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The three types of oxyacetylene flames are neutral, carburizing (also called reducing) and oxidizing. You get each one by changing the mix of oxygen and acetylene at the torch, and the flame you pick changes how the weld or braze behaves.
A neutral flame burns equal volumes of oxygen and acetylene and suits most steel welding. A carburizing flame has extra acetylene, and an oxidizing flame has extra oxygen. Each one gives the inner cone a different shape, so you can read it by eye.
We cover how each flame looks, its temperature, which metals it suits, and how to set the torch to each one.
The 3 Types of Oxyacetylene Flames at a Glance
Weld Guru lists the gas ratio and inner cone temperature for each flame. Use this table as a quick reference before you light the torch.
| Flame | Oxygen to acetylene | Inner cone tip temperature | How it looks | Common uses |
|---|---|---|---|---|
| Neutral | About 1:1 | About 5,850°F (3,232°C) | Sharp, bluish-white inner cone, no feather | Mild steel, stainless, cast iron, copper, aluminum |
| Carburizing (reducing) | Slightly less than 1 part oxygen | About 5,700°F (3,149°C) | Whitish acetylene feather around the inner cone | High carbon steel, hard facing, nickel and Monel |
| Oxidizing | Slightly more than 1 part oxygen | About 6,300°F (3,482°C) | Short, pointed, slightly purple cone with a hiss | Zinc, copper, manganese steel, torch brazing of steel and cast iron |
The outer envelope of a neutral flame runs much cooler, at about 2,300°F (1,260°C), per Weld Guru. The hottest point sits just past the tip of the inner cone.
Neutral Flame
A neutral flame burns 1 volume of oxygen with 1 volume of acetylene at the torch. It has a clean, well-defined inner cone that is bluish-white, with no feather trailing off it.
This flame adds nothing to the weld pool and takes nothing away. That’s why Weld Guru lists it for mild steel, stainless steel, cast iron, copper and aluminum. It is the default setting for most oxyacetylene welding.
The OpenWA welding textbook describes the neutral point as the moment the inner cone stops shrinking and settles into a small, bright blue point at the tip.
Carburizing (Reducing) Flame
A carburizing flame has a little less oxygen than acetylene. You can spot it by the acetylene feather, a whitish zone that wraps the inner cone and stretches past it.
Its inner cone tip runs at about 5,700°F, the coolest of the three. The extra acetylene can add carbon to steel, so Weld Guru reserves it for high carbon steel and hard facing, plus nonferrous alloys such as nickel and Monel.
A strongly carburizing flame looks bright orange and throws smoke and soot, per OpenWA. That is a sign to add oxygen.
Oxidizing Flame
An oxidizing flame has a little more oxygen than acetylene. The inner cone turns short, pointed and slightly purple, and the torch makes a distinct hissing sound.
It is the hottest of the three at about 6,300°F at the inner cone tip. The extra oxygen burns into steel, so Weld Guru says it should not be used for welding steel.
A slightly oxidizing flame does have a place. Weld Guru lists it for torch brazing of steel and cast iron, and for zinc, copper and manganese steel.
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How to Adjust an Oxyacetylene Torch to Each Flame
These steps follow the OpenWA welding textbook. Always check your torch maker’s manual for the exact lighting order and pressures for your tip size.
Step 1: Light the Fuel
Crack the acetylene valve on the torch and light it with a friction striker. Add fuel until you have a strong, feathery flame that no longer smokes much.
Step 2: Add Oxygen Slowly
Open the torch oxygen valve a little at a time. A long feather around a bright inner cone means the flame is still carburizing.
Step 3: Stop at Neutral
Keep adding oxygen until the feather disappears and the inner cone stops shrinking. That sharp, clean cone is a neutral flame.
Step 4: Shift to the Flame You Need
For a carburizing flame, cut back oxygen until a short feather returns. For an oxidizing flame, add a touch more oxygen until the cone gets shorter, sharper and a little dimmer.
Step 5: Fix an Accidental Oxidizing Flame
If the cone turns pointed and the torch starts to hiss when you wanted neutral, OpenWA says to reduce oxygen or add more fuel.
Which Flame to Use for Each Job
| Job | Flame | Why |
|---|---|---|
| Mild steel welding | Neutral | Adds no carbon and burns no metal |
| Stainless steel, copper, aluminum | Neutral | Listed by Weld Guru for these metals |
| High carbon steel | Carburizing | Extra acetylene keeps carbon in the steel |
| Hard facing | Carburizing | Listed by Weld Guru for hard facing work |
| Torch brazing steel or cast iron | Slightly oxidizing | Listed by Weld Guru for torch brazing |
| Silver brazing and soft soldering | Carburizing, outer flame only | Weld Guru says only the intermediate and outer cones are used |
Heat also depends on tip size and distance. The oxy acetylene flame temperature at the work drops fast once you move the work past the inner cone tip.
Reading Flame Mistakes
- Black soot and an orange flame: far too much acetylene. Add oxygen.
- Hissing torch and a sharp purple cone on steel: too much oxygen. Weld Guru says this flame should not be used on steel.
- Feather that won’t go away: keep opening the torch oxygen valve slowly, then check the oxygen regulator setting.
Safety Note
OSHA 29 CFR 1910.253(a)(2) says acetylene must never be used at a pressure over 15 psig. The acetylene gauge in the OpenWA setup guide carries a red line past 15 psi to mark that limit.
Store oxygen cylinders at least 20 feet from fuel-gas cylinders, or behind a noncombustible barrier at least 5 feet high, per OSHA 1910.253(b)(4)(iii). Keep oil and grease away from oxygen fittings.
Oxygen hoses are green and fuel hoses are red, and fuel fittings use left-hand threads. Wear shaded goggles, gloves and flame-resistant clothing every time you light the torch.
Frequently Asked Questions
What Are the 3 Types of Oxyacetylene Flames?
The three types are neutral, carburizing and oxidizing. A neutral flame burns about equal oxygen and acetylene. A carburizing flame has extra acetylene and shows a feather. An oxidizing flame has extra oxygen, a short pointed cone and a hissing sound. Neutral is the default for most welding.
Which Oxyacetylene Flame Is the Hottest?
The oxidizing flame is the hottest. Weld Guru puts its inner cone tip at about 6,300°F, against about 5,850°F for neutral and 5,700°F for carburizing. The extra heat comes with extra oxygen, which burns into steel, so it is not used for welding steel.
What Is a Reducing Flame in Welding?
A reducing flame is another name for a carburizing flame. It burns slightly more acetylene than oxygen and shows a whitish feather around the inner cone. It suits high carbon steel and hard facing work, where the extra acetylene keeps carbon in the metal rather than burning it out.
Which Flame Is Used for Brazing?
It depends on the job. Weld Guru lists a slightly oxidizing flame for torch brazing steel and cast iron. For silver brazing and soft soldering, it says to use only the intermediate and outer parts of a carburizing flame. Follow the filler maker’s data sheet for the final call.
How Do You Know When a Flame Is Neutral?
Watch the inner cone as you add oxygen. The feather shrinks and then disappears, and the cone settles into a sharp, bright blue point that stops getting smaller. Add oxygen past that point and the cone gets shorter and starts to hiss, which means it is oxidizing.
Final Words
Neutral, carburizing and oxidizing flames all come from the same torch. The only change is the oxygen to acetylene mix, and the inner cone tells you which one you have.
Set a neutral flame first, then shift toward a feather or a sharper cone only when the metal or filler calls for it. Keep acetylene under 15 psig and your cylinders stored apart.
