Are hardned nuts necessary?

[SPYDERLK]

I don't think I've ever seen a bolt pull the thread out of a nut so, is there an advantage of putting a hardened nut on a hardened bolt? Is it OK to save a few cents and use the mild steel nuts?

There is a good reason for what you have noticed. Think of the thread roots in the bolt vs the nut. The nuts threads make a longer path and hence provide more shear area per turn than do those of the bolt. Forget stripping ... Once youve got a nut as tall as the minor diameter of the bolt you have a nut that will pull on the bolt til it snaps. If you use an equal grade nut you are more likely to snap the bolt a little sooner because the bolt stretches elastically during tension. If a nut of equal strength steel to the bolt is used, the elastic tension/stretch of the bolt forces huge stress on the ~1st thread of engagement. The nut thread, have superior area, is stronger, more rigid, and can damage the bolt here. - Very slight damage to that 1st bolt thread but much more danger caused by the abrupt stress riser in the root of the thread/minor bolt diameter. This is where you see a bolt break.
,,, OTOH, use of a nut the next grade down acts to spare the bolt stress by allowing its elastic stretch to go a little further into the nut. -The 1st threads in the nut displace plastically very slightly to accomodate this, giving more equal thread loading/sharing within the entire engagement. This works great for initial and repeated assembly with the condition that the nut load face is always the same. This complied with you will find that repeated full [bolt grade] torque fastening with a slightly softer nut will endure through more tightenings than if a hard nut is used. I have seen this occur on a puller I devised for a special task

I use Gr5 nuts on Gr8 bolts. Std nuts on Gr5 bolts.
larry

 

[Soundguy]

i usually change nuts at least, if not most fasteners on critical items if I can. no need to save a buck and have soemthing hundreds of dollars fly apart ( think mowers.. etc.. ) head studs / nuts.. etc.