It stands to reason that if you want stronger structures you need to build them with stronger components, such as high strength bolts. That means bolts manufactured with some grade of steel, chosen for the tensile strength generally required for the application and industry in which the bolt is typically used. And this is where it can start to get confusing. 

There are two major standards organizations that categorize bolts by their mechanical properties and each differs from the other in how they are designated. The American Society for Testing and Materials (ASTM) assigns high strength, also known as structural, bolts to one of three designations: ASTM A563, Al94 and F3125 (formerly A325, which, for familiarity’s sake, we’ll continue to use in this post).The Society of Automotive Engineers (SAE) has identified seven different grades of structural bolts, numbered from 2 to 8. To keep this as simple as possible, we’re going to limit our discussion to the specific grades typically used for structural bolting projects. 

ASTM A325 and SAE Grade 5 are the two most popular bolt grades for structural bolting. Grade 5 bolts are manufactured with a medium carbon zinc-plated alloy steel while A325s are commonly made with a medium carbon or alloy steel, or boron steel (designated as Type 1), a low carbon martensitic steel (Type 2), or a weathering steel (Type 3). The general properties as well as the physical and chemical strengths of the two bolts are basically the same, but engineers usually recommend ASTM A325 bolts for heavy construction projects, while SAE Grade 5s are more popular for OEM equipment assembly. 

We should note here that while the highest SAE standard structural bolt grade is 8, there is actually one more grade reserved for the heaviest-duty assemblies – Grade 9 – one of the strongest structural bolts available today. You’ll find it often used in assembling heavy machinery, such as bulldozers.

While there are some standard rules to consider when choosing a high strength bolt for a specific application, what’s really important is having a good understanding of the specific material and tensile strength requirements, as well as any other appropriate considerations, depending on your particular need. Unique circumstances may call for a unique bolt. 

If you have questions about selecting a high strength bolt, or simply want to learn more, contact the bolt experts here at Electronic Fasteners for more information.