AMS 2759 – Carbon and low alloy heat treating (UTS below 220Ksi)
Product types: Type 1, 2, 3, 4
Type 1: Parts with 0.020” or more to be machined off all surfaces after HT and parts with as-forged, as-
cast or hot-finished mill surface at time of HT with all surfaces to be machined off.
Type 2: Forgings, castings, sheet, strip, plate, bar, rod, tubing and extrusions with hot-finished surfaces
at time of HT and will remain on finished part.
Type 3: Parts with finished machined surfaces or surfaces with less than 0.02” to be machined off any
surface after HT and parts with protective coating on all surfaces.
Type 4: Parts that are partially machined with both unmachined, as-forged, as-cast, or hot-finished mill
surfaces and finished machined surfaces or machined surfaces with less than 0.020” to be machined
after HT.
Atmosphere classes: Class A, Class B, Class C
Class A: Argon, hydrogen, helium, nitrogen, nitrogen-hydrogen blends, vacuum, or neutral salt. Nitrogen
from dissociated ammonia is not permitted.
Class B: Endothermic, exothermic, or carbon-containing nitrogen-base
Class C: Air or products of combustion.
Heating Environment:
Parts shall be controlled by type, and heat treated in the class of atmosphere permitted in Table 1 for
that type when heating above 1250 °F (677 °C). When heating parts at 1250 °F (677 °C) or below, Class
A, B, or C atmosphere may be used.
Preheating:
Preheating to 900-120F(482-649C) until furnace stabilization is recommended before heating above
1300F(704C) if
- parts have previously been HT’ed to hardness greater than 35 HRC,
- or have abrupt section thickness changes,
- or have sharp reentrant angles,
- or have finished machined surfaces,
- or have been welded,
- or have been cold formed or straightened,
- or have holes,
- or have sharp or only slightly rounded notches or corners.
Soaking
- Heating shall be controlled either heating medium or part temperature
- Maintain at set temperature for soak time specified
- Parts coated with copper plate or similar reflective coatings that tend to reflect radiant heat shall
have their soak time increased by at least 50%, unless load thermocouples are used.
Annealing:
- Annealing heating temperature and soak time specified below; cooling to below temperature
also specified, then air cooling to ambient temperature.
Subcritical Annealing:
- Prior to hardening, heating to 1150-1250F (621-677C), soaking time specified.
Pre-hardening Stress Relieving:
- Prior to hardening, heating to 1000-1250F (538-677C), soaking for not less than the time
specified.
Normalizing:
- Heating to temperature and soaking time specified, cooling in air or ambient temperature.
- Circulated air or atmosphere is recommended for thickness great than 3”.
- Normalizing may be followed by tempering or subcritical annealing.
Hardening (Austenitizing and Quenching):
- Welded parts, and brazed parts with a brazing temperature above the normalizing temperature,
shall be normalized before hardening.
- Damage tolerant, maintenance critical or fracture critical parts shall be normalized before
hardening.
- Heating to austenizing temperature and soak time specified, quenching as specified.
- Parts shall be cooled to or below the quenchant temperature before tempering.
Tempering:
- Heating quenched parts to the temperature required to produce the specified properties.
- Parts shall be tempered within two hours of quenching.
- Suggested tempering temperature for specific strengths for each alloy and quenchant are
specified.
- Soaking time shall be not less than two hours plus one hour additional for each inch of thickness
or fraction therof greater than one inch.
- Multiple tempering is allowed; parts shall be cooled to ambient temperature between
tempering treatments.
- Prior to final tempering, parts may be snap tempered for two hours at a temperature of 400F
(204C), that is lower than tempering temperature.
Straightening:
- For parts have min. tensile strength below 180Ksi (1240Mpa), straightening may be
accomplished cold without stress relieving.
- For parts have min tensile strength over 180Ksi, straightening may be accomplished at either
ambient temperature during tempering, or by heating to not higher than 50F (28C) below the
tempering temperature; ambient temperature straightening or hot or warm straightening after
tempering shall be followed by stress relieving.
- It is permissible to retemper at a temperature not higher than the last tempering temperature
after straightening during tempering.
Post-Tempering Stress Relieving:
- When required, parts shall be stress relieved by heating parts to 50F(28C) below tempering
temperature and soaking for not less than one hour plus one hour per inch of thickness or
fraction thereof greater than one inch after operations which follow hardening and tempering.
