What is Welding Flame?- Neutral, Carburizing & Oxidizing

What is Welding Flame?

Welding flame is used to heat metal or thermoplastics, fusing them as they cool. Most gas welding processes use oxyfuel welding. It’s one of the oldest welding processes, first developed in 1903. With oxyfuel welding, which is also called oxyacetylene welding, you need a liquid fuel or gas, such as acetylene. The gas is combined with oxygen to increase the temperature of the flame.

The torch has hoses that connect to gas tanks. When I’m ready to start welding, I open the valves and ignite the gas as it exits the torch. I can then adjust the valves to adjust the flow of each gas, altering the gas ratio.

Each flame also has several zones called cones. The inner cone is the hottest part of the flame. It’s where acetylene and oxygen combine. The outer cone is cooler as it gets more oxygen from the surrounding air. It’s also commonly called the outer envelope or sheath.

Welding flame is used to heat metal or thermoplastics, fusing them as they cool. Most gas welding processes use oxyfuel welding.

Types of Welding Flames

There are three basic flame types: neutral (balanced), excess acetylene (carburizing), and excess oxygen (oxidizing) as shown below.

  • A neutral flame is named neutral since in most cases will have no chemical effect on the metal being welded.
  • A carburizing flame will produce iron carbide, causing a chemical change in steel and iron. For this reason, a carburizing flame is not used on metals that absorb carbon.
  • An oxidizing flame is hotter than a neutral flame and is often used on copper and zinc.
There are three basic flame types: neutral (balanced), excess acetylene (carburizing), and excess oxygen (oxidizing) as shown

1. Neutral Flame

An oxyfuel gas flame that is neither oxidizing nor reducing. It is a quiet and clean flame obtained by burning approximately 50% acetylene and 50% oxygen.

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The neutral flame has a one-to-one ratio of acetylene and oxygen. It obtains additional oxygen from the air and provides complete combustion. It is generally preferred for welding. The neutral flame has a clear, well-defined, or luminous cone indicating that combustion is complete.

Neutral welding flames are commonly used to weld:

  • Mild steel
  • Stainless steel
  • Cast Iron
  • Copper
  • Aluminum

The welding flame should be adjusted to neutral before either the carburizing or oxidizing flame mixture is set.

There are two clearly defined zones in the neutral flame.

  • The inner zone consists of a luminous cone that is bluish-white.
  • Surrounding this is a light blue flame envelope or sheath.

This neutral flame is obtained by starting with an excess acetylene flame in which there is a “feather” extension of the inner cone. When the flow of acetylene is decreased or the flow of oxygen increased the feather will tend to disappear. The neutral flame begins when the feather disappears.

The neutral or balanced flame is obtained when the mixed torch gas consists of approximately one volume of oxygen and one volume of acetylene. It is obtained by gradually opening the oxygen valve to shorten the acetylene flame until a clearly defined inner cone is visible.

For a strictly neutral flame, no whitish streamers should be present at the end of the cone. In some cases, it is desirable to leave a slight acetylene streamer or “feather” 1/16 to 1/8 in. (1.6 to 3.2 mm) long at the end of the cone to ensure that the flame is not oxidizing.

This flame adjustment is used for most welding operations and for preheating during cutting operations. When welding steel with this flame, the molten metal puddle is quiet and clear. The metal flows easily without boiling, foaming, or sparking.

In the neutral flame, the temperature at the inner cone tip is approximately 5850ºF (3232ºC), while at the end of the outer sheath or envelope the temperature drops to approximately 2300ºF (1260ºC). This variation within the flame permits some temperature control when making a weld. The position of the flame to the molten puddle can be changed, and the heat is controlled in this manner.

The Carburizing Flame

A reducing oxyfuel gas flame in which there is an excess of fuel gas, resulting in a carbon-rich zone extending around and beyond the cone. A carburizing flame is used in hard facing and similar processes to obtain fusion between the base metal and weld metal without deep melting of the base metal.

The carburizing flame has excess acetylene, the inner cone has a feathery edge extending beyond it. This white feather is called the acetylene feather. If the acetylene feather is twice as long as the inner cone it is known as a 2X flame, which is a way of expressing the amount of excess acetylene. The carburizing flame may add carbon to the weld metal.

Reducing or carburizing welding flames are obtained when slightly less than one volume of oxygen is mixed with one volume of acetylene. This flame is obtained by first adjusting to neutral and then slowly opening the acetylene valve until an acetylene streamer or “feather” is at the end of the inner cone.

The length of this excess streamer indicates the degree of flame carburization. For most welding operations, this streamer should be no more than half the length of the inner cone.

The reducing or carburizing flame can always be recognized by the presence of three distinct flame zones. There is a clearly defined bluish-white inner cone, a white intermediate cone indicating the amount of excess acetylene, and a light blue outer flare envelope. This type of flare burns with a coarse rushing sound. It has a temperature of approximately 5700ºF (3149ºC) at the inner cone tips.

When a strongly carburizing flame is used for welding, the metal boils and is not clear. The steel, which is absorbing carbon from the flame, gives off heat. This causes the metal to boil. When cold, the weld has the properties of high carbon steel, being brittle and subject to cracking.

A slight feather flame of acetylene is sometimes used for back-hand welding. A carburizing flame is advantageous for welding high carbon steel and hard-facing such nonferrous alloys as nickel and Monel. When used in silver solder and soft solder operations, only the intermediate and outer flame cones are used. They impart a low temperature soaking heat to the parts being soldered.

