Hardened tool steel can span a pretty wide range of alloys.

Exactly what do you need to put together and what does it have to do?

But pretty much a small hardened piece of steel being welded to another small hardened piece of tool steel means some careful preheat (so the pieces don't crack while being welded), and then some careful post-heat (so the pieces don't crack after being welded) and then most likely being completely heat treated again (temper and quench as appropriate) because all the preheat and post-heat and welding heat will usually totally screw things up.

It is sometimes do-able, but it isn't necessarily easy even on a good day.

 

RedneckJazz, I've never welded tool steel in that small a section, hard or annealed.

H-12 is the usual "safe" filler for most tools steels if you are sure the parts are tool steel.

I think that with sections that thin you're going to have to clamp them to a larger backing plate heated to a lower tempering temp (maybe 350-450°F). The Tig proceedure is not normal, you don't start the puddle in the parent material, you stick the filler down from the top and wet out the edge from the center if you are working with hardened material at low temps. If you weld it Ok and let it cool down with the backing plate the weld should be in the 54-58Rc range. If the other parts soften very much away from the weld they are something other than AISI tools steel.

Does the weld have to be hard or resist impact/abrasion?

If you could explain the purpose I could help more.

You can also look here (from Ed Craig), it covers about everything big I can think of; http://www.weldreality.com/ToolstlsH.htm

Good Luck

Matt

 

Try Cronatron 345T (www.lawsonproducts.com). I don't know how you are welding the parts together, but I do small pieces of .050 thick tool steel and clamp them to copper or brass, where possible, to keep the non-welded areas as "cool" as possible. The 345T as welded finishes at Rc 54-58. 346T as welded finishes at Rc 56-52. They are a little pricey but get the job done. I have had good success with Cronatron fillers, both MIG and TIG.

Another option is their 340T meant for chrome-moly and the 400 series SS. It works well on some tool steels, as it has chrome, nickel and molybdenum the primary alloying elements.

I have no affiliation with either Cronatron or Lawson; I just like their products.

 

Well, I tried some test pieces and after some heat adjustments and start/stop practice, as well as filler rod manipulation I gave it a go. (filler was stainless wire feed wire)
I tried two small deposits, and the first went very well.
Then I went on to the other, and had trouble getting the bead to form very well, let the heat build a bit too much, and I destroyed the workpiece.
I needed a much larger and more efficient heat sink. Was using steel. Next time, copper or aluminum.
Learned some lessons, and still cobbled the part back together anyway.
Surprisingly, the hardness didn't suffer as much as I thought it would. It was still pretty tough to gouge with a file. (the workpiece) The welds also held and were fairly hard, but nowhere near the workpiece hardness. If I hadnt wrecked/deformed it with the excess heat, it would have been a successful patch.
By the way, it was a broken trigger sear from an old rifle that I bought at a garage sale.
The rifle is way too far gone to fire safely anyway, and I can hunt the web for a replacement sear.
Just a conversation piece.
Thanks for the advice guys!

 

Sear might be casehardened, and not just some tool steel.

That could explain the file almost skating off the surface in your quick 'file check'.

Maybe just build up a little hardface onto the sear and then carefully file/grind back into shape.

Either that, or build up with plain mild steel filler, shape as desired, and then re-caseharden the sear.

Or, if it is just a wall-hanger conversation piece, take the sear out and it is then a non-firing wall-hanger conversation piece.

(case hardening was often used for several reasons, chief among them to get a v-e-r-y hard surface to get the wear characteristics desired yet still have a softer and tougher core to resist cracking and fracturing.)

 

I use the 345T as well as the 340T all the time on firearms parts (I am an FFL 'smith, mainly pistols and revolvers. I do this part-time but will do it full time once I retire). I also make obsolete parts for firearms, so if you are trying to restore the gun and can't come up with the part, let me know and I'll try to help you out. Save the old part, no matter the condition. Just for your info, I have a website, www.TotalAutomation.us.