AMS 2759/2
Heat Treatment of Low-Alloy Steel Parts, Minimum Tensile Strength 220 ksi (1517 MPa) and Higher
Heat treatment of low-alloy steel parts (4340, 300M, D6AC, 4330V, Hy-Tuf and similar) to minimum tensile strengths of 220 ksi (1517 MPa) and higher.
What AMS 2759/2 specifies
AMS 2759/2 covers the ultra-high-strength airframe and landing-gear steels: 4340 at 260–280 ksi, 300M at 280–305 ksi, D6AC, 4330V (Hy-Tuf) and similar grades at 220 ksi and above. The tables give per-alloy normalizing, austenitizing, quenchant and tempering data for each tensile class, and the hardness bands used for lot acceptance (220–240 ksi is about 45–49 HRC). Because these strengths are sensitive to hydrogen, decarburization and residual stress, the sheet is stricter than /1: austenitizing must be done in a protective atmosphere, neutral salt or vacuum, decarburization limits are tighter, tempering must start promptly after the quench, and 300M and high-strength 4340 are double tempered with cooling to room temperature between tempers. Straightening after hardening is restricted, plated parts must be baked per AMS 2759/9, and ground surfaces are normally checked for grinding burns by temper etch (AMS 2649). Acceptance is by hardness on every lot plus tensile tests when specified.
AMS 2759/2 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Requirement / test |
|---|---|---|
220–240 ksi | Minimum tensile 220 ksi (1517 MPa), maximum 240 ksi (1655 MPa); D6AC, 4330V / Hy-Tuf and 4340 tempered to the lower end of the sheet. Hardness equivalent about 45–49 HRC. | Hardness per ASTM E18; tensile per ASTM E8 when specified |
260–280 ksi | Minimum tensile 260 ksi (1793 MPa), maximum 280 ksi (1931 MPa); the standard class for 4340 landing-gear and actuator parts, also used for D6AC. | Hardness per ASTM E18; tensile per ASTM E8 when specified |
280–305 ksi | Minimum tensile 280 ksi (1931 MPa), maximum 305 ksi (2103 MPa); 300M (AMS 6257 / AMS 6417), double tempered at about 575 °F (300 °C), 2 h + 2 h. | Hardness per ASTM E18; tensile per ASTM E8 when specified |
Tests and the drawing call-out
Tests the document imposes
- Rockwell C hardness per ASTM E18 on each lot after the final temper
- Tensile test per ASTM E8/E8M when specified (common for landing-gear forgings)
- Decarburization depth by metallographic cross-section (ASTM E1077)
- Temper etch inspection per AMS 2649 for grinding burns after finish grinding
- Hydrogen embrittlement bake records per AMS 2759/9 after plating or pickling
- Magnetic particle inspection per ASTM E1444 when the drawing requires it
How to cite it
Drawing call-out
HEAT TREAT PER AMS 2759/2, 4340, 260–280 KSI; DOUBLE TEMPER; HARDNESS TEST PER ASTM E18; BAKE PER AMS 2759/9 AFTER PLATINGAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2759/2
| Treatment | Role | Cost |
|---|---|---|
Martempering is an interrupted quench that equalizes austenitized steel just above its martensite-start temperature before cooling through martensite formation. | Governing | $$$ |
Quench and temper transforms hardenable steel to martensite and then tempers it to the required balance of strength and toughness. | Governing | $$ |
AMS 2759/2 — frequently asked
Why is a double temper required?
After the first temper some retained austenite transforms to fresh, untempered martensite on cooling; a second temper at the same temperature tempers that martensite and stabilises dimensions and toughness. 300M is always double tempered under the sheet (2 h + 2 h at about 575 °F / 300 °C) and the two cycles must be separated by cooling to room temperature.
Can an air furnace be used?
Not for austenitizing. Above 220 ksi the sheet requires a protective atmosphere, neutral salt or vacuum, because even partial decarburization removes strength exactly where fatigue cracks start and the allowance for later stock removal is small.
How soon after the quench must tempering start?
Promptly. Untempered martensite above 50 HRC can crack spontaneously from quench stress and hydrogen, so the sheet sets a maximum delay and does not allow parts to sit overnight before tempering. Sub-zero treatment, when specified, is done between the quench and the first temper.
What about straightening a warped part?
Straightening after hardening is restricted at these strengths because cold straightening leaves residual stress that promotes hydrogen and stress-corrosion cracking. Where it is allowed it must be followed by a stress relief below the tempering temperature and recorded on the certification; fixture quenching or hot straightening at the temper is preferred.
AMS 2759/2 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 ↗