Materials Used to Make SAE Bolts

Steel comes in a variety of grades for bolts. The torque that the bolt can withstand and the tightness of the fastened joint increase with the tensile strength of the steel. The Society of Automotive Engineers developed a marking system that uses raised dashes on the bolt head to signify strength, and these standards for bolt strength grades are determined in accordance with this marking method. Other techniques mark the bolt head with raised numerals to identify the grade of the bolt.

Grade dictates how many raised dashes should be used on the bolt head to indicate the bolt’s strength and torque capabilities. SAE bolts come in two basic categories (sometimes referred to as machine screws). The UNF (fine thread pitch) bolt has more threads per inch than the UNC bolt of the same diameter (coarse thread pitch). SAE sets maximums for nut dimensions. Dimensions for the normal size washer (vs. narrow or wide). Every measurement is in inches.

Grade 2 is represented by a bolt head with no marks at all, according to the SAE marking standard. The minimum tensile strength for a grade 2 bolt manufactured of low-carbon steel is 64,000 pounds per square inch. The amount of force a bolt can endure before breaking is known as its tensile strength.

An SAE grade 5 bolt of tempered medium carbon steel with a tensile strength of at least 105,000 pounds per square inch is identified by a bolt head with three raised dashes in a radial pattern. The grade 8 bolt, identified by six elevated dashes, is the strongest commercial-quality bolt; it is made of medium-carbon alloy steel that has undergone quenching and tempering to increase its tensile strength to 150,000 psi. Bolt shear strength is the amount of tension a bolt can withstand before it shears or breaks under pressure.

The stronger the bolt, the higher the ISO numbers. An SAE Grade 2 bolt and an ISO class 6.8 bolt are essentially equivalent in strength. SAE Grade 5 bolts are generally equivalent to bolts of ISO class 8.8 and class 9.8, which are significantly stronger. In general, an SAE Grade 8 bolt and an ISO Class 10.9 bolt are equivalent.

Among the materials used to make fasteners are alloy steel, aluminum, brass, silicon bronze, and stainless steel. Each fastener’s strength and ideal application are determined by the interaction of its material, treatment, hardening, and coating (also known as plating).

Medium-carbon steels are typically used to create SAE bolts. Medium-carbon steels are comparable to low-carbon steels, with the exception that they contain manganese between 0.60 and 1.65 percent and carbon between 0.30 and 0.60 percent. Medium-carbon steels can be employed in the quenched and tempered state by increasing the carbon percentage to roughly 0.5 percent while also increasing the manganese concentration. For SAE bolts, the tensile strengths range from 35,000 psi for carbon steel with 30% manganese to 75,000 psi for grades with high manganese content.

Bolt grade can be quickly identified by looking at the marks on the bolt’s head. The most valuable bolts are those with clean heads and little markings. Some bolts may have additional markings that identify the maker. The most popular zinc-plated alloy steel grades are Grade A and Grade 2 steel with case hardening that is low or medium carbon. No head markings (manufacturers mark may be included).

In addition to copper, silicon, and other alloys like zinc, tin, iron, and manganese, silicon bronze fasteners are also created. According to the amount of copper in the fastener, silicon bronze’s color may change. 316 stainless steel cannot match the corrosion resistance of silicon bronze, which is why it costs more than other fasteners. In addition to being used in corrosive, high heat, and marine environments, silicon bronze is also appropriate for use in conventional applications for aesthetic reasons. Silicon bronze fasteners can also be found in tattoo machines and power plants, and they are frequently utilized in plumbing and electrical applications. Sometimes used in finish applications for the color, silicon bronze has a hue comparable to that of copper.

A mixture of copper and zinc is called brass. Depending on the zinc percentage, brass can range in hue from dark to light; brass with a higher zinc content is lighter. Because of its softness, brass is not appropriate for many applications even though it is valued for its ability to resist corrosion. In addition to being a good heat conductor, brass also conducts electricity. Pipes, weather stripping, trim, radiators, musical instruments, and weapons frequently employ it. Brass fasteners that are machined (such as screws, nuts, and bolts) are manufactured of alloy 360 brass, while non-machined components like washers are made of alloy 270 brass.

Fasteners are typically made of alloy steel, which is the most prevalent type of material. To increase their resistance to corrosion, alloy steel fasteners are frequently treated, coated, or plated with zinc. To generate a firmly bonded alloy finish, alloy steel is heated in a bath of molten zinc before being hot-dipped galvanized. Grades A, C, 2, 5, and 8 alloy steel are the most common grades to be found. Alloy steel fasteners are dark in color when untreated.

When compared to aluminum, titanium exhibits greater strength-to-weight ratio and exceptional corrosion resistance. Titanium has a minimum tensile strength of 150,000 PSI and a maximum of 180,000 PSI. They are widely utilized by the aerospace sector and perform well in hot situations. Their great resistance to corrosion and oxidizing agents makes them a popular choice in the chemical industry as well.

In the field of engineering, bolts, nuts, and fasteners play a crucial role. They are literally what holds diverse machinery together, making them essential for a variety of sectors. They are made using different kinds of steel and varying in strength and costs. The SAE, or Society of Automotive Engineering is only one of the many standards that have been formed by the engineering community. 

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