Aerospace Heat Treatment: Pyrometry, Process Specs and Callouts
Aerospace heat treatment is controlled by two layers of specification: AMS 2750 governs the furnace, thermocouples and instruments, and an alloy family spec such as AMS 2770, AMS 2774 or AMS 2759 governs the cycle. Most primes also require the heat treater to hold Nadcap accreditation for the processes it runs.
The short version for a drawing or PO
Call out the alloy family process specification and the final condition, and let that spec pull in pyrometry. A note such as heat treat per AMS 2770 to T6 for a 6061 detail, or per AMS 2759/3 to H1025 for a 17-4PH part, tells the processor the cycle family, the testing it owes you and the paperwork it must produce. The family specs in turn require furnaces and instruments controlled to AMS 2750.
What the drawing should not do is specify a furnace temperature and time in isolation. Soak times depend on section thickness and load, quench delay limits depend on alloy, and the family specs already handle both. Writing your own cycle invites a conflict between your note and the spec the heat treater is audited against.
Two items do belong on the PO: the final condition you will accept, stated as a temper designation or a minimum tensile strength or hardness, and any requirement that the processor be Nadcap accredited for that commodity. If the part will be plated after heat treatment, also tell the processor the final strength so a hydrogen embrittlement relief bake can be planned. AMS 2759/9E requires the purchaser to supply the final heat treat condition, minimum tensile strength or minimum hardness, to the plater.
AMS 2750: what pyrometry actually controls
AMS 2750 is the pyrometry specification. It does not tell anyone how to heat treat a part. It defines how temperature is measured and proven: sensor types and calibration, instrument accuracy, system accuracy tests (SAT) that check the whole measurement chain, and temperature uniformity surveys (TUS) that prove the qualified work zone stays within tolerance.
Each furnace is assigned a class based on the uniformity it can demonstrate, and an instrumentation type, A through E, based on how many control, recording and load sensors it carries. Class and type together set the survey intervals. Revision G is the current revision cited in industry articles, so check which revision your customer flows down.
| Item | What it means | Published example |
|---|---|---|
| Furnace class | Allowed temperature spread across the qualified work zone | Six classes spanning ±5 °F (±3 °C) at Class 1 to ±50 °F (±28 °C) at Class 6 |
| Class in practice | A zone surveyed at 1,887 to 1,913 °F around a 1,900 °F setpoint (±13 °F) | Qualifies as Class 3 |
| Instrumentation type | Sensor arrangement, A is the most instrumented, E the least | Defined in the specification |
| TUS interval | Driven by class and type | Class 2 with Type B: monthly. Class 5 with Type B: quarterly |
| Interval extension | Reward for consecutive passing surveys | Class 2, Type B moves from monthly to quarterly after four consecutive passes |
The class you need is set by the alloy spec, not by preference. Aluminum solution treatment has a narrow window between incomplete solutioning and incipient melting, so aluminum process specs require tight furnace classes. Many steel tempering operations tolerate a wider band. When you audit a heat treater, compare the furnace class on its latest TUS report with the class your process spec requires, and confirm the survey is current.
The family specifications
Each alloy family has its own process specification. The table below lists the ones engineers meet most often and what the published scopes say they cover.
| Spec | Covers | Notes from the published scope |
|---|---|---|
| AMS 2770 | Wrought aluminum parts | Applies to fabricators and their subcontractors, and to distributors that change raw material temper. Lists alloys including 2014, 2024, 2219, 6061, 7050, 7075 and 7475. Castings are handled by the procurement spec |
| AMS 2774 | Wrought nickel and cobalt alloys | Revision G scope includes wrought or additively manufactured nickel or cobalt alloy parts and fabricated assemblies |
| AMS 2759 | Steel parts, general requirements | Parent document. Slash sheets carry alloy specific cycles |
| AMS 2759/1 | Carbon and low alloy steel | Minimum tensile strength below 220 ksi (1517 MPa) |
| AMS 2759/3 | PH stainless, maraging, secondary hardening steels | Used with AMS 2759 |
| AMS 2759/5 | Martensitic corrosion resistant steels | Used with AMS 2759 |
| AMS 2759/7 | Carburizing grade steels | Excludes pack carburizing |
| AMS 2759/9 | Hydrogen embrittlement relief baking | Revision E. Purchaser supplies final strength or hardness |
| AMS 2801 | Titanium alloy parts | Commonly cited titanium heat treat spec. Confirm scope and revision with SAE |
| ASTM F3301 | Powder bed fusion metal parts | Thermal post processing including HIP for Ti-6Al-4V, CP Ti, Co-28Cr-6Mo, 718, 625, 316L and AlSi10Mg |
Revision matters. Several AMS 2770 revisions are withdrawn, and a supplier certifying to an old letter is a common receiving inspection finding. Put the revision on the PO if your quality system requires it, or state the latest revision in effect on the PO date.
