AMS 2759/9
Hydrogen Embrittlement Relief (Baking) of Steel Parts
Baking of steel parts to relieve hydrogen absorbed during electroplating, electroless plating, acid pickling, cathodic cleaning, chemical stripping and similar wet processes, keyed to the part's tensile strength or hardness.
What AMS 2759/9 specifies
AMS 2759/9 sets the post-process bake that drives out hydrogen absorbed by steel parts during plating, acid pickling, cathodic cleaning or chemical stripping — hydrogen that otherwise causes delayed brittle fracture in high-strength parts. The bake is normally 375 ± 25 °F (191 ± 14 °C) and must start within a limited time after the hydrogen-introducing process (4 h is the usual limit after plating), before any load is applied. Minimum time at temperature rises with strength class: the table runs from a few hours for the lower classes to 23 h for steels at 220 ksi (1517 MPa) and above. The bake temperature must stay below the part's tempering or ageing temperature so hardness is not affected, and lower-temperature, longer-time schedules are given for coatings and parts that cannot tolerate 375 °F. Parts below the lowest tabulated strength class are exempt unless the drawing requires baking. Certifications record the bake start time, temperature and duration for each lot.
Tests and the drawing call-out
Tests the document imposes
- Bake start-time verification against the end of plating or pickling (traveller / log)
- Furnace temperature record for the full bake; pyrometry per AMS 2750
- Hardness check per ASTM E18 when the bake temperature approaches the tempering temperature
- Sustained-load hydrogen embrittlement test per ASTM F519 for process qualification when specified
- Certification of strength class, bake temperature and duration per lot
How to cite it
Drawing call-out
BAKE PER AMS 2759/9 WITHIN 4 H AFTER PLATING, 375 ± 25 °F, <HOURS> H MIN; PART STRENGTH <TENSILE CLASS OR HRC>Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
AMS 2759/9 — frequently asked
Why does the bake have to start so quickly?
Hydrogen from plating sits in the surface and diffuses inward at room temperature; once it reaches a stress concentration in a high-strength part a crack can start before the bake is run, and a late bake cannot heal it. The sheet fixes a maximum delay (commonly 4 h after plating) and forbids loading, torqueing or straightening in between.
Which parts need baking?
Any steel part in the sheet's strength classes that has been through an aqueous cathodic process or acid: plated fasteners, chromium-plated landing-gear cylinders, pickled springs, stripped and re-plated parts. Parts below the lowest tabulated class are exempt unless the drawing says otherwise; austenitic stainless and aluminum do not need it.
Can I bake at a lower temperature?
Yes, when the part or coating cannot stand 375 °F — parts tempered or aged near that temperature, or coatings with low melting points. The sheet gives lower-temperature alternatives with longer minimum times; pick the schedule that matches the part strength and put it on the drawing so the plater and heat treater agree.
Does baking remove all the hydrogen?
No. It lowers mobile hydrogen to a level at which the steel is no longer embrittled under design loads and lets the remainder redistribute to harmless traps. Process qualification is proven with ASTM F519 notched-specimen sustained-load tests (200 h at 75 % of notched fracture strength), not by measuring hydrogen content.
AMS 2759/9 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 ↗