Structural steel is mostly welded with four arc processes: shielded metal arc welding (SMAW, or stick), flux-cored arc welding (FCAW), gas metal arc welding (GMAW, or MIG, along with its metal-cored cousin MCAW) and submerged arc welding (SAW). TIG (GTAW) is a structural process too, but it is slow and seldom used on building steel. What makes a weld structural is not the process. It is the procedure, the welder's qualification and the company's certification, which in Canada means CWB certification and welding to CSA W59.
This post compares the processes. For what structural welding is and where it happens, see what is structural welding and our post on shop vs field structural welding.
The processes at a glance
| Process | Common name | Shielding | Where it is used on structural steel | Main limits |
|---|---|---|---|---|
| SMAW | Stick | Flux coating on the electrode | Field welding, repairs, short or awkward welds, all positions | Slow, frequent electrode changes, slag to remove |
| FCAW-S | Self-shielded flux core | Flux in the wire | Field welding, outdoor work, erection connections | Fumes and slag; consumable choice matters for toughness |
| FCAW-G | Gas-shielded flux core, dual shield | Flux plus shielding gas | Shop fabrication, out-of-position welds | Gas needs protection from wind |
| MCAW | Metal core | Shielding gas, metal powder in the wire | Shop fabrication, high-production fillet welds | Gas needs protection from wind |
| GMAW | MIG | Shielding gas | Shop fabrication, lighter sections | Gas sensitive to wind; transfer mode matters |
| SAW | Submerged arc | Granular flux blanket | Long shop welds on plate girders and built-up members | Flat and horizontal positions only, mechanized |
| GTAW | TIG | Shielding gas | Thin material, stainless, pipe, root passes | Very slow for structural sizes |
CWB welder qualification covers all of these processes, and each welder is tested for the specific process and position they will weld in. Our post on CWB vs AWS certification explains how those tests work.
Shielded metal arc welding (stick)
SMAW uses a consumable electrode coated in flux. The arc melts the rod and the base metal, and the coating burns to form a gas shield and a layer of slag over the weld.
Why it is still everywhere on structural work:
- Simple, portable equipment. A power source and leads. No gas bottles, no wire feeder.
- Tolerates wind better than gas-shielded processes, because the shield comes from the electrode coating.
- Works in every position with the right electrode, which suits field connections, column splices and repairs.
The trade-offs are speed and handling. Deposition is slow, the welder stops to change rods, and slag has to be cleaned between passes. Structural stick welding mostly uses low-hydrogen electrodes (the familiar E7018, designated E4918 in metric), which must be kept dry, usually in a heated rod oven once the can is opened. Damp low-hydrogen rods are a known cause of hydrogen cracking.
Flux-cored and metal-cored welding
FCAW feeds a tubular wire filled with flux through a gun. It deposits metal much faster than stick and is the workhorse of many structural shops and erection crews. There are two families:
- Self-shielded (FCAW-S). The flux in the wire makes its own shield, so no gas bottle is needed. That makes it a strong choice for field welding, deck-level work and anywhere wind is a factor.
- Gas-shielded (FCAW-G, often called dual shield). The wire's flux works together with an external shielding gas. It gives good weld quality and handles vertical and overhead welds well, which is why it is common in the shop.
Metal-cored wire (MCAW) has metal powder in the core instead of flux, runs with shielding gas, and leaves little slag. It suits long production fillet welds in the shop.
Flux-cored consumables are not interchangeable. Where a project has toughness requirements (for example on seismic or low-temperature work), the consumable has to be chosen to meet them, and that choice is written into the welding procedure.
Gas metal arc welding (MIG)
GMAW feeds a solid wire and relies entirely on a shielding gas. It is clean, fast and easy to learn, which is why people ask "is MIG welding structural?" The answer is yes, in the right conditions, with two cautions.
Wind. With no flux, the only protection for the molten weld is the gas. A breeze on site can strip it away and leave porosity. That is the main reason GMAW and the other gas-shielded processes live mostly in the shop, or behind screens and enclosures when used outdoors.
Transfer mode. GMAW can transfer metal across the arc in different ways: short circuit, globular, spray and pulsed spray. Short-circuit transfer runs cool, which is good for thin sheet but prone to lack of fusion on thicker structural sections. AWS D1.1 does not treat short-circuit GMAW as prequalified, and in Canada the transfer mode is something the accepted welding procedure has to address. Spray and pulsed transfer are the modes used for heavier structural fillets and grooves in the flat and horizontal positions.
