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Bolt Materials and Terms

Aircraft quality fasteners, Inconel fasteners, yield and tensile strengths - what do they all mean?

Courtesy of ARP

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This article was first published in 2008.

Selecting the optimum material for use in a given fastener application is of paramount importance. That's why high performance fastener manufacturers use a wide assortment of materials...ranging from popular stainless steel and 8740 chrome moly to exotic alloys that have been developed to handle space travel. You should also know that there are grades within specific alloys. For example, 8740 is available in four grades:

  1. SDF (guaranteed seamless and defect free)
  2. CHQ (cold head quality)
  3. Aircraft
  4. Commercial

Top quality automotive bolts are made from only the first two (SDF and CHQ), even though they cost more than double "Aircraft" quality.

Materials

And what about the other materials? Here's a quick guide.

Stainless Steel: Ideally suited for many automotive and marine applications because stainless is tolerant of heat and virtually impervious to rust and corrosion. Tensile strength is typically rated at 170,000 psi.

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8740 Chrome Moly: Until the development of today's modern alloys, chrome moly was popularly considered a high strength material. Now viewed as only moderate strength, 8740 chrome moly is seen as a good tough steel, with adequate fatigue properties for most racing applications, but only if the threads are rolled after heat treatment. Typically, chrome moly is classified as a quench and temper steel, that can be heat treated to deliver tensile strengths between 180,000 and 210,000 psi.

ARP2000: Exclusive to fastener company ARP, this is a hybrid-alloy developed to deliver superior strength and better fatigue properties. While 8740 and ARP2000 share similar characteristics, ARP2000 is capable of achieving clamp loads in the 215,000-220,000 psi range. ARP2000 is used widely in short track and drag racing as an up-grade from 8740 chrome moly in both steel and aluminium rods. Stress corrosion and hydrogen embrittlement are typically not a problem, providing care is taken during installation.

L19: This is a premium steel that is processed to deliver superior strength and fatigue properties. L19 is a very high strength material compared to 8740 and ARP2000 and is capable of delivering clamp loads in the 230,000-260,000 psi range. It is primarily used in short track and drag racing applications where inertia loads exceed the clamping capability of ARP2000. Like most high strength, quench and temper steels, L19 requires special care during manufacturing to avoid hydrogen embrittlement. This material is easily contaminated and subject to stress corrosion. It must be kept well-oiled and not exposed to moisture.

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Aermet 100: With a typical tensile strength of 280,000 psi, Aermet 100 is a new martensitic super-alloy that is stronger and less expensive than the super-alloy austenitic materials that follow. Because it is capable of achieving incredibly high clamping loads, it is ideal for short but extreme environments like top fuel, funny car and some short track applications. Although Aermet 100 is a maraging steel that is far superior to other high strength steels in its resistance to stress corrosion, it must be kept well-oiled and not exposed to moisture.

Inconel 718: A nickel-based material that is in the high temperature, super-alloy class, it is found to be equally suitable in lower temperature applications. This material delivers tensile strengths into the 220,000 psi range and exhibits improved fatigue properties. Best of all, Inconel 718 is completely immune to hydrogen embrittlement and corrosion.

ARP3.5 (AMS5844): While similar to Inconel 718, these super-alloys are found in many jet engine and aerospace applications where heat and stress attack the life of critical components. The high cobalt content of this alloy, while expensive, delivers a material with superior fatigue characteristics and typically tensile strength in the 270,000 psi range. The immunity to hydrogen embrittlement and corrosion of these materials is a significant design consideration. These materials are primarily used in connecting rods where extremely high loads, high RPM and endurance are important factors-Formula 1, Winston Cup and CART applications.

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Custom Age 625+: This newly formulated super-alloy demonstrates superior fatigue cycle life, tensile strength and toughness, with complete resistance to atmospheric corrosion and oxidation. Best of all it is less expensive and expected to soon replace MP-35 as the material of choice in the high strength, super-alloy field. Typical tensile strength is 260,000 psi.



Quick Reference Guide To Materials Used In Fasteners

Material Use? Yield Strength Tensile Strength Used For
Grade 5 No 90,000 psi 120,000 psi Accessory bolts and studs
Grade 8 No 120,000 psi 150,000 psi Accessory bolts and studs
"Stainless 300" Yes 140,000 psi 170,000 psi Accessory bolts and studs, head studs
ARP Custom 450 Yes 150,000 psi 180,000 psi Head bolts, accessory bolts
8740 chrome moly Yes 160,000 psi 190,000 psi Rod bolts, head and main studs and bolts
A286 Yes 170,000 psi 200,000 psi Head bolts, connecting rod bolts
ARP2000 Yes 180,000 psi 215,000-220,000 psi Connecting rod bolts
L19 Yes 200,000-230,00 psi 230,000-260,000 psi Connecting rod bolts
Inconel 718 Yes 190,000-210,000 psi 220,000-240,000 psi Connecting rod bolts
Custom Age 625+ Yes 235,000-255,000 psi 250,000-280,000 psi Head studs, connecting rod bolts
ARP 3.5 Yes 220,000-250,000 psi 250,000-280,000 psi Connecting rod bolts
Aermet 100 Yes 258,000 psi 300,000 psi Connecting rod bolts

Glossary of Fastener Terms

Austenitic: Refers to the atomic arrangement of some metals, such as nickel-based alloys, and some steels with about 18% chromium. This atomic arrangement is called "face centred cubic." Austenitic steels cannot be heat treated, but can be strengthened by cold working.

