Plugs welds & B7 threaded rod

I have had an interesting time with plug welding B7 rod through 0.25 plate.

I was asked to plug weld, using ER70S6 wire 0.5" rod to make a mounting plate that will support a dead load of 160 lbs.

Everything weld up beautifully.....until the hanger plate was bolted up. The B7 studs snapped off like ice sickles!!!!!!

I look further into the stats on threaded rod and found that B7 has a tensile strength of 128,000 psi whereas mild steel has 58,000psi.

So.....I tried a plate using mild steel and everything worked great. The studs didn't break and appear find.

I was of the school that stronger is better. Obviously, I was wrong.

The reason for the plug weld is to allow the mounting plate to attach flush to bottom of an I beam, bar joist, etc. Bolt heads would be a problem.

Long story short......the mild steel takes a weld while B7 doe not!

Thanks Guys

Tom

B7 is about the same thing as 4140. Takes some pre heat and post heat to get crack free welds. The chrome and medium carbon in the material is causing air hardening.

John