ceramic temperature-measurement devices

25.10

25.60

26.10

26.60

27.10

27.60

790 840 890 940 990 1040 1090 1140

Dist

ance

, mm

Temperature, ˚C

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to remove the binder used during forming, bound-chemical water and other volatiles, resulting in a product with no contami-nate evolution on subsequent exposure to temperature. Each batch is then analyzed to develop a unique temperature look-up table (Fig. 3) to account for the shrinkage characteristics in the raw materials. The ceramic-disc location within a load or furnace can be marked with an identifi er simply by using a high-temperature mark-ing pen or in many cases a Sharpie®. Once the heat-treating process is complete, the discs are collected and measured (in milli-meters to two decimal places) using a digi-tal micrometer and a gauge fi xture (Fig 4). The measurements are taken across the widest dimension of the disc and then con-verted to a TempTab temperature using the look-up table supplied with each batch. The accuracy of temperature can be measured within +/-10°F (5.5°C). Therefore, varia-tions in temperature within the furnace can easily be detected and quantifi ed. TempTab discs are so easy to use, the furnace operator is able to run temperature uniformity checks

with each load, when they are experiencing problems or before a formal temperature uniformity survey. Statistical-process-con-trol charts can be created for each furnace, for each part number run or for each type of load. In this way, the discs serve as an early warning device to detect temperature varia-tion and to plot trends (Fig. 5). TempTab discs, however, are not intended to be used as thermocouple-calibration devices. The fact that these ceramic discs can be run with or alongside the work is a real convenience because temperature unifor-mity survey checks can be conducted with-out interrupting normal production. To conduct a temperature uniformity check, one needs only to place the TempTab discs in locations of interest within the furnace, such as where you would normally place thermocouples if conducting a survey.

Equipment DiversityTempTab discs have been successfully used to conduct temperature profi les and temperature uniformity surveys in mesh-belt conveyor furnaces (Fig. 6). They have

also been used for brazing steel, stainless steel, copper and even brass alloys in vari-ous atmospheres, including dissociated ammonia and various nitrogen/hydrogen atmospheres supplied from cylinders, tube trailers and cryogenic tanks. These ceramic shapes have been test-ed in integral-quench furnaces, which were hardening, carburizing and carboni-triding gears and other types of critical components. They are used to provide an indication of the uniformity of tempera-ture within a large batch load. Ceramic discs are well suited to the sintering of powdered-metal products and were fi eld tested in mesh-belt conveyor furnaces running at a variety of belt speeds. This provided, for example, an indication of temperature uniformity across the belt as well as peak part temperature. Used in pusher-style furnaces, these discs help determine the heat work associated with the proper delivery of thermal energy over time. The discs have also proven to be reli-able in temperature uniformity surveys in single-, dual- (Fig. 7) and multi-chamber vacuum furnaces for various applications, including low-pressure vacuum carburiz-ing of automotive parts with a high-pres-sure nitrogen quench.

Application DiversityCase Study 1 – Brazing and Annealing of Brass, Copper and Stainless SteelIn a large annealing and brazing opera-tion with 22 atmosphere mesh-belt fur-naces, TempTab discs and other ceramic temperature-measurement shapes were

