Just leaving a gap between otherwise-unprepared edges does help penetration, but not much. Despite all the noise and fury, an arc does not dig very deep. Check this for yourself by welding a series of butt joints in 1/4" (6mm) plate, leaving an increasingly large gap between the plates each time. Weld one side of the joint only, and then break it open in the vise and look for penetration. You’ll probably conclude that the arc needs help to penetrate. The best way forward is to bevel the plate edges with a grinder or gas cutter.
Lower Limit
As material drops to 1/8" (3mm) or below, blowing holes in the work gets increasingly likely. For most mortals, 1/8" (3mm) sheet is SMAW’s lower working limit.
1.27 lays out the general idea. A total angle of 60º (that is, 30º on each plate) is the best compromise between accessibility and gap size, but in practice grind a little more to allow yourself plenty of elbow room. A wider Vee makes the first (or ‘root’) run easier: if the angle is too tight it can be a slag trap generator. Dodgy technique (insufficient current, travelling too fast, wrong rod angle or too short or too long an arc) is magnified when welding in a Vee.
1.27. To get to the bottom of a butt joint in material over 1/8" (3mm) thick, first get rid of the edges with a grinder or gas cutter (dotted lines). Then fire in the first run with good penetration (1).
MAKING THE WELD 3
Different joint types
Here’s a run-through of things to think about when SMAW-welding various joints.
First, the basic butt weld of prepared-edge plate (1.27, 1.28 and 1.29). Try really hard to get good fusion and penetration in the first (or root) run (1.27). If the Vee is tighter than 60º, open it out before you start welding. If this can’t be done, dial in a little more current than usual to minimize the chance of slag traps. Where the job allows, boost penetration by leaving a gap between the plates’ bottom edges, though don’t go wider than electrode thickness.
Whack in the first run with a small electrode; 3/32" (2.5mm) is usually fine. Using a thick rod at this stage can be counter-productive as it can’t reach into a Vee bottom as well as a slimmer one. Having said that, a thicker rod’s extra metal and current can help stop slag traps. Try it and see, but watch for lack of penetration.
1.28. Fill the Vee using overlapping runs – 1.30 shows the technique.
With a good root run in place, fill the Vee using the basic overlap technique in 1.30. Place each weld so it either overlaps or stays well clear of its neighbour, as the last thing you want is a narrow, slag-trapping gap between adjacent runs (1.28). Chip off all slag and exercise the wire brush after each pass — good ‘weld hygiene’ leaves no bits of slag behind to form weakening inclusions in the next run.
1.29. Finish a wide Vee with a weaved capping run – 1.37 shows ways to weave.
1.30. Overlap technique. Start with a single run, then build a pad by adding more runs so each half-overlaps its predecessor. Angle the rod at 45º and fire the arc at the spot where weld and plate meet. This basic method is brilliant for building up worn parts or re-surfacing.
1.31. In T-joints or fillets, weld throat depth (1) needs to at least equal the thinnest plate. Equal leg lengths (2) give balanced strength, and come from a 45º rod angle.
1.32. Often you’ll need more than one run to achieve enough throat depth. Sequence these as shown, using a shallower rod angle for the final pass (3) to keep it pushed well up.
Joint width grows as you fill it, allowing the use of thicker rods. Switch to a bigger diameter rather than going more slowly with a small rod, or slag traps will result where there’s not enough filler metal. If appropriate, finish off the joint with a weaved capping run (page 27). This will blend the weld’s edges into the original plates and ‘normalize’ the preceding passes (page 11). If you didn’t manage to achieve full penetration from the first run, shame on you — but you won’t be the first or the last. In such cases, turn the work over, grind the joint line into a U-shape then seal it with a single pass.
To form a strong T-joint or fillet, the weld’s throat depth needs to be equal to the thinnest plate (1.31). So, when joining 0.2 to 3/8" (5 to 10mm) plate, the throat should be at least 0.2" (5mm) deep. Equal leg lengths give balanced strength: achieve them by angling the rod so it splits the joint in two, i.e. forms a 45º angle. See also page 109.
Good fillet root fusion comes from a mix of careful plate alignment and steady work with a small diameter rod. If plates aren’t parallel, one inevitably takes more heat than the other, the gap between them varies and it’s easy to make a slag trap at the start. The best way around this is to let weld metal fully fill the joint before moving off, keeping speed down if necessary to be sure it does. Once fusion spreads across the joint, move off. Keep travel speed steady and arc length relatively short to make certain the joint is filled and free from slag traps.
Thick plates will need more than one pass to get the required throat depth.
1.33. The theory of 1.32 turned into metal. Starting off slowly let the three overlapping runs fuse full-width at the beginning of the joint (far right). See how they then blend into each other and the parent plates?
In a three-run joint, sequence the passes as shown in 1.32: see how number 2 gives number 3 a ‘shelf’ to sit on? Keep fresh weld metal well up on the vertical plate during pass 3 by using a shallower rod angle. And where necessary, go up one rod diameter for runs 2 and 3 to fill the space.
Run Number Alert
Don’t go overboard when making a multi-pass fillet. Use the minimum number of runs needed to build the right throat depth. Why? Over-heavy weld build-up often makes a joint weaker rather than stronger, as the extra heat can affect material properties. Stress is also concentrated at the edges of the weld.
If slag traps are a problem in fillet welds, check the four variables. Look at current first, as low setting is a prime cause of traps. Welding with limited amps (=limited heat) is always dodgy, as the bottom plate in a T-joint acts like a big sink, draining heat from the weld. On fillets, use 10-15% more current than for the equivalent butt joint, but don’t overdo it: check the vertical plate for undercut at the bead edge, as significant notching produces a weak area.
Where current