Heat Treatment in Critical Forgings: Process Control, AMS 2750, and Qualification

August 10, 2026


Heat treatment — the controlled heating and cooling of forged components to develop specified mechanical properties is the most consequential manufacturing step after forging itself. The same Inconel 718 disc forging can achieve tensile strength of 800 MPa (inadequate for aeroengine disc service) or 1,000 MPa (meeting AMS 5663 requirements) depending solely on the heat treatment cycle applied. AMS 2750 (Pyrometry requirements for thermal processing equipment) is the aerospace and critical industry standard for heat treatment furnace qualification it defines how furnaces must be calibrated, surveyed, and loaded to ensure that every component in every furnace load receives the intended heat treatment. This blog explains what AMS 2750 requires and how Vinir applies it.

Heat Treatment ProcessMaterialCritical ParametersStandard
Quench and temper (Q&T)4340, CrMoV alloy steelAustenitise temp, quench delay, temper temp/timeAMS 2759/1, AMS 2750
Solution treat and age (STA)Ti-6Al-4V, Inconel 718ST temp/time, quench rate, age temp/timeAMS 2801 (Ti), AMS 5663 (IN718)
Normalise and temperStructural steel, S355NLNormalise temp, cooling rate, temper tempEN 10025-3, AMS 2750
Carburise and case hardenEN36, 18CrNiMo7-6Carbon potential, case depth, core hardnessAMS 2759/7, AMS 2750
Solution anneal (stainless)Duplex 2507, 316LAnneal temp, hold time, quench rateASTM A182, NORSOK M-630

AMS 2750 divides heat treatment furnaces into classes based on their temperature uniformity — the range of temperatures measured at different points within the furnace working zone during a temperature uniformity survey (TUS). Class 5 (±28°C uniformity) is the least stringent; Class 1 (±3°C uniformity) is the most demanding. Aerospace rotating component heat treatment (Inconel 718 double ageing, Ti-6Al-4V STA) requires Class 2 (±6°C) or Class 3 (±8°C) uniformity. The TUS which involves placing multiple calibrated thermocouples throughout the furnace working zone and measuring the actual temperature distribution at the specified setpoint is the definitive verification of furnace temperature uniformity capability.

Thermocouple calibration under AMS 2750 is a rigorous requirement that extends beyond simply having calibrated instruments.
AMS 2750 distinguishes between: load thermocouples (placed in or on the actual component load to monitor its temperature directly), control thermocouples (connected to the furnace controller that maintains the setpoint), and recording thermocouples (connected to the chart recorder or data acquisition system that documents the time-temperature profile). All three types must be within calibration, and the calibration intervals are more frequent than commercial calibration practice typically 3 months for expendable load thermocouples and 6 months for semi-permanent control thermocouples.

The System Accuracy Test (SAT) — a check performed periodically between TUS surveys verifies that the temperature reading at the control point remains consistent with a calibrated reference. SAT is performed monthly or before each heat treatment operation for the most demanding AMS 2750 applications. The SAT result is recorded in the furnace log and becomes part of the heat treatment record for every production lot processed in that furnace. For aerospace customers performing NADCAP audits, the SAT records for the period covering each production lot’s heat treatment are subject to review a missing or out-of-tolerance SAT fails the NADCAP audit.

Quench systems — the water tanks, polymer quench systems, or forced-air cooling arrangements used to cool forgings from the austenitising temperature are also subject to AMS 2750 requirements for agitator performance and quench media temperature monitoring. An inadequate quench system produces inconsistent hardness through-section because the outer surface cools faster than the core creating a hardness gradient that means the reported surface hardness may not represent the core hardness where the component actually carries stress. Vinir’s quench tanks are equipped with mechanical agitators and quench media temperature monitoring per AMS 2750 ensuring consistent through-section cooling for all quenched forgings.

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Vinir Engineering — Capability for this Market

  1. AMS 2750 Class 2 and Class 3 certified heat treatment furnaces for aerospace and critical-industry forgings.
  2. TUS performed per AMS 2750 at required intervals — records available for NADCAP audit review.
  3. SAT performed per AMS 2750 for aerospace programmes.
  4. Load thermocouple use for critical aerospace heat treatment cycles.
  5. Quench tanks with mechanical agitation and temperature monitoring per AMS 2750.
  6. Carburising furnaces for EN36 and 18CrNiMo7-6 case-hardening forgings.
  7.  Solution annealing with rapid water quench for duplex and super duplex stainless forgings.
  8. NABL accreditation covers all post-heat-treatment mechanical testing referenced to heat treatment records by furnace chart identifier.
  9. Heat treatment records retained per AS9100D record retention requirements.

