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A T-joint joins the edge of one piece of steel to the face of another at 90 degrees. On 1/8-inch mild steel, it is a common MIG welding exercise and a useful real-world joint for brackets, frames, tabs, and light repairs. The main challenge is balancing enough heat for fusion without burning through the vertical or horizontal plate.

Tools and materials

Use a MIG welder capable of running solid wire with shielding gas. A 120-volt machine can handle 1/8-inch steel for short welds, but a 240-volt unit gives you more duty cycle and a wider adjustment range. For a clean, controlled setup, a 180-amp MIG welder is a practical choice.

• ER70S-6 solid MIG wire, preferably 0.030 inch for general 1/8-inch work
• 75/25 argon-carbon dioxide shielding gas
• Clean steel, clamps, an angle square, and a wire brush or grinder
• Auto-darkening helmet, gloves, flame-resistant clothing, and safety glasses
• Ventilation or fume extraction

Remove mill scale, paint, oil, and rust from both plates near the joint. Contamination causes porosity, spatter, and an inconsistent arc. A wire wheel and angle grinder are useful, but do not grind so aggressively that you thin the steel at the weld area.

Prepare and fit the T-joint

Set the upright plate on the flat plate and check it with a square. A tight fit is best for this thickness. Gaps make burn-through more likely and can pull the upright out of square as the weld cools.

Clamp the pieces securely, then place tack welds at both ends of the joint. For a long joint, add tacks every 2 to 3 inches. Check the angle again after tacking. If the parts move, correct them before welding the full joint; trying to force a distorted T-joint straight afterward may crack the weld or damage the steel.

Decide whether the joint needs one fillet weld or two. A single fillet is often adequate for light brackets where the load direction is predictable. Welding both sides improves strength and stiffness, but it adds heat and increases distortion. For structural or safety-critical work, use a qualified procedure rather than relying on a general setting.

Starting MIG settings

Use the machine’s chart as a starting point, then verify the settings on scrap of the same thickness. The exact voltage and wire speed vary between machines, wire brands, and gas flow.

These figures are only a starting range. The correct arc should sound fairly steady, with a controlled frying or sizzling sound. Set gas flow with the regulator while the trigger is released, and keep the nozzle close enough to shield the puddle without dragging it through the weld.

Weld the fillet

Hold the gun at approximately a 45-degree work angle in the corner of the T. Use a slight 5- to 15-degree travel angle, normally pushing the gun for a flatter bead and better visibility. A small travel angle is less likely to trap slag-like oxides or create excessive spatter.

For 1/8-inch steel, begin with a short straight bead rather than a large weave. Aim the arc at the root of the joint, but divide your attention between both plates. The puddle should wash onto the horizontal and vertical surfaces and visibly fuse with each one. A fillet leg size around 1/8 inch is suitable for many light-duty joints, but the required size depends on the design and load.

Run a 1- to 2-inch test weld, stop, and inspect it. If the bead sits like a rope on top, lower your travel speed slightly or increase heat. If the puddle becomes excessively wide, the edges sag, or a hole starts forming, move faster or reduce heat. For longer welds, use short sections and alternate sides to limit distortion.

A short back-and-forth motion can help distribute heat, but avoid a wide weave. Excessive weaving makes the weld larger without necessarily improving penetration. Pause briefly at each toe only if the edges are not fusing. Do not pause at the center of the puddle, where excess filler can build up.

Diagnose common failures

Cold lap or poor fusion: The bead overlaps the plate without melting into it. The usual causes are low voltage, excessive travel speed, a dirty surface, or aiming the arc at only one plate. Clean the joint, slow down slightly, and direct the arc into the root.

Burn-through: A hole forms or the back of the joint sags badly. Reduce voltage or wire speed, increase travel speed, and use shorter weld sections. A tighter fit also helps. Do not simply pile more metal over a hole; stop, clean the area, and repair it properly.

Porosity: Small holes appear in the bead. Check for wind, an empty or closed gas cylinder, a blocked nozzle, excessive spatter, or a loose gas connection. Gas flow that is too high can also create turbulence and draw air into the shielding area.

Uneven leg lengths: The gun angle or travel angle is inconsistent. Practice on scrap, keep the nozzle distance steady, and watch both toes of the bead instead of following only the shiny center.

Inspect and finish the weld

Let the joint cool naturally. Inspect both toes for continuous fusion, visible cracks, pinholes, undercut, and abrupt changes in bead size. Lightly wire-brush the weld and look again. Do not grind a questionable weld smooth just to make it look better; grinding can hide undercut and lack of fusion.

For noncritical work, a visual inspection and a bend or destructive test on a duplicate sample may be enough. If the joint will support people, vehicles, lifting equipment, pressure, or significant impact loads, have the design and weld procedure reviewed by a qualified professional. Select your helmet carefully as well: a basic auto-darkening welding helmet is adequate for occasional work, while a clearer, faster-reacting helmet is easier on the eyes during repeated welding.