Fasteners come in a wide variety of specifications, but whether you are planning to use a bolt, screw, rivet, peg, or clamp, selecting material suitable for the intended application is an important concern. For example, choosing a fastener made of steel rather than aluminum can greatly affect the quality and duration of the joint it forms. Numerous factors, such as environmental circumstances, presence of corrosive elements, physical stress requirements, and overall structural stability, can similarly influence material effectiveness.
The majority of industrial fasteners are produced from aluminum, brass, certain synthetic non-metals, and various grades of steel and stainless steel. In addition, metals such as titanium are also valuable for building fasteners, although they have very specific applications in a narrow range of fields. Generally, fastener materials are judged on their mechanical properties, potential for post-fabrication treatments, cost-efficiency, and several other secondary criteria.
Steel is the most commonly used material in fastener production, constituting nearly 90 percent of all fasteners manufactured annually. This metal’s popularity stems from its high degree of formability coupled with tensile strength and durability. Compared to other metal stock, steel is also relatively inexpensive to fabricate. It is frequently processed with zinc or chrome plating, but can also be formed without any surface treatments.
Carbon steel is the most common type of steel used in fastener production. Grades 2, 5, and 8 are typically the standard for carbon-steel based screws and bolts, with alloyed carbon steel being a higher-end variation on these metals. Their mechanical strength ranges from approximately 50 ksi (kilo-pound per square inch) up to 300 ksi in a finished product. Material properties for these grades include:
• Grade 2: This is a low carbon category that features the least expensive, but also least durable, types of steel. Grade 2 material is highly workable, and forms the bulk of steel grade fasteners.
• Grade 5: Grade 5 steels are produced from unalloyed medium carbon groups, such as type 1038, and are usually work-hardened to improve their strength. This is the most common grade used in automotive applications.
• Grade 8: These steels are typically medium carbon alloys, such as types 4037 and 4340. They are work-hardened to a high degree, making them stronger and better-suited for mechanically straining applications, like vehicle suspension systems.
• Alloy Steel: This is an alloy formed with high-strength carbon steel that can be thermally treated up to 300 ksi. Alloy steel has low corrosion resistance and typically benefits from additional coating. These steels are extremely strong, but can be rigid and brittle.
Stainless Steel Fasteners
Stainless steel is an alloy that combines the properties of low carbon grades with certain percentages of chromium and nickel. Its chromium component lends stainless steel a high degree of corrosion resistance that does not decrease from deformation or long-term use. However, the low carbon content prevents it from being effectively hardened, making the metal stronger than most grade 2 steels, but weaker than many hardened grade 5 and 8 varieties. The final strength of most stainless steel grades ranges from around 70 to 220 ksi, depending on the ratio of metals in the alloy. Stainless steel fasteners are also less magnetic than their standard steel counterparts. The two main categories of stainless steel fastener materials are:
• Martensitic Stainless Steel: The martensitic group includes strong, durable stainless steels that can be further strengthened through heat treatments. They are more magnetic than other types of steel, but have lower corrosion resistance.
• Austenitic Stainless Steel: The vast majority of stainless steel fasteners are produced with metals from the austenitic family. Their high levels of chromium and nickel provide tough corrosion resistance and the ability to withstand considerable physical strain without fracturing, albeit at a higher cost than the martensitic varieties.
Bronze and Brass Fasteners
The bronze used in fastener production is an alloy primarily consisting of tin and copper. With its high corrosion resistance, bronze is well-suited for aquatic applications, such as shipbuilding or underwater construction.
It shares copper’s reddish color, but is relatively expensive compared to other fastener materials. Brass, an alloy of copper and zinc, is similar to bronze in its anti-corrosive and electric conductivity properties, but it has lower tensile strength and is a relatively soft metal. Part of brass’s appeal as a fabricating material lies in its yellowish golden color.
Nylon is a lightweight synthetic plastic material used for specific fastener applications. It is corrosion resistant, has high electrical and thermal insulating properties, and can be easily dyed to meet aesthetic requirements, such as those necessary for fastener replacement. However, nylon is subject to severe deterioration under elevated temperatures and may become weakened in low temperature conditions. In addition, its comparatively low tensile strength makes it less effective for applications with demanding physical stress requirements.
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