Gas welding uses an oxygen-acetylene flame to melt and join metal, and it remains one of the most accessible welding processes for beginners. The equipment is relatively affordable, the technique is forgiving on thin materials, and the same torch setup works for welding, brazing, and cutting. Learning it well comes down to understanding your equipment, setting the right flame, and controlling the puddle.
Equipment You Need
A standard oxy-acetylene rig has two gas cylinders (oxygen and acetylene), a regulator on each cylinder, color-coded hoses, a torch body with control valves, and interchangeable welding tips. The regulator is the critical control piece: it steps down the high storage pressure inside the cylinder to the much lower working pressure your torch needs. Each regulator has two gauges, one showing how much gas remains in the cylinder and one showing the outlet pressure feeding the torch.
Hoses are color-coded so you never mix them up: red for acetylene, blue for oxygen (in the US, green is used for oxygen). As an extra safeguard, the fittings on acetylene hoses use left-hand threads while oxygen fittings use standard right-hand threads, making it physically impossible to connect a hose to the wrong regulator.
Welding tips (also called nozzles) are sized by bore diameter, and you choose one based on the thickness of the metal you’re joining. A size 1 nozzle handles sheet steel under 1mm, while a size 35 nozzle covers steel around 10mm thick. Using a tip that’s too large overheats thin metal and burns through; too small, and you can’t get enough heat into the joint. Your torch manufacturer’s chart will match tip numbers to metal thickness ranges.
You’ll also need a filler rod matched to your base metal, a striker or flint lighter (never a cigarette lighter), and the safety gear covered below.
Safety Gear and Cylinder Handling
Gas welding produces less UV radiation than arc welding, but the intense yellow-white flame still requires eye protection with the correct filter lens. OSHA sets minimum shade numbers based on material thickness: shade 4 for steel under 1/8 inch, shade 5 for 1/8 to 1/2 inch, and shade 6 for anything thicker. Industry recommendations from ANSI and AWS run one to two shades darker than those minimums. A good rule of thumb is to start with a shade that’s too dark to see through, then step down until you can clearly see the puddle without going below the OSHA minimum. Welding goggles or a flip-front shield both work; standard sunglasses do not.
Beyond eye protection, wear leather gloves, closed-toe boots, and a long-sleeve cotton or leather jacket. Synthetic fabrics melt into skin. Keep a fire extinguisher within reach and work on a fireproof surface away from flammable materials.
Acetylene cylinders must always be stored and used valve-end up. OSHA regulations require this because acetylene is dissolved in a solvent inside the cylinder, and tilting the tank can allow that solvent to leak out, creating a fire hazard. Both cylinders should be secured upright with a chain or strap so they can’t fall. Keep oxygen and acetylene cylinders at least 20 feet apart in storage, or separated by a fireproof barrier.
Flashback Arrestors and Check Valves
A flashback occurs when the flame travels backward into the hose or even the regulator. Check valves stop reverse gas flow, but they won’t stop a flame front. Flashback arrestors will. Install combination flashback arrestor and check valves on the outlet of both regulators, and ideally on the torch inlets as well. Most modern units combine both functions in a single body. Never skip these: a flashback reaching the acetylene cylinder can cause an explosion.
Preparing the Metal
Clean metal is essential for a sound weld. Oil, dirt, paint, and rust all contaminate the weld pool and create porosity or weak joints. Wipe the joint area with a rag and acetone to remove surface grime. For mill scale on hot-rolled steel, use a grinding disc or wire wheel to get down to bare metal. Aluminum has a tough oxide layer on its surface that melts at a much higher temperature than the aluminum underneath; remove it with a stainless steel wire brush dedicated to aluminum only, since cross-contamination from a carbon steel brush will weaken the joint.
For steel thicker than about 1/4 inch, you’ll want to bevel the edges of the joint. Grinding a 60-degree included angle between the two pieces lets the flame and filler rod penetrate fully through the joint instead of only fusing the surface. Leave a root opening of 1/16 to 3/16 inch between the beveled edges to allow complete penetration at the bottom of the weld.
Setting Gas Pressures
Before opening any cylinder valve, make sure the regulator adjusting screws are fully backed out (no tension). Stand to the side of the regulator, not in front of or behind it, and slowly open the oxygen cylinder valve until the high-pressure gauge reads its maximum. Open the acetylene cylinder valve no more than one full turn so you can shut it off quickly in an emergency.
