Hi everyone，just joined and happy to find this forum.
please see the attached photos，it is manual TIG autogenous welding on 304 stainless steel，
the welds are always with "black slag” which could be grinded off，we do clean the base metal before welding.
have you ever met similar problem and what causes it？
You brushed the weld before taking the picture. Hard to tell what's going on when people do that. We are fellow welders giving advice, not the customer....so no need to hide the flaws when you are seeking advice on how to get rid of them.
The black spots are oxide deposits that, in your case, are floating up into the puddle and solidifying on the surface. In your case, probably due to argon levels being too low, or a small gap in the fit up without controlling rod feed and heat input/travel speed.
Yake wrote: ↑Sun Jul 25, 2021 11:12 pm
Hi everyone，just joined and happy to find this forum.
please see the attached photos，it is manual TIG autogenous welding on 304 stainless steel，
the welds are always with "black slag” which could be grinded off，we do clean the base metal before welding.
have you ever met similar problem and what causes it？
first thing is those pipes look like they are fairly well corroded.
i suspect they might be still be contaminated by what ever they had running through them.
if they are new they are junk, find a new supplier.
otherwise the two main causes of black slag is either lack of gas coverage or using mig gas instead of argon.
Yake wrote: ↑Sun Jul 25, 2021 11:12 pm
Hi everyone，just joined and happy to find this forum.
please see the attached photos，it is manual TIG autogenous welding on 304 stainless steel，
the welds are always with "black slag” which could be grinded off，we do clean the base metal before welding.
have you ever met similar problem and what causes it？
first thing is those pipes look like they are fairly well corroded.
i suspect they might be still be contaminated by what ever they had running through them.
if they are new they are junk, find a new supplier.
otherwise the two main causes of black slag is either lack of gas coverage or using mig gas instead of argon.
what it looks like may because after welding we brushed the welds and do pickling/passivation . could shielding gas lead to this type of slag, or have you ever saw the similar imperfection
Spartan wrote: ↑Mon Jul 26, 2021 9:36 pm
You brushed the weld before taking the picture. Hard to tell what's going on when people do that. We are fellow welders giving advice, not the customer....so no need to hide the flaws when you are seeking advice on how to get rid of them.
The black spots are oxide deposits that, in your case, are floating up into the puddle and solidifying on the surface. In your case, probably due to argon levels being too low, or a small gap in the fit up without controlling rod feed and heat input/travel speed.
ok，next time I would post a photo of weld before brush and pickling/passivation.
Yake wrote: ↑Sun Jul 25, 2021 11:12 pm
Hi everyone，just joined and happy to find this forum.
please see the attached photos，it is manual TIG autogenous welding on 304 stainless steel，
the welds are always with "black slag” which could be grinded off，we do clean the base metal before welding.
have you ever met similar problem and what causes it？
first thing is those pipes look like they are fairly well corroded.
i suspect they might be still be contaminated by what ever they had running through them.
if they are new they are junk, find a new supplier.
otherwise the two main causes of black slag is either lack of gas coverage or using mig gas instead of argon.
what it looks like may because after welding we brushed the welds and do pickling/passivation . could shielding gas lead to this type of slag, or have you ever saw the similar imperfection
if its black slag thats really hard its probably mig gas.
there is a few threads on here where people have used 75/25 on stainless mig instead of the stainless gas mix.
pickling/passivation won't cause all that pitting on the stainless tube.
i've got some of that tubing thats been sitting outside in the rain for the last 10 years or so and it still looks better than that.
if your dealing with 2nd hand steel it pays to know whats been through it as that can be soaked into the steel. it can come back out when welding and if its toxic.........
i've done parts where i have boiled it to get the stuff out.
Spartan wrote: ↑Mon Jul 26, 2021 9:36 pm
You brushed the weld before taking the picture. Hard to tell what's going on when people do that. We are fellow welders giving advice, not the customer....so no need to hide the flaws when you are seeking advice on how to get rid of them.
The black spots are oxide deposits that, in your case, are floating up into the puddle and solidifying on the surface. In your case, probably due to argon levels being too low, or a small gap in the fit up without controlling rod feed and heat input/travel speed.
Hi，please see the picture which could see the“black slag” clearly，still autogenous welding and this time without any treatment
ok, so its not slag but rather black specks.
that rules out mig gas.
one thing you can try is to see how hard that black speck is. if its super hard it could be carbide.
tweake wrote: ↑Mon Aug 02, 2021 4:21 am
ok, so its not slag but rather black specks.
that rules out mig gas.
one thing you can try is to see how hard that black speck is. if its super hard it could be carbide.
usually what could lead to black speck？ and is black speck a type of weld imperfection？
if its carbide, its caused by carbon and oxygen getting into the weld.
ie from shielding/purging gas problem, contaminated gas (hence previous mig gas remark), oil on the part, grease/oil on tig torch threads, type of coolant used by band saws.
