High Performance Fastener Alloys for Critical Applications
When an application calls for fasteners that can withstand atmospheric or chemical corrosion, elevated temperatures or a combination of both, fastener makers find themselves dealing with specifications and availability issues that aren’t a factor with the more common carbon steels. And to make matters worse, the fasteners are often the most critical parts to the final application. Their properties must often exceed those of the materials being joined because the joint areas may be the most vulnerable to failure.
Bolts, nuts, washers, rivets and other fasteners for jet engines, gas turbines, superchargers, afterburners, petrochemical processing parts and other critical aerospace equipment are made from special alloys that can withstand demanding conditions of heat, stress and corrosive environments. Applications like these are becoming more common as designers push the envelope for better performance and greater durability under very demanding conditions.
Fortunately, there are a number of alloys in the commercial mainstream that have proven themselves in these critical applications. At Ulbrich, buyers can obtain just-in-time deliveries when required and select from generous inventories covering more than 165 grades of stainless steel, nickel alloys, cobalt alloys and titanium and titanium alloys in a range of thicknesses, shapes, sizes and tempers. The following alloys are in stock at Ulbrich service centers serving the aerospace market. They include foil and strip in a variety of gauges, as well as wire, round bar, centerless ground bar, hex bar, rod, sheet and plate from other Ulbrich divisions.
Precipitation Hardening Alloys
Most of the fastener alloys in demand today are of the precipitation hardening variety. What makes them different from other grades of stainless and nickel-based alloys is the addition of small amounts of copper, aluminum, phosphorous or titanium to their matrix.
With these alloys, cold forming is done in the relatively soft solution annealed condition. After fabrication, the parts are given an age-hardening treatment in which the added elements precipitate as hard intermetallic compounds that significantly increase hardness and strength.
Despite their more complex metallurgy, PH alloys are not necessarily more costly than many non-age-hardenable alloys. In fact, performance may be substantially higher in PH alloys without a cost penalty. Although corrosion resistance is decreased (or possibly increased) during the aging cycle, it is only by a slight amount.
Because they are very similar, these alloys can often be used interchangeably depending upon their availability in the sizes, tempers and shapes required. In the case of strip, adjustments in metallurgy can be made through rerolling and heat treatment to arrive at the desired solution annealed condition.
PH Stainless Steel
A-286® Iron Base Alloy
The widest used fastener grade steel among the PH alloys is A-286. This heat and corrosion resistant alloy is found in applications requiring high strength and corrosion resistance up to 1300°F, and for lower stress applications at higher temperatures. It also performs well in extremely low temperatures where a ductile, non-magnetic high strength material must withstand temperatures down to -320°F.
This alloy has a chromium, nickel and molybdenum content similar to many of the austenitic stainless steels, so its aqueous corrosion resistance is very comparable to those alloys. In the elevated temperature service up to 1300°F, its corrosion resistance is excellent. A-286 is available from Ulbrich in the form of strip, sheet, wire and both solution-annealed and aged bars.
Formed A-286 alloy parts attain their high strength in a three-step process that includes solution anneal at 1800°F, followed by forming or fabricating to shape, and then an aging heat treatment. The solution anneal provides superior creep and stress rupture properties, better ductility and higher hardness after aging.
In the solution treated condition, A-286 alloy can be cold formed much like standard austenitic stainless steels. It is somewhat stronger than these alloys, however, and may require higher loads to deform the material. During cold working, solution treated A-286 sheet has a work hardening rate about the same as that of Type 310 stainless steel.
The normal aging cycle consists of heating to 1300/1400°F, holding 12 to 16 hours at heat, followed by air cooling. For an even higher aged strength, solution treated material may be cold worked prior to the aging treatment.
Another precipitation-hardening stainless steel, 17-7PH provides high strength and hardness, excellent fatigue properties, good corrosion resistance and has minimum distortion from heat treatment. Ulbrich stocks 17-7PH in the forms of strip, wire and bar.
The exceptionally high strength of 17-7PH offers many advantages where limited ductility and workability are permissible. In the age-hardened condition, it possesses excellent mechanical properties at temperatures up to 900°F and its corrosion resistance is superior to that of most hardenable chromium alloys, approximating that if the austenitic chromium-nickel stainless steels in same environments. Fabrication practices for 17-7PH stainless steel are similar to those used for other chromium-nickel stainless steels.
There are three levels of age hardening, each providing a slightly different mix of properties. In one, the alloy is solution annealed at 1950°F for forming and fabricating, then given a three-part aging process 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, held for 10 minutes; cooled to -100°F, held for eight hours; heated to 950°F, held for 60 minutes and air cooled.
The third level offers the highest mechanical properties. In this treatment, the solution annealed material is transformed to martensite by cold reduction, heated to 900°F, held for 60 minutes, then air cooled.
PH Special Metals
This nickel chromium alloy exhibits high strength and excellent corrosion resistance up to 1300°F with slightly diminished properties as temperatures rise. The strong point of Inconel 718 is in its weldability, especially its resistance to post-weld cracking, in any comparison among PH alloys.
Strength can be maximized by work hardening, and the alloy may be used in the cold worked and aged condition in certain instances. However, aging response is considered minimal when compared to typical age-hardening alloys reviewed earlier. A 1900°F - 1950°F anneal followed by aging is preferred to achieve the best transverse ductility in heavy sections, impact strength, and low-temperature notch tensile strength. For this condition, aging is at 1400°F/10 hours, lowering to 1200°F and holding at that temperature for total aging time of 20 hours. Inconel 718 is available in strip, sheet, wire and bar.
