Cutting and Gouging by the Numbers
Chapter 4 of 10 · 10 min read
Back-gouging and defect removal
Back-gouge double-sided welds to sound metal, size the groove and the weld metal to refill it, compare gouging with grinding, and excavate cracks with tapered ends and checks until the defect is gone.
Most gouging in fabrication and repair has one purpose: to reach sound metal. When a double-sided butt weld is made, the root of the first side may contain slag, lack of fusion or cracks, so the second side begins by gouging the root out until clean weld metal shows. When a weld or casting is cracked, the crack is gouged out until no trace of it remains, and the groove is then welded. In both cases, too little gouging leaves the defect in the joint, and too much wastes time, weld metal and heat. The gouge is therefore a measured operation, with a clear target, a check and a written record.
This chapter explains why and how welds are back-gouged, calculates the groove and the weld metal it costs, compares gouging and grinding for the job, and sets out how cracks and defects are excavated and checked. Weld Repairs That Hold covers the repair welding that follows.
Why back-gouge
A full-penetration butt weld welded from both sides, on plate too thick to penetrate fully from one side, has a root where the two sides meet. The first side’s root pass is made against an open joint or a root face, and its underside is often irregular, with slag trapped at the root face or areas where the root did not fuse fully. If the second side is welded directly over it, those defects stay inside the finished weld, at its middle, where examination may find them and where they weaken the joint. Back-gouging the second side removes the first side’s root down to sound weld metal, and leaves a clean, U-shaped groove that the second side’s welding can fuse into completely.
Welding procedures for double-sided joints usually specify the back-gouging: the method, the depth to sound metal, the groove’s shape, and the cleaning and checking before the second side is welded.
Sizing the back-gouge
The back-gouge must be deep enough to reach sound metal, which depends on how well the first side was welded, and its shape must let the second side fuse fully at the bottom and sides. A U-shaped groove about as wide as the electrode diameter plus a few millimetres, and deep enough to pass through the first side’s root, is typical.
- Groove
- about 10 mm wide, 6 mm deep, about 48 mm²
- Seam
- 6 m
- metal removed ≈ 48 × 6,000 ≈ 288,000 mm³, about 2.3 kg
- gouging at about 400 mm/min ≈ 15 minutes of arc time
- the second side needs about 2.3 kg more weld metal than if the root had needed no gouge
The back-gouge’s weld metal is a real cost, about 2.3 kg of extra weld metal on the 6 m seam here, plus its welding time and heat. A root that is better made on the first side, with good fit-up and a sound root pass, needs a shallower back-gouge; a groove gouged deeper and wider than needed wastes metal twice, once in gouging and again in welding. Some procedures replace back-gouging altogether with a backing bar, a ceramic backing tape, or a root made by a process that gives reliable full penetration from one side, which avoids the gouge, its noise and fume, and its extra weld metal.
Gouging or grinding
Both arc-air gouging and grinding can back-gouge, and the choice depends on the length and depth of the groove and the material.
| Removal rate | Groove quality | Best for | |
|---|---|---|---|
| Air carbon arc | about 10 kg/h of arc time | smooth U; thin layer to grind off | long, deep grooves in carbon and low-alloy steel |
| Plasma gouging | lower than heavy arc-air | smooth; little contamination | stainless, aluminium; lighter grooves; quieter work |
| Grinding | about 0.5 kg/h or less with a disc | precise, clean | short or shallow grooves; final cleaning; sensitive materials |
On the 6 m seam above, gouging removes the 2.3 kg in about 15 minutes of arc time; grinding at about 0.6 kg an hour would take nearly four hours. Gouging is the obvious choice for long, deep back-gouges in carbon steel, followed by a short grind to clean the groove. For short grooves, shallow roots and stainless steel, grinding or plasma gouging is often better, because the set-up for arc-air, the cleaning afterwards and its noise and fume outweigh its speed.
Checking the back-gouge
Before the second side is welded, the back-gouged groove is checked: visually for any remaining slag line or unfused root at its bottom, and, where the procedure requires or the first side was difficult, with dye penetrant or magnetic particle testing. A dark line along the bottom of the groove usually means the gouge has not yet reached sound metal; a bright, even groove bottom means it has. The groove’s depth and width are checked against the procedure, because a groove much deeper than intended changes the weld metal and heat of the second side.
Excavating a crack
A crack in a weld or a casting must be removed completely before it is welded; welding straight over a crack simply leaves it in place, often to grow from the new weld’s shrinkage. The crack is gouged or ground out along its length and to its full depth, and the excavation is shaped so that it can be welded soundly.
- Find the endsdye penetrant or magnetic particle shows the crack's full length, beyond what the eye sees
- Stop the endsdrill small holes just beyond each end where the procedure calls for them
- Gouge along the crackto a depth where it is no longer visible; preheat first if the material needs it
- Check and repeatclean, test again; gouge deeper or longer where the crack still shows
- Shape the grooveboat-shaped, with tapered ends and sides open enough to weld the bottom
- Recordlength, depth and shape of the excavation; test results; then weld
The excavation is commonly carried a little beyond the visible ends of the crack, because cracks taper to fine tips that neither the eye nor a test may show until the surrounding metal is removed. Its ends are tapered, not cut off square, so that the repair weld can start and finish on a slope rather than against a wall, where lack of fusion and cracking are likely. Its sides are opened enough for the electrode or torch to reach and fuse the bottom of the groove.
The volume of an excavation
An excavation’s volume sets the weld metal and time of the repair.