i'm not up on the play with it all, i was hoping some others would pitch in, but afaik it can cause weld fractures and corrosion issues.
tweake wrote: ↑Tue Aug 03, 2021 12:35 am
if its carbide, its caused by carbon and oxygen getting into the weld.
ie from shielding/purging gas problem, contaminated gas (hence previous mig gas remark), oil on the part, grease/oil on tig torch threads, type of coolant used by band saws.
i'm not up on the play with it all, i was hoping some others would pitch in, but afaik it can cause weld fractures and corrosion issues.
what concerns me on the last pic is the heat tint, especially as it seams to be on one side of the weld.
that suggests an issue with purging.
i usually get spotless back side and i'm crude and rough using a hose and tin foil.
may be flow rate or simply not waiting for all the air to clear out before welding. or taking purge off to early after welding.
the other thing to look at is how the tacks are done. trying to do big tacks without purge can end up with sugar on the back, which later on gets welded over and the sugar comes to the top.
tweake wrote: ↑Wed Aug 04, 2021 12:11 am
what concerns me on the last pic is the heat tint, especially as it seams to be on one side of the weld.
that suggests an issue with purging.
i usually get spotless back side and i'm crude and rough using a hose and tin foil.
may be flow rate or simply not waiting for all the air to clear out before welding. or taking purge off to early after welding.
the other thing to look at is how the tacks are done. trying to do big tacks without purge can end up with sugar on the back, which later on gets welded over and the sugar comes to the top.
based on the discoloration is light straw and before welding we carefully check the argon filling in，i think it is ok with back purging.
but the "tacks" you mentioned I never think of，because welders tack without purge argon.
maybe this black spots appear at where the tacks are？
and when you tack welds do you purge shielding gas？
Try using a clean, sharp file and scraper for the final joint face preparation ?
I'm thinking back to the recommendation in Jody's video on a sheared edge prep for an aerospace aluminum butt weld test to eliminate inclusions in the weld.
tweake wrote: ↑Wed Aug 04, 2021 12:11 am
what concerns me on the last pic is the heat tint, especially as it seams to be on one side of the weld.
that suggests an issue with purging.
i usually get spotless back side and i'm crude and rough using a hose and tin foil.
may be flow rate or simply not waiting for all the air to clear out before welding. or taking purge off to early after welding.
the other thing to look at is how the tacks are done. trying to do big tacks without purge can end up with sugar on the back, which later on gets welded over and the sugar comes to the top.
based on the discoloration is light straw and before welding we carefully check the argon filling in，i think it is ok with back purging.
but the "tacks" you mentioned I never think of，because welders tack without purge argon.
maybe this black spots appear at where the tacks are？
and when you tack welds do you purge shielding gas？
its not the color of the discoloration, its the fact that its there at all. its just something that can be improved. you may have to much of an exhaust and the flow past the weld is sucking air in.
tacking is something i screw up especially as i'm usually dealing with poor fit up.
if its something i can clamp together well, i will purge when tacking. that way i can do big tacks.
otherwise its a couple of tiny blast tacks. which are super weak but its enough to hold it so i can get clamps third hand etc to hold it while it get the purge setup.
kiwi2wheels wrote: ↑Wed Aug 04, 2021 5:25 am
Try using a clean, sharp file and scraper for the final joint face preparation ?
I'm thinking back to the recommendation in Jody's video on a sheared edge prep for an aerospace aluminum butt weld test to eliminate inclusions in the weld.
aluminium is a bit different as its the oxide layer on the edge, where the arc can't get to it and clean it, that ends up in the weld and can cause cracking.
i doubt that applies to stainless.
tweake wrote: ↑Wed Aug 04, 2021 12:11 am
what concerns me on the last pic is the heat tint, especially as it seams to be on one side of the weld.
that suggests an issue with purging.
i usually get spotless back side and i'm crude and rough using a hose and tin foil.
may be flow rate or simply not waiting for all the air to clear out before welding. or taking purge off to early after welding.
the other thing to look at is how the tacks are done. trying to do big tacks without purge can end up with sugar on the back, which later on gets welded over and the sugar comes to the top.
based on the discoloration is light straw and before welding we carefully check the argon filling in，i think it is ok with back purging.
but the "tacks" you mentioned I never think of，because welders tack without purge argon.
maybe this black spots appear at where the tacks are？
and when you tack welds do you purge shielding gas？
its not the color of the discoloration, its the fact that its there at all. its just something that can be improved. you may have to much of an exhaust and the flow past the weld is sucking air in.
tacking is something i screw up especially as i'm usually dealing with poor fit up.
if its something i can clamp together well, i will purge when tacking. that way i can do big tacks.
otherwise its a couple of tiny blast tacks. which are super weak but its enough to hold it so i can get clamps third hand etc to hold it while it get the purge setup.
I noticed that welders always place TIG nozzle on the cooling welds for support when weave welding with pipe rotating，and the nozzle looks dirty，do you think it may lead to the black speck？
