Air carbon arc gouging and back-gouging

On Tank 14, the insert plate has been rolled, fitted into its opening and welded from the outside: the root and the first fill passes on the outside V of the double-V groove are in. Before the inside V can be welded, the root has to be back-gouged from the inside. The first passes of a weld from one side seldom fuse completely at the root of a double-V joint; there is often a line of incomplete fusion, slag or porosity at the very bottom of the first pass. Back-gouging removes that root from the second side, down to sound weld metal, so that the second-side welding fuses with clean metal and the joint is completely penetrated. On Tank 14, Dale does it with an air carbon arc gouging torch, the tool boilermakers reach for to remove weld metal fast.

How air carbon arc gouging works

Air carbon arc cutting and gouging, CAC-A, uses an arc between a copper-coated carbon electrode and the work to melt the metal, and a jet of compressed air blowing along the electrode to blow the molten metal out of the groove. Unlike oxyfuel, it does not depend on burning the metal, so it works on stainless steel, cast iron and other metals as well as carbon steel. It removes metal very fast and leaves a smooth, U-shaped groove that is ready for welding after light cleaning, which is why boilermakers use it for back-gouging, for removing defects found by radiography, for taking out old welds and for removing hard-faced or clad layers.

The equipment is a constant-current welding power source, usually used on DC electrode positive, a gouging torch with jaws that hold the electrode and air holes that point along it, and a supply of clean, dry compressed air at the pressure and volume the torch maker specifies, typically around 80 to 100 psi. The air volume matters more than the pressure: a small compressor that holds pressure but cannot deliver enough volume gives a weak jet, and the molten metal stays in the groove as a carbon-rich deposit.

Running a good gouge

Gouging is a skill of angle, speed and steadiness. The electrode sticks out about 6 in from the jaws, the air jet is on the underside of the electrode, between it and the work, and the torch is pushed, with the electrode pointing in the direction of travel, at a shallow angle to the surface. A steeper angle cuts a deeper, narrower groove; a shallower angle cuts a wider, shallower one. Travel speed sets the depth too: faster travel, shallower groove. The arc is short, and the operator keeps it short by feeding the electrode forward steadily as it burns away.

Carbon left in the groove is the fault that matters most on pressure work. Molten metal that is not blown clear picks up carbon from the electrode, and if it freezes in the groove, it leaves a hard, brittle layer that can crack when welded over. That is why many WPSs for alloy steels and stainless steels require the gouged surface to be ground to bright metal, removing a set depth, before welding, and why every competent gouger grinds the groove lightly after gouging even on carbon steel, to remove the thin oxide and any carbon pickup.

Back-gouging to sound metal

The purpose of a back-gouge is to reach sound metal, not to make a groove of a set depth. Dale starts the gouge on the inside of the Tank 14 seam with a shallow first run along the root, looks into the groove for the dark line or pinholes of the incomplete root, and makes a second, deeper run where he sees them. When the groove bottom is bright and uniform, he stops, lets Kiera grind it clean, and then examines it. On Tank 14, the back-gouged groove gets a visual check under good light and a magnetic particle examination; a line indication means the gouge has not reached sound metal yet. Harjit records the check on the traveler as a hold point before the inside welding starts.

A back-gouge should also make a good weld preparation. The groove should be a smooth U with a rounded bottom, wide enough for the electrode the welder will use on the inside root pass, and with no sharp corners where slag could be trapped. A groove that is too narrow and deep is as bad as one that does not reach sound metal.

Kiera works out the gouging time, the electrodes and the air for the inside root of the insert plate.

The section of a rounded U gouge of width w and depth d is about three-quarters of the rectangle that contains it; multiplied by the joint length and the density of steel, it gives the weight of metal removed.

