4140 is different than mild steel, but you knew that already.

When welding 4140 or any other of the heat-treatables-the big question is whether the weldment will be heat treated before or after welding.

If heat treatment is required post welding, than a 4140 type electrode or filler would be best as they will heat treat to approximately the same mechanical condition.

That said, if no post weld heat treat is anticipated, a 110 or 120 class electrode or wire usually works well.

I've even had luck with E71T-1 on 4140 journal ends, although the hardness was off a bit.

As far as your question...

I usually suggest that your weld needs to be no stronger than the weaker material-the mild steel.

This case may be different as your 70S hard wire is certainly picking up a bunch of carbon from the .40 carbon 4140. This extra carbon can make welds brittle.

Stick and gas shielded flux core are much more forgiving in this app.

It is not wise to qualify hard wire (E70S) to hi carbon material with dynamic mechanical properties being important. Mechanical properties (mostly yield and impact) with hard wire are not always the best.The additional carbon makes this situation worse.

If, after all this, you are still getting failures-welding with 309 or 312 stainless wire will cure the problem, but at much greater expense.

Hope this helps a little.
Keep in touch, let us know how you make out!

Good Luck

brande

How about making the chain link plate out of mild steel, but thicker. This way, you can achieve the strength you need and don't have to worry about weld procedures for welding 4140