Technical Crash course in metallurgy,,,

You might try leaving a small gap between the pieces to be welded (CLEAN is the key word here!especially TIG)
308L tig wire is for stainless to stainless but......is harder than 70S2.
Build a nice filet weld around the piece and cool it slooooowly.
Plenty of 'heat' and penetration is a must.
6sally6

 

The air gap between the two pieces will result a very quick failure. The pounding effect the impact driver would put on the tool would be completely on the weld and the weld effected areas on both pieces of the shaft. The first impact will try to close the gap, and each impact after that will increase the pressure, at 3500 impacts per minute, if the gap lasts a minute, it did good. The weld effected area doesn't stand a chance. That is experience talking, been there, done that.

The failures are caused by the constant pounding effect. The single piece tool survived because the impact at one end is transferred through the solid tool piece to the other end where it applies the impact to the object you want changed. The weakest point moves, you want that to be what ever the tool is striking.

I have welded a few two piece air hammer tools together that, with a couple rounds of welding, lasted long enough to get a job done, but you have to be prepared to be ready to stop at any point. Then each time it was welded back together, its life was shorter then the time before.

 

That’s been the exact same events here.
This is for a repeated job, there’s already been many hours of trial and error attempts in the process. Maybe more is needed.
The air hammer and Delrin yield the best results.

I’ve seen a welded tool for a 60Lb jack hammer field designed for railroad spikes last for tens of thousands of spikes. I know this particular tool was sent for treating process but no idea what was done to it. It seems to be secret squirrel mission when I ask about it. It’s definitely a machined part welded to the large hex shaft. The prototype proof of concept tool broke quickly.
There’s only 1 issue with it, the hammer has a T handle configuration and a hex tool holder. The business end of the tool that engages the spikes is off square to the handle.

I wouldn’t be opposed to having a tool to suit my purposes manufactured and treated.