Aluminum: solution, quench and age
Aluminum heat treat failures usually come from the quench, not the furnace. A long quench delay or a quenchant that is too warm leaves thick sections under strength and can lower corrosion resistance. Distortion from the quench is the reason many shops rough machine, heat treat, then finish machine. If your part is thin walled, discuss fixturing and straightening allowances with the processor before release, because straightening after age can introduce residual stress.
Verification is by hardness and electrical conductivity testing, with tensile tests where the spec or PO requires them. Conductivity is a fast screen for incorrect temper in 7xxx alloys. Nadcap treats hardness and conductivity testing as its own sub-checklist under the heat treating criteria, so ask whether the processor is accredited for the testing as well as the furnace work.
For material choice and temper behavior, see the aluminum alloys page.
Steels and stainless: hardness, decarb and embrittlement
For low alloy steels such as 4130 and 4340, the risks are decarburization, quench cracking and missing the hardness window. Vacuum or protective atmosphere furnaces control surface chemistry, and the spec will set limits on decarburization depth that are checked metallographically on test coupons or sacrificial parts.
For PH stainless such as 15-5PH and 17-4PH, the aging condition (H900, H1025, H1150 and so on) sets the strength and toughness balance. Higher strength conditions carry more risk of stress corrosion and hydrogen embrittlement, so many programs prefer the overaged conditions for structural parts. The stainless and specialty steels page covers the tradeoffs.
Any plated steel part above the threshold in AMS 2759/9 needs an embrittlement relief bake soon after plating. The revision D scope applied to carbon, low alloy and martensitic stainless steel heat treated to 180 ksi (1241 MPa) minimum or higher. Sequence the bake before any later conversion coating or paint.
Nickel alloys and titanium
Nickel superalloys such as 718 are solution treated and double aged per AMS 2774. Furnace atmosphere and cooling rate control matter because surface oxidation and slow cooling through sensitive ranges degrade properties. Additively built 718 normally gets a stress relief and a HIP cycle before solution and age. ASTM F3301 covers that thermal post processing for powder bed fusion parts, and the AMS 2774 revision G scope explicitly includes additively manufactured nickel and cobalt alloy parts. See the metal additive page.
Titanium is heat treated in vacuum or inert atmosphere because it absorbs oxygen at temperature and forms alpha case, a hard brittle surface layer that must be removed by machining or chemical milling. If a titanium part is heat treated after final machining, the drawing should say how alpha case is verified as absent. The titanium alloys page has more on this failure mode.
Nadcap and supplier selection
Heat treatment is a special process in AS9100 terms because its result cannot be fully verified by inspecting the finished part. That is why primes approve heat treaters by process and why Nadcap accreditation under the AC7102 heat treating criteria is a common flow down. The baseline criteria carry sub-criteria by process and alloy family, for example aluminum, carburizing, nitriding, and hardness and conductivity testing, so an accreditation certificate lists a specific scope.
When qualifying a source, check four things: the Nadcap certificate scope covers your alloy and process, the customer approval list (if your prime keeps one) names the processor for that spec, the furnace class on the current TUS meets the spec, and the certification will state the spec, revision, cycle, load or furnace identification, and test results.
