Alloy A286 (UNS S66286): Chemistry, Heat Treatment and AMS Conditions
A286: an Iron-Base Superalloy for High-Temperature Fasteners
Alloy A286 is a precipitation-hardenable iron-base superalloy, UNS S66286, built around roughly 25 % nickel and 15 % chromium with titanium and molybdenum additions. It sits between the austenitic stainless steels and the nickel-base superalloys: stronger than 316 above 540 C, considerably cheaper than a nickel-base grade, and formable with conventional shop equipment in the solution treated condition. The aerospace fastener and gas turbine industry uses it in the largest volumes, and the same combination of strength, oxidation resistance and fatigue behaviour has made it a standard choice for offshore oil and gas hardware exposed to heat and pressure.
Chemistry and Product Standards
The grade is controlled by several documents depending on the product form. Bar and forging stock are covered by ASTM A638, where A286 appears as Grade 660, and by ASTM A453 for high-temperature bolting, again as Grade 660 with type designations. Aerospace material specifications split by form and condition: AMS 5731 for solution treated bar, AMS 5732 for solution treated and aged bar, AMS 5737 for the aged condition in specific product forms, and AMS 5525 for sheet, strip and plate. Typical specified chemistry is carbon 0.08 % max, manganese 2.00 % max, silicon 1.00 % max, phosphorus 0.025 % max, sulfur 0.025 % max, chromium 13.5-16.0 %, nickel 24.0-27.0 %, molybdenum 1.00-1.50 %, titanium 1.90-2.35 %, vanadium 0.10-0.50 %, aluminum 0.35 % max and boron in the 0.001-0.010 % range.
Heat Treatment: Solution Treat, Then Age
| Step | Practice | Effect |
|---|---|---|
| Solution treatment | 982 C (1800 F) plus or minus 14 C, 1 hour, water or oil quench | Produces the highest creep rupture strength in the aged material |
| Alternative solution treatment | 900 C (1650 F), rapid cool | Improves ductility and room temperature strength, lowers creep rupture strength |
| Age hardening | 720 C (1325 F) for 16 hours, air cool | Precipitates the gamma prime phase that carries the strength |
| Governing documents | AMS 5731 (solution treated), AMS 5732 (solution treated and aged), ASTM A638 Grade 660 | Certificate must state the condition delivered |
The 1650 F and 1800 F solution treatments are not interchangeable. The selection is a design decision: bolts that must survive long-term loading at temperature are generally specified with the higher temperature solution treatment, while components where ductility and room temperature strength dominate use the lower one. Aged material is commonly released against minima in the region of 896 MPa (130 ksi) tensile strength and 586 MPa (85 ksi) 0.2 % yield strength, with the exact values taken from the governing specification.
Where A286 Performs
A286 is specified for turbine and compressor fasteners, afterburner components, exhaust hardware, supercharger parts, and high-temperature bolting in chemical and petrochemical plants. In the offshore sector it appears in valve and wellhead hardware where stainless fasteners would lose strength at service temperature. Its limits are equally well defined: it should not be used for sustained service in strong oxidising acids or in concentrated chloride environments, where higher-molybdenum nickel-base grades are required, and it is not a substitute for a cobalt-base alloy in metal-to-metal wear at very high temperature. Elevated temperature properties also depend strongly on the product form and grain size, so a fastener specification should be matched to the bar specification rather than treated as equivalent.
Frequently Asked Questions
Q: What is the difference between AMS 5731 and AMS 5732?
A: AMS 5731 covers the solution treated condition, and AMS 5732 covers the solution treated and aged condition. For a finished fastener, the aged condition is what delivers the high strength, so the certificate must state which condition was supplied.
Q: What is the aging treatment for A286?
A: A common schedule is 720 C (1325 F) for 16 hours followed by air cooling, after a solution treatment at 982 C (1800 F) for 1 hour with a water or oil quench. The lower 900 C (1650 F) solution treatment is used when ductility and room temperature strength are the priority.
Q: Is A286 stainless or a superalloy?
A: Both descriptions appear in trade use, but technically it is a precipitation-hardenable iron-base superalloy. It has more nickel than a conventional stainless steel but is not one of the nickel-base alloys, which is why it costs less than a grade such as a nickel-chromium-molybdenum superalloy.
Q: Which standards apply to A286 bar?
A: ASTM A638 Grade 660 covers bar and forging stock, ASTM A453 Grade 660 covers high-temperature bolting, and the aerospace conditions are covered by AMS 5731, AMS 5732 and AMS 5737, with AMS 5525 for sheet, strip and plate.
Q: Can A286 be welded?
A: Yes, in the solution treated condition, using a matching filler metal. Welding in the fully aged condition is not recommended, and the component normally requires a post-weld heat treatment to restore the precipitation structure in the weld zone.
Q: What operating temperature range suits A286?
A: It is typically selected for service up to about 650 C, where it retains useful strength and oxidation resistance. Above that range, design strength falls away and nickel-base or cobalt-base grades become the practical choice.