The Oxidizing Flame

In various burners, the oxidizing flame is the flame produced with an excessive amount of oxygen. When the amount of oxygen increases, the flame shortens, its color darkens, and it hisses and roars.

With some exceptions (e.g., platinum soldering in the jewelry), the oxidizing flame is usually undesirable for welding and soldering, since, as its name suggests, it oxidizes the metal’s surface. The same principle is important in firing pottery see Reducing atmosphere.

The reducing flame is the flame with low oxygen. It has a yellow or yellowish color due to carbon or hydrocarbons which bind with (or reduce) the oxygen contained in the materials processed with the flame. The reducing flame is also called the carburizing flame since it tends to introduce carbon into the molten metal.

The neutral flame is the flame in which the amount of oxygen is precisely enough for burning, and neither oxidation nor reduction occurs. A flame with a good balance of oxygen is clear blue. The reducing and neutral flames are useful in soldering and annealing.

An oxidizing flame can also be recognized by its distinct hissing sound. The temperature of this flame is approximately 6300ºF (3482ºC) at the inner cone tip.

When applied to steel, an oxidizing flame causes the molten metal to foam and give off sparks. This indicates that the excess oxygen is combining with the steel and burning it. An oxidizing flame should not be used for welding steel because the deposited metal will be porous, oxidized, and brittle. This flame will ruin most metals and should be avoided.

A slightly oxidizing flame is used in torch brazing of steel and cast iron. A stronger oxidizing flame is used in the welding of brass or bronze. In most cases, the amount of excess oxygen used in this flame must be determined by observing the action of the flame on the molten metal.

What Type of Gas Should You Use for an Ideal Flame?

Acetylene is the most used gas for producing the types of gas welding flames discussed but it’s not the only option. MAPP and hydrogen are often listed as alternatives to acetylene. Acetylene has a “triple bond” that uniquely bonds carbon atoms.

When other gases reach their ignition temperatures, the bond breaks. The gases then absorb energy. When the bond breaks in acetylene, it releases energy. This allows acetylene to achieve higher temperatures.

Compared to other gases, acetylene also has fewer oxidizing characteristics. However, it also ignites easily. MAPP was created as a safer option. MAPP gas is a liquified petroleum gas combined with propane and acetylene. It can be shipped in smaller containers compared to standard acetylene, has a higher ignition temperature, and works with much higher pressures.

The drawback to MAPP is the temperature. The flames produced using MAPP achieve lower temperatures compared to acetylene flames. It’s not suitable for use with most steels but may work well for aluminum. Hydrogen is another gas that welds aluminum easily. As with MAPP, hydrogen flames reach lower temperatures and work with higher pressures.

Unless you plan on fusing aluminum, stick with acetylene. The low temperature and withdrawal rate keep MAPP and hydrogen from properly fusing harder metals. While MAPP and hydrogen aren’t the best choices for gas welding, they’ve become popular options for gas cutting. When used with a high-pressure torch, MAPP and hydrogen provide cleaner cutting.

The lower temperatures also make MAPP and hydrogen common choices for heating, bending, and brazing.

Common Ratios for Producing Oxyacetylene Flames

Neutral gas welding flames have an equal mixture of oxygen and gas. Carburizing flames have less oxygen while oxidizing flames contain more oxygen. So how do you determine the ratio?

No matter the project, start with a neutral flame. The carburizing flame and oxidizing flame are created by increasing the release of acetylene or oxygen after achieving a neutral flame.

I created the following list to break down the typical ratio of oxygen to acetylene for each flame:

  • Carburizing flame: 0.8 to 1.0
  • Neutral flame: 0
  • Oxidizing flame: 1.0 to 2.5

As you increase the flow of acetylene, the distinct feather starts to extend from the inner cone. The feather should reach about two or three times the length of the inner cone. Preventing the gas from fully combusting also lowers its temperature.

If you need an oxidizing flame, you increase the flow of oxygen instead of increasing the flow of acetylene. The extra oxygen produces the oxidizing effect and allows the gas to combust faster, resulting in higher temperatures.

How Do You Create a Neutral Flame for Gas Welding?

As the neutral flame is the starting point for creating other flames, it’s the first flame that I learned how to produce. Start by adjusting the regulators. The oxygen cylinder and acetylene cylinder each have a regulator with two gauges. One gauge tells you the remaining pressure while the other displays the working pressure.

Adjusting the screw on the regulator adjusts the working pressure, allowing you to increase or decrease the flow of oxygen or gas. Before lighting the torch, stand away from the front of the regulators and slowly open the oxygen cylinder and then the acetylene cylinder. Turn the regulator screws to adjust the pressure settings.

With the pressure on the regulators set, you can light and adjust the torch. Open the acetylene valve a quarter turn and ignite the torch. Slowly open the oxygen valve until you see three distinct zones. You should see the inner cone, the feather-shaped acetylene cone, and the outer envelope. Continue to slowly open the oxygen valve until the feather disappears into the inner cone. You now have a neutral flame.

To create a carburizing flame, slowly open the valve on the acetylene cylinder until the feather reaches two to three times the length of the inner cone. To create an oxidizing flame, increase the flow of oxygen until the inner cone is about a quarter of its original size. You should also hear the distinct hissing sound.