Frequently Asked Questions

1.What is AMS 2750 and why is it the standard for aerospace and critical-industry heat treatment?+–
AMS 2750 (Pyrometry Requirements for Thermal Processing Equipment Used for Metallic Materials) was developed by SAE International and is the globally recognised standard for aerospace and critical-industry heat treatment furnace qualification. It is referenced by NADCAP AC7102 (heat treatment audit checklist), by Boeing D1-4426, by GE Aerospace GESR, and by virtually every aerospace OEM quality requirement document for heat treatment. AMS 2750 is rigorous enough to ensure that heat treatment produces consistent, repeatable results which is the fundamental requirement for reproducible mechanical properties in safety-critical forged components. The alternative simply controlling the furnace to a temperature without independently verifying uniformity produces variable results that depend on furnace loading, ageing of heating elements, and thermocouple drift.
2.What is a Temperature Uniformity Survey (TUS) and how is it performed?+–
A TUS is performed by placing a grid of calibrated thermocouples throughout the furnace working zone (the volume within the furnace where production loads are placed) and heating the furnace to its specified operating temperature without a production load. The temperature at each thermocouple location is recorded at steady state typically after 30 minutes at setpoint to ensure thermal equilibrium. The maximum temperature spread (difference between the hottest and coldest thermocouple readings) determines the furnace’s AMS 2750 class. For a Class 3 furnace at 720°C (Inconel 718 ageing setpoint), all thermocouple readings must be within 720 ± 8°C i.e., between 712°C and 728°C. Any reading outside this range fails the TUS, requiring furnace adjustment and re-survey before the furnace can be used for Class 3 applications.
3.What is the difference between Type A, Type B, and Type C furnaces in AMS 2750?+–
AMS 2750 classifies heat treatment equipment by instrumentation sophistication. Type A furnaces (most demanding) require load thermocouples placed in or on the actual production load not just control and recording thermocouples on the furnace itself. Type A is required for the most critical aerospace heat treatment (Inconel 718 rotating component ageing, Ti-6Al-4V STA). Type B furnaces use over-temperature control but do not require load thermocouples in every load a higher-criticality process than Type C but less demanding than Type A. Type C (least demanding) is the standard for non-critical structural heat treatment where temperature uniformity requirements are relaxed. Most critical aerospace forging heat treatment requires Type A or Type B furnace instrumentation.
4.How does quench delay affect the mechanical properties of quenched and tempered alloy steel forgings?+–
Quench delay the time elapsed between removing a forging from the austenitising furnace and immersing it in the quench tank is critical because the forging begins to cool by radiation and convection immediately on furnace exit. If the quench delay is too long, the surface temperature falls below the martensite start temperature (Ms) before the quench begins allowing partial pearlite or bainite transformation that reduces final hardness and strength. AMS 2759/1 specifies maximum quench delay times for different alloy steel grades and section thicknesses typically 15–30 seconds maximum for medium alloy steels like 4340. For the heaviest forgings (above 1,000 kg), the minimum quench delay is also managed too rapid a temperature change on a large forging can cause thermal shock cracking at stress concentrations.
5.What records must a heat treatment record contain for aerospace and nuclear forging supply?+–
A compliant heat treatment record for aerospace or nuclear forging supply must include: furnace identification number (traceable to its TUS and SAT records), load charge record showing component serial numbers loaded, thermocouple calibration certificate numbers for all thermocouples used, the actual time-temperature chart (chart recorder trace or digital equivalent showing the complete heating, soak, and cooling cycle), furnace operator identification, quality inspector witness record (for hold-point-controlled critical heat treatments), and any non-conformances identified during the cycle with their disposition. The time-temperature chart must be continuous showing every moment from furnace loading to final temperature measurement with no unexplained gaps or anomalies. For nuclear NQA-1 programmes, these records are classified as quality records and must be retained for the life of the nuclear plant plus 10 years.