2018 ARP Catalog rev4
23 800-826-3045 FASTENER TECH GLOSSARY OF TECH 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 centered cubic.” Austenitic steels can not be heat treated, but can be strengthened by cold working. 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: The tensile load a fastener generates when torque is applied. Also referred to as preload. Torque Cycle or Pull: A torque cycle or pull is described as one tight- ening and one loosening (ON/OFF) process of a fastener and is the same as one installation and one removal of a fastener. 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 ARP300 and A286 are far superior. Hydrogen Embrittlement: This condition results from the accumula- tion 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. ARP employs a baking process that purges all hydrogen gas from the steel. 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 centered 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. 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. OAL: Means “Over All Length.” Clamp Load Scatter: A term used to describe the variation in clamp load a fastener generates due to fluctuating levels of friction from one torque cycle to subsequent cycles. Clamp load scatter generally reduces after multiple cycles because the friction levels out and becomes more consistent. Qualified Products List: A government requirement that simply man- dates that bolts be manufactured only by companies which have qualified by making bolts that have been submitted for testing and approval to a government agency. ARP has qualified for this list. Quench & Temper: A method of heat-treating martensitic steels. The parts are heated into the austenitic range (usually above 1450˚F) 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 350˚F and 1200˚F), depending upon the steel and strength desired. Reciprocating Load: The acceleration force exerted on a connecting rod due to the up and down motion of the piston and its 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. 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. This is typically the highest grade available, and is the only steel used by ARP. 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. Ultimate Tensile Strength: The maximum stress that a particular material can support without breaking. It is expressed in terms of lbs. per square inch, and is measured by means of a tensile test. The maximum force (lbs.) that a test specimen can support is divided by the cross-sec- tional area (square inches) of the specimen, the result is ultimate tensile strength in psi. 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). UHL: Means “Under Head Length.” The distance as measured from tip of the fastener to a place directly at the base of the head. Yield Strength: The stress at which a given material or component exhibits a permanent deformation (i.e. “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, the fastener is ruined and must be replaced.
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