High-Performance Fastener Alloys for Critical Applications

When applications require fasteners to withstand atmospheric or chemical corrosion, high temperatures, or both, their performance must often exceed that of the materials being joined, since the joined area may be most susceptible to failure.

Bolts, nuts, washers, rivets, and other fasteners used in jet engines, gas turbines, superchargers, afterburners, petrochemical processing components, and other critical aerospace equipment are made from special alloys that can withstand demanding heat, stress, and corrosive environments. Such applications are becoming more common as designers push the limits to achieve better performance and increased durability under very demanding conditions.

Precipitation Hardening Alloys:

Most fastener alloys in demand today are precipitation hardening alloys. They differ from other grades of stainless steel and nickel-based alloys by the addition of small amounts of copper, aluminum, phosphorus, or titanium to their matrix.

For these alloys, cold forming is accomplished in a relatively soft solution annealed condition. After fabrication, the parts are age hardened, where the added elements precipitate as hard intermetallic compounds that significantly increase hardness and strength.

Despite the more complex metallurgy, PH alloys are not necessarily more expensive than many non-age-hardenable alloys. In fact, the performance of PH alloys can be much higher without the added cost. Although corrosion resistance decreases (or may increase) during aging cycles, the decrease is minimal.

Because they are so similar, these alloys are often used interchangeably, depending on their availability in the required size, temper, and shape. For strip, metallurgical adjustments can be made by rerolling and heat treating to achieve the desired solution annealed condition.

PH Stainless Steels:

A-286® Iron-Based Alloys

The most widely used fastener grade of the PH alloys is A-286. This heat- and corrosion-resistant alloy is used in applications requiring high strength and corrosion resistance up to 1300°F, and for applications with lower stresses at higher temperatures. It also performs well at very low temperatures, where a ductile, non-magnetic, high-strength material must withstand temperatures as low as -320°F.

The chromium, nickel, and molybdenum contents of this alloy are similar to those of many austenitic stainless steels, so its resistance to aqueous corrosion is very similar to those alloys. Its corrosion resistance is excellent in high temperature service up to 1300°F.

Heat Treatment

Formed A-286 alloy parts achieve high strength through a three-step process that includes solution annealing at 1800°F, followed by forming or fabrication into shape, and then aging heat treatment. Solution annealing provides excellent creep and stress rupture properties, improved ductility, and increased hardness after aging.

Formability

In the solution treated condition, A-286 alloy can be cold formed like the standard austenitic stainless steels. However, it is slightly stronger than these alloys, and higher loads may be required to deform the material. During cold working, solution treated A-286 plate will work harden at about the same rate as Type 310 stainless steel.

Aging

The normal aging cycle consists of heating to 1300/1400°F for 12 to 16 hours, followed by air cooling. To achieve higher aged strength, solution treated material may be cold worked prior to aging.

17-7 PH Another precipitation hardening stainless steel, 17-7PH offers high strength and hardness, excellent fatigue properties, good corrosion resistance, and minimal heat treatment distortion. Ulbrich stocks 17-7PH in strip, wire, and bar stock.

The exceptionally high strength of 17-7PH provides many advantages while allowing for limited ductility and machinability. In the age-hardened condition, it has excellent mechanical properties up to 900°F, and its corrosion resistance is superior to most hardenable chromium alloys and approaches that of the austenitic chromium-nickel stainless steels in the same environment. Fabrication practices for 17-7PH stainless steel are similar to those for the other chromium-nickel stainless steels.

Heat Treatment/Aging

Age hardening is divided into three levels, each providing a slightly different combination of properties. In it, the alloy is solution annealed at 1950°F for forming and fabrication, followed by a three-part aging treatment consisting of austenite conditioning at 1400°F, holding for 90 minutes, and air cooling.

To create an alloy with higher room temperature and elevated temperature strength, the 1950°F solution annealed material is heated to 1750°F and held for 10 minutes; cooled to -100°F and held for eight hours; heated to 950°F, held for 60 minutes and air cooled.

Level 3 provides the highest mechanical properties. In this treatment, the solution annealed material is transformed to martensite by cold rolling, heated to 900°F, held for 60 minutes, and air cooled.

PH Special Metals:

Inconel 718 This nickel-chromium alloy exhibits high strength and excellent corrosion resistance at temperatures up to 1300°F, but properties decrease slightly as temperature increases. Inconel 718's strengths are its weldability, especially its resistance to post-weld cracking, compared to the PH alloys.

Strength can be maximized by work hardening, and in some cases the alloy can be used in the cold worked and aged condition. However, the aging reaction is considered minimal compared to the typical age-hardenable alloys reviewed previously. Aging after annealing at 1900°F - 1950°F is preferred to obtain the best transverse ductility, impact strength and low temperature notched tensile strength in thick sections. For this case, aging is at 1400°F/10 hours, ramping down to 1200°F and holding at that temperature for a total aging time of 20 hours.