Temp C mm Temp C mm Temp C mm Temp C mm Temp C mm Temp C mm1150 25.45 1150 25.25 1150 25.21 970 26.74 970 26.57 970 26.531145 25.55 1145 25.28 1145 25.25 965 26.76 965 26.60 965 26.561140 25.60 1140 25.31 1140 25.28 960 26.79 960 26.64 960 26.591135 25.66 1135 25.34 1135 25.32 955 26.82 955 26.66 955 26.611130 25.72 1130 25.37 1130 25.36 950 26.84 950 26.69 950 26.641125 25.78 1125 25.43 1125 25.39 945 26.87 945 26.72 945 26.671120 25.85 1120 25.49 1120 25.43 940 26.90 940 26.74 940 26.701115 25.92 1115 25.53 1115 25.47 935 26.92 935 26.77 935 26.721110 26.00 1110 25.57 1110 25.50 930 26.95 930 26.81 930 26.751105 26.03 1105 25.61 1105 25.54 925 27.00 925 26.84 925 26.781100 26.06 1100 25.66 1100 25.58 920 27.05 920 26.87 920 26.811095 26.10 1095 25.72 1095 25.62 915 27.11 915 26.91 915 26.831090 26.13 1090 25.80 1090 25.65 910 27.16 910 26.95 910 26.861085 26.16 1085 25.86 1085 25.69 905 27.21 905 26.98 905 26.891080 26.20 1080 25.92 1080 25.73 900 27.26 900 27.06 900 26.921075 26.25 1075 26.00 1075 25.76 895 27.32 895 27.15 895 26.951070 26.30 1070 26.04 1070 25.80 890 27.37 890 27.19 890 26.971065 26.32 1065 26.09 1065 25.84 885 27.38 885 27.23 885 27.001060 26.35 1060 26.13 1060 25.87 880 27.39 880 27.28 880 27.031055 26.38 1055 26.17 1055 25.91 875 27.40 875 27.31 875 27.061050 26.40 1050 26.21 1050 25.95 870 27.41 870 27.34 870 27.081045 26.42 1045 26.26 1045 25.98 865 27.42 865 27.37 865 27.111040 26.44 1040 26.28 1040 26.02 860 27.43 860 27.41 860 27.141035 26.46 1035 26.29 1035 26.06 855 27.45 855 27.44 855 27.171030 26.48 1030 26.31 1030 26.09 850 27.46 850 27.45 850 27.201025 26.50 1025 26.33 1025 26.13 845 27.47 845 27.46 845 27.221020 26.52 1020 26.35 1020 26.17 840 27.48 840 27.47 840 27.251015 26.54 1015 26.37 1015 26.21 835 27.49 835 27.48 835 27.281010 26.56 1010 26.39 1010 26.24 830 27.51 830 27.49 830 27.311005 26.59 1005 26.41 1005 26.28 825 27.52 825 27.50 825 27.331000 26.61 1000 26.43 1000 26.32 820 27.53 820 27.52 820 27.36995 26.64 995 26.45 995 26.35 815 27.54 815 27.53 815 27.39990 26.66 990 26.47 990 26.39 810 27.55 810 27.54 810 27.42985 26.68 985 26.49 985 26.43 805 27.56 805 27.55 805 27.44980 26.70 980 26.52 980 26.46 800 27.57 800 27.56 800 27.47975 26.72 975 26.54 975 26.50 795 27.58 795 27.57 795 27.50

Time at Temperature Time at Temperature

10 Minutes 30 Minutes 60 Minutes 10 Minutes 30 Minutes 60 Minutes

Fig. 4. Digital dimension measurement tool Fig. 3. Batch-specifi c table for relating fi nal dimension to temperature

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placed on the belt alongside the product as it was going through the furnace (Fig. 8). The dwell time in the furnace was ap-proximately 40 minutes with the parts be-ing exposed to maximum temperature for approximately 10 minutes. Depending on belt width, either two or three discs were placed on the belt – left side, right side and center, if necessary. Once through the process, the discs were measured and the look-up table used to determine the maximum (peak) temperature they were exposed to as a function of location. As expected in a well-run operation, the temperature uniformity of many of the furnaces was relatively close, within ±10°F (5.5°C). These tests revealed, how-ever, that several furnaces had large tem-perature differences across the belt, in one case as much as 122°F (50°C). Finding this large difference told the process engineer he needed to immediately work with his maintenance group to fi nd and correct the source of the temperature variation, and he alerted the quality engineer to conduct more extensive testing of product being produced from those specifi c furnaces. Running discs at the beginning of ev-

ery shift provided a record of the thermal energy being applied

to the product, and this was used as a diagnostic tool to locate problems. The use of this product can alert personnel to a problem with temperature or assist when trying to solve problems with higher than normal reject rates.

