Fasteners are manufactured from a variety of materials such as steel, stainless steel, aluminum, brass, bronze, copper, nickel, titanium and other non-ferrous metals, plastics and exotic materials. Material selection should be based on considerations such as – strength required, stresses, corrosive environment, weight, magnetic properties, electrical conductivity, coatings / platings required, reusability and expected life.
Over 90% of fasteners are made from steel because of its inherent strength properties, excellent workability and relative cheapness as compared to other materials. Steel that is used to manufacture fasteners falls into 3 types – low carbon, medium carbon and alloy steel.
LOW CARBON STEEL generally contains less than 0.25% carbon. it has outstanding ductility and toughness, is easily machined and welded and is relatively inexpensive to produce. lt has a tensile strength of between 60,000 psi to 80,000 psi ( 410MPa to 550MPa). SAE Grade 2 (Metric class 4.6, 4.8 & 5.8) fasteners are generally made out of low carbon steel with AISI grades 1006, 1010, 1016, 1018, 1022 & 1035 falling into this category.
MEDIUM CARBON STEEL has a carbon content of between 0.25% to 0.60% It is easily heat treated and has a tensile strength of between 100,000 psi to 120,000 psi. (690MPa to 830MPa). SAE Grade 5 (Metric class 8.8) fasteners are generally made out of medium carbon steel with AISI grades 1038, 1541, 5132 & 5135 falling into this category.
ALLOY STEEL is carbon steel that has additives e.g. boron, manganese, chromium, silicon etc. Additions of these elements improve the capacity of the alloys to be heat-treated, giving rise to a wide variety of strength to ductility combinations. Alloy steels have a tensile strength in excess of 150,000 psi (1 034MPa). SAE Grade5 (Metric class 1 0.9, 12.9) fasteners are generally manufactured out of alloy steels with Al SI grades 4137, 4140, 4340 & 5140 falling into this category.
STAINLESS STEELS are alloy steels that have a minimum of 10.5 % chromium content. The presence of chromium creates an invisible surface film (chromium oxide) that resists oxidation and makes the metal « passive » or corrosion resistant. If the surface layer is damaged, it rebuilds itself (self-repairs) in the presence of oxygen. it is very important to understand this self-healing process because stainless steel when used in a low-oxygen or oxygen-free surrounding, is susceptible to aggressive influences, if the protective surface layer gets damaged. Stainless Steels are divided into 3 classes – Austenitic, Martensitic and Ferritic.
AUSTENITIC STAINLESS STEEL have a chromium content of between 15% to 20% and nickel of between 5% to 19% and offers a higher degree of corrosion resistance than the other two types of stainless steels. The tensile strength of austenitic stainless steel varies between 72,000 psi to 115,000 psi (500 MPa to 800 MPa). 18-8 stainless steel is a type of austenitic stainless steel that contains approximately 18% chromium and 8% nickel. This category includes AISI grades 302, 303,304, 304L, 316, 32, 347 & 348.
MARTENSITIC STAINLESS STEEL contain between 12% to 18% chromium, can be hardened by heat treatment, have poor welding characteristics and are considered magnetic. The tensile strength of Martensitic stainless steel varies between 72,000 psi & 160,000 psi ( 500 MPa & 1100 MPa). This type of stainless steel should only be used in mild corrosive environments. Martensitic stainless steels include AIS\ grades 410, 416, 420 & 431.
FERRITIC STAINLESS STEEL contain between 15% to 18% chromium and is a non-heat-treatable, magnetic and has very poor weld characteristics. The tensile strength of ferritic stainless steel is about 65,000psi to 87,000 (450MPa to 600 MPa). It should not be used in situations where the likelihood of corrosion is high and includes grades 430 & 430F.
ALUMINUM is a lightweight metal that has a high strength to weight ratio, good corrosion resistance in most environments, excellent electrical and thermal conductivity, is easily cold formed or hot forged and easily machinable. This is why aluminum is the most popular choice for fasteners amongst non-ferrous metals. The tensile strength of aluminum is between 13,000psi (90MPa) for pure aluminum up to 60,000psi (414MPa). Aluminum fasteners are usually manufactured from AIS\ grades 2024, 6061, 6262 & 7075.
