What is the Difference in Type 1 vs. Type 3 Structural Steel?

When we’re talking about structural bolts, particularly ASTM F3125 Grades A325 and A490, there are two types of structural steel: Type 1 and Type 3. On the outside, they look the same, but there’s one key difference: Type 3 structural steel is designed to weather, or rust, as a means of protection. Type 1 bolts can be galvanized to protect the bolts from corrosion. There are advantages to each type of steel in regards to structural bolts. Here, we’ll discuss their make-up, what types of coatings they can have, and their differences.

Type 1 Medium Carbon and Alloy Steel

According to the RCSC, Type 1 covers medium carbon steel for F3125 Grade A325 bolts and alloy steel for F3125 Grade A490 bolts. Type 1 Grade A325 bolts can be mechanically coated or hot-dip galvanized to protect them from corrosion.  Type 1 Grade A490 bolts on the other hand are only approved for F1136 Grade 3 and F2833 Grade 1 coatings per ASTM F3125 and the IFI 144 standard. Unlike Type 3 weathering steel, any rust on Type 1 bolts will not act as a protective coating to the bolt. Though Type 1 A325 bolts can have a protective coating (mechanical or hot-dipped), it is extremely important to ensure proper jobsite storage throughout your project to prevent any corrosion.

Type 3 Weathering Steel

The alternative to Type 1 steel is Type 3 weathering steel. These high strength fasteners have atmospheric corrosion resistance and weathering properties. This means that Type 3 structural bolts are designed to weather and rust over time. Unlike Type 1, their rust acts as a protective barrier or “coating” that seals the bolt.