A bogie hearth furnace is a large thermal system. Temperature gradients are inevitable — but they can be managed. The Temperature Uniformity Survey (TUS) is the test that proves the management is working.
MONTE INTELLIGENCE performs TUS testing as part of furnace commissioning and recommends periodic retesting throughout the furnace life. This article explains the TUS requirements under AMS 2750F, the most widely referenced standard for thermal processing equipment in aerospace, automotive, and general heat treatment.
AMS 2750F classifies furnaces by temperature uniformity tolerance. Class 1 requires ±3°C uniformity — the tightest tolerance, used for critical aerospace components like turbine disks and structural forgings. Class 2 requires ±6°C — the standard for most aerospace heat treatment. Class 3 requires ±8°C. Class 4 requires ±10°C. Class 5 requires ±14°C. Class 6 requires ±28°C — the loosest tolerance, used for stress relieving and non-critical applications.
For a bogie hearth furnace, achieving Class 2 (±6°C) is a demanding but achievable target. Class 1 (±3°C) is extremely difficult for a furnace of this type because of the physical scale involved — a 10-meter-long furnace chamber has inherent temperature gradients from burner placement, flue gas flow patterns, and door heat loss. Class 1 TUS on a bogie hearth typically requires electric heating elements with very fine zone control rather than gas burners.
The TUS procedure under AMS 2750F requires placing an array of thermocouples throughout the furnace working zone. The working zone is the volume within which the workload must be placed to receive the specified thermal treatment — it excludes areas near the walls, door, and back wall where temperature is likely to be less uniform. For a bogie hearth furnace, the working zone is typically defined as the volume that is at least 300 mm from the walls, ceiling, and door, and at least 150 mm from the hearth surface.
The number of TUS thermocouples required depends on the working zone volume. For volumes up to 0.085 cubic meters (3 cubic feet), 9 thermocouples are required — one at each corner of a rectangular grid plus one in the center. For volumes between 0.085 and 6.4 cubic meters, the number increases to a 4x4x4 grid or 15-40 thermocouples depending on the aspect ratio. For a large bogie hearth furnace with a working zone of 30-50 cubic meters, the standard requires a grid with enough thermocouples that no two adjacent measurement points are more than 0.6 meters apart.
The thermocouple array is mounted on a test frame — a lightweight structure, typically made of stainless steel tubing, that positions the thermocouples at their specified locations without significantly affecting the airflow or heat transfer in the furnace. The test frame is placed on the bogie car, and the car is moved into the furnace for the survey.
The survey is run at the minimum and maximum operating temperatures of the furnace, and typically at one or more intermediate temperatures as well. For a furnace rated at 500-1100°C, the survey would be run at 500°C, 800°C, and 1100°C — three complete survey cycles. At each temperature, once the furnace has stabilized (all survey thermocouples within ±3°C of the setpoint for 30 minutes), data is collected for a minimum of 30 minutes.
The acceptance criterion is that all survey thermocouples must remain within the specified uniformity tolerance band during the entire data collection period. If any thermocouple drifts outside the tolerance, the survey fails at that temperature. Failed surveys require adjusting the furnace controls — typically the zone temperature offsets in the controller — and retesting.
Zone temperature offset adjustment is the primary tool for improving uniformity. If the TUS shows that the front of the furnace (near the door) runs 8°C colder than the setpoint while the back runs at the setpoint, the front zone controller can be programmed with a +8°C offset. After adjustment, the TUS is rerun to verify the improvement. Most furnaces can achieve Class 2 uniformity with proper zone offset tuning, even if the initial survey shows ±10°C or worse.
The survey interval depends on the furnace class and the criticality of the work. For Class 2 furnaces processing aerospace components, the TUS must be repeated every 6 months per AMS 2750F. For Class 4 furnaces processing commercial heat treatment, annual survey is typical. Any major furnace modification — burner replacement, control system upgrade, refractory reline — triggers a requalification survey.
The relationship between TUS and SAT (System Accuracy Test) is often confused. The TUS measures the temperature distribution throughout the working zone. The SAT measures the accuracy of the furnace control and recording thermocouples against a calibrated reference. Both tests are required for AMS 2750 compliance, but they serve different purposes. A furnace can pass the SAT (the control thermocouple reads accurately) but fail the TUS (some parts of the working zone are too hot or too cold relative to the control point).
Documentation is the deliverable of a TUS. The survey report must include the furnace identification, the test date, the test temperatures, a diagram showing thermocouple locations, the raw temperature data at all thermocouples for the entire data collection period, the calculated temperature uniformity, and a pass/fail determination. This report becomes part of the furnace quality record and is reviewed during customer audits and NADCAP accreditation.
MONTE INTELLIGENCE includes initial TUS as part of furnace commissioning and can provide periodic resurvey services. We also supply the thermocouple mounting hardware, data acquisition systems, and reporting software.
For TUS services or furnace commissioning support, contact helenxu@cnlymonte.com.