For most welding with small to medium tips, acetylene pressure runs between 3 and 5 psi, and oxygen runs at roughly the same pressure when welding (not cutting). The detailed data from TWI shows that for steel up to about 3mm, both gases run at around 2 psi (0.14 bar), increasing to about 9 psi (0.63 bar) for steel in the 10 to 25mm range. Your tip manufacturer’s chart is the most reliable guide. For oxy-fuel cutting with a dedicated cutting attachment, oxygen pressure jumps significantly, often to 25 to 40 psi, while acetylene stays in the 3 to 7 psi range.
Lighting the Torch and Setting the Flame
Open the acetylene valve on the torch body about a quarter turn and ignite the gas with a striker. You’ll see a yellow, smoky flame. Slowly increase the acetylene flow until the smoke clears and the flame just barely leaves the tip. Then gradually open the oxygen valve. The flame will change from a long, feathery cone to a shorter, more defined shape.
There are three flame settings, and choosing the right one matters:
- Neutral flame: Equal parts oxygen and acetylene. The inner cone is smooth, bright, and clearly defined with no feathery edges. This is the standard setting for welding mild steel and most common metals.
- Carburizing (reducing) flame: Excess acetylene. You’ll see a secondary feather or “acetylene feather” extending beyond the inner cone. This flame adds carbon to the weld pool and is used for hardfacing or welding high-carbon steel.
- Oxidizing flame: Excess oxygen. The inner cone is shorter, more pointed, and hisses noticeably. This flame burns hotter and is used primarily for brazing and welding brass, but it will cause oxidation and brittleness in steel.
For nearly all steel welding, you want a neutral flame. Adjust the oxygen valve until the acetylene feather just disappears into the inner cone. That’s your neutral point.
Torch Technique and Filler Rod Control
Hold the torch at an angle between 30 and 60 degrees from the surface of the metal. The tip of the inner cone should hover about 1/16 to 1/8 inch above the workpiece. Too close and you’ll pop the flame or melt through. Too far and you lose heat concentration.
There are two main techniques:
Forehand welding is the most common approach, especially for thinner material. You point the flame in the direction you’re traveling, pushing the puddle forward. The filler rod stays ahead of the torch tip, and you dip it into the leading edge of the molten puddle as you move. This produces a wide, shallow bead with good fusion on sheet metal and tubing.
Backhand welding is better for thicker stock. Here, the flame points back toward the completed weld, and the filler rod sits between the flame and the finished bead. This concentrates more heat into the joint, providing deeper penetration. The bead tends to be narrower and more built-up.
Regardless of technique, move at a pace that keeps a consistent puddle size. If the puddle grows too large, you’re moving too slowly and risk burning through. If it shrinks or you see the edges of the joint not melting, speed up or pause briefly to let heat build. Dip the filler rod into the puddle with a steady rhythm rather than holding it in the flame continuously, which just wastes rod and creates spatter.
Shutting Down Safely
The shutdown sequence matters more than most beginners realize. Getting it wrong can damage equipment or create a flashback hazard.
Close the acetylene valve on the torch first. If you shut off the fuel valve second instead, the momentary oxygen-rich flame can pop and blow carbon soot back into the torch, gradually clogging internal passages and flashback arrestors. After the acetylene valve, close the oxygen valve on the torch.
Next, close both cylinder valves tightly. Then reopen the torch valves one at a time to bleed the remaining pressure from the hoses and regulators. Watch the gauges drop to zero. Close the torch valves again, and finally back out the regulator adjusting screws completely so no spring tension remains on the diaphragm. This full bleed-down protects the regulators and ensures no pressurized gas sits in the hoses while the equipment is idle.
Common Mistakes to Avoid
Using too large a tip is probably the most frequent beginner error. It feels like more heat means faster welding, but oversized tips make the puddle uncontrollable and burn through thin stock in seconds. Start with a tip one size smaller than you think you need and move up if the puddle forms too slowly.
Dirty base metal is the second most common problem. Even a thin layer of mill scale or oil will cause porosity, poor fusion, and ugly beads. If you’re frustrated with your weld quality and your technique feels right, the answer is almost always more surface prep.
Finally, watch your flame throughout the weld. As cylinders deplete, pressures can shift slightly, and your neutral flame may drift toward carburizing or oxidizing. A quick glance at the inner cone every few inches of welding keeps you on track.