Furnace Temperature Uniformity Survey (TUS): Procedure and Class Certification
A temperature uniformity survey (TUS) measures how uniform the temperature is across the working zone of a heat-treatment furnace — the volume in which parts are actually placed for processing. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. The survey is the evidence that every point in the load reaches and holds the specified temperature within a stated tolerance. Without a current survey, a heat treater can describe the furnace cycle and produce a chart showing the control thermocouple hit setpoint, but cannot demonstrate that the furthest corner of a 50-ton load actually saw the required soak temperature. For aerospace and regulated heat treatment, the TUS is required by AMS 2750 at defined intervals; for heavy-industrial heat treatment outside aerospace, the TUS is an optional but increasingly common quality practice that customers reference to select between vendors and to establish what tolerance the furnace can reliably deliver. This article covers the TUS procedure, survey-point placement, the relationship between survey results and furnace class, and what a TUS result actually tells you about your parts.
What does a temperature uniformity survey measure, and what does it not measure?
A TUS measures the temperature at a defined set of points distributed throughout the working zone of the furnace, under empty-furnace or surrogate-load conditions, during a stabilized soak at each of several setpoints across the furnace's working range. The measurement is taken with calibrated thermocouples and recorded through an independent instrumentation channel (not the furnace's own control loop). The survey result is reported as the maximum range of temperatures observed across all survey points during the stabilized soak — a ±10 °F survey result means the hottest and coldest points differed by no more than 20 °F total, reported as ±10 °F around the setpoint. What the survey measures is the capability of the empty furnace to hold uniform temperature across the working zone; what it does not directly measure is the temperature at any specific point in a loaded furnace on a specific cycle. Those two measurements are linked — a furnace that holds ±10 °F empty will hold somewhere between ±10 °F and roughly ±25 °F loaded, depending on load size, thermal mass, and burner-gas flow obstruction — but the loaded performance is not identical to the empty survey result. Load thermocouples (part-mounted) on individual cycles are the complementary measurement that ties the survey-demonstrated capability to the specific job. A heat treater who can produce both a current TUS certificate and thermocouple records from each cycle has the complete documentation chain: the survey proves the furnace can deliver the tolerance, and the load thermocouples prove the specific cycle did deliver it (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
How is a TUS performed, and what is the survey-point pattern?
The AMS 2750 TUS procedure places survey thermocouples in a rectangular array throughout the working zone of the furnace. For a working zone of defined dimensions, the minimum number of thermocouples is specified by the standard — typically 9 thermocouples for working zones up to 3 cubic feet, 13–20 thermocouples for larger working zones up to several hundred cubic feet, with the count increasing as the working zone grows. The thermocouples are placed at the corners and at the center of the working zone, with intermediate points along the edges and faces for larger zones. The survey is performed with the furnace either empty or loaded with a thermal-mass surrogate (a load of steel blanks or a specified fixture arrangement that approximates service conditions) — AMS 2750 defines both "empty" and "loaded" TUS variants, and the choice depends on the intended use of the survey certificate. The survey thermocouples are routed through a calibrated data logger (or through the furnace's own recorder if it has been calibrated as a survey instrument) and the furnace is cycled to each of several setpoints spanning the furnace's operating range — typically the low, mid, and high ends of the range for which certification is sought. At each setpoint, the furnace is allowed to stabilize for a defined minimum time (typically 30 minutes after the slowest thermocouple reaches setpoint), then all thermocouples are read simultaneously for a defined recording period (typically 30 minutes to 1 hour). The maximum-to-minimum temperature spread across all survey points during the stabilized period at each setpoint is the survey result for that setpoint. The furnace is certified to the class whose tolerance is met at all surveyed setpoints within the certified operating range (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
What are the AMS 2750 furnace classes, and what applications does each serve?
