​Different Types and Grades of Bolts and Their Uses

​Different Types and Grades of Bolts and Their Uses

Several considerations come into play when you’re planning and carrying out a project. You have to determine what you’ll be using the finished product for, the correct dimensions of the structure or item you’re building, how to approach the project, and which materials to use to name a few of the main considerations. While the figures, your skills, and the materials will factor into how sturdy and reliable the end results are, they’re only as good as the bolts you use to put everything together.

Taking a Closer Look at Bolts
Before diving into the nuts and bolts of the matter, it’s important to cover just what bolts are. By definition, they’re fasteners with threaded shafts. Once they’re inserted into the materials they’ll hold together, a nut is threaded onto the shaft to provide sturdiness and prevent movement. Washers are sometimes added to the mix to increase stability as well.

Bolts shouldn’t be confused with screws, which create their own holes and don’t require nuts or washers. When comparing the two, bolts inevitably give you more strength and resistance to damage. Numerous types and sizes of bolts are available, each of which has its own benefits and applications.

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Common Bolt Materials
Bolts can be made of a range of materials, including aluminum, brass, bronze, steel alloys, and stainless steel. Not all of them work well in every situation. Some just don’t have all the qualities you need for certain applications.

Aluminum and Aluminum Alloys
Aluminum bolts are lightweight, aesthetically pleasing options that provide ample corrosion resistance under some circumstances. That said, this material can corrode when it’s paired with certain other materials.
Other metals are often added to aluminum to increase bolts’ strength and resistance to extreme temperatures. Bolts made of aluminum aren’t the strongest choices, but they’ll hold up in lightweight applications.

Brass
Made by combining copper and zinc, brass is a beautiful metal for bolts and other types of hardware. It’s resistant to corrosion and offers electrical conductivity, but it’s fairly soft. That means it doesn’t provide much in the way of tensile strength.

Bronze 
Bronze is composed of copper with tin and silicon added to the mix. Bronze bolts are often used in applications where they’ll be exposed to salt water or spray because of their superior corrosion resistance. Though bronze is stronger than brass, it’s still not the prime choice for most heavy-duty uses.

Steel and Steel Alloys
Steel is one of the most popular and versatile materials for bolts. It’s strong, durable, corrosion-resistant, and capable of holding up to extreme conditions. Steel bolts are often treated or coated to enhance their positive qualities. You might find them in galvanized or zinc- or chrome-plated varieties. An array of steel alloy bolts is also available for various purposes.

Stainless Steel 
Made of low-carbon steel and chromium, stainless steel gives you strength, durability, corrosion resistance, and many other highly sought-after qualities. Though most stainless alloys can’t be heat-treated, some can be hardened to further boost their tensile strength and endurance. Stainless steel bolts are used in a number of applications.

Consider those points when deciding which types of bolts will work best for your project. While each material offers corrosion resistance and a certain amount of strength, aluminum, brass, and bronze are only appropriate for relatively lightweight applications. Stainless steel is more versatile, and steel bolts are likely to serve you well in virtually any situation.

Covering the Different Grades of Bolts
Now that we’ve gone over the basics of bolts and the different materials they may be made of, let’s move along to the various grades of bolts on the market. Three distinct grading systems are used for bolts: SAE, Metric, and ASTM. Higher grades can withstand greater force and weight, and higher numbers translate to higher grades.

SAE Grading System
The SAE grading system was developed by the Society of Automotive Engineers. Several grades are distinguished in this system, but the three most common are 2, 5, and 8. You can determine the grade by looking at the head of the bolt.

If an SAE bolt has no markings on its head, it’s a grade 2, meaning it has a tensile strength of about 60,000 pounds per square inch. This is your average hardware bolt.

Grade 5 bolts are set apart with 3 raised marks on their heads. They’re stronger than grade 2 bolts and can withstand about 100,000 PSI of force. These bolts are often used in the automotive field.

Grade 8 bolts are heat-treated and have a tensile strength of 150,000 PSI. They’re best for heavy loads and high stress. They have 6 raised marks on their heads. Essentially, you count the raised marks on the heads of SAE bolts and add 2 to get their grades.

Metric Grading System
Metric bolts have two numbers on their heads separated by a decimal point, and they’re graded according to the International Standards Organization, or ISO. Their numbers represent their property class. The number on the left side of the decimal point is the load strength whereas the one on the right tells you the spread between the breaking and bending loads. You’ll see certain common classes of bolts in the metric grading system.

Class 8.8
Steel metric bolts rated 8.8 are made of medium carbon steel that has been quenched and tempered. Those measuring less than 16 millimeters are rated for a proof load of 580 MPa with a minimum yield strength of 640 MPa and a minimum tensile strength of 800 MPa. These types of bolts ranging from 16 to 72 millimeters have a proof load of 600 MPa, a minimum yield strength of 660 MPa, and a minimum tensile strength of 830 MPa.

Class 10.9
If you see a steel bolt with 10.9 on its head, it’s made of a quenched and tempered steel alloy. Bolts in this category range from 5 to 100 millimeters and will withstand a maximum tensile force of 830 MPa. They have a minimum yield strength of 940 MPa and a minimum tensile strength of 1,040 MPa.

Class 12.9
Class 12.9 steel bolts range in size from 1.6 to 100 millimeters and stand up to a proof load of 970 MPa. Their minimum yield strength is 1,100 MPa, and their minimum tensile strength is 1,220 MPa. They’re heavier-duty bolts and useful for many purposes.

ASTM Grading System
ASTM grades were developed by the American Society for Testing and Materials. They’re characterized by the letter A followed by a series of 3 numbers. They’re comparable to SAE bolts. An A325 bolt offers close to the same properties as an SAE grade 5. An A490 corresponds to an SAE grade 8 bolt. The ASTM equivalent of a grade 2 SAE bolt would be A307.

Keep in mind, these are the more widespread bolt grades. Others are available as well. As mentioned, higher numbers indicate higher strength regardless of the grading system used. Now, with general bolt grades explained, we can focus specifically on stainless steel bolts.

Covering Common Stainless Steel Bolt Grades
Stainless steel bolts are often chosen because of their strength, longevity, and corrosion-resistant nature among other strong suits. If they happen to obtain surface damage, those nicks and scratches won’t compromise the qualities of the metal. Various types of stainless steel are available, and they have different grade markings than other types of bolts.

Austenitic stainless steel contains 15 to 20 percent chromium and 5 to 19 percent nickel. Bolts made of this type of stainless steel offer the highest level of corrosion resistance. They have a tensile strength of 80,000 to 150,000 PSI. Austenitic stainless steel bolt grades include 302, 303, 304, 304L, 316, 320, 347, and 348.

Martensitic stainless steel is magnetic but doesn’t weld very well. It offers an intrinsic tensile strength of 180,000 to 250,000 PSI and, unlike some other types of stainless, can be heat-treated for further reinforcement. Common bolt grades in this type of stainless steel are 410, 416, 420, and 431.

Consisting of 15 to 18 percent chromium, ferritic stainless steel has a tensile strength of 65,000 to 87,000 PSI but can’t be heat-treated or welded. Though ferritic stainless steel bolts can resist corrosion, they’re not recommended for applications where they’re likely to be exposed to highly corrosive substances. Popular stainless steel bolts in this category include 430 and 430F.

Wrapping Things Up
In a nutshell, numerous types and grades of bolts are at your disposal. While aluminum, brass, and bronze are great for aesthetic appeal and lightweight applications, steel and stainless steel are best for heavy-duty loads. Consider the grades of the metal you’re going to use when choosing the right bolts for your project as well. ​