AMS 2769
Heat Treatment of Parts in a Vacuum
Requirements for heat treating metal parts in vacuum furnaces — leak rate, operating pressure, partial-pressure backfill, gas quenching, cleanliness and surface condition — used together with the alloy-specific AMS heat-treatment specifications.
What AMS 2769 specifies
AMS 2769 covers what the alloy-specific specifications assume when they permit "vacuum" as an atmosphere: how the vacuum furnace itself must perform. It sets the maximum leak rate, the vacuum level to be reached before heating and the operating pressure ranges allowed for each material group, with tighter levels for titanium and other reactive alloys. It permits partial-pressure backfill of argon or nitrogen during high-temperature soaks to suppress evaporation of chromium, manganese and other volatile elements from stainless and nickel alloys, and controls the purity and dew point of backfill and quench gases. Gas quenching (nitrogen, argon or helium, from atmospheric to high pressure) and integral oil quenching are addressed as quench media. Parts must arrive clean and dry — no oils, inks or plating residues — and leave bright and free of discoloration beyond the limits the sheet allows; pyrometry follows AMS 2750 with load thermocouples where the slash sheet requires them.
Tests and the drawing call-out
Tests the document imposes
- Furnace leak-rate test (pressure rise over time with the pumps valved off) at the specified interval
- Ultimate vacuum and operating pressure records for each load
- Backfill / quench gas purity and dew-point certification
- Surface discoloration inspection of parts and coupons after the cycle
- Temperature uniformity survey and system accuracy test per AMS 2750 under vacuum conditions
- Hardness / property tests as required by the alloy-specific heat-treatment specification
How to cite it
Drawing call-out
HEAT TREAT PER AMS 2759/5 IN VACUUM PER AMS 2769; SURFACE BRIGHT, NO DISCOLORATIONAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2769
| Treatment | Role | Cost |
|---|---|---|
High-temperature tempering reheats quenched steel around 500–650 °C to produce tempered martensite with high toughness and moderate hardness. | Alternative | $ |
Low-temperature tempering reheats freshly quenched steel at about 150–250 °C to relieve stress while retaining high martensitic hardness. | Alternative | $ |
Medium-temperature tempering reheats quenched steel around 300–450 °C to trade some hardness for strength, elasticity, and improved toughness. | Alternative | $ |
Solution annealing dissolves detrimental precipitates and restores a single-phase matrix in austenitic stainless and many nickel alloys. | Related | $$$ |
AMS 2769 — frequently asked
Why specify vacuum for stainless and tool steel?
It prevents oxidation and decarburization without carburizing or nitrogen-bearing atmospheres, leaving finished surfaces bright and dimensions untouched; 440C, 17-4PH, H13 and D2 are almost always vacuum hardened with gas quench for that reason. The trade is slower quenching than oil, so the section size that will through-harden is smaller.
What is partial pressure and why backfill during a vacuum cycle?
At high vacuum and 1900–2200 °F, chromium and manganese evaporate from stainless and nickel alloys, depleting the surface and coating the hot zone; backfilling with a small controlled pressure of argon or nitrogen suppresses this. AMS 2769 states when partial pressure is required and which gas is acceptable for each alloy group — nitrogen is excluded for titanium and some nickel alloys.
Does a vacuum gas quench harden like oil?
High-velocity nitrogen or helium at several bar approaches slow oil on small sections but cannot match oil or water on heavy sections, so the alloy-specific specification decides whether gas quench is permitted for the strength class. Vacuum furnaces with an integral oil quench exist for 4340 and carburized steels.
What does the leak rate mean for my part?
Air leaking into a hot furnace oxidises and decarburises parts even at low absolute pressure; the sheet limits the leak rate and requires periodic testing so a "bright" vacuum load stays bright. Discoloured parts after a vacuum cycle are the classic symptom of a leak or contaminated backfill gas.
AMS 2769 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 ↗