Preparing a Joint for Welding

A weld can only be as good as the joint under it. Dirt turns into porosity, a bad gap turns into a burn-through, and a poor tack turns into a crack. Experienced welders often spend more time preparing the joint than welding it.

What You Will Be Able to Do

• Clean each common metal the right way.
• Decide when a joint needs a bevel, and set a root face and gap.
• Fit parts and check them before you tack.
• Place tacks that hold and that weld over cleanly.
• Use simple methods to keep parts from pulling out of shape.

Why Cleaning Comes First

Mill scale, rust, oil, paint and moisture all put unwanted material into the weld pool. As they burn, they make gas. That gas gets trapped as the metal freezes and leaves small holes, called porosity. Contaminants can also stop the weld from fusing to the base metal, and oil and moisture add hydrogen, which can cause cracks.

Clean the weld area on both sides of the joint. A good rule is at least 1 inch back from the joint edge, and your procedure may give a specific distance.

Remove oil before you grind or brush. If you do it the other way round, you drive the oil into the metal.

Aluminum Oxide

Aluminum has a tough oxide skin. The oxide melts at about 3,700 F, while the aluminum under it melts at about 1,220 F. The metal can melt under the skin before the skin breaks, which blocks fusion. Use a stainless wire brush kept only for aluminum, and weld soon after brushing because the oxide grows back.

Warning: Never clean a joint with a chlorinated solvent near where you will weld. Arc light can turn the vapor into phosgene, a highly toxic gas. Let solvents evaporate fully, and keep rags and cans out of the work zone.

When a Joint Needs a Bevel

A square-edged butt joint only fuses so deep. On thicker metal, you cut a bevel so the arc can reach the root. As a general starting point, plate over about 3/16 inch needs a bevel for a full penetration butt joint. The exact limit depends on the process and the procedure.

Typical details for a single-V groove:

These are common values only. Your welding procedure or the joint detail on the print sets the real numbers, and a code may set tight tolerances on them.

A narrow angle saves filler but makes it hard to reach the root. A thin land burns through, while a thick land stops the arc from getting through.

Bevels can be cut with a grinder, a beveling machine, plasma or an oxy-fuel torch. After thermal cutting, grind the surface to clean metal. Oxide and slag left on a torch-cut edge cause porosity and poor fusion.

Fitting the Parts

Fit-up is how the parts sit together before welding. A good fit gives you the same gap and alignment from one end of the joint to the other. If the gap changes, your heat and travel speed have to change too, and that is where defects start.

1. Check the print for dimensions, angles and any marks.
2. Clean the joint and the surfaces that touch.
3. Set the parts on a flat surface or in a fixture.
4. Clamp them, using C-clamps, locking pliers, magnets or a fixture.
5. Set the gap with a gauge. A piece of filler wire or rod of the right diameter works as a spacer.
6. Check square, level and alignment.
7. Check the fit again after clamping, because clamps can shift parts.

For fillet welds, keep the parts tight. A gap at the root of a tee joint weakens the weld, and some codes make you increase the weld size if the gap is too large. For butt joints, check for misalignment, also called hi-lo, where one piece sits higher than the other. Do not hammer a poor fit into place without asking. If the fit is out of tolerance, stop and tell your lead before you weld.

Tack Welding

A tack weld is a short weld that holds parts in place until the final weld is made. A tack becomes part of the finished weld on most jobs, so it has to be sound.

• Use the same process and filler as the final weld, unless the procedure says otherwise.
• Follow the same preheat as the final weld.
• Keep tacks small enough to weld over: about 1/4 to 3/8 inch long on thin metal, and up to about 1 inch on heavy plate.
• Space tacks about every 4 to 6 inches on thin metal and 6 to 12 inches on thicker metal, and closer if the part is pulling.
• Tack the ends of a joint first, then the middle, and work back and forth to balance the pull.
• Clean each tack before you weld over it.

Grind the ends of a tack to a taper on critical work, so the root pass can tie in with no bump. This is called feathering.

If a tack cracks, grind it out and redo it. Welding over a cracked tack leaves the crack in the weld.

Controlling Distortion

Weld metal shrinks as it cools, and it pulls the parts with it. You can reduce this by planning before you weld.

Worked Example: Presetting a Tee Joint

You are welding a 1/4 inch fillet on one side of a 12 inch tee joint, and a test piece pulled up about 2 degrees. For the real part:

1. Tack the upright piece about 2 degrees past square, leaning away from the weld.
2. Clamp the base to the table.
3. Run the weld and let it cool in the clamps.
4. Check it with a square. It should pull close to 90 degrees.

If it is still off, adjust your preset on the next part. Note the amount so you do not start over next time.

Before You Strike the Arc

Run through this list on every joint:

• The material and thickness match the print.
• The procedure, if there is one, is at your station.
• The joint is clean to the required distance on both sides.
• The bevel, land and gap are in tolerance.
• The parts are square, aligned and clamped.
• The tacks are sound and clean.
• Preheat is reached, if required.
• Your work clamp is on clean metal, near the joint.

Common Mistakes

• Skipping the back side of the joint. Contamination there still reaches the weld.
• Using a carbon steel brush on stainless. It leaves iron that rusts and ruins the finish.
• Welding on a torch-cut edge without grinding. Oxide causes porosity and poor fusion.
• Bridging a big gap with extra weld. It adds heat, distortion and defects. Fix the fit.
• Tacking a long joint from one end. The gap closes up ahead of you, and the far end ends up wrong.
• Leaving a cracked tack in place. It remains in the weld as a crack.

Check Your Understanding

You are about to weld a plate with light oil on it. You plan to run a wire wheel over it to clean it. What is wrong with that?

A wire wheel smears oil into the metal. Remove the oil first with a suitable solvent, let it dry, then brush or grind.

Your gap is 1/8 inch at one end and 1/4 inch at the other. The procedure calls for 1/8 inch. What do you do?

Stop and fix the fit. A changing gap means changing heat, and a wide root can blow through. If you cannot bring it into tolerance, tell your lead before you weld.

A beam-to-plate tack cracked while it cooled. Can you weld over it?

No. Grind the tack out fully and replace it, and find out why it cracked. Missing preheat is a common cause.

You are welding a long seam on thin sheet and it keeps bowing. Name two changes you could make.

Weld in short back-step or skip sections, and clamp it to a heavy table or use a backing bar. Keeping the weld to the required size also helps.

Key Takeaways

• Clean first, and remove oil before you grind or brush.
• Every metal has its own cleaning method. Keep stainless and aluminum tools separate.
• Bevels, lands and gaps come from the procedure or print, not from habit.
• A joint with an even gap and good alignment is easier to weld well.
• Tacks are part of the weld. Grind out any that crack.
• Plan for shrinkage before you weld, with clamps, balance, sequence and preset.