AMS 2750 defines six furnace classes by allowable temperature uniformity:
| Class | Max temperature spread | Typical applications |
|---|---|---|
| Class 1 | ±5 °F (±2.8 °C) | Aerospace nickel superalloy age, titanium solution treatment, specialty aerospace alloy work |
| Class 2 | ±10 °F (±5.6 °C) | Critical aerospace alloy-steel heat treatment, precision aerospace castings |
| Class 3 | ±15 °F (±8.3 °C) | Aerospace alloy-steel quench and temper, some PWHT of critical welded assemblies |
| Class 4 | ±20 °F (±11.1 °C) | Standard alloy-steel quench and temper, PWHT on non-critical pressure vessels |
| Class 5 | ±25 °F (±13.9 °C) | Stress relief, annealing, normalizing, most industrial heat treatment processes |
| Class 6 | ±50 °F (±27.8 °C) | Rough warming cycles, pre-heat before forging, non-critical low-accuracy applications |
Class 5 and Class 6 cover the bulk of non-aerospace heavy-industrial heat treatment — stress relief of weldments, annealing of castings, normalizing of forgings, quench and temper of heavy industrial components — where the thermal processes themselves are not sensitive to ±25 °F variation at the soak temperature. Class 3 and Class 4 are where non-aerospace work gets tight: pressure vessel PWHT to ASME Section VIII, some structural PWHT to AWS D1.1, and industrial heat treatment of critical components where the customer specification drives a tighter tolerance than ASME's code minimums. Class 1 and Class 2 are aerospace territory — the combination of tight uniformity, tight pyrometry instrumentation requirements, and documented Nadcap accreditation that typically accompanies these classes is a specialty market rather than a general heavy-industrial one. Heat treatment work that does not have explicit class requirements on the drawing is routinely processed in Class 5 furnaces to Class 5 tolerances, with the survey certificate serving as evidence that the furnace meets a recognized industry standard even without a contractual class requirement (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
How often must a TUS be performed, and what triggers a re-survey?
AMS 2750 specifies TUS intervals by furnace class: Class 1 and 2 furnaces require surveys at intervals ranging from weekly to monthly depending on pyrometry instrument type; Class 3 and 4 furnaces typically survey quarterly to semi-annually; Class 5 and 6 furnaces typically survey semi-annually to annually. The exact interval depends on the AMS 2750 instrumentation type (Types A through E), with higher instrumentation redundancy extending the allowable interval between surveys. Beyond the scheduled interval, any of the following events triggers a re-survey: a major repair or rebuild of the furnace (new burners, new refractory, new control thermocouples in primary control loops); relocation of the furnace; a change in the normal operating range that includes temperatures not covered by the most recent survey; or evidence that the furnace has drifted out of tolerance (system accuracy test failures, cycle records showing unusual temperature excursions, or customer complaints tied to heat-treatment quality). A re-survey may be conducted at reduced scope — surveying only the setpoints affected by the change rather than the full operating range — with documentation justifying the reduced scope. For heat treaters operating a furnace without formal AMS 2750 compliance, a voluntary TUS at roughly annual intervals is a reasonable quality practice that produces the same documented tolerance evidence at a manageable cost. UTEC Industrial's approach to the 6' × 10' × 17' car-bottom furnace is to document cycles through calibrated control and load thermocouples on every job, with survey-style uniformity characterization performed on a periodic basis rather than on a contractual AMS 2750 schedule — appropriate for the heavy-industrial, non-aerospace work that comprises the furnace's typical load (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
How does loaded performance differ from empty-furnace survey results?
An empty furnace reaches uniform temperature more readily than a loaded one, because there is no thermal mass absorbing heat from one region faster than another. A furnace that surveys at ±10 °F empty may operate at ±15 to ±25 °F loaded, with the larger spread driven by three effects. First, large parts block radiant heat from reaching surfaces in their "shadow" — the part-facing surfaces of the load are heated directly by radiation, while the opposite surfaces are heated primarily by convection, which takes longer. Second, thick sections of a load heat more slowly than thin sections, so at the moment the thinnest section reaches setpoint, the thickest section may still be 50–100 °F below setpoint. Third, burner-gas flow patterns around a loaded car differ from the empty-car pattern — gas may accelerate through narrow gaps between stacked parts and slow in regions where parts restrict flow, producing local hot and cold zones that did not appear in the empty survey. The AMS 2750 framework addresses this by allowing both empty and loaded TUS variants and by requiring load thermocouples on production cycles to document actual part temperatures. For heat treaters without formal TUS compliance, a practical approach is to develop a family of "typical load" thermocouple placement patterns based on experience — for a stacked-crane-wheel load, for a structural weldment load, for a crusher-liner load — and to place load thermocouples at locations where prior cycles have demonstrated the slowest warm-up. The placement becomes a firm-specific best practice rather than a contract requirement, but the resulting data quality is similar (AMS 2750; ASM Handbook, Vol. 4B, ASM International, 2014; Totten, Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006).