A hobby MIG machine running short-circuit on a beam connection is not structural welding, however neat the bead looks.
Submerged arc and TIG
Submerged arc welding (SAW) is a mechanized process. The arc runs under a blanket of granular flux, so there is no visible arc, very little spatter and very high deposition. It is used in shops for long, straight welds such as the web-to-flange welds on plate girders and built-up columns. It is limited to flat and horizontal fillet positions and needs the piece set up in a fixture or on a carriage, so it is not a field process.
Gas tungsten arc welding (GTAW, or TIG) uses a non-consumable tungsten electrode and adds filler by hand. It gives excellent control and clean welds. Is TIG welding structural? It is a recognized process for welder qualification, but on structural steel it is far too slow for most connections. It turns up on thin material, stainless steel, small tubing and pipe, and sometimes for root passes.
Two other processes are worth knowing by name:
- Arc stud welding fixes headed shear studs to beams through steel deck for composite floors, and anchors to embed plates. It has its own qualification requirements.
- Electroslag and electrogas welding are specialist processes for very thick joints in heavy fabrication. You will rarely meet them on building work.
Shop vs field: which process ends up where
The process is usually decided by where the weld is made and what the joint needs.
| Situation | Typical processes |
|---|---|
| Shop fillet welds on beams, plates, clips and stiffeners | FCAW-G, MCAW, GMAW (spray or pulsed) |
| Long shop welds on built-up girders | SAW |
| Field welding outdoors, column splices, erection connections | SMAW, FCAW-S |
| Field repairs and small additions | SMAW |
| Thin, stainless or decorative work | GMAW, GTAW |
| Shear studs on composite floors | Arc stud welding |
Weather, access and position count as much as the process. Codes and procedures also have requirements on preheat, minimum temperature, moisture on the steel and protection from wind. Those values depend on steel grade, thickness and process, so we don't quote them here; they come from CSA W59 and the accepted procedure. For why most welding belongs in the shop in the first place, see bolted vs welded connections.
What makes a weld structural in Canada
Any of the processes above can make a sound structural weld, and any of them can make a bad one. The things that turn a weld into a structural weld are:
- An accepted procedure. The company's welding procedure data sheets set the process, consumable, joint, position and parameters. They are submitted to the CWB Group, which accepts them, asks for revisions, or requires qualification testing. Our post on welding procedures (WPS and WPDS) covers how they work.
- A qualified welder. Each welder is tested for the process, position and type of joint they will weld, while working for a CWB-certified company.
- A certified company. CSA S16, the steel design standard the Ontario Building Code references, requires fabricators and erectors doing the welding to be certified to CSA W47.1 (Division 1 or 2). See why hire a CWB-certified fabricator.
- The right weld, inspected. The weld type and size come from the engineer's drawings, and workmanship and inspection follow CSA W59. Our guide to structural weld defects and inspection shows what inspectors look for.
So when someone asks whether flux core or MIG is "strong enough", the better question is whether the weld was made to an accepted procedure by a qualified welder in a certified shop. If it was, the process has already been matched to the job.
Welded steel from a CWB-certified shop
Our shop welding is certified by the Canadian Welding Bureau, and we fabricate beams, columns, posts, plates and connections to your shop drawings, so most of the welding is done in the shop before the steel reaches site. If your drawings call for a particular process or field weld, tell us when you send them and we will tell you how we would handle it.
Send your drawings for a quote, or see our services for fabrication, shop drawings, delivery and installation across the GTA and Ontario.
Frequently asked questions
Is MIG welding structural?
It can be. Gas metal arc welding (MIG) is used for structural welds in fabrication shops when the welder is qualified, the procedure is accepted and the transfer mode suits the joint. Its weak points are wind, which blows away the shielding gas, and short-circuit transfer, which can leave lack of fusion on thicker steel.
Is TIG welding structural?
TIG (GTAW) makes high-quality welds and is a recognized process for welder qualification, but it is slow and rarely used for building steel. You see it on thin material, stainless, pipe and root passes rather than on beam connections.
Is structural welding stick welding?
Stick welding (SMAW) is one of the main structural processes, especially in the field, but it is not the only one. Shops lean on flux-cored, metal-cored, MIG and submerged arc welding because they deposit metal faster.
Is flux core welding strong enough for structural steel?
Yes. Flux-cored arc welding is one of the most common processes on structural steel in shops and on site. Like any process, the weld has to be made to an accepted procedure, with the right consumable, by a welder qualified for that process and position.
