After this, lap joints are relatively easy (1.34). Tack plates together on the non-welded side where possible, and truck along at a speed such that the upper plate’s top edge just melts into the weld pool (arrow). If the joint looks ‘hollow’ or won’t fill properly, try the next-fatter rod. As with T-joints, slag traps come from too little current, travelling too fast or the wrong arc length.
Open corner joints — ones where plates meet to make a ‘roof’ and are welded on the outside — are just as straightforward as laps. Take great care over set-up; this is half the battle. Tack the joint halves so their edges are parallel, leaving a small penetration gap in material over 1/8" (3mm) thick. Travel along the joint at a speed that sees both outer plate edges melting into the weld pool. Use multi-runs to fill the Vee in thick material, sequencing passes as in 1.29.
Turning over an open corner joint leaves a trough. For practice weld this just as you would fill a prepared butt, using overlapping runs to build thickness as required. Deep Vees attract slag traps like a dead rabbit attracts flies, so use a little more current than normal to cut the risk.
1.34. Start a lap joint by tacking it so the plates are in good contact. Any gap risks the top plate’s exposed edge burning back. Travel at a speed that just melts-in the upper plate edge (arrow).
1.35. To keep a horizontal–vertical joint tidy, use a short arc, minimum current and don’t hang about. Angle the rod as shown to keep the bead pushed up into the joint.
Horizontal–vertical joints are those that run horizontally between vertical plates, as 1.35 shows. Welds in this position can end up with globs of weld metal to rival Babylon’s Hanging Gardens. Too much heat – from high current or travelling too slowly – is the usual cause, made worse where a long arc lets weld metal drop out of the joint. Dial in only enough current to give penetration and a smooth-running arc. Angle the rod to keep the bead pushed up into the joint, keep a constant and shortish arc. Don’t travel too slowly.
Last but never least, welding thin sheet to thick plate is not as hard as it might seem if the two are kept in close contact. Pic 1.36 shows 1/16" (1.6mm) sheet lap-jointed to 1/4" (6mm) plate. The corners were tacked to hold the plates tight together then each side was welded. The arc was aimed so that most of its heat went into the thick plate, leaving just enough warmth to fuse in the thin sheet’s edge (arrow). Watch the sheet during operations; if it starts to buckle, it’ll burn back quickly and you’ll end up in a pickle. Stop, dress the sheet back down and continue.
1.36. Welding thin sheet to thick plate is easy as long as the pair stay in close contact. Fire the arc mainly into the bottom plate, letting arc heat just melt the sheet metal into the bead.
MAKING THE WELD 5
Stops, starts and other business
Here are several techniques which will make the finished job stronger and neater…or get you out of trouble. The principles apply to GMAW work too.
Weaving is a useful trick used to spread heat and metal. You’ll need to weave in all sorts of places: making a ‘capping’ run on a substantial multi-pass joint; when putting down a layer of hard-facing metal; when welding uphill or downhill and when bridging gaps between plates. Whatever the job, the important thing is to follow a definite pattern and work very deliberately. Pics 1.37 and 1.38 show some patterns.
Whatever the purpose, weaving is best kept simple. Firstly, because that’s easier (never a bad thing) and secondly, to lessen the chance of trapping slag in the weld. Either a crescent weave or Z-shaped zig-zag serves most purposes other than vertical work.
How wide should you go? A rough rule of thumb allows up to three times electrode diameter, otherwise the weld’s edges may cool too much and you’ll get slag traps and/or ‘cold shuts’. In a cold shut, fresh metal lies over (rather than fuses with) the stuff you’ve just laid down, forming a weak point. The motto is ‘Take care of the edges and the middle looks after itself’, which means making a deliberate pause at each side to let metal flow, fuse and build before moving smartly across the weld face.
1.37. Either the crescent weave (as here) or a zigzag pattern is fine for capping runs. Be very sure to pause significantly at each edge (white dots), letting weld metal fill and fuse. Slag traps are likely if you rush or wave the rod tip about recklessly.
1.38. Where a simple weave would be overstretched, filling between two beads is the answer.
When you’re weaving make very deliberate rod tip movements. Waving the rod about pushes heat and metal around, inviting slag traps. Keep those at bay by weaving tight into the face of the weld, i.e. not travelling too far forward for each traverse. On jobs where a conventional weave would be overstretched, infill weaving between two beads can help (1.38).
First cousin to weaving is building up a thickness of weld metal. You’ll do it while filling a deep prepared-edge joint, when remaking worn bits or when hardfacing soil-engaging parts. Pic 1.30 (page 24) has the basics. First lay down a single bead, using a chalked guide line if it helps. Then add successive runs so that each overlaps the one before. Use a 45º rod angle, and fire the arc right into the place where the last run meets the plate. Adjust speed of travel so the new metal half-covers the old. Chip off, wire brush and inspect after each run. Repeat until the thickness needed is achieved, watching out for distortion through heat build-up.
A small snag with SMAW is that you have to keep changing rods. How you manage stops, starts and restarts has a big impact on weld strength and appearance.
Let’s start with stops. As a rod burns back keep an eye on the flux coating, particularly when using high currents. If flux starts to fly away from the tip in droplets, stop the weld right there; protection from the coating is disappearing. Plan on finishing all runs anyway when the rod burns to a 2" (50mm) stub. It’s never worth trying to get to the end of a joint before the rod expires: somehow you never make it and end up leaving a poor stretch of weld.
1.39. To set things up for a clean re-start, break the arc cleanly as the old rod burns back to 2" (50mm) length. This will leave a hollow pad (arrow).
As the old rod expires, set things up for a good restart by breaking the arc cleanly. Whisk the tip away to one side, leaving a shallow crater (1.39). Wire-brush the area clean.
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