COPPER is used in fasteners that require a high degree of electrical & thermal conductivity and corrosion resistance. lt is non-magnetic and cannot be hardened ether than by cold working. The tensile strength of pure copper is about 30,000 psi (207MPa).
BRASS which is a composition of copper and zinc, is the most common copper based alloy. It is highly corrosion resistant and electrically conductive, however, its use as a fastener is somewhat limited due to its relative softness. Used primarily for its appearance.
SILICON BRONZE often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Manganese or aluminum is added for strength. Lead is added to make it free machining. Depending on its composition, silicon bronze possesses high tensile strength and is used primarily in marine applications for its high corrosion resistance, non-magnetic properties, rich appearance and ability to resist the corrosive action of salt water.
NICKEL & nickel base alloys have excellent strength properties, exceptional toughness and ductility and perform well in both high and low temperature extremes. Fasteners made from nickel-copper alloys have a tensile strength of 80,000 psi ( 550 MPa) whilst those made from nickel-copper-aluminum alloys have a tensile strength of 1130,000 psi (896MPa). Nickel and high nickel alloy fasteners offer excellent performance and oxidation resistance at high temperatures but their use is restricted by the high cost.
TITANIUM as compared to aluminum has superior strength to weight ratios, excellent corrosion resistance, good high temperature performance, and is therefore extensively used in the aerospace industry. Titanium is highly corrosion resistant to chemical agents and aggressive oxidizing substances used in the chemical industry. Fasteners manufactured from titanium can have tensile strengths in excess of 150,000psi (1034 Mpa).
The following is a brief on some of the common special proprietary materials that are used for the manufacture of fasteners for special applications. These materials are often referred to as super alloys because of their ability to perform in extremes of temperature or posess extremely high strength qualities as compared to standard fastener materials. This list is intended as a guide only and it is recommended that further technical advice be sought from manufacturers for individual requirements.
MONEL 400~(UNS N04400) is a nickel-copper alloy that is hardened by cold working only. Monel has low corrosion rate in flowing seawater, therefore it is widely used in marine applications. lt has excellent resistance to stress corrosion cracking in most freshwaters and can be used in temperatures up to 1000 °F. The alloy has great mechanical properties at subzero temperatures.
MONEL K500 (UNS N05500) is a nickel copper alloy with the same corrosion resistance and characteristics as the Monel 400. lt has a greater strength and hardness then the 400, as a result of added aluminum and titanium.
INCONEL 600 (UNS N06600) is a standard engineering material and has a great resistance to heat and corrosion. It’s also has high strength and can be easily formed and can be hardened and strengthened only by cold work.
INCONEL 718 (UNS N07718l is a precipitation hardenable nickel-based alloy designed to display exceptionally high yield, tensile and creep-rupture properties at temperatures up to 1300″F (704″C). The age-hardening response of Alloy 718 permits annealing and welding without spontaneous hardening. It has excellent weldability and is often used for high tensile fasteners subject to elevated temperatures.
INCONEL X-750 (UNS N07750) is a nickel-chromium alloy used for its corrosion and oxidation resistance and high strength at temperatures to 1300°F. Because it is available in all standard mill forms, it is used for a wide variety of special fastener applications.
INCOLOY 825 (UNS N08825) is a nickel-iron- chromium alloy with titanium,copper, and molybdenum. It’s chemical composition provides great resistance to many corrosive environments, such as pitting, crevice corrosion, intergranular corrosion, and stress- corrosion cracking. lncoloy 825 has good mechanical properties from moderate to high temperatures. The hot- working range is 1600 to 2150° F and the material has good weldability by all conventional processes.
HASTELLOY B-2 (UNS N1 0665) is a nickel-base wrought alloy with excellent resistance to hydrochloric acid at all concentrations and temperatures. Therefore, it has great resistance to stress-corrosion cracking and pitting. lt is used for most chemical process applications in the welded condition.
HASTELLOY » C Series alloys are versatile nickel-chromium-molybdenum alloys with excellent resistance to both oxidizing and reducing acids and chloride solutions over a wide range of temperatures. lt can be heat treated to obtain high strength properties and more importantly, it’s corrosion resistance and ductility remain excellent when in the high strength condition.