Cutting and gouging review

Mateo teaches cutting and gouging in week two. It is the topic he likes best, because he spent his first winter at Hartwell on the burning table and the beveler, and it is the one where the group's first timed set showed overconfidence: Devon, Aisha and Jun all rated themselves strong, and all three missed questions on flame chemistry and air carbon arc settings. Every test coupon Devon will weld starts with a cut and a bevel, and a bad bevel can fail a test before the first bead. Mateo has a cutting tip, a plasma consumable set and a gouging rod on the table, and a stack of practice questions written from WLD-110 lessons 7 to 11.

How oxyfuel cutting works

Oxyfuel cutting is not melting. The preheat flame, a mixture of fuel gas and oxygen, heats the steel to its kindling temperature, a bright cherry red around 1,600 °F. Then a jet of pure oxygen is turned on, and the hot iron burns in it: iron combines with oxygen, the reaction gives off heat, and the oxygen jet blows the molten oxide, the slag, out of the kerf. Because the cut depends on iron burning, oxyfuel cuts carbon and low-alloy steels well, but not stainless steel, aluminum or cast iron. Stainless steel and aluminum form oxides that melt at a higher temperature than the metal and protect it; cast iron's high carbon content keeps it from burning cleanly.

This is where most exam questions on oxyfuel come from. Which metals can be cut? Why does stainless not cut? What does the preheat flame do and what does the cutting oxygen do? Why must the cutting oxygen be pure? A drop of oxygen purity of even a percent or two slows the cut noticeably, because less iron burns. Mateo writes the answer on the board in one line: preheat heats, oxygen burns, the jet clears.

Fuel gases differ in flame temperature and in how the heat is delivered. Acetylene gives the hottest flame, with most of its heat in the small inner cone, which makes it fast to start a cut and good for piercing and beveling. Propane and natural gas give a cooler inner flame but more heat in the outer flame, are cheaper, and need more oxygen for each volume of fuel. They take longer to start a cut but give a smooth edge on thicker steel. Tips are made for one fuel; a propane tip has a different preheat pattern and must not be used with acetylene. WLD-110 lessons 7 and 8 cover equipment and technique.

Reading a cut

A good cut has a square top edge with little rounding, a smooth face with fine drag lines that are nearly vertical, a square bottom edge and slag that falls away or knocks off easily. Each fault points to a cause. Drag lines that curve back strongly mean the travel was too fast for the oxygen, so the bottom of the cut lags the top. Gouges in the face come from too fast travel, a dirty tip or interruptions in the cut. A rounded top edge comes from a preheat flame that is too hot or held too close. Heavy slag stuck to the bottom edge comes from too slow travel or too little cutting oxygen. A bevel face that is not straight means the torch angle wandered.

Plasma arc cutting

Plasma cutting melts the metal rather than burning it. An arc between a tungsten or hafnium electrode in the torch and the work passes through a narrow nozzle with a flow of gas. The constricted arc heats the gas into a plasma, an electrically conducting gas at a very high temperature, and the high-speed jet melts the metal and blows it out of the kerf. Because it does not depend on oxidation, plasma cuts stainless steel, aluminum and other conductive metals as well as carbon steel. It is fast on thin and medium plate and leaves a narrow heat-affected zone, but it produces intense ultraviolet light, noise and fume, and on some machines a bevel on one face of the cut.

Plasma questions usually test the parts and the choices. The consumables are the electrode, the swirl ring, the nozzle and the shield, and they wear together: a worn electrode or a damaged nozzle opening shows up as a wider, beveled kerf and arc instability. Current is chosen by thickness, with a higher current and larger nozzle for thicker metal. Air is the common plasma gas for carbon steel in small shops; nitrogen, argon-hydrogen mixes and oxygen are used on larger systems for better cut quality on particular metals. A pilot arc starts the process inside the torch so the torch can cut expanded metal and painted surfaces. WLD-110 lesson 9 covers it.

Air carbon arc gouging

Air carbon arc gouging uses an arc between a copper-coated carbon electrode and the work to melt the metal, and a jet of compressed air from the electrode holder to blow it away. It is the standard way to back-gouge the root of a double-sided groove weld, remove defects for repair and prepare joints in the field. It works on most metals. Electrodes are usually run on direct current with the electrode positive, which gives a smooth, efficient gouge. The air jet must be on before the arc is struck and must hit the molten metal behind the electrode, with the electrode pointing in the direction of travel and the air on the side of the electrode toward the work.

Settings and technique decide the gouge's shape. A steeper electrode angle gives a deeper, narrower groove; a flatter angle gives a shallower, wider one. Travel speed sets depth too: slower is deeper. A common error is too little air, which leaves carbon and molten metal in the groove and can pick up carbon into the steel, hardening it. Another is touching the electrode to the work, which leaves carbon deposits. Before welding over a gouged groove, the groove is ground clean of the carbon-rich skin, and for back-gouging the inspector checks that sound weld metal has been reached, often with a visual check or magnetic particle test. WLD-110 lesson 10 and WLD-230 lesson 11 cover gouging and repairs.

