ASTM F3125 Grade A325 bolts and A490 bolts are two of the main types of structural bolts we sell, and they are the two you will most often see on a jobsite. The difference in grades A325 and A490 may seem simple. A490 is stronger than A325. That said, plenty of projects call for A325 bolts. Let’s take a look at why this is, what are the other key differences, and why you would use one over the other.
In a lot of ways, grades A325 and A490 are similar. Both of these F3125 bolts have a heavy hex head configuration which gives a larger bearing surface to distribute the load. These structural bolts have a specific thread length and shank length. In structural bolting applications, this allows the bolt's threads to not be in the shear plane (where the two plates meet up). The threaded section of the bolt is the weakest, thus a longer shank length makes for a stronger tensile connection. While these similarities make these grades of bolts seemingly identical, there are some fundamental differences that make grades A325 and A490 different.
Grade A325 bolts are commonly used in bridge and highway construction projects. They can be made of Type 1 medium carbon steel or Type 3 weathering steel. An important distinction between A325 bolts and A490 is that it A325 Type 1 bolts can be galvanized. A325 bolts have a minimum tensile strength of 120,000 PSI for one inch or less diameters and 105,000 PSI for over one inch. This makes them weaker than A490, certain jobs do not require a structural bolt as strong as Grade A490.
Grade A490 bolts can be made of Type 1 higher carbon alloy steel or Type 3 weathering steel. Unlike Grade A325, Grade A490 bolts cannot be galvanized. This is due to the potential for hydrogen embrittlement, which could happen during the hot dip or mechanical galvanizing process. As mentioned earlier, Grade A490 requires a higher minimum tensile strength (150,000 PSI), making it stronger than Grade A325. Therefore, jobs that require a stronger bolt typically use Grade A490. That said, their strength can often be an issue. Higher strengths of steel are more susceptible to stress corrosion or hydrogen cracking. Because of this, A490 bolts are not as commonly used for bridge and highway construction compared to A325 bolts.
Ultimately, your engineers will specify which grade of F3125 structural bolts you will need to use, but it is important to know the distinction between grades A325 and A490. Grade A490 is stronger than Grade A325, but strength isn't the only factor in making a decision for bolts. A490 bolts cannot be hot dip or mechanically galvanized. Grade A325 is not as strong, but it is a lower cost bolt that can be galvanized to avoid corrosion.
Author’s Note: In 2016, ASTM A325 and A490 were replaced by ASTM F3125. A325 and A490 are now grades under the F3125 specification. When reading this blog, please note that when we discuss “A325” and “A490” structural bolts, we are specifically discussing the grades of F3125 structural bolts. This blog is for informational purposes only. For more information on the ASTM, visit their website.