Bolts serve as foundational mechanical fasteners, and their material composition is rarely simple. The choice of material is directly tied to the intended application and the performance requirements of the final product. A bolt’s ability to resist tension, shear forces, corrosion, and extreme temperatures is determined by its chemical composition and manufacturing process. Understanding these materials is the first step in ensuring a connection is structurally sound and durable.
The Foundation: Carbon and Alloy Steels
The majority of bolts used in construction and manufacturing are made from steel, an alloy of iron and carbon. The amount of carbon introduced is the primary factor dictating the bolt’s strength and ductility. Low-carbon steel bolts, sometimes called mild steel, offer moderate strength and are used for general, non-structural purposes.
Medium-carbon steels are heat-treated through quenching and tempering, forming the basis for stronger bolts like those found in the SAE Grade 5 or Metric Class 8.8 standard. These grades balance tensile strength and ductility, allowing them to withstand significant pulling force while exhibiting some stretch before failure. Alloy steels, which include elements like chromium, manganese, or vanadium, create the highest-strength fasteners, such as SAE Grade 8 and Metric Class 10.9.
The grading system directly communicates the bolt’s mechanical properties. For instance, a Metric Class 8.8 bolt has a nominal tensile strength of 800 megapascals (MPa), while a Class 10.9 bolt is formulated for 1000 MPa. While higher grades offer superior strength, the trade-off is often a reduction in ductility, making them more susceptible to brittle fracture if improperly tightened or subjected to extreme stress.
Specialized Corrosion Resistance: Stainless Steel
When corrosion resistance is required, stainless steel is selected due to the inclusion of chromium. This element reacts with oxygen to form a thin, self-repairing layer of chromium oxide on the surface, which provides passive protection against rust and corrosive agents. Stainless steel bolts are part of the ferrous family.
The most common stainless steel types are the austenitic 300 series, specifically 304 and 316. Type 304 stainless steel contains approximately 18% chromium and 8% nickel, making it suitable for general-purpose applications where moderate corrosion exposure is expected. For more demanding environments, such as marine or chemical processing, Type 316 is preferred.
The superior resistance of Type 316 is due to the addition of molybdenum, typically around 2% to 3%, which enhances its ability to resist pitting and corrosion from chlorides and saltwater. Although both 304 and 316 bolts have comparable mechanical strength, the 316 variant is the choice for harsh environments because of its higher degree of environmental tolerance.
Non-Ferrous and Non-Metallic Options
When steel’s properties are unsuitable due to weight, conductivity, or specific chemical exposure, manufacturers turn to specialty materials. Aluminum bolts are valued for their light weight and are frequently used in aerospace and automotive applications where mass reduction is paramount. However, aluminum is generally lower in strength compared to steel and requires careful design to prevent thread damage.
Copper alloys, such as brass and bronze, are utilized when electrical conductivity or non-sparking properties are required. Brass bolts, an alloy of copper and zinc, are commonly found in electrical hardware and plumbing due to their corrosion resistance. Bronze, typically copper alloyed with tin, offers higher strength and resistance in marine environments.
Non-metallic options, primarily nylon and other plastics, serve niche applications where insulation or chemical inertness is required. These fasteners are lightweight, non-magnetic, and act as electrical insulators, making them ideal for circuit board mounting and low-stress assemblies. While their mechanical strength is significantly lower than their metal counterparts, they offer resistance to many chemicals that would rapidly degrade steel.
Surface Protection: Coatings and Finishes
Most carbon steel bolts require a surface coating to provide a barrier against the environment. Zinc plating is the most common and inexpensive method, applying a thin layer of zinc using an electroplating process. This coating provides sacrificial protection, meaning the zinc corrodes before the underlying steel, but its thinness (typically 5 to 25 micrometers) limits its effectiveness to dry or indoor applications.
A drawback of electroplating is that the chemical processes can introduce hydrogen into the metal structure, leading to hydrogen embrittlement in high-strength steel bolts. For outdoor and structural use, hot-dip galvanizing is applied by submerging the bolt in molten zinc. This creates a much thicker, rougher layer that provides superior, long-term corrosion resistance but necessitates that mating nuts be tapped oversize to accommodate the coating thickness.
In cases where tight tolerances must be maintained, a black oxide finish is a preferred alternative because it is a chemical conversion that barely changes the bolt’s dimensions. Black oxide creates a thin, black magnetite layer on the surface, which offers only mild corrosion resistance but is often sealed with oil for improved performance. Because black oxide is not an electroplating process, it eliminates the risk of hydrogen embrittlement, making it a safer choice for high-strength fasteners like Metric Class 10.9 or higher.