# Cutting and gouging review: cost per length, gouge time and kerf yield
# 1. Oxyfuel cutting cost per length
speed = 18            # cutting speed, length per minute
oxygen = 160             # oxygen use, volume per hour (preheat + cutting)
fuel = 21              # fuel gas use, volume per hour
o_cost, f_cost = 0.035, 0.26        # cost per unit volume
wages = 60.0               # wages and overhead per hour
per_hour = oxygen * o_cost + fuel * f_cost + wages
length_per_hour = speed * 60
print(f"oxyfuel: {per_hour:.2f} per hour, {length_per_hour:.0f} in per hour, {per_hour / length_per_hour:.3f} per in of cut")

# 2. Air carbon arc gouge: time to back-gouge a seam
seam = 720               # seam length
rate = 24               # gouging travel rate, length per minute
rods_per_len = 0.03       # electrodes used per unit length
print(f"back-gouge {seam} in at {rate} in/min: {seam / rate:.0f} min of arc time, about {seam * rods_per_len:.0f} electrodes")

# 3. Kerf loss when cutting strips from a plate
plate_w = 60
strip_w = 5.5
kerf = 0.12
n = int((plate_w + kerf) // (strip_w + kerf))
waste = plate_w - n * strip_w
print(f"{n} strips of {strip_w} from a {plate_w} plate with a {kerf} kerf; {waste:.1f} lost to kerf and offcut")

oxyfuel: 71.06 per hour, 1080 in per hour, 0.066 per in of cut
back-gouge 720 in at 24 in/min: 30 min of arc time, about 22 electrodes
10 strips of 5.5 from a 60 plate with a 0.12 kerf; 5.0 lost to kerf and offcut

The cost of the cut is mostly labor, not gas, which is why speed matters: a cut done at the right speed is both better and cheaper. The back-gouge of a 720 in seam takes about half an hour of arc time and around 22 electrodes. The strip problem shows how kerf adds up: ten strips of 5.5 in from a 60 in plate with a 0.12 in kerf leave a small remainder that becomes scrap. Prices in the program are illustrative.

Mechanical cutting and edge preparation

Not every cut is thermal. Shears, saws, abrasive cutoff wheels, nibblers, beveling machines and grinders all appear in the trade, and the exam expects the right tool for the job and its hazards. A band saw gives a square, clean cut with no heat-affected zone, which is why some procedures require sawn or machined edges. A pipe beveler gives an accurate bevel and land for a pipe joint; grinding by hand rarely gives as consistent a land. Abrasive wheels are rated for a maximum speed, must match the tool, and must be the right type: cutting discs are thin and must not be side-loaded.

Edge preparation is where cutting meets welding. Thermally cut edges must be cleaned of slag and oxide before welding, and many codes limit the roughness and notches allowed on a cut edge and require hard edges on high-strength steel to be ground. Stainless steel and aluminum need tools kept for those metals only, because steel particles from a shared grinder will rust on stainless and contaminate aluminum welds. The bevel angle, root face and root opening on a test coupon or production joint come from the procedure, and getting them right is the welder's first quality check. WLD-110 lesson 11 and WLD-320 lesson 3 cover preparation.

Mateo ends with a quick round of what-would-you-use. A 1 in hole in 3/8 in plate: drill, or plasma with a pierce if the edge quality allows. A 12 in cut in 1/4 in stainless sheet: plasma or a saw, never oxyfuel. Removing a cracked fillet weld from a beam flange: air carbon arc gouge, then grind and test. Beveling an NPS 6 pipe for a test coupon: pipe beveler or a machined end, then file the land by hand. Each answer has a reason, and the reason is what the exam marks.

Cutting safety in review

Cutting carries its own safety questions. Flashback arrestors on the regulator end of each hose stop a flame traveling back into the regulator and cylinder; check valves stop reverse gas flow. Hoses are checked for cracks and leaks with an approved leak-test solution, never a flame. The torch is lit with a striker, fuel gas valve first, and shut down oxygen first on most torches, following the maker's instructions. Plasma units have their own hazards: high open-circuit voltage at the torch, intense ultraviolet light, noise above hearing protection thresholds on heavy cutting and fume from coatings. Gouging adds noise, a shower of molten metal and a lot of fume, so it calls for full leather, hearing protection and local exhaust. WLD-110 lessons 7 to 10 list the steps.

Mateo's last practice question of the night asks what to do when a torch backfires repeatedly while cutting, with a pop and the flame going out. The answer is to shut it down and find the cause: a tip held too close to the work, a dirty or damaged tip, too low a gas flow for the tip size, or an overheated tip from cutting in a corner. Relighting and carrying on without finding the cause risks a flashback, which is a sustained burn back into the torch with a hiss or squeal, and that means shutting off the oxygen at once and then the fuel.