AMS 2750
Pyrometry
Pyrometric requirements — temperature sensors, instrumentation, thermal-processing equipment, system accuracy tests and temperature uniformity surveys — for equipment used in the heat treatment of metals and other thermal processing.
What AMS 2750 specifies
AMS 2750 is the pyrometry standard every AMS heat-treatment specification (AMS 2759, 2770, 2774, 2801 and others) invokes, and the basis of Nadcap heat-treat audits. It classifies furnaces by temperature uniformity into Classes 1–6 — ±5 °F (±3 °C), ±10 °F (±6 °C), ±15 °F (±8 °C), ±20 °F (±10 °C), ±25 °F (±14 °C) and ±50 °F (±28 °C) — and by instrumentation into Types A through E, from Type A (control, recording, load and high/low sensors) down to Type E (a single control sensor with recorded output). It sets accuracy for thermocouples (base metal K, N, J, E and noble metal R, S, B), their calibration and usage limits, instrument accuracy (±2 °F / ±1 °C or 0.2 % of reading for control and recording instruments), system accuracy test (SAT) intervals and temperature uniformity survey (TUS) intervals that depend on class and type, survey method and record retention. Revision H (2024) is the current issue.
AMS 2750 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Requirement / test |
|---|---|---|
Class 1 | Temperature uniformity ±5 °F (±3 °C) throughout the qualified work zone; the tightest class, required by some aluminum and precipitation-ageing specifications. | TUS at the class tolerance; SAT at the interval for the class / type |
Class 2 | Uniformity ±10 °F (±6 °C); typical for aluminum solution treating and ageing and for precipitation-hardening stainless ageing. | TUS at the class tolerance; SAT at the interval for the class / type |
Class 3 | Uniformity ±15 °F (±8 °C); common for tempering and ageing of steels. | TUS at the class tolerance; SAT at the interval for the class / type |
Class 4 | Uniformity ±20 °F (±10 °C); common for austenitizing, carburizing and nitriding of steel. | TUS at the class tolerance; SAT at the interval for the class / type |
Class 5 | Uniformity ±25 °F (±14 °C); annealing, normalizing and stress relief. | TUS at the class tolerance; SAT at the interval for the class / type |
Class 6 | Uniformity ±50 °F (±28 °C); preheating, hydrogen-embrittlement baking and other low-criticality processes. | TUS at the class tolerance; SAT at the interval for the class / type |
Type A | Control sensor, recording sensor, high and low (over/under-temperature) sensors and load sensors in the work zone; the most complete instrumentation. | SAT on each control and recording channel |
Type B | Control sensor, recording sensor and load sensors. | SAT on each control and recording channel |
Type C | Control sensor, recording sensor and high / low sensors, no load sensors. | SAT on each control and recording channel |
Type D | Control sensor plus a separate recording sensor. | SAT on each control and recording channel |
Type E | A single control sensor whose output is recorded; the minimum instrumentation permitted. | SAT on the control channel |
Tests and the drawing call-out
Tests the document imposes
- Temperature uniformity survey (TUS) with nine or more test sensors in the qualified work zone (more for large volumes) at the extremes of the qualified operating range
- System accuracy test (SAT) comparing each control / recording channel against a calibrated test sensor at the working temperature
- Thermocouple calibration / certification traceable to national standards, with usage and expiry tracking
- Instrument calibration (control, recording and field test instruments) at fixed intervals
- Record review and retention (Nadcap AC7102 checklist)
How to cite it
Drawing call-out
FURNACE PYROMETRY PER AMS 2750 CLASS <1–6>, INSTRUMENTATION TYPE <A–E> (stated in the heat-treatment specification or purchase order, not on the part drawing)Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2750
| Treatment | Role | Cost |
|---|---|---|
Aluminum O-temper annealing removes strain hardening and produces the softest standard wrought condition. | Related | $$ |
Carbonitriding diffuses both carbon and nitrogen into low-carbon steel in the austenitic range, then quenches and tempers the surface to martensite. | Related | $$ |
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case. | Related | $$$ |
Cryogenic treatment cools a previously hardened alloy below room temperature to transform retained austenite and improve dimensional stability. | Related | $$ |
Gas nitriding diffuses nitrogen into finished ferrous parts below the transformation temperature, producing a hard compound layer and diffusion zone without quenching. | Related | $$$$ |
High-temperature tempering reheats quenched steel around 500–650 °C to produce tempered martensite with high toughness and moderate hardness. | Related | $ |
Hot isostatic pressing applies high-temperature inert gas pressure uniformly to cast, sintered, and metal powder-bed-fusion parts. | Related | $$$$ |
Low-temperature tempering reheats freshly quenched steel at about 150–250 °C to relieve stress while retaining high martensitic hardness. | Related | $ |
Medium-temperature tempering reheats quenched steel around 300–450 °C to trade some hardness for strength, elasticity, and improved toughness. | Related | $ |
Ferritic nitrocarburizing diffuses nitrogen and carbon into ferrous surfaces below the transformation temperature. | Related | $$$ |
Plasma nitriding uses a glow discharge to clean, heat, and diffuse nitrogen into ferrous surfaces. | Related | $$$$ |
Solution annealing dissolves detrimental precipitates and restores a single-phase matrix in austenitic stainless and many nickel alloys. | Related | $$$ |
Stress relief reduces residual stress from welding, machining, forming, casting, or additive manufacture without intentionally changing the bulk microstructure. | Related | $ |
AMS 2750 — frequently asked
Which furnace class do I need?
The processing specification sets it, not the part drawing. As a rule of thumb aluminum solution-treating and ageing furnaces run at Class 2 (±10 °F) or tighter, steel austenitizing and carburizing furnaces at Class 4 (±20 °F) or tighter, and annealing or stress-relief furnaces may qualify at Class 5 or 6.
How often are TUS and SAT run?
Intervals depend on class and instrumentation type: SATs run roughly weekly to quarterly, TUS monthly to annually, with extension of the TUS interval permitted after a run of consecutive passing surveys and a mandatory re-survey after repairs, relocation or a failed SAT. The heat treater's records are what an auditor checks first.
What is the difference between a control thermocouple and a load thermocouple?
The control sensor reads the furnace hot zone and drives the controller; a load sensor sits on or in a part or dummy block and records what the metal actually saw. Type A and B instrumentation require load sensors, and many specifications count soak time from the load thermocouple reaching set-point, not from the furnace reading it.
Why do thermocouples have usage limits?
Base-metal thermocouples (Type K, N) drift as their wires oxidise and change composition at temperature; AMS 2750 limits how many times and how long expendable and non-expendable base-metal sensors may be used above set temperatures and requires noble-metal or freshly calibrated sensors for TUS reference. Nadcap auditors check the usage log.
AMS 2750 is copyrighted by SAE International. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