Types of Welding Flames (Neutral, Carburizing & Oxidizing)

Today we will learn about the types of welding flames used in gas welding. In my last post, we have discussed gas welding. In this welding process gases, and fuels are burned and produce high-temperature flames which are further used to create a weld joint.

The flame plays a lead role in creating weld joints and the weld properties are highly dependent on it. There are three types of flames natural flame, carburizing flame, and oxidizing flame. Natural flame has a synchronized mixture of fuel and oxygen, carburizing flame has more fuel and oxidizing flame has more oxygen. Different materials used different flames according to weld condition.

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.

Types of Welding Flames

There are three types of flames natural flame, carburizing flame, and oxidizing flame.

  • Natural flame has a synchronized mixture of fuel and oxygen,
  • The carburizing flame has more fuel and,
  • An oxidizing flame has more oxygen.

As we know there are three basic welding flames. These areas follow.

1. Natural Flame

As the name implies, this flame has an equal amount of oxygen and gases fueled by volume. This flame burns fuel completely and does not produce any chemical effect on the metal to be welded. It is mostly used for welding mild steel, stainless steel, cast iron, etc. It produces little smoke.

This flame has two zones. The inner zone is white and has a temperature of about 3100 degrees centigrade and the outer zone has a blue color and has a temperature of about 1275 degrees centigrade.

2. Carburizing Flame

This flame has an excess of fuel gas. This flame chemically reacts with metal and forms metal carbide. Due to this reason, this flame does not use metal which absorbs carbon. It is a smoky and quiet flame. This flame has three regions.

The inner zone is white, the intermediate zone is red in color and the outer cone has a blue color. The inner cone temperature is about 2900 degrees centigrade. This flame is used to weld medium carbon steel, nickel, etc.

3. Oxidizing Flame

When the amount of acetylene reduces from natural flame or the amount of oxygen increases, the inner cone tends to disappear and the flame obtained is known as oxidizing flame. It is hotter than natural flame and has clearly defined two zones.

The inner zone has a very bright white color and has a temperature of about 3300 degrees centigrade. The outer flame is blue. This flame is used to weld oxygen-free copper alloys like brass, bronze, etc.

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 regulator 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.