AMS 2759/7
Carburizing and Heat Treatment of Carburizing Grade Steel Parts
Carburizing (gas, vacuum/low-pressure or pack) and subsequent hardening, sub-zero treatment and tempering of carburizing-grade steel parts such as 9310, 8620, 4320, 3310 and Pyrowear 53.
What AMS 2759/7 specifies
AMS 2759/7 is the aerospace carburizing sheet for gears, bearings and splines in 9310 (AMS 6265), 8620, 4320, 3310, Pyrowear 53 and similar low-carbon alloy steels. Carburizing runs in gas, vacuum/low-pressure or pack processes at about 1550–1750 °F (845–955 °C) with the carbon potential controlled to give a near-eutectoid surface carbon and no continuous carbide network; parts are then hardened by direct quench or by reheat and quench, sub-zero treated when specified (about -100 °F / -73 °C) to transform retained austenite, and tempered at about 275–350 °F (135–175 °C). The drawing states the effective case depth after finishing, the surface hardness (typically 58–62 HRC), the core hardness range and the areas to be masked by copper plating or stop-off paint. The sheet limits retained austenite, carbide morphology and intergranular oxidation, and requires a coupon of the same heat and condition to be sectioned for case depth by microhardness traverse (SAE J423 / ASTM E384) with each lot.
Tests and the drawing call-out
Tests the document imposes
- Effective case depth (to 50 HRC / 513 HV equivalent) by microhardness traverse per ASTM E384 following SAE J423
- Surface hardness per ASTM E18 (HRC, or HR15N / HR30N on thin cases) after finish grinding
- Core hardness per ASTM E18 at the drawing location
- Microstructure: retained austenite, carbide network, intergranular oxidation and decarburization by metallography (ASTM E3 / E407)
- Surface carbon by combustion analysis or step-turning when specified
- Pyrometry per AMS 2750 and carbon-potential records
How to cite it
Drawing call-out
CARBURIZE AND HARDEN PER AMS 2759/7, 9310, EFFECTIVE CASE DEPTH 0.030–0.045 IN AFTER GRIND, SURFACE 58–62 HRC, CORE 34–40 HRC; COPPER MASK AREAS SHOWNAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2759/7
| Treatment | Role | Cost |
|---|---|---|
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case. | Governing | $$$ |
AMS 2759/7 — frequently asked
How is effective case depth defined?
Per SAE J423 for carburized cases it is the depth to 50 HRC (about 513 HV / 542 HK500) on a microhardness traverse of a sectioned coupon or part, distinct from total case depth (to core carbon). The drawing must say whether the depth applies before or after grinding, because gear flanks lose 0.005–0.015 in in the finish grind.
How much stock does carburizing add?
It adds no coating, but the surface grows slightly and the part distorts during the hardening quench; expect to grind 0.003–0.010 in per surface on precision gears. Order the case depth "after finish grind" so the heat treater can add the allowance.
Why sub-zero treat?
A 9310 case carburized to 0.8–1.0 % carbon retains 15–30 % austenite after the oil quench, which is soft and transforms in service; holding at -100 °F (-73 °C) or colder for about an hour before tempering converts most of it, raising surface hardness one to two HRC points and stabilising dimensions. Bearings and thin-web gears almost always specify it.
Can areas be kept soft?
Yes: copper plating or stop-off paint prevents carbon pickup, or the area is machined off after carburizing and before hardening (carburize, machine, harden). The drawing must show the masked zones with tolerances; threaded holes and press-fit bores are the usual masked features.
AMS 2759/7 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 ↗