Case Study 2 – Brazing of Stainless SteelProduct-contamination tests were con-ducted on small stainless steel compo-nents run on graphite boards through mesh-belt conveyor furnaces in which the operating dew point of the dissociat-ed-ammonia atmosphere was in the -50 to -60°F (-45 to -50°C) range. In addition to determining the peak temperature achieved during the brazing operation, part contamination was carefully evalu-ated using X-ray photoelectron spec-troscopy (XPS). This analysis method revealed that the TempTab discs did not cause any type of part surface contamina-tion to occur.

Case Study 3 – Annealing of Stainless SteelCeramic discs were tested in a commer-cial heat-treat facility in a vacuum furnace dedicated to bright annealing of stainless steel automotive parts. The discs were run along with survey thermocouples attached to a data logger at several temperatures, and the resulting data showed good correlation against thermocouple readings. The discs were wired in place in and around the parts in a stack of fi ve wire trays used to hold the parts to be annealed. The batch furnace was brought to temperature and the parts were loaded into the vacuum chamber. Once evacuated, the parts were exposed to tem-peratures of 1740°F (950°C) for three hours.

Fig. 5. Typical TempTab® position in a furnace load Fig. 6. Typical mesh-belt brazing furnaces (Photograph courtesy of C.I.Hayes)

Fig. 7. Typical vacuum-furnace application (Photograph courtesy of Specialty Heat Treating, Inc.)

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The temperature was quickly lowered using a 2-bar nitrogen quench. Temperature dif-ferences were identical from both testing methods, and a 30°F differential was found from one location to another.

Case Study 4 – Hardening of Tool SteelsTests were run in two identical single-chamber batch vacuum furnaces process-ing M-series (M2, M3 Type 1 & 2, M7,

M42), T-series (T-1, T-15) and specialty tool steels (ASP 23, ASP 30, CPM M4, REX 23, REX 76) at temperatures between 2050°F (1120°C) and 2315°F (1270°C) to check disc integrity as a function of pro-

Fig. 8. Various ceramic temperature de-vices running with production parts Fig. 9. TempTab® discs placed in proximity to workload thermocouples

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cess recipe, load confi guration, ramp rate, partial-pressure settings, austenitizing temperature and quench severity (5- and 6-bar nitrogen). The discs were unaffected by changes in any of the process variables. The results from these tests proved that running a temperature uniformity survey before its scheduled date was unnecessary, and a good correlation was achieved with the two load thermocouples routinely run with each workload.

Case Study 5 – Sintering of Powder MetalA major producer of automotive powder-metal engine components ran tests to check the side-to-side temperature uni-formity as well as peak temperature values in multiple mesh-belt conveyor furnaces having both open-chamber and muffl e-type designs. The impact of delubrication and changes in furnace-atmosphere dew

point as a function of location within the furnace were evaluated. The discs were unaffected, and comparison to standard temperature uniformity surveys showed excellent correlation.

ConclusionThe importance of temperature control in the heat-treatment, brazing and sintering industry cannot be overstated. Furnace temperature uniformity surveys have and always will be necessary, but they have never been as simple as now. TempTab® discs offer the fi rst truly simple and af-fordable way to monitor thermal process-ing on a daily basis. Early detection of problems allows more time for thought as to the best path for corrective action and promotes preventative maintenance. Although these discs are not a com-plete replacement for temperature unifor-mity surveys, they are very capable of pro-

viding a record of the temperature history within a furnace without interrupting production schedules and without incur-ring additional cost. IH

References1. The Edward Orton Jr. Ceramic Foundation

(www.ortonceramic.com)2. TempTab® ceramic discs (www.temptab.

com)

For more information: Contact Jim Litz-inger, director of business development for The Edward Orton Jr., Ceramic Foundation, 6991 Old 3C Highway, Westerville, OH 43082; tel: 614-895-2663; fax: 614-895-5610; e-mail: info@temptab.com; web: www.temptab.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: temperature uniformity survey, AMS 2750D, Nadcap, annealing, hardening