What does a TUS certificate typically include, and how should a buyer interpret it?
A TUS certificate issued to a recognized format (AMS 2750 or equivalent) typically includes: the furnace identification; the survey date; the survey-point layout diagram with thermocouple positions labeled; the thermocouple type, calibration status, and traceable serial numbers; the test instrumentation used and its calibration status; the setpoints surveyed and the operating range certified; the maximum-to-minimum temperature spread observed at each setpoint; the class certification result (e.g., "Class 5, ±25 °F, across the operating range 800–1,800 °F"); and any deviations or corrective actions taken during the survey. For a buyer evaluating a heat treater's certificate: verify that the survey is current (within the class's required interval, typically a year or less for Class 5 commercial work); verify that the certified operating range covers the intended process temperature (a furnace surveyed only at 1,100–1,400 °F cannot be contractually used for a 1,550 °F austenitize cycle); verify that the class meets the specification's requirement (if the drawing calls for Class 3 pyrometry, a Class 5 certificate is not sufficient); and verify that the thermocouple types and instrument calibrations referenced on the certificate are standard industry practice. A certificate with missing elements, an expired interval, or an operating range that does not cover the work is either incomplete evidence or a sign that the furnace is not the right tool for the job. For specifications that do not explicitly require AMS 2750 class certification, the certificate remains useful as evidence of the furnace's demonstrated capability — a ±15 °F survey result on a Class 5 commercial furnace indicates better-than-required uniformity, which is useful context for quoting tighter-tolerance work (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
How does UTEC Industrial's car-bottom furnace operate relative to TUS practice?
UTEC Industrial operates its 6' × 10' × 17' / 1,800 °F / 50-ton car-bottom furnace under a documented pyrometry practice that includes calibrated Type K control and recording thermocouples, load thermocouples placed on every customer job at coolest-expected locations, and strip-chart records of each cycle delivered with the finished parts. The furnace handles the range of non-aerospace heavy-industrial processes — stress relief, annealing, normalizing, quench and temper, PWHT — for which Class 5 pyrometry (±25 °F) is appropriate and for which individual-cycle load thermocouple data establishes the actual per-part soak temperature. The furnace is not certified to AMS 2750 Class 1, 2, or 3 nor held to aerospace Nadcap pyrometry requirements; work requiring those certifications is directed to specialty heat treaters that carry them. For customer work that calls out tight uniformity requirements (±15 °F or less across a large load), UTEC discusses the requirement against the furnace's demonstrated performance on representative loads and either confirms capability or declines the work to direct it to a more appropriate facility. The documentation package delivered with every job — programmed cycle setpoints, actual thermocouple trace, hardness results where applicable, and cycle completion statement — is the per-cycle evidence of what the specific load experienced, complementing the general capability demonstrated in periodic uniformity characterization (AMS 2750, current revision; ASM Handbook, Vol. 4B, ASM International, 2014).
- Thermocouples and Furnace Pyrometry: Accuracy, Calibration, and Placement — the thermocouple system that underlies every TUS measurement
- Car-Bottom Furnaces: Design, Operation, and Applications — the furnace type typically surveyed for heavy-industrial work
- Ramp-and-Soak Cycles: Programming Controlled Thermal Processing — the programmable cycle execution that operates within the uniformity the TUS documents
- Heat Treatment Documentation: What to Request on Every Order — the documentation package that ties survey-class capability to specific-cycle evidence
References
- AMS 2750: Pyrometry (current revision). SAE Aerospace.
- ASM International. (2014). ASM Handbook, Volume 4B: Steel Heat Treating Technologies. ASM International.
- ASTM E230: Standard Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples. ASTM International.
- ASME Boiler and Pressure Vessel Code, Section VIII Division 1 (current edition). American Society of Mechanical Engineers. UW-40, Appendix R.
- ASM International. (1995). Heat Treater's Guide: Practices and Procedures for Irons and Steels (2nd ed.). ASM International.
- Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
Need In-House Heat Treating for Heavy Industrial Parts?
UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.
Questions? Call (509) 922-1832 or email sales@utec.co