Bolt grades include a grading of 2, 5, 8 and 18-8 stainless in the U.S. grade systems. The grade is determined by the material used to construct the bolt and the material properties of the bolt.
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A grade 2 bolt has no markings on the head of the bolt and is made of low or medium carbon steel. In 1/4" to 3/4" bolts the proof load -- the pressure the bolt can withstand -- is 55,000 psi (pounds per square inch), while in 3/4" to 1-1/2" bolts the proof load is 33,000 psi.
Grade 5 bolts are marked with three radial lines on the head of the bolt. The bolt is made of medium carbon steel that is quenched and tempered. In 1/4" to 1" bolts the proof load is 85,000 psi. In 1" to 1-1/2" bolts the proof load is 74,000 psi.
Grade 8 bolts are marked with six radial lines on the bolt head. It is made of medium carbon alloy steel that is quenched and tempered. In 1/4" to 1-1/2" bolts the proof load is 120,000 psi.
Grade 18-8 Stainless
Markings vary on this bolt. It is made with steel alloy with 17-19 per cent chromium and 8-13 per cent nickel. In 1/4" to 5/8" proof load is 80,000 to 90,000 psi. In 3/4" to 1" proof load is 45,000 to 70,000 psi.
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