Waspaloy® is a precipitation hardening, nickel-based alloy that successfully resists intergranular oxidation from combustion products in gas turbines and aircraft jet engines at temperatures up to 1600°F. This alloy is available in strip, wire, sheet and bar at Ulbrich.
The best stress rupture and creep properties in this alloy are obtained through very specific high temperatures solution anneals. A typical anneal is 1975°F will result in coarse grain size, low tensile yield strength and low tensile ductility. However, if the alloy is treated below 1825°F, the as-hot-worked grain size will be retained and high tensile yield and tensile ductility will result, but with some loss in stress-rupture properties.
A practical compromise for adequate rupture properties, acceptable tensile properties and moderate grain growth is to solution anneal just above 1875/1900°F.
The normal aging treatment for Waspaloy® is to stabilize at 1550°F for four hours, air cool, hold at 1400°F for 16 hours, then air cool. However, if solution treating is done at 1975°F, the 1550°F stabilization aging time is increased to 24 hours.
This cobalt, nickel, chromium alloy has excellent resistance to hot corrosive atmospheres typical of jet engine operation, qualifying L-605 for the most critical types of fasteners.
It resists oxidation for intermittent service up to 1600°F, continuous service up to 2000°F and shows particularly good qualities under extreme oxidizing conditions. It also successfully resists chemical oxidizing agents, hydrochloric and nitric acids at certain concentrations and temperatures, and salt spray corrosion.
This non-magnetic alloy also possesses high strength properties at elevated temperatures and remains non-magnetic in the work-hardened condition. L-605 alloy can be supplied in the easier-to-form solution-annealed or cold worked condition depending on section size and application. Strength can be maximized by work hardening, and the alloy may be used in the cold worked and aged condition in certain instances. However, aging response is considered minimal when compared to typical age-hardening alloys.
Another popular material for high performance fasteners is titanium bar, particularly Grade 6AL-4V. The most widely used of the titanium alloys, 6AL-4V has a minimum room-temperature tensile strength of 130,000 psi, yield at 120,000 psi, and it retains 70% of its yield strength at 750°F.
It is fully heat treatable in section sizes up to one inch. This process increases both hardness and strength by about 20%. After aging, typical yield strength of a one-inch bar is 150,000 psi, and hardness is 360 Bhn.
Less Popular Alloys
What if the fastener specification calls for a less popular or more exotic alloy? Or you need it in a gauge or temper that your suppliers don’t carry? There are two solutions, both very doable.
First ask your buyers to be more flexible in their specifications. Usually there are any number of alloys that might be considered for the job. With more choices you increase your chances of finding the alloy you need. The precipitation hardening alloys mentioned above can often be used interchangeably, depending on availability. The exception to this rule are fasteners that must combat specific conditions, such as ion attack or stress corrosion cracking. Then the specifications should be followed to the letter.
But even if the first option doesn’t work, you have another alternative. If your starting material is strip, then you can use a specialty outsource called a reroller. These operations are a combination of service center and production shop. They use powerful cold rolling mills to reduce strip to the desired thickness and at the same time work harden it to the temper required.
Rerollers do more than roll metals to precise thicknesses and temper. With their combination of rolling mills and bright anneal furnaces, they can adjust properties such as yield strength, hardness and elongation to meet a particular specification or requirement.
As you might expect, exercising this type of control requires a knowledgeable staff of metallurgists. If your company is regularly in the market for high performance alloys, the most valuable part of dealing with a development partner and reroll source such as Ulbrich may be the technical assistance you can get from these experts. They can assist your engineers in writing the specification. Factors such as tensile or yield strength, elongation, corrosion resistance, hot tensile properties, grain size, packaging requirements, coil sizes and many others can be addressed in this process.
Rerollers exist throughout the metal industry. But, in the stainless, nickel and cobalt alloy market, they are especially important because these higher priced metals are expected to meet more stringent specifications. Minute changes in metallurgy or surface finish can be key factors in the success of many fastener applications.
Whether your needs can be met “off-the-shelf” or with the help of a secondary re-roller like Ulbrich, fastener industry engineers can be assured the answer to their specification can be found readily and within reasonable cost parameters from service centers specializing in these alloys.
A Reliable Source for Fastener Alloys
Fastener applications involving corrosive conditions and high temperatures require material specifications that can involve stainless, nickel, cobalt and chromium alloy steels. Many of the precipitation hardening alloys can be used interchangeably.
As a major supplier to aircraft and aerospace parts manufacturers, Ulbrich has accumulated a great deal of experience in processing these alloys to meet the industry’s high standards. We also provide a number of special products and services for fastener producers. These include:
- Bare, copper coated or soap coated rod, bar and wire for cold heading operations
- Centerless ground bar up to ½ inch diameter
- Annealed, percent cold reduced, or other special tempers or precipitation hardening alloys
- Hex shapes in many alloys
Because of the important jobs they do in maintaining the structural integrity of assembled parts, fasteners are among the more critical applications for alloys supplied by Ulbrich Stainless Steels & Special Metals, Inc. To meet the demands of this market, our manufacturing operations and service centers follow strict quality control guidelines. We carry large inventories of these and other high-performance alloys, and process them to precise gauges or diameters, cut them to size, and package them to accommodate your needs. Contact us today to discuss your next project!