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CHQ: A term used to grade heading wire and stands for "cold heading quality." This grade is superior to both Commercial and Aircraft quality.

Clamp Load: This is the force exerted by a tightened bolt and is the same as preload.

Fatigue: The process by which failure is caused after many repetitions of loads smaller than the ultimate strength of the material.

Ferritic: Refers to steels with an atomic arrangement different from austenite and martensite. These steels are not strong and the widest use is in steam power plants and accessory fasteners made by some companies, because they are able to withstand wet environments. Newer steels such as A286 are far superior.

Hydrogen Embrittlement: This condition results from the accumulation of hydrogen gas in the atomic structure of the metal. This gas flows to the point of high stress (stress risers) and causes microscopic cracks. The hydrogen then flows to the "new" crack tip and causes it to crack further. In this way the crack moves across the part, because the crack-tip is the stress riser. Finally the crack gets so large that the section is not large enough to support the load. No hydrogen embrittlement can take place without tensile stress.

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Knurling: A process of creating serrations in a part by rolling a die, under pressure, against the part. Normally these serrations are very sharp and can create cracks and are stress risers. The process is used on knobs so the user can get a firm grip. But in the case of fasteners, the body can be knurled so the part can be forced into and retained in an irregular hole - stress risers and all.

Maraging: Refers to steels that are a low carbon version of martensitic steels, specially alloyed so that the martensite is not hard. These steels can be worked in the quenched condition and then be hardened by low temperature aging. The strength comes from the formation of complex metal carbides.

Martensitic: Refers to atomic arrangement and in the case of steels, is a modified body centred cubic structure. These steels can be heat treated because martensite is iron carbide, which is very hard. However, these steels can be hydrogen embrittled and will rust. Generally, martensite normally refers to metal structures which are formed by quenching from high temperature.

MS21250: A military specification for a 12-point, 180,000 psi bolt which specifies the fatigue load required for testing every size.

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Notch Sensitivity: Refers to the ability of a metal to withstand the increased stress at a notch. Some materials, such as glass, crack very easily if notched. While others, such as soft gold or tin stretch out under stress - even with a notch. Normally, the stronger the steel, the more likely it is to break quickly at the notch. "Toughness" is wanted because this is associated with opposite of notch sensitivity. Austenitic metals are usually less notch sensitive than martensitic steels of the same strength levels.

O.A.L.: Means "Over All Length."

Preload: The force in a bolt when it is installed with a torque greater than simply hand tight. Preload can be established by measuring torque or bolt stretch or by the less than accurate "turn-of-the-nut" method.

Quench & Temper: A method of heat-treating martensitic steels. The parts are heated into the austenitic range (usually above 790 degrees C) then quenched into water or oil. This leaves the part in a very hard martensitic condition which then must be tempered by heating at lower temperatures (between 180 degrees C and 650 degrees C), depending upon the steel and strength desired.

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Reciprocating Load: The acceleration force exerted on a connecting rod due to the up and down motion of the piston and it's associated mass ie, wrist pin, rings, small end of the rod.

Stretch: The increase in length of a bolt when installed with a preload.

Stress: The load applied to a part divided by the cross-sectional area of the part, usually expressed in pounds per square inch (psi).

Stress Corrosion: This is a special form of hydrogen embrittlement in which the metal is attacked while under stress. Without the stress the crack will not move. But under stress the crack moves and corrosion takes place at the freshly opened crack face.

Stress Ratio: The ratio of the minimum stress to the maximum stress in a structure which is subject to fluctuating loads.

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Stress Riser: You have a notch, ding or some change in section size, so now the stress at these points is increased above nominal stress. Compare this kind of stress to the flow of water in a river. When the river hits a narrow point it flows faster. Perhaps there is a rock in the middle - the river flows faster around the rock. The stress at these points can be so high that the part will fail - even though the average stress on the part never exceeded the tensile strength of the part.

S.D.F.: Seam and defect free. A designation for premium steel, typically the highest grade available.

Thread Engagement: This refers to the number of threads engaged in a nut or threaded hole. Full engagement, meaning all the female threads are engaged, is a desirable configuration to maximize fatigue strength.

Torque Angle: A method of tightening a fastener relative to the amount of degrees turned once the underside of the bolt head or nut face contacts the work surface. This procedure is suitable for engine assembly only when the installation has been calibrated in terms of bolt stretch relative to the exact application (the amount of compression of the clamped components is critical).

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U.H.L.: Means "Under Head Length." The distance as measured from tip of the fastener to a place directly at the base of the head.

Ultimate Tensile Strength: The maximum stress that a particular material can support without breaking. It is expressed in terms of pounds per square inch, and is measured by means of a tensile test. The maximum force (pounds) that a test specimen can support is divided by the cross-sectional area (square inches) of the specimen, the result is ultimate tensile strength in psi.

Yield Strength: The stress at which a given material or component exhibits a permanent deformation (ie "takes a set"). When the load that caused the stress is removed, the part will not return to its original dimensions. If you exceed the yield strength of a fastener (tighten it until it feels funny and then back it off a bit) the fastener is ruined and must be replaced.

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