Why are larger grade 2 and grade 5 bolts less strong than smaller diameter bolts?
The reason is similar but different for the drop in strength of grade 2 and grade 5 fasteners.
For J429 grade 2 fasteners, the drop is due to the manufacturing process. Small, mass produced cap screws are cold formed by drawing and forming wire. The wire, as it is cold formed, work hardens which increases its tensile and yield strength. Fasteners that are larger are typically less strong for two reasons. First, is that they are more likely to be hot formed, which does not increase the tensile and yield. Second is that the cold/work hardening effects do not penetrate all the way through, so as the bolt gets larger, the percentage of the steel affected by work hardening gets smaller and smaller. The authors of the J429 standard decided to draw that line at ¾” diameter. In addition, long bolts, those longer than 6” are also more likely hot formed. For those reasons, any grade 2 bolt above ¾” diameter, and any diameter of grade 2 bolt longer than 6” are governed by the mechanical requirements of J429 grade 1, which is also very similar to ASTM A307A.
As for grade 5 fasteners, the reasoning is similar. Grade 5 fasteners are made from medium carbon steel, and are heat treated in order to achieve their higher mechanical properties. In the same way that a large cookie is still gooey in the middle, but a small one is fully cooked, a large grade 5 fastener has a hard time getting completely hardened in the middle, but a smaller grade 5 fastener does not. The authors, in writing grade 5, and I’m sure after some diligent research, drew the line at 1”. Bolts 1” and smaller have higher mechanical properties then their larger counterparts. SAE J429 only covers fasteners up to 1-1/2”, but in similar, larger standards lie ASTM A449, there is another step down in mechanical properties above 1-1/2” as well.
The reason you don’t see this same step down in grade 8 fasteners is that they are made from alloy steel, which heat treats more thoroughly and evenly, so the need for a step down is not there.
The IFI, in their Fastener Technology Handbook, 2010 summarizes this:
Because heat treat response is due solely to carbon content without the booster effect of alloying elements, the fastener’s end properties are subject to size effect. This means that using the same material analysis and the same processing, the strength properties of the fastener decrease as its size increases.
J429 Mechanical Properties
|Grade||Nominal Size, inches||Full Size Proofload, psi||Yield Strength, min, psi||Tensile Strength, min, psi||Elong, min, %||RA, min, %||Core Hardness, Rockwell||Tempering Temperature, min|
|1||1/4 thru 1-1/2||33,000||36,000||60,000||18||35||B7 to B100||N/A|
|2||1/4 thru 3/4||55,000||57,000||74,000||18||35||B80 to B100||N/A|
|Over 3/4 thru 1-1/2||33,000||36,000||60,000||18||35||B70 to B100|
|5||1/4 thru 1||85,000||92,000||120,000||14||35||C25 to C34||800F|
|Over 1 thru 1-1/2||74,000||81,000||105,000||14||35||C19 to C30|
|8||1/4 thru 1-1/2||120,000||130,000||150,000||12||35||C33 to C39||800F|
|Grade 2 requirements for sizes 1/4" thru 3/4" apply only to bolts 6" and shorter, and to studs of all lengths. For bolts longer than 6", Grade 1 requirements shall apply.|