A few days ago, I was welding some tooling parts made from 4140 low alloy steel. The parts were small and were basically intended to be an extension of a torque wrench
4140 is very similar to 4130 chromoly but with a bit more carbon and manganese.
The extra carbon allows for more hardenability.
P 0.035 (max)
S 0.04 (max)
P 0.035 (max)
S 0.04 (max)
(4140 is also known as 4140 alloy steel, 4140 chromoly, chromium-molybdenum steel, and all kinds of other slang terms)
Remember in the metals and how to weld them series when I did the demonstration where I dipped a red hot piece of 4130 in a bucket of water?
Remember what happened? It got brittle to the point where it easily broke by bending it with vise grips.
Thats because the way 4140 steel hardens is by heating up past a transition temperature and then cooling quickly....or quenching.
Make sense? So when an arc strike happens on a piece of hardenable steel like 4140 steel, would that be a good thing or a bad thing?
An arc strike is an exaggerated situation. The metal reaches a temperature past its melting point in a microsecond... and then cools extra quickly by the surrounding cool metal. That is extremely quick heating and quenching.
So What happens?
The little area that arcs, becomes a very hard little blemish...a bump or tit... Its a stress riser. And depending on how highly the area is stressed, an arc strike can become a crack.
On the 4140 parts I was welding that day, the arc strike was more like a cosmetic problem. It was removed by blending and then repolished.
But... It occurred to me that I could remedy the situation for future jobs, so that is what this page is about.
I was using a homemade tool called a "third hand" to hold the parts still while I tacked and welded them.
A third hand is just a name for a tool welders make to hold small or odd shaped parts still.
My third hand was made from scrap steels and the sharpened steel tip that holds the piece in place, put an arc strike on the nice polished piece because the piece was picking up the ground from the sharp tip of the third hand.
Sure, I could always hook up a separate ground or jumper clamp... but for really small parts, that is just not practical...especially when I have lots of them to weld.
So here is how I fixed it.
I figured that if I welded a little monkey fist on the tip of my third hand using aluminum bronze tig rod, the aluminum bronze would be such an excellent conductor that it would never make an arc.
I was right. It worked great. And was easier than any of the other solutions I was thinking of like making a set of small alligator clamp jumper cables ....
You could also take this a step further and just use a copper grounding rod for the parts of the third hand that transfer the current.
But if you think about it, the place where the rubber meets the road is all that matters.
So all it took for me to improve my third hand tool was a few drops of aluminum bronze filler rod ...added to the tip along with some bare braided copper wire wrapped around the base.
silicon bronze or even pure copper wire from some household wiring could also work.
No more arc marks... exit welding tools the third hand mod, and go to home page