As an illustration, a crack 300 mm long and 15 mm deep in a thick casting is excavated to a boat-shaped groove about 360 mm long, including tapers of about 30 mm at each end, about 25 mm wide at the top and 15 mm deep. Taking its volume as about 0.45 of length times width times depth, to allow for the boat shape, it is about 0.45 × 360 × 25 × 15 ≈ 61,000 mm³, about 0.48 kg of metal. Refilling it with about 10% reinforcement needs about 0.52 kg of weld metal, which the repair procedure’s deposition rate turns into an arc time. The excavation is always measured again after it is finished, because its real size often differs from the first estimate once the crack’s full extent is found.
Defects in new welds
The same practice applies to defects found by examination in new welds: porosity, slag, lack of fusion or cracks reported by radiography or ultrasonic testing. The defect’s position along the weld and its depth from the surface, from the examination report, guide the excavation; it is gouged or ground out from the side nearer the defect, checked, and rewelded, then examined again. Pipe Welding by the Numbers notes that a repair costs several times the original weld; much of that cost is the excavation, its checking and the second examination.
Preheat and gouging
Gouging puts concentrated heat into a small region and then lets it cool quickly, which can harden or crack hardenable and crack-sensitive materials. Where the repair procedure calls for preheat before welding, it usually calls for the same preheat before gouging, and for gouging soon after preheating while the part is still hot. On large castings and heavy sections of alloy steel, gouging cold can create new cracks at the edges of the groove, which then have to be gouged out in turn.
Removing old repairs
Older equipment often carries old repairs, some of them poor. Before a new repair is made at the same place, the old weld is often gouged out completely, because it may contain defects, the wrong filler, or hard and cracked metal. Identifying the old weld’s extent, by etching or by the colour and texture of the gouged metal, and removing it to sound parent metal is part of the repair’s preparation. The repair bay records every old repair it finds, with photographs and notes on what it found inside, because old repairs explain many of the cracks it sees and help predict where the next one will appear.
Cleaning the groove
Whatever method made the groove, it is cleaned before welding. Arc-air grooves carry a thin layer of oxidised, possibly carbon-enriched metal and loose slag, which a short pass with a grinding disc or a burr removes to bright metal. Plasma-gouged grooves need less cleaning but are still brushed or lightly ground. The groove and a band each side are cleaned of spatter, oil, paint and moisture, as for any weld preparation. The groove’s profile is checked too: ridges and steps left by an uneven gouge are ground smooth, because a welder filling a ridged groove can trap slag in the valleys, and the bottom of the groove must be rounded, not a sharp V that the root pass cannot fuse.
Distortion from gouging
A long, deep gouge removes metal from one side of a part and heats it, and the part can move, much as it does when welded on one side. A plate back-gouged along a long seam may bow towards the gouged side; a thin-walled casting can warp. The second side’s weld often reverses some of the movement, but not always all of it. Where distortion matters, the gouging is done in stages, alternating sides or areas, or the part is restrained while it is gouged and welded. Welding Distortion by the Numbers covers the shrinkage and restraint involved.
Rivets, bolts and lugs
Gouging also removes things: rivet heads before the rivets are punched out, seized bolts and studs, temporary lifting lugs and attachments, and old wear plates. The aim is to remove the unwanted metal without gouging into the parent part beneath. A skilled gouger washes a rivet head off almost flush with a low electrode angle, leaving a thin skin that is then ground or chipped away, rather than gouging into the plate; a temporary lug is cut off a few millimetres proud of the surface and the remainder ground flush. Any gouge or nick into the parent part is a defect in its own right on a load-bearing part, and is repaired and checked as the procedure requires.
The running case: a dragline bucket
This is an illustration. A dragline bucket arrived with a crack about 450 mm long along a heavy fillet weld joining a lip casting to the bucket’s floor plate. The repair bay found its ends with magnetic particle testing, preheated the area as the repair procedure required, gouged the crack out with arc-air in three stages, testing after each, and found that it ran about 60 mm further at one end than the visible crack. The final excavation was about 560 mm long and up to 20 mm deep. The groove was ground clean, tested clear, measured and recorded, and welded to the procedure. The bucket returned to service the following week, and the excavation record went into its repair history.
A failure: the shallow gouge
This is an illustration. Under pressure to return a machine to service, a gouger back-gouged a double-sided weld only until the groove looked clean, without the dye penetrant check the procedure required, and the second side was welded. Ultrasonic testing of the finished joint found lack of fusion along about a third of its length at mid-thickness: the back-gouge had not reached the first side’s unfused root over that length.
The joint had to be gouged out again from one side, much more deeply than the original back-gouge would have needed, and rewelded. The repair bay now treats the check of the back-gouge as a hold point that a supervisor signs before the second side is welded.
Exercise: back-gouge or backing?
A girder has 40 m of double-V butt welds in 25 mm plate. Back-gouging to a groove about 12 mm wide and 7 mm deep, with an area of about 0.8 of width times depth, would need refilling. How much weld metal does the back-gouge add? At a deposition rate of 4 kg/h and an operator factor of 40%, how many paid hours does refilling add, before the gouging itself?
The groove’s area is about 0.8 × 12 × 7 ≈ 67 mm². Over 40 m it removes, and the second side must refill, about 67 × 40,000 ≈ 2.7 million mm³, about 21 kg of weld metal.
At 4 kg/h, refilling needs about 21 ÷ 4 ≈ 5.3 hours of arc time, and at a 40% operator factor about 13 paid hours, before the gouging, grinding and checking. That is the saving a joint design or root procedure that avoids back-gouging would offer, against whatever the alternative costs in fit-up, backing or a more demanding root.
