Alloy A286 (UNS S66286) is a precipitation-hardening superalloy that develops its strength via gamma-prime Ni₃(Ti,Al) precipitation. The two-step heat-treatment sequence, high-temperature solution treatment followed by lower-temperature aging, is mandated by ASTM A453 grade 660, AMS 5731 / 5732 / 5737, and DIN 17240.
- Temperature: 1800 °F ± 25 °F (980 °C ± 14 °C) per AMS 5731 and ASTM A453 grade 660 Class A.
- Hold time: 1 hour per inch (25 mm) of section thickness, minimum 1 hour.
- Atmosphere: air, neutral salt or controlled-atmosphere furnace. Vacuum at 10⁻³ mbar for aerospace bar-stock and finished components.
- Quench medium: oil for sections > 25 mm; water for thin sheet and small fasteners.
- Result: single-phase austenite with all gamma-prime dissolved, hardness 140-180 HBW.
- Temperature: 1325 °F ± 25 °F (720 °C ± 14 °C) per AMS 5732 and AMS 5737.
- Hold time: 16 hours, regardless of section thickness.
- Cooling: air-cool to room temperature; no slow furnace-cool required.
- Result: fine, coherent gamma-prime Ni₃(Ti,Al) precipitates 8-15 nm in size; hardness 277-302 HBW (30-32 HRC), tensile > 895 MPa (130 ksi).
- ASTM A453 grade 660 Class B (solution-treated only): 980 °C / oil-quench, no aging. Used where tensile > 690 MPa (100 ksi) is sufficient.
- ASTM A453 grade 660 Class C: 900 °C solution-treat + 720 °C / 16 h age. Higher yield (760 MPa) but lower stress-rupture life.
- ASTM A453 grade 660 Class D: double age, 720 °C / 16 h + 650 °C / 16 h. Maximum stress-rupture strength for high-temperature service.
Solution-treatment in air produces a thin chromium-rich oxide scale that is removed by pickling (HNO₃ / HF) or grit-blasting. Vacuum or argon atmosphere keeps surfaces bright and dimensionally stable. Aging causes negligible distortion (< 0.05 %) because no phase transformation occurs, only precipitation. Machining is therefore typically completed after solution treatment but before aging, with final tolerances finished post-aging.
Welded A286 assemblies should be solution-treated and re-aged to restore full strength in the heat-affected zone. Filler metal AMS 5853 matches base-metal aging response; preheat is not required. Strain-age cracking can occur on rapid heating to aging temperature in heavily restrained welds, mitigation is to ramp from 540 °C to 720 °C over > 2 hours.
Each A286 specification defines a specific heat-treatment cycle. The two principal classes are sol-treated-only (downstream aging by user) and sol-treated+aged (mill-aged, ready for use). Per-spec heat-treatment reference:
- AMS 5731, Solution-treated 1800 °F (982 °C), oil-quench. NOT aged at mill.
- AMS 5732, Solution-treated 1650 °F (899 °C) + precipitation-aged 1325 °F × 16 h.
- AMS 5737, Same cycle as AMS 5732 with ±5 °C aging temperature control for hardness uniformity.
- AMS 5853, Solution-treated only (NOT aged), aging done post-weld with full assembly.
- ASTM A453 grade 660, Class A: sol 1650 °F + aged. Class B: sol 1800 °F + aged. Class C: sol only. Class D: alternate sol.
- ASTM A638 grade 660, Type 1: sol 1650 °F + aged. Type 2: sol 1800 °F + aged.
Solution-treatment temperature and aging time directly drive A286 mechanical properties. Higher solution temperature (1800 °F) dissolves more gamma-prime forming elements, producing higher stress-rupture strength but lower room-temperature yield. Lower solution temperature (1650 °F) retains more precipitates in solution, producing higher room-temperature tensile and yield. Aging at 1325 °F for 16 hours is the standard precipitation-hardening cycle, develops gamma-prime Ni₃(Ti,Al) precipitates that strengthen the austenitic matrix. Full A286 mechanical-property data per heat-treatment condition is on the dedicated mechanical properties page.