# Back-gouging the Tank 14 insert plate from the inside (US units)

# carbon electrode: diameter -> (amps, gouge width, electrode used per minute of arc) - illustrative
elec = {0.25: (300, 0.38, 4.0), 0.3125: (420, 0.47, 4.5), 0.375: (500, 0.56, 5.1)}
d = 0.3125
amps, width, burn = elec[d]
depth = 0.31                           # gouge to sound weld metal at the root
speed = 18                           # travel speed for that depth
length = 301.7                            # joint length to gouge (perimeter of the insert)
usable = 9                            # usable length of one electrode before the stub
arc_min = length / speed
used = arc_min * burn
area = 0.75 * width * depth               # gouge section, a rounded U
removed = area * length * 0.2836
print(f"{d} in electrode at {amps} A: gouge about {width} in wide, {depth} in deep")
print(f"arc time {arc_min:.0f} min, electrode used {used:,.0f} in -> {used / usable:.0f} electrodes")
print(f"metal removed about {removed:.1f} lb")
air = 30                               # compressor delivery at the torch
need = 25
print("air supply", "OK" if air >= need else "TOO LOW", f"({air} cfm available, {need} cfm needed)")

0.3125 in electrode at 420 A: gouge about 0.47 in wide, 0.31 in deep
arc time 17 min, electrode used 75 in -> 8 electrodes
metal removed about 9.3 lb
air supply OK (30 cfm available, 25 cfm needed)

The whole perimeter of the insert is gouged in about 17 minutes of arc time, using eight electrodes. That is the reason crews choose air carbon arc for back-gouging: grinding the same groove by hand would take most of a shift. The metal removed is about 9.3 lb, most of it blown out as molten droplets and slag that land on the floor plates of the tank, where the fire watch and the clean-up crew deal with them.

Removing defects

When a radiograph or ultrasonic examination finds a defect in a weld, the defect must be removed and the area rewelded. Gouging is fast, but on thin tube walls it is often too fast; a gouge on a 1/4 in tube wall can go through the wall in a moment. For tube welds, defects are usually removed by grinding with small carbide burrs and discs, which lets the welder find the exact end of the indication and stop there. On thicker plate, such as a coker drum shell or a vessel nozzle, gouging is used to remove most of the metal and grinding to finish. The rule is to excavate until the defect is gone, confirm it with penetrant or magnetic particle examination, make the cavity a smooth weldable shape, and then reweld to the WPS. Lesson 11 follows a repair through this sequence.

On Northgate's spring work, Kiera sees the other side of gouging. A crack has been found by ultrasonic examination in an old repair weld on a drum nozzle. The crew gouges out the old repair from the outside, in steps, with the inspector watching, stopping at each step to check with magnetic particle examination, until the last trace of the crack is gone. The cavity is nearly 1 in deep when it is clean.

Gouging safely

Air carbon arc gouging is one of the noisiest and dirtiest jobs on a site. Noise from a gouging torch at high current is well above the level that requires hearing protection, and double hearing protection, plugs under muffs, is common. The molten metal blown from the groove travels several meters and can start fires and burn people, so the area in front of the gouge is cleared, protected with fire blankets and watched. The fumes contain metal oxides and, from the electrode coating, copper; in a confined space like a tank, local exhaust is placed at the gouge and the gouger wears respiratory protection. The arc is as bright as a heavy welding arc and needs a dark filter lens. And the heavy current through the electrode heats the torch and the leads; worn leads and loose connections can overheat.

In Tank 14, the gouging is done inside the tank, which is a confined space under its entry permit. The tank's manways are open, a fan is blowing fresh air in through one, an exhaust hose with a capture hood is at the gouge, and Imran Qureshi has added the gouging noise and fume to the entry permit's hazard list.

Gouging without air carbon arc

Not every job allows air carbon arc gouging. Inside a boiler during an outage, when other crews are working in the same furnace, the noise, fume and spray of molten metal may make it a poor neighbor, and some owners restrict it in certain areas. On stainless and some alloys, the carbon pickup risk leads owners to prefer other methods. The alternatives are slower but cleaner. Plasma gouging uses a plasma torch with gouging consumables and makes a cleaner groove with less noise and no carbon. Grinding with a heavy grinder and a gouging disc is slow but controllable, and is used for short lengths and thin material. Oxyfuel gouging, with a special gouging tip, works on carbon steel and was common before air carbon arc became widespread; it is still used where electrical equipment cannot be brought in. Whichever method is used, the result must be the same: a smooth, clean groove down to sound metal, confirmed by examination before welding. On Tank 14, air carbon arc is the right tool because the tank is empty, the crew is alone in it and the job is long.