Tig weld S7 tool steel

gamble · Tue Jul 15, 2014 5:03 pm

What filler would you use? S7 tool steel must be high strength and heat treated. From my understanding. It's for a machine like a cnc lathe.

Cricket · Wed Jul 16, 2014 1:24 pm

I welded tool steel and it is not a reliable operation. I usually preheat the piece to 600-800F at least and use the same steel for the filler. If you cannot get S7 wire - cut a strip from the sheet. Some steel decomposition\degragation in the bead is unavoidable. So the joint will be somewhat different than the rest of the piece. I am not saying "weaker", it might behave differently in the following HT process.
Put the piece into the annealing furnace immidieately after the welding; DO NOT LET IT COOL DOWN AT ALL and make a full annealing cycle.

gamble · Wed Jul 16, 2014 1:59 pm

they might scrap it. not sure yet. what about a bucket of sand?

Cricket · Thu Jul 17, 2014 1:39 pm

gamble wrote:what about a bucket of sand?

Sand? What for?

gamble · Thu Jul 17, 2014 3:13 pm

to control the slow down cooling?
i think they scrapped the piece but curious to know anyway

79jasper · Thu Jul 17, 2014 5:21 pm

Cricket wrote:
gamble wrote:what about a bucket of sand?
Sand? What for?

He's saying to burry it in sand, after welding.
Personally, I don't think it would work that well. I mean yes, it'll work, but not good enough.

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Braehill · Thu Jul 17, 2014 6:03 pm

I can't speak to welding tool steel, but I will say that burying welded parts in sand works surprisingly well. I used to weld cast iron safety levers for gasoline trailers and then stuff them down in a drum of sandblasting sand and when you retrieved them the next day, they were still too hot to touch. Only had one break after I welded them and I did maybe 8-10 of them.

I also have a cast iron compressor housing that was done the same way that's still in service after 16 years. Don't discount the simple, inexpensive stuff, sometimes it's just what the doctor ordered.

Len

79jasper · Thu Jul 17, 2014 6:14 pm

I've heard of it before, just didn't know if I'd trust it on something really important.
But if it works, it works.

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Thu Jul 17, 2014 6:55 pm

"Vermiculite" from the local garden center is (when dry) a MUCH better insulator for a slow-cool than sand, when cool-rate is critical.

Before fiberglass insulation was invented, Vermiculite was the insulation of choice for houses, though it would tend to settle.

Steve S

Braehill · Thu Jul 17, 2014 8:44 pm

I just used the sand because we always had a drum full of sandblast sand in the shop and it was handy.

Len

Cricket · Thu Jul 17, 2014 10:35 pm

The thing is - you need to bring the steel to/above critical temp, soak it for a certain time and than slowly cool down. Than you need to HT (heat treat) it again. Sand\vermiculite\whatever will let you cool down slowly. May be even slow enough. But it will miss the critical temp and soak for sure. Temperatures and times could be found in the steel datasheet.
During the welding in the bead zone steel is overheated. That creates stress and coarse\rough steel structure. To make the steel grain fine again you need or work it mechanically (forging\rolling) or thermocycle it several times( not that simple actually).
If the goal is to weld without major cracks - sand might work for the simple steels like S7, W1, O1. But the "tool" qualities of the steel most likely will suffer.
If you try to weld something like D2 or M4 no sand will save it.

By the way, if the tool steel is weldable it will be described in the datasheet.
Here it is:
http://www.simplytoolsteel.com/S-7-tool ... sheet.html

It says that welding is possible for small repairs AWAY from the working surfaces and MAY be satisfactory...