What the cert should show
- Part number, revision, quantity and your PO number
- Process specification and revision, and the final condition achieved
- Furnace identification and cycle record or chart reference
- Hardness, conductivity or tensile results with the test method
- Nadcap or customer approval reference where the PO requires it
- For plated steel, the bake time and temperature and the time from plating to bake
Missing furnace identification or a cert that states "heat treated per AMS 2759" with no slash sheet or condition should be rejected at receiving. The inspection and metrology page covers how these records feed a first article report.
Get heat treat sourced and documented
If you have parts that need heat treat to AMS 2770, AMS 2774 or the AMS 2759 family, send us the drawing and spec callouts. We source the work from qualified processors, flow down the revision and accreditation requirements, check the cert against the PO before shipment, and a person replies within one business day. A mutual NDA comes before files. If the drawing is export controlled, read how we handle controlled programs first, because that data never goes through the website. Related reading: the processes hub, surface finishing and nondestructive testing.
Questions
Does AMS 2750 tell the heat treater what temperature to run?
No. AMS 2750 controls how temperature is measured and proven: thermocouples, instruments, system accuracy tests and temperature uniformity surveys. The cycle itself comes from the alloy family specification, such as AMS 2770 for wrought aluminum, AMS 2774 for nickel and cobalt alloys or the AMS 2759 slash sheets for steels. Those family specs require furnaces controlled to AMS 2750, so calling out the family spec brings pyrometry with it.
What furnace class do I need?
The process specification sets it, based on how sensitive the alloy is to temperature. AMS 2750 defines six classes from ±5 °F (±3 °C) at Class 1 to ±50 °F (±28 °C) at Class 6. Aluminum solution treatment needs a tight class because the window between full solutioning and incipient melting is narrow. Check the furnace class on the processor current survey report against your spec rather than asking for the tightest class by default.
Should I require Nadcap heat treat accreditation?
For flight hardware, usually yes, and many primes require it in their purchase terms. Nadcap AC7102 accreditation shows an independent audit of the processor pyrometry, procedures and records. Read the certificate scope, because accreditation is granted by process and alloy family. A shop accredited for steel hardening may not be accredited for aluminum solution treatment or for hardness and conductivity testing.
Why do plated steel parts need a bake?
Electroplating and some chemical processes put hydrogen into high strength steel, which can cause delayed brittle cracking under load. AMS 2759/9 sets the relief bake requirements, and the plater needs the final heat treat condition, minimum tensile strength or hardness, to choose the cycle. The bake must be done within the time limit the spec sets after plating and before any conversion coating or paint.
What is alpha case and how is it controlled?
Alpha case is an oxygen enriched, hard and brittle layer that forms on titanium heated in the presence of oxygen. It reduces fatigue life and can crack in service. Heat treating in vacuum or inert gas limits it, and any layer that forms is removed by machining or chemical milling. If titanium is heat treated after final machining, the drawing or PO should state how absence of alpha case is verified.
What should a heat treat certification include?
At minimum: your part number, revision and quantity, the process specification and its revision, the final condition, the furnace and cycle identification, and the hardness, conductivity or tensile results with test methods. Where the PO requires Nadcap or a customer approval, the cert should reference it. A cert that names only the parent specification with no slash sheet or condition is not enough to accept the parts.
Related
Sources
- Ipsen: furnace classifications under AMS 2750
- Thermal Processing: TUS practice for AMS 2750E
- ANSI Webstore: SAE AMS 2770 heat treatment of wrought aluminum parts
- ANSI Webstore: SAE AMS 2774G heat treatment of nickel and cobalt alloy parts
- SAI Global: SAE AMS 2759F heat treatment of steel parts
- DIN Media: SAE AMS 2759/3 PH, maraging and secondary hardening steels
- SAE: AMS 2759/9E hydrogen embrittlement relief baking of steel parts
- ASTM F3301 thermal post processing of powder bed fusion metal parts
- ASM International: heat treatment standards for powder bed fusion parts
- Performance Review Institute: Nadcap
Ready to
source it?
A person replies within 1 business day. NDA first.
Request a quote