Welding process
Oxyfuel gas welding (OFW)
A flame produced by burning fuel gas in oxygen melts the joint while filler rod is added by hand, the oldest fusion welding method still valued for its total control of heat.
How it works
Fuel gas and oxygen are mixed in the torch and burned at the tip. With acetylene the reaction happens in two stages: a bright inner cone where acetylene and oxygen combine to produce intense heat, and a larger outer envelope where the products burn further in surrounding air, consuming oxygen and shielding the pool from the atmosphere.
Adjusting the oxygen-to-fuel ratio changes the flame's chemistry as well as its heat. A neutral flame with balanced proportions is used for most steel welding. Excess fuel gives a carburising flame that can add carbon to the pool, and excess oxygen gives an oxidising flame that burns out alloying elements but suits some copper alloys.
Heat is delivered by conduction and convection from the flame rather than by an electric arc, so the heat is far less concentrated. The welder can move the flame nearer or further from the joint to change heat input continuously, and can preheat, weld, and post-heat with the same tool, which is why the process survives in repair and artistic work.
Strengths
- Complete, continuous control of heat input by simply moving the torch closer or further away
- Equipment is portable and needs no electrical supply at all
- The same torch can preheat, weld, braze, cut, and post-heat, making it versatile for repair work
- Excellent for thin sheet, tube, and repair welding where an arc would blow through
Limitations
- Very slow compared with any arc process, so it is uneconomical on production work
- The wide, diffuse heat produces a broad heat-affected zone and significant distortion
- Not suitable for reactive metals or for most thick sections
- Compressed fuel gas and oxygen bring flashback, cylinder, and fire hazards that need strict handling discipline
Key variables
- Flame setting
- Neutral, carburising, or oxidising. The ratio of oxygen to fuel decides both flame temperature and how the flame chemically affects the pool.
- Tip size
- Sets the volume of gas burned and therefore total heat available, matched to material thickness rather than adjusted with pressure.
- Torch angle and distance
- Holding the inner cone just off the surface delivers maximum heat; pulling back reduces it smoothly without changing any setting.
- Filler rod addition
- Rate and rod diameter determine bead size, and the rod tip must stay within the flame envelope to avoid picking up oxide.
- Travel speed
- Balances against tip size and flame heat to control penetration and how much the wide heat pattern distorts thin material.
What an inspector watches for
- Check the flame setting, since a carburising or oxidising flame changes the deposit chemistry in ways that show up as brittleness or oxide inclusions
- Look for oxidised, scaly bead surfaces that indicate the outer envelope was not covering the pool
- Expect and check for distortion, because the wide heat pattern moves thin material more than arc processes do
- Verify hose, regulator, flashback arrestor, and cylinder condition, since safety compliance is a routine part of examining this process