TIG vs MIG welding: which process a plant job needs

The rest of this guide explains why, in terms a plant engineer or buyer can put into a specification or a conversation with a fabricator.

What is the difference between TIG and MIG?

The difference is where the filler metal comes from. In TIG welding the arc burns from a pointed tungsten electrode that is not consumed, and the welder adds filler separately, by hand, from a rod. In MIG welding the wire is fed continuously through the torch and is both the electrode and the filler: the arc melts the wire into the joint as it feeds.

Both shield the molten weld from the air with gas. TIG uses an inert gas, and TWI, the welding institute based in the UK, lists argon as the usual choice for steels and stainless. MIG uses inert gas or, for steels, active mixtures of argon with carbon dioxide or oxygen. Strictly, that version is called MAG (metal active gas), and TWI notes the shielding gas is the only thing separating MIG from MAG. On site, you will hear MIG used for both.

Everything else follows from that difference in how the metal goes in. TIG gives the welder separate control of heat and filler, which is why it is precise. MIG feeds metal continuously, which is why it is productive.

When is TIG the right process?

TIG suits work where control matters more than speed. TWI describes its small, intense arc as ideal for thin material and for controlled penetration "in the root run of pipe welds".

On a process plant, that points to:

• Root runs on pipework, where the inside of the joint is the surface the product touches.
• Thin stainless tube and sheet, where a small, controllable arc is easier to manage.
• Joints where the inside matters, for hygiene or for corrosion, because the root is the part of the weld that forms it.

The trade-off is speed. Because the filler goes in from a separate rod, TWI notes that deposition can be quite low, and that MIG or manual arc "may be preferable for thicker material and for fill passes in thick-wall pipe welds".

When is MIG the better choice?

MIG suits work with a lot of metal to put down. TWI describes it as suitable for both thin sheet and thick sections, and productive because the wire is continuously fed.

That makes it a natural fit for heavier fabrication: structural steel, platforms and supports, tank and hopper work, and fill and cap passes on thicker joints. TWI also notes it lends itself to mechanisation.

It has limits that matter on a plant:

• Wind. TWI is direct about it: MIG cannot be done outdoors without enclosures, because the shielding gas has to be protected from the wind. On site that means screens, a sheltered position or a different process.
• Surface condition. TWI notes that rust has to be removed from the workpiece before welding starts. Old carbon steel on a maintenance job needs proper preparation first.
• Fusion. In its low-heat short-circuit mode, TWI notes that skill is needed to avoid lack of fusion, where the weld metal has not properly joined the metal around it. TWI also says MIG demands a high degree of manipulative skill, so speed and ease are different things.

Why would a joint use both?

Because each process is strongest at a different part of the joint. A pipe weld is built up in passes: a root run that closes the joint from the inside, then fill passes to build up thickness, then a cap. The root needs control; the fill needs metal.

That is how our own work is sequenced. On a fabricated pipe spool, Austek's process runs cut and prep, fit-up and tack, then a TIG root pass, purged where the specification calls for it, followed by fill and cap with heat input kept under control, and inspection and sign-off before it goes live. The steps are set out in the process section of our homepage.

Which process fills the joint after the TIG root depends on the thickness, the material, the position and what the welding procedure for that joint says. That is the fabricator's call to make and to write down, and it is a fair question to ask.

Does stainless change the answer?

Stainless raises the stakes on the root, which is where TIG is strongest. What welding does to stainless, and how a fabricator puts the corrosion resistance back, is covered in our guide to 304 vs 316 stainless: purging the inside of the joint, controlling heat input, cleaning off heat tint, and why the low-carbon L grades matter for welded work.

TIG also relies on a gas shield, so the same care over draughts applies to it on an open site, and so does any purge gas inside the pipe. Ask how both will be protected where the joint actually sits.

What should you put in the specification?

The choice of process can sit with the fabricator's welding procedure. What you need to know is which process is being used, and that the welder and the procedure are qualified for it.

Worth settling before work starts:

1. Is there a welding procedure for this joint, and which process does it name for the root and for the fill? The procedure is the written recipe; the process is part of it.
2. Is the welder qualified on that process? A welder's certificate names the process tested, and each process is tested separately. Our guide what is a coded welder? explains how to read the ranges on a certificate.
3. Where will the joint be welded: in the workshop or in position on the plant? On site, ask how the gas shield and any purge will be protected from wind and draughts.
4. What does the inside of the joint have to look like? For hygienic or corrosive service, say so. It decides how the root is welded and how it is inspected.
5. How will the finished weld be checked? Our own sign-off includes a visual inspection and a dimensional check; anything beyond that should be agreed before the job is quoted.

Where Austek fits

Austek lists MIG, TIG and arc welding on stainless and carbon steel among its services, for process pipework, hygienic plant, boilermaking and structural work across Gauteng from our base in Brakpan. If you are not sure which process your job calls for, send the drawing or describe the joint, the material and where it sits on the plant, and we will tell you how we would weld it and why.

Shorter answers to common questions are on the frequently asked questions page. If the work falls inside a planned outage, see how to prepare for a plant shutdown.