Fasteners are made from a broad range of materials, ranging from common steel to plastic, titanium, or other exotic materials. Several materials are further subcategorized in order to differentiate between different grades which can help to describe specific hardening processes, alloy mixtures, etc. In addition to this, some materials are offered in a wide selection of platings or coatings which are used to improve its corrosion resistance or used to change the appearance of the fastener itself. Fastener material, grade, and coating may be critical when selecting a aircraft fastener due to variations between materials in strength, corrosion resistance, brittleness, galvanic corrosion properties, and cost.
Generally speaking, whenever replacing old fasteners, it is encouraged to best match whatever fastener you are replacing. It is not always safe to replace a bolt with a stronger bolt fastener, as harder bolts are often more brittle, causing it to potentially fail in specific applications. Some equipment is specifically designed for bolts to fail before critical or more expensive items are damaged.
“Steels alloyed with molybdenum, nickel, and chromium (AISI 4037, 4130, 8630) are best if strength is required. These steels also have good cold-forming properties when annealed and can be heat-treated for the best combination of strength, toughness, and shock resistance,”
suggests David Zimmermann from Machine Design. Regular steel fasteners are more magnetic than most stainless steel fasteners, though some grades of stainless steel fasteners may be slightly magnetic.
“Fine-grain, fully killed (i.e., completely deoxidized) basic steel with no alloying agent. Low-carbon steels have from 0.06 to 0.22% carbon content (AISI 1008, 1010, 1018, and 1022) and exhibit good ductility for cold-forming. Medium-carbon steels contain 0.30 to 0.50% carbon content (AISI 1038, 1045) and are stronger but less ductile,”
“These steels respond well to quench and temper. High-carbon steels contain 0.50% or more carbon (AISI 1066, 1095). They are difficult to cold-form unless annealed. They do, however, have high strength and can be heat-treated.”
When determining what grade a bolt is, it is best to reference the head of the fastener bolt where it is typically marked for its grade. Many bolts are also marked with a manufacturer’s mark in addition to the grade marking.