FIGURE 1. This stainless-steel weld appears clear, apart from these small black spots. These are silica islands, and addressing easy methods to deal with them upfront, earlier than a job is accepted, can save loads of time and bother. Northern Manufacturing
The request got here in on a Wednesday morning. A buyer’s high quality inspector had reviewed images of a stainless weldment we shipped and located small black spots on among the welds. May we hit them with a wire wheel or a wire brush? He advised that, from a beauty standpoint, it might be higher to take away them.
The small black spots are generally generally known as silica islands (see Determine 1). We’d been down this street with one other buyer within the current previous, and that point it become an costly nightmare. However we realized lots in that course of, and earlier than anybody picked up a grinder, I sat down and wrote the shopper an extended electronic mail. That electronic mail turned this text.
Why These Spots Matter
Let me be clear, I’m not complaining concerning the buyer. The request sounds fully affordable. Black spots on vibrant stainless appear to be one thing went flawed, and our buyer is attempting to keep away from costly website rejections by its personal buyer. The issue is that nearly no one on both aspect of the acquisition order is aware of what the spots are, what the codes truly say about them, or what “simply clear them off” will value. We didn’t know both the primary time round.
Silica islands are a pure a part of gasoline metallic arc welding (GMAW, or MIG) on stainless, and the welding wire is the supply of the black spots. ER308L and ER316L carry silicon and manganese on goal, as deoxidizers designed to seize oxygen within the molten puddle earlier than it might turn into porosity or oxide inclusions contained in the weld metallic. What they type whereas doing that job is generally manganese silicate. It’s lighter than molten metal, so it floats, and when the puddle freezes it’s sitting proper on prime: small glassy deposits on the weld toes and on the crater the place the bead ties off. On the job that prompted this text, I discovered a silica island on the finish of just about each sew weld.
You’ve in all probability seen these on carbon metal a thousand occasions. They’re brown or amber, paint covers them anyway, and no one says a phrase. On stainless towards a vibrant pickled floor, the identical deposit reveals up as a bit of black spot—similar chemistry on a special backdrop, with a very completely different expectation.
Are they slag? No, and the excellence issues. AWS A3.0 defines slag because the nonmetallic product of flux-based processes, and solid-wire MIG has no flux. ASME BPE-2024 (for bioprocessing tools) attracts the identical line in its Nonmandatory Appendix A, which calls these deposits high-melting-point nonmetallic oxides, “usually known as oxide islands,” and separates them from slag.
Why care? As a result of the structural codes require slag removing, and I’ve seen CWIs and different inspectors lump silica islands in with slag. However the definitions don’t help it. Are they a power downside? Additionally no. The islands sit on the floor of the solidified bead. They don’t put inclusions within the weld metallic. Are they a corrosion downside? That’s a greater query. Reasonably than guess, we paid a lab to reply it.
Remove the Root Trigger
We first thought the issue was the 308LSi wire we have been utilizing. We switched it out and anticipated an enormous discount in silica islands. The change was virtually unnoticeable. Then we tried all the pieces: completely different wire, completely different gasoline blends, completely different pulse settings, completely different method, completely different welders. You may change what number of islands you get and the place they land, however you’ll be able to’t get to zero, as a result of the gasoline gained’t allow you to.
Why? Finest follow for MIG on stainless is an argon plus CO2 or an O2 mix. We run 98% Ar, 2% CO2. Pure argon provides you an erratic, wandering arc. And whether or not you employ CO2 or O2, the CO2 breaks down within the weld plasma and frees oxygen, and that oxygen causes the islands, regardless of how ideally suited your welding course of and procedures are. The identical oxygen that stabilizes your arc feeds the response with the silicon and manganese in your wire. A store promising silica-free MIG welds on stainless is promising one thing the chemistry doesn’t permit.
Gasoline tungsten arc welding (GTAW, or TIG) can get rid of them with good weld procedures and method, because it makes use of 100% argon, an inert gasoline. Even then, the ASME BPE code concedes {that a} small black spot at a weld termination is “usually unavoidable” on stainless. TIG works nice, particularly for thin-gauge or high-purity purposes.
Right here’s the catch: Making a TIG weld bigger than about 3/16 in. in a single move isn’t typical. TIG is already slower than MIG, and a ¼-in. or bigger TIG weld requires a number of passes. For big structural weldments, MIG is usually a lot more economical; its greater deposition price yields quicker journey speeds and achieves bigger weld sizes in fewer passes. Convert a weldment filled with 1/4-in.-and-up welds to TIG and also you’ve signed up for a big value enhance. Generally that’s the best reply. On hygienic course of surfaces, it may be price each greenback. On a wastewater help body? Maintain studying.
FIGURE 2. This bead-on-plate coupon, 304/304L base metallic with ER308/308L, was despatched to a lab (TMR Stainless) for testing. Northern Manufacturing
Looking the Code Books
When the primary dispute began, we assumed the world had already settled this, and we went searching for the requirement we have been supposedly violating. We searched each code doc and contractual requirement we may get our fingers on. We couldn’t discover one.
It’s not in AWS D1.6, the structural welding code for chrome steel. The visible acceptance standards cowl cracks, fusion, craters, profiles, undercut, and porosity. Oxide and silica islands don’t seem as a discontinuity in any respect. Sure, the code requires slag removing, however as famous, no flux means no slag. An proprietor can completely impose one thing stricter, but it surely must be written into the contract paperwork, not learn into the code after the components are on the dock able to ship.
It’s not in ASTM A380, the descaling (pickling) follow, both. A380 covers free iron, scale, warmth tint, and contamination—nothing about silica or oxide islands. Prospects assume pickling removes all of the black warmth tint when it’s performed correctly, however the nitric-hydrofluoric acid chemistry doesn’t contact the silica islands. A weldment may be pickled and passivated to the letter of A380 and nonetheless carry each island the welding course of left behind. But the default buyer assumption is that “pickled and passivated” means silver and spotless.
