What Does Bolt Grade Mean?

February 28, 2018

Bolts really are fabricated from differently graded materials, which is why today's post is all about bolt grade. What is it that's being graded? Well, as any capable engineer knows, the greater a fastener's material strength, the more mechanical stress it'll support. Established at the manufacturing stage, that tensile strength can be expressed as an easy to interpret integer. Look lively, here's where we decipher those numbers.

Bolt Grades: Bigger Numbers Indicate More Strength

Starting with a Grade 2 fastener, let's check out each grade:

Grade 2: Manufactured from low or medium carbon steel, the minimum tensile strength of this bolt family sits at around 57000-PSI. That's assuming a nominal size range from 1/4-inch to 3/4-inch. If that size range enters the 3/4-inch to 1/2-inch area, then the graded bolt family has access to a 60000-PSI strength rating.

Grade 5: Moving up on the grading scale, here's a bolt type that's made from medium carbon steel. The alloy is tempered and quenched, and it's equipped with more holding strength. Broken down into two size ranges, 1/4-inch to 1-inch and then 1-inch to 1 1/2-inch, Grade 5 bolts deliver a proof loading capability that begins at 85000-PSI and rises to 74000-PSI.

Grade 8: Medium carbon alloy steels support extra-heavy loads. The tensile strength of this size 1/4-inch to 1 1/2-inch group is set at 150000-PSI, the yield strength is 130000-PSI, and the proof load is locked in at 120000-PSI. Grade 8 bolts are obviously designed to handle larger loads

Grade S: Way up at with the load supporting giants, this bolt type endures when 49.9 Metric Tons of stress is applied to a mere 6.45 cm3 of object-to-object surface area. Grade S bolts are heavy-duty fasteners. Incredibly durable, they're employed in numerous high-load applications.

Noticing the Radial Markings

Pay attention to any and all head markings. Located on the flat face of the hex heads, there are no markings on Grade 2 bolts. However, Grade 5 bolts are etched with 3 radial lines. Similarly, Grade 8 bolts are marked with 6 radial lines.

The numbers and letters etched on bolts are tied to certain engineering factors. For instance, the tensile strength of a fastener tells us how much stress it will endure before it fails. Yield strength is similar. If this loading force is exceeded, the bolt stretches permanently. As for proof load, this is the threshold force level at which plastic deformation first occurs. If torque stress passes this last limit, then the bolt shaft stretches.

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