AMS 2750 Furnace Class and Temperature Uniformity Reference
AMS 2750 is the pyrometry specification used across aerospace heat treating. A furnace class tells you how tightly temperature must be held across the working zone, and this table shows the six classes side by side.
| Furnace class | Uniformity (+/- F) | Uniformity (+/- C) | Total range (F) | Reading note |
|---|---|---|---|---|
Class 1 | 5 | 3 | 10 | Tightest class, used where a specification demands it |
Class 2 | 10 | 6 | 20 | Common for aluminum solution treatment and precipitation hardening steels |
Class 3 | 15 | 8 | 30 | Frequently used for many steel heat treatments |
Class 4 | 20 | 10 | 40 | General heat treatment where tolerance allows |
Class 5 | 25 | 14 | 50 | Wider tolerance applications |
Class 6 | 50 | 28 | 100 | Widest class, for non-critical processes |
Values are the +/- uniformity tolerances published for AMS 2750 furnace classes. Which class and instrumentation type a part needs is set by the heat treat specification and the customer, not by this table. Confirm against your current AMS 2750 revision.
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How to read the table
Find the class named by the heat treat specification or purchase order. The uniformity column is the allowed variation above and below the setpoint within the qualified working zone, measured by a temperature uniformity survey. The computed total range doubles the tolerance so you can compare spans. A Class 2 furnace has a ten degree F plus or minus tolerance, a total spread of twenty degrees F.
Class versus instrumentation type
Do not confuse furnace class with instrumentation type. The class sets uniformity tolerance, while instrumentation types, labelled A to D in the standard, describe the controlling, recording and monitoring instruments and their calibration and test frequencies. A supplier needs both the right class and a suitable instrumentation type for the specification called out.
What to check at a heat treat supplier
Request the current temperature uniformity survey report, the furnace class and instrumentation type being claimed, the system accuracy test dates and the calibration status of thermocouples. Check that survey intervals match the standard for the class and that the load being processed sits inside the surveyed working zone.
- Survey date and furnace class on the report
- Thermocouple calibration traceable to a recognized body
- Load configuration inside the qualified zone
Common mistakes
Assuming any furnace is acceptable for any alloy, ignoring survey expiry, and mixing up Fahrenheit and Celsius tolerances are common. Always use the units given on the report and verify the rounding used in the standard.
Frequently Asked Questions
What is a temperature uniformity survey?
A test using thermocouples placed through the working zone to confirm temperature stays within the class tolerance. It must be repeated at defined intervals and after major maintenance.
Which class do I need?
The one called out by the heat treat specification or your customer. Some materials, such as certain aluminum alloys, usually require a tighter class than general steel treatment.
Does the class apply to the part temperature?
It applies to the qualified furnace working zone as measured in the survey. Part temperature during processing is controlled through the process specification and monitoring.
Are these figures exact?
They are the commonly published class tolerances. Confirm against the current revision of AMS 2750 since revisions can refine requirements.
We can help track supplier survey dates and approvals against purchase orders in your ERP so expired pyrometry evidence gets flagged.
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