The one trade spec we discovered that mentions silica oxide islands by title is ASME BPE-2024. Its visible examination tables for vessels, tanks, pipe, and tube welds all carry the identical entry: Oxide islands are permitted on course of contact and nonprocess contact surfaces alike, so long as they’re adherent to the floor. The one place BPE says “none allowed” is course of contact surfaces after postweld ending, that means surfaces which can be getting polished to a spec end anyway.
BPE’s adherent-versus-loose distinction can also be the one which issues in your store. Loosely adhered islands come off with a nonwoven abrasive pad in seconds. Tightly adhered ones are a special animal. We tried far more aggressive abrasives, usually spending 5 minutes and extra per spot and risking eradicating sound weld metallic. The aftermath is ugly: The grinding leaves a shiny mark at each location that appears cosmetically unprofessional. Do this just a few hundred occasions and the weldment usually appears worse than if you began. In our expertise, the nonwoven pad clears many of the seen islands shortly. Preventing the final 10% to 25% is the place the cash goes.
What the Lab Stated
Throughout the first dispute, we did one thing I’d suggest to any store that lives on stainless: We give up arguing about principle and despatched samples out (see Figures 2 and 3). TMR Stainless, a metallurgical consultancy in Pittsburgh, ran two sequence of checks on MIG bead-on-plate coupons we welded, 304/304L base metallic with ER308/308L filler.
The primary check used a scanning electron microscope (SEM) and power dispersive spectroscopy on the islands themselves. Every of the 4 islands examined got here again as manganese-silicon-rich oxide, roughly 16% to 35% silicon, 10% to 37% manganese, and 18% to 43% oxygen by weight, with minor chromium, titanium, and aluminum. That’s the deoxidation story, confirmed by instrument as a substitute of inference—not slag, not contamination, however the wire doing its job.
The second check was the aggressive ASTM G48 Methodology A corrosion check. One coupon, carrying a big silica island on the weld crater plus a scattering of small ones alongside the bead, went into ferric chloride at 10 levels C for twenty-four hours. G48-A is a brutal pitting display, far harsher than something a 304 help body sees in service. The consequence was particular: Pitting initiated adjoining to the big crater island. The small islands? No assault in any respect, confirmed below SEM after publicity.
One coupon is directional proof, not a design rule. However the path is evident, and it matches the sphere: Small adherent islands shrugged off a extreme chloride publicity, and the large island on the termination was the weak level. In case your software includes aggressive chlorides or a really corrosive atmosphere, spec the BPE standards or require GTAW on the method contact surfaces. For extra regular purposes, blanket removing of each seen speck doesn’t appear to purchase a lot corrosion resistance. That matches our 75-year historical past. We’ve sometimes had a buyer query the islands at inspection, however we’ve by no means acquired a guaranty declare the place the islands have been the reason for a corrosion failure within the subject.
The Inspection Entice
On that first challenge, we wished to maintain the shopper blissful and maintain the job shifting, and the removing determination received made mid-job with no written acceptance criterion. So, “take away the silica islands” had no definition. We floor each inch of weld and eliminated all the pieces seen.
FIGURE 3. Assessments of the silicon islands revealed they have been manganese-silicon-rich oxides. TMR Stainless
Then the query turned what is nice sufficient? Inspecting and reinspecting each inch of weld at that degree of element is staggeringly time consuming, and every spherical added grind marks, recleaning, and hours no one had quoted. No one was performing in dangerous religion; there was simply no clear place to cease.
The repair is just not higher grinding. The repair is written standards agreed to earlier than work begins: viewing distance, lighting, whether or not adherent islands are acceptable, and which surfaces it applies to. AWS already handles weld porosity this fashion, the place a sure quantity under a sure measurement is appropriate. The ASME BPE code’s framework exists as a result of the bioprocessing trade went via the identical loop and wanted a defensible stopping level. For those who don’t wish to write your personal, borrow it.
Settle It within the Quote
In case your buyer can’t settle for silica islands on a challenge, that data is price severe consideration at RFQ time. Estimators can value the TIG welding or the MIG-plus-cleanup-plus-inspection prices up entrance. What no one can value is a brand new requirement that arrives after the welds are performed. What follows are 4 positions that work, most cost-effective first:
- All silica islands accepted – That is the usual for nearly all of our clients in all industries.
- Silica islands acceptable in the event that they move PT (dye penetrant) testing and don’t maintain dye – This consists of the price of PT testing and related repairs for giant silica islands.
- Adherent islands accepted – This references BPE-style standards: oxide islands permitted the place adherent, loosely adhered islands knocked off with a nonwoven abrasive pad. A small price is added for touching each weld.
- TIG on uncovered welds – This emphasizes prevention as a substitute of cleanup, priced actually, with the pass-count math proven so the shopper is aware of what it’s shopping for.
Any of the 4 is quotable. For its open contracts, our buyer settled on possibility 3—grinding the unfastened islands off—and that was ample. Then the shopper did the genuinely good factor: It modified its personal high quality requirements to permit silica islands and put that language in its contract paperwork so no future challenge would relitigate it.
Silica islands on GMAW stainless are regular. They’re the anticipated results of a sound weld made with the best wire and the best gasoline. They let you know the deoxidizers did their job. They exist alongside good workmanship, correct weld procedures, and correct pickling.
To my information, no related welding code explicitly says a silica island is not a defect, at the very least at this writing. To stop confusion, deal with the problem up entrance, earlier than the job begins. In any other case, it’s solely matter of time earlier than your cellphone rings with clients asking about these “black spots” once more.
