As the fundamental component for mechanical connections, the performance of fasteners is closely linked to the materials selected. Under different operating conditions, the requirements for fasteners—such as strength, corrosion resistance, wear resistance, and lightweight design—vary considerably; thus, it is essential to match the appropriate materials accordingly. From general-purpose applications to extreme environments, fastener materials encompass a wide range of categories, including carbon steel, alloy steel, stainless steel, non-ferrous metals, and special functional alloys. Selecting the right material appropriately is crucial for ensuring connection reliability and extending service life. The following sections will provide a detailed breakdown of the characteristics, classifications, and application scenarios of commonly used fastener materials.
 

1. Carbon Steel/Alloy Steel: The Mainstream Choice for General Applications

Due to their moderate cost and adjustable strength, carbon steel and alloy steel have become widely used materials in the fastener industry, covering a variety of applications ranging from ordinary machinery to heavy-duty equipment. The key difference between them lies in their carbon content and the addition of alloying elements.

Ordinary carbon steel, exemplified by Q235 (commonly known as A3 steel), has a relatively low carbon content, moderate strength, good machinability, and low cost. It is suitable for general-purpose applications with light loads and no special corrosion-resistant requirements—for instance, furniture assembly, ordinary bolts, nuts, and washers used in simple machinery. Medium-carbon steel, such as No. 45 steel, has a moderate carbon content. After undergoing heat treatments like quenching and tempering, its strength and toughness are significantly enhanced, making it appropriate for fasteners in mechanical equipment that bear medium loads—such as bolt connections for gearbox housings or fixing screws for machine tool worktables.

Alloy steels are carbon steels to which alloying elements such as chromium, manganese, molybdenum, and titanium have been added, further enhancing their strength, toughness, and wear resistance. They are commonly used materials for high-strength fasteners. Common grades include 40Cr (chromium steel), 35CrMo (chromium-molybdenum steel), and 20MnTiB (manganese-titanium-boron steel). Among these, 40Cr is suitable for heavy-duty applications with moderate strength requirements, such as automotive chassis bolts; 35CrMo, with its excellent high-temperature strength and fatigue resistance, is often used for engine connecting-rod bolts and high-strength bolts in construction machinery; and 20MnTiB, thanks to its superior hardenability, is widely employed in high-strength bolts for steel structures, making it ideal for large-scale load-bearing equipment such as bridges and tower cranes.

II. Stainless Steel: The Preferred Material for Corrosive Environments

The advantage of stainless steel lies in its excellent corrosion resistance. The passivation film formed on its surface effectively isolates the substrate from contact with air and moisture. Depending on its microstructure, stainless steel can be classified into types such as austenitic and martensitic, making it suitable for applications with varying levels of corrosion resistance and strength requirements.

Austenitic stainless steels, exemplified by grades 304 and 316, are non-magnetic, exhibit excellent rust resistance, and offer superior machinability. Grade 304 stainless steel is suitable for humid environments and mildly corrosive conditions, such as outdoor household appliances, food-processing machinery, and architectural hardware. Grade 316 stainless steel, enriched with molybdenum, boasts enhanced resistance to salt spray and acid-alkali corrosion, making it ideal for applications with high anti-corrosion requirements—such as marine environments, chemical processing equipment, and medical devices. Examples include deck fasteners for ships and fasteners used in chemical pipeline connections.

Martensitic stainless steels such as 410 and 420 possess the characteristic of being heat-treatable for strengthening, while also offering a balance between rust resistance and high strength. They are suitable for mechanical transmission applications that require wear resistance and moderate corrosion protection—for example, stainless steel fasteners used in gearboxes and tool-related fasteners. However, their corrosion resistance is weaker than that of austenitic stainless steels, making them unsuitable for use in environments with severe corrosion.

III. Nonferrous Metals: The Ideal Choice for Specialized Needs

Non-ferrous metals include copper and copper alloys, aluminum alloys, and others. Although their cost is higher than that of carbon steel, they offer unique advantages such as excellent electrical and thermal conductivity, as well as lightweight properties, making them suitable for applications with specific functional requirements.

The commonly used copper and copper alloys include red copper (pure copper) and brass (a copper-zinc alloy). They exhibit excellent electrical and thermal conductivity, strong corrosion resistance, and an attractive appearance. Red copper is particularly suitable for electrical equipment applications involving conductive connections, such as wire terminal fasteners and transformer terminal bolts. Brass, on the other hand, boasts superior machinability and relatively lower costs, making it widely used in bathroom hardware and air-conditioning pipe connections. Additionally, thanks to its non-magnetic properties, brass is well-suited for precision equipment that is sensitive to magnetism.

The core advantage of aluminum alloys is their lightweight nature—its density is only about one-third that of steel—and they also exhibit a certain degree of corrosion resistance. As a result, aluminum alloys are well-suited for applications where weight is a critical factor, such as aerospace components, lightweight automotive bodies, and new-energy equipment. Examples include aluminum bolts used in automobile engine compartments and fasteners for drone structures. However, aluminum alloys have relatively low strength and are not suitable for heavy-load applications. Typically, their wear resistance and corrosion resistance are further enhanced through surface treatments such as anodizing.

IV. Special-Function Alloys: Dedicated Materials for Extreme Operating Conditions

Under extreme operating conditions such as high temperature, high pressure, and strong corrosion, ordinary materials often fail to meet the requirements, necessitating the use of special-function alloys. Although these materials are costly, they can ensure the reliability of fasteners even in harsh environments.

High-temperature alloys, exemplified by Inconel and Hastelloy, can maintain stable strength and corrosion resistance in high-temperature environments above 600°C. They are particularly suitable for fasteners used in high-temperature equipment such as aircraft engines, boilers, and gas turbines. Titanium alloys, on the other hand, combine high strength, lightweight properties, and excellent corrosion resistance. Their strength is comparable to that of steel, yet their weight is only about half as much. Moreover, they exhibit superior resistance to acid, alkali, and seawater corrosion compared to stainless steel. Titanium alloys are primarily employed in high-end applications such as aerospace, medical devices (including implantable fasteners), and deep-sea exploration equipment. However, their relatively high cost currently limits their widespread use in civilian applications.

In summary, the selection of fastener materials must precisely match the operational requirements: For general applications, carbon steel or alloy steel is preferred; in corrosive environments, stainless steel is the better choice; if electrical conductivity and lightweight design are required, non-ferrous metals can be considered; and in extreme operating conditions, fasteners made from special functional alloys are necessary. At the same time, it is crucial to comprehensively weigh factors such as cost budget, processing difficulty, and compatibility with surface treatments in order to fully harness the fasteners’ connection capabilities and ensure the overall operational stability of the equipment.