34 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 tightening 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 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. 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 FASTENER TECH
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