SAE AMS 5589G Standard

Chemical Composition

Composition is determined by wet chemical methods per ASTM E354, or by other analytical methods acceptable to the purchaser. All values are percentages by weight, with iron as the remainder.

Three elements do the strengthening work. Niobium at 4.75–5.50 % forms the gamma double-prime phase that gives 718 its strength, while titanium at 0.65–1.15 % and aluminium at 0.20–0.80 % form gamma prime. The ageing cycle in this specification is designed around precipitating those phases in a controlled sequence.

Check analysis variations shall meet the requirements of AMS2269, Chemical Check Analysis Limits for Nickel, Nickel Alloys and Cobalt Alloys. The phosphorus and sulphur caps at 0.015 % each are tighter than most industrial 718 specifications allow.

Inconel 718 seamless tube supplied to industrial ASTM specifications shares this nominal chemistry, though the aerospace route adds the melting practice, grain size and stress-rupture requirements that follow.

Melting Practice and Condition

The alloy shall be produced by multiple melting using consumable electrode practice in the remelt cycle, or shall be induction melted under vacuum. Where consumable electrode remelting is not performed in vacuum, the electrodes used for remelting must themselves have been produced by vacuum induction melting.

That rule closes the loop on cleanliness. Either the remelt happens under vacuum, or the starting electrode was made under vacuum, so there is no route to AMS5589 material that skips vacuum processing entirely.

The delivery condition is cold drawn or pilgered, solution heat treated, and descaled. Tubing is produced by a seamless process, and surface finish may be produced by pickling, bright annealing or any method that gives the required surface condition without affecting wall thickness limits or corrosion resistance.

One finishing method is prohibited outright: a centerless-ground finish is not acceptable. A light polish to improve external appearance may be applied after solution heat treatment, and bore conditioning is permitted after final anneal provided the tubing is not sized by metal removal and is passivated afterwards.

Heat Treatment: Solution and Precipitation Cycles

Two separate cycles appear in the specification. The producer performs the solution treatment; the precipitation treatment is defined so that properties can be verified, and in service it is usually applied by the customer after forming or welding.

The two-stage ageing cycle is the classic 718 treatment. Stage one at 1325 °F precipitates the coarser gamma double-prime; the furnace cool and hold at 1150 °F completes precipitation at a finer scale. Total precipitation time is 18 hours, not 8 plus 18; the stage two hold runs until the cumulative total reaches 18 hours.

Solution heat treatment must be carried out in a protective atmosphere, and pyrometry shall be in accordance with AMS2750. That pyrometry reference brings furnace survey, thermocouple and instrument calibration requirements into the specification by reference.

The specification requires properties to be met after re-solution and precipitation as well as after precipitation alone. That second route matters for fabricators who re-solution treat an assembly after welding before ageing it, and the tubing must satisfy the requirements by either path.

Tensile Properties as Solution Heat Treated

The as-supplied tensile requirements are maximums, not minimums. The tubing must be soft enough to form and weld, which is the reverse of how most tensile tables are read.

Tubing that arrives stronger than 155 ksi tensile or 95 ksi yield is rejectable, because excess strength in the solution treated condition signals incomplete solutioning or unintended ageing, and it makes forming harder. The 30 % minimum elongation is the only minimum in this table.

Two further requirements apply in the solution treated condition. Average grain size shall be ASTM No. 5 or finer per ASTM E112, which controls both formability and the creep behaviour the aged material will show later.

Flarability Test

Tubing from 0.188 to 2.000 inches (4.78 to 50.80 mm) inclusive in nominal OD, with an OD to wall thickness ratio of 10:1 or less, must withstand flaring at room temperature without cracks or other visible defects.

The test forces the specimen axially with steady pressure over a hardened and polished tapered steel pin having a 74-degree included angle, producing a permanent expanded OD not less than 1.20 times the original nominal OD. The end to be flared must be cut square, smooth and free from burrs, but not rounded.

Properties After Precipitation Heat Treatment

After ageing, the numbers invert. Minimums replace maximums, and the material delivers the strength the application was designed around.

The strength gain from ageing is substantial. Tensile moves from a 155 ksi ceiling to a 185 ksi floor, and yield from a 95 ksi ceiling to a 150 ksi floor, a gain of at least approximately 58 % in yield strength, paid for by elongation dropping from 30 % to 12 %.

The hardness requirement carries an unusual escape clause. Tubing shall not be rejected on hardness if the tensile requirements are met, determined on specimens from the same sample as the nonconforming hardness, or from another sample with similar nonconforming hardness. Hardness conversion tables are in ASTM E140.

Stress-Rupture at 1300 °F (704 °C)

A tensile specimen held at 1300 °F ± 3 °F (704 °C ± 2 °C) under a continuously applied load producing an initial axial stress of 72.5 ksi (500 MPa) shall not rupture in less than 23 hours. The test continues to rupture without change of load, and elongation after rupture measured at room temperature shall be not less than 5 % in 4D.

Testing follows ASTM E139. The specification permits two alternative loading approaches, both of which must still meet the 23-hour and 5 % elongation requirements.

• Higher constant load: a load above that producing 72.5 ksi may be used, but the load shall not change while the test is in progress
• Incremental loading: the 72.5 ksi load runs to rupture or 23 hours, whichever comes first, then stress increases in 5,000 psi (34.5 MPa) increments at intervals of 8 hours minimum

Revision G added a provision for small sizes. Where the finished tube size precludes obtaining a suitable test specimen, the stress-rupture sample may be taken from the same tubing stock earlier in the process, and the test report must note that specimens were extracted prior to finished dimensions.