The bolt’s job is to clamp two or more joint members together, and that the joint designer’s job is to select bolts strong enough to do this. The strength of these bolts will depend upon their size, geometry, and on the strength of the materials of which they are made.
The selection of materials for bolts and nuts is based on maximum strength, high ductility, and high notch strength. Other choices are based on the environmental aspects of bolt and nut applications. Fastener materials are initially selected for engineering products. However, the fabrication and performance requirements of the fastener govern the ultimate choice. One objective of fastener design is to achieve a threaded product with properties that match those of the base material.
Material selection is further based on capability of being fabricated and the material’s response to heat treating and cold work strengthening mechanisms.
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.
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. Read More
Medium Carbon Steel
It contains 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
These are 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 (1034MPa). SAE Grade5 (Metric class 10.9, 12.9) fasteners are generally manufactured out of alloy steels with AlSI grades 4137, 4140, 4340 & 5140 falling into this category.
Stainless Steel
Stainless Steels that 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
Having 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
It 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
It 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
It 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 & 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).
Steel Fasteners for The Temperature Range Between –50°C and +150°C
| Strength class | Material and heat treatment | Chemical composition (molten mass analysis %)a | Tempering temperature | ||||
| C | P | S | Bb | °C | |||
| min. | max. | max. | max. | max. | min. | ||
| 4.6c, d | Carbon steel or carbon steel with additives | | 0.55 | 0.050 | 0.060 | not stipulated | |
| 4.8d | |||||||
| 5.6c | 0.13 | 0.55 | 0.050 | 0.060 | |||
| 5.8d | | 0.55 | 0.050 | 0.060 | |||
| 6.8d | 0.15 | 0.55 | 0.050 | 0.060 | |||
| 8.8f | Carbon steel with additives (e.g. B or Mn or Cr), hardened and tempered or | 0.15e | 0.40 | 0.025 | 0.025 | 0.003 | 425 |
| Carbon steel, hardened and tempered or | 0.25 | 0.55 | 0.025 | 0.025 | |||
| Alloy steel, hardened and temperedg | 0.20 | 0.55 | 0.025 | 0.025 | |||
| 9.8f | Carbon steel with additives (e.g. B or Mn or Cr), hardened and tempered or | 0.15e | 0.40 | 0.025 | 0.025 | 0.003 | 425 |
| Carbon steel, hardened and tempered or | 0.25 | 0.55 | 0.025 | 0.025 | |||
| Alloy steel, hardened and temperedg | 0.20 | 0.55 | 0.025 | 0.025 | |||
| 10.9f | Carbon steel with additives (e.g. B or Mn or Cr), hardened and tempered or | 0.20e | 0.55 | 0.025 | 0.025 | 0.003 | 425 |
| Carbon steel, hardened and tempered or | 0.25 | 0.55 | 0.025 | 0.025 | |||
| Alloy steel, hardened and temperedg | 0.20 | 0.55 | 0.025 | 0.025 | |||
| 12.9f, h, i | Alloy steel, hardened and temperedg | 0.30 | 0.50 | 0.025 | 0.025 | 0.003 | 425 |
| 12.9f, h, i | Carbon steel with additives (e.g. B or Mn or Cr or molybdenum), hardened and tempered | 0.28 | 0.50 | 0.025 | 0.025 | 0.003 | 380 |
| a In case of arbitration, the product analysis applies. b The boron content may reach 0.005%, provided that the non-effective boron is controlled by additions of titanium and/or aluminium. c In case of cold-formed screws in strength classes 4.6 and 5.6 heat treatment of the wire used for cold forming or the cold formed screw may be necessary to achieve the required ductility. d Free-cutting steel with the following max. sulphur, phosphorous and lead shares is permissible for these strength classes: sulphur 0.34%; phosphorous 0.11%; lead 0.35%. e A manganese content of not less than 0.6% for strength class 8.8 and 0.7% for strength classes 9.8 and 10.9 must be present in simple carbon steel with boron as an additive and a carbon content under 0.25% (molten mass analysis). f Materials in these strength classes must be sufficiently hardenable to ensure that there is a martensite share of roughly 90% in the hardened state before tempering in the microstructure of the core in the threaded part. g Alloy steel must contain at least one of the following alloying components in the given minimum amount: chromium 0.30%, nickel 0.30%, molybdenum 0.20%, vanadium 0.10%. If two, three or four elements are ascertained in combinations and have smaller alloy shares than those given above, the threshold value to be applied for the classiscation is 70% of the sum of the individual threshold values given above for the two, three or four elements concerned. h In case of strength class 12.9/12.9 a metallographically detectable white layer enriched with phosphorous is not permissible. This must be verifed with a suitable test procedure. i Caution is necessary when strength class 12.9/12.9 is used. The suitability of the screw manufacturer, the assembly and the operating conditions must be taken into account. Special environmental conditions may lead to stress corrosion cracking of both uncoated and coated screws. |
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