AMS 2759/4
Heat Treatment, Austenitic Corrosion-Resistant Steel Parts
Annealing, stabilization annealing and stress relief of austenitic corrosion-resistant steel parts such as 302, 304/304L, 316/316L, 321, 347, 309, 310 and the nitrogen-strengthened 21-6-9 / Nitronic grades.
What AMS 2759/4 specifies
AMS 2759/4 covers the heat treatment of austenitic stainless steel parts, which cannot be hardened by heat treatment and are processed to restore corrosion resistance, ductility or dimensional stability. It specifies annealing (solution) temperatures per alloy, generally 1850–2050 °F (1010–1120 °C), followed by cooling fast enough to keep chromium carbides in solution — water, forced gas or air depending on section — and warns that holding unstabilized grades in the 800–1500 °F (425–815 °C) range sensitizes them to intergranular corrosion. Stabilized grades 321 and 347 may receive a stabilization anneal near 1550–1650 °F (845–900 °C). Stress-relief options are given for machined or welded parts. Furnace atmospheres must not carburize (carbon pickup is prohibited) and parts must be free of sulfur- and halide-bearing residues; air-annealed parts are descaled and passivated afterwards. Acceptance is by hardness, and by ASTM A262 intergranular-corrosion tests when the drawing specifies them.
Tests and the drawing call-out
Tests the document imposes
- Hardness per ASTM E18 (Rockwell B) or ASTM E10 to confirm the annealed condition (for example 304 annealed at 92 HRB maximum)
- Intergranular corrosion susceptibility per ASTM A262 Practice A (oxalic-acid etch screen) or Practice E (copper–copper sulfate–sulfuric acid) when specified
- Tensile test per ASTM E8/E8M when specified
- Visual check for scale, carbon pickup and discoloration; passivation per AMS 2700 after descaling
- Furnace TUS / SAT per AMS 2750
How to cite it
Drawing call-out
ANNEAL PER AMS 2759/4, 304L, 92 HRB MAX; INTERGRANULAR CORROSION TEST PER ASTM A262 PRACTICE E WHEN SPECIFIEDAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to AMS 2759/4
| Treatment | Role | Cost |
|---|---|---|
Solution annealing dissolves detrimental precipitates and restores a single-phase matrix in austenitic stainless and many nickel alloys. | Governing | $$$ |
AMS 2759/4 — frequently asked
Why heat treat a steel that does not harden?
To dissolve chromium carbides formed by welding or hot working, to soften cold-worked material for further forming, or to relieve machining stress before a tight-tolerance finish. Annealing at 1850–2050 °F restores the single-phase austenite that gives 304 and 316 their corrosion resistance and a typical 80–92 HRB hardness.
Can 304 be stress relieved without sensitizing it?
Only by staying below the sensitization range: a low stress relief around 750–800 °F (400–425 °C) removes part of the residual stress without forming carbides, while a full anneal above 1850 °F removes it all but changes dimensions. Holding plain 304 between 800 and 1500 °F for more than a few minutes risks sensitization; 304L, 316L, 321 and 347 are tolerant.
Does the sheet allow air furnaces?
Yes. Austenitic grades are often air annealed and then pickled or blasted to remove the oxide, followed by passivation; vacuum or hydrogen/argon furnaces are used when a bright, scale-free surface is required or pickling would attack thin sections. Carbon-bearing atmospheres (endothermic gas) are prohibited because carbon pickup destroys corrosion resistance.
When is an ASTM A262 test required?
When the drawing or material specification invokes it, typically for welded 304/316 pressure or chemical-service parts. Practice A is a quick oxalic-acid etch screen; Practice E is the boiling copper–copper sulfate test with a bend; both check that the anneal left no continuous carbide network at the grain boundaries.
AMS 2759/4 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 ↗