Filling voids in SS weld - micro welding?

szymon_woj · Tue Nov 29, 2016 5:08 am

Hi!

I have this problem (see the picture). I designed this piece, and sent it to our shop for welding. The plane of the piece have to be flat and smooth. The hole had been precision grind to fit a piston. Unfortunately the guys at the shop did terrible job, as you can see at the picture. Upon lathing and grinding the planar surface multiple voids did appear for poorly done weld. Re-lathing the weld, and filling with another weld might be a solution, but as the part it made from SS 304 it would probably bend beyond repair.

I am thinking about using the pointiest electrode and filling the voids (TIG) as they are with a thinnest wire I can find, and hoping for the best. Any tips on that? As I sad, the hole and piston are precision grind to fit. I can preheat the part to any temperature, no problem. Maybe soldering would be a better option? This part does not carry any force, it is that massive only to have high heat capacity.

I could ask the guys at shop to redo the part, but it was I kind of favor, and since I am learning TIG welding myself I thought I give it a try.

exnailpounder · Tue Nov 29, 2016 8:52 am

It wouldn't take much to fill those little voids, just a quick zap with a little filler and you'd be good. I am a little appalled that they would hand you you're part back with those pits in it though. Welcome to the forum!

kiwi2wheels · Tue Nov 29, 2016 9:08 am

You have two ground surfaces in stainless. If you don't have any concerns about structural integrity or leakage, I would leave it alone.

You don't know what's under the voids and could be opening a can of worms.

Arno · Tue Nov 29, 2016 10:14 am

kiwi2wheels wrote:You don't know what's under the voids and could be opening a can of worms.

That would be my worry too.. You'd really want to 'V' out a groove in the surface (eg. on a lathe) to clean up the porosity and fill it in and then re-machine/re-surface for a consistent and strong repair, but it may expose more gaps and holes hiding under there

Bye, Arno.

AndersK · Tue Nov 29, 2016 10:57 am

Yes, absolutely v-groove. If you have hidden porosity just below surface it can blister during heat load. Might not be an issue for your applikation but Ive seen it happen on castings.

Poland308 · Tue Nov 29, 2016 6:32 pm

I'd v groove it as well. My concern about not repairing it would be the likelihood that the heat stress from use would cause expansion that would crack the two parts worse and cause the center to drop out.

Rick_H · Tue Nov 29, 2016 9:43 pm

Poland308 wrote:I'd v groove it as well. My concern about not repairing it would be the likelihood that the heat stress from use would cause expansion that would crack the two parts worse and cause the center to drop out.

I'm with Josh, this is what I would recommend as well. Would not take much to physically weld and fill the joints, preheat will help since the part is so large.

szymon_woj · Wed Nov 30, 2016 4:18 pm

Thanks for info. V-groove would be perfect if one would want to do this part justice, but filling this groove would bend the part for sure. I think I will go with "quick zap". High current, preheat to 200*C, a lot of argon, maybe etching with acid those pits before the job. If I could give it a pass I would be the most happy to, but I use it to extrude 0,5 mm polymer films (with the piston) and those voids ruin the film surface and make the analysis difficult. After quick zap, and grinding/polishing with sand paper on glass surface it should be fine.

Will post the results.

Poland308 · Wed Nov 30, 2016 8:21 pm

I think you misunderstand you are getting experience from people who use stainless!

dave powelson · Wed Nov 30, 2016 10:46 pm

szymon_woj wrote:Thanks for info. V-groove would be perfect if one would want to do this part justice, but filling this groove would bend the part for sure. I think I will go with "quick zap". High current, preheat to 200*C, a lot of argon, maybe etching with acid those pits before the job. If I could give it a pass I would be the most happy to, but I use it to extrude 0,5 mm polymer films (with the piston) and those voids ruin the film surface and make the analysis difficult. After quick zap, and grinding/polishing with sand paper on glass surface it should be fine.

Will post the results.

1-what acid are you're musing to use on SS porosity?

2-As others have alluded to--expecting to get ALL of the porosity out with a "quick sap"....isn't really what happens.
Some to a lot will remain, new will appear.....ad infinitum; until one v-grooves, then fills....and one still may have to contend with residual contamination during assembly of the sleeve to the plate, coming back up thru the vee puddle.

3-The image you posted kinda, sorta looks to show distinct line between the piston sleeve and the plate. If this really is the case, then there's very little, original weld left; which possibly may only have been a poor, porous, fusion weld with little or no filler addition or bevel prep, using the "quick zap" method and that's been machined off.

4-Anyhew, give it your best shot.

szymon_woj · Thu Dec 01, 2016 5:22 am

When you put it like this it is much more convincing... To the soldering then with soft solder containing Ag (for stainless). Acid, preheat, then torch soldering. I use HF and HNO3 mix. Ratio depends what I want to achieve and how lazy I am.

Thanks for holding me down on this one.