Types of Welding Flames

Updated: April 03, 2022

The welding flame is one of the most, if not the most important part of the welding process. The flame is what actually allows the weld to occur, as it heats the metal or filler so that it can be used to combine the materials together.

Clearly, this is the most integral part of the welding process itself. If the welding flame does not generate enough heat or does not blend the materials together, it can cause the welded area to become compromised, maybe even leading to severe damage. This is why it is essential that the flame be of the right quality and temperature to create a proper weld.

What Types of Welding Joints Are Best Suited for Different Types of Welding Flames?

When it comes to welding, understanding the weld types and joint options is crucial for achieving strong and long-lasting connections. Different types of welding flames are better suited for specific welding joints. For instance, a T-joint typically requires a neutral flame, while a lap joint benefits from a slightly carburizing flame. By selecting the appropriate welding flame for each joint, welders can ensure precise and efficient welds.

3 Different Types of Welding Flames

While the purpose of the flame is the same no matter what type you are using, the type of flame that is produced can be different. There are three different types of flames, these include:

• Neutral flame
• Carburizing flame
• Oxidizing flame

These flames get their names based upon the chemical effect they have on the material that is being welded. For example, an oxidizing flame produces an oxidation, and this is where its name is derived.

Neutral Flame

A neutral flame gets its name from the fact that it usually has no chemical effect on the materials that are being welded. To create this type of flame, there is a one-to-one ratio of oxygen to acetylene. The additional oxygen that is needed to produce the flame is acquired from the air, which enables complete combustion during the welding process. This is generally considered the prefer method of welding. This type of welding flame is most commonly used with the following materials:

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

Just as a reference, if a person is going to use either of the two other types of welding flames, they must first adjusted to neutral before readjusting the mixture for either oxidizing or carburizing. Within the neutral flame, there are two clearly defined zones. These are easily distinguishable when viewing the welding flame. They include:

• The bluish-white area located inside the luminous cone. This is known as the inner zone.
• The area surrounding the light blue flame envelope or sheath.

How this type of frameworks is quite simple. Because of the excess amount of acetylene that is added to the flame, there is what is referred to as a “feather” extension added to the inner core. As the flow of acetylene is decreased or the amount of oxygen is increased, this feather disappears. When the feather has disappeared, the neutral flame begins. This neutral flame is achieved when there is an equal amount of oxygen and acetylene gas that is used. It is important to note that this does not have to be an absolute one-to-one match. It must be close, however. To achieve this match, it is common to increase the opening of the oxygen valve slowly until the match has been created. For those looking for a perfect neutral flame, there should be no whitish looking streamers present at the end of the column. Some prefer to leave a slight acetylene streamer, that is as much as 1/16 to 1/8 of an inch long. This helps to ensure that the flame is not oxidizing. The adjustment of the flame is performed most often when preheating the flame just prior to the cutting operation. When one is welding using a neutral flame, the molten metal puddle is quiet and clear. The metal should flow easily without foaming, sparking, or boiling. The neutral flame usually reaches a temperature around 5850°F with the end of the outer sheath dropping in temperature to about 2300°F. This variation in flame allows for some degree of temperature control during the weld.

Carburizing Flame

In the carburizing flame, there is an increased amount of acetylene. This gives the inner core a feather edge. In this case, the white feather is referred to as “acetylene feather.” There are some terms that are important to understand related to the carburizing flame. The first of these relates to the size of the feather. When it is twice as long as the inner core, then it is known as a 2X flame. When referred to in this way, it tells the welder that there is twice as much acetylene that is being used. When using a carburizing flame carbon may be added to the weld metal. A carburizing or reducing welding flame is obtained when there is slightly less than one full volume of oxygen mixed with one volume of acetylene. This is why even when the welder is looking for a neutral flame, if there is a larger amount of acetylene in comparison to the oxygen and reducing welding flame is obtained. As mentioned in the previous section, this flame is obtained by first adjusting the welding flame to reach a neutral position. Then, the amount of acetylene is slowly increased by turning the valve to allow more of the gas to enter. This creates the feather at the end of the inner core. The length of the streamer gives you the degree of the flame carburization. In most welding operations using this type of flame, the streamer should be no more than half the length of the inner core. While the neutral flame had two distinct zones, a carburizing flame has three. The first of these is the bluish-white inner core, followed by a light intermediate cone that indicates that there is an excess amount of acetylene. The final area is a light blue outer flare envelope. The amount of heat generated by this type of welding flame reaches to about 5700°F at the inner core tip. When this type of flame is used, the metal boilers but is not clear. When steel is used, it is absorbing carbon from the flame, giving off heat in the process. This causes the metal to begin to boil. When the metal cools, it has the same properties of high carbon steel, which can mean they will be subject to cracking and brittleness. This type of welding flame is the perfect choice for those who are welding high carbon steel or who are hard facing non-ferrous alloys, such as nickel. When this flame is used with silver solder or soft solder operations, only the intermediate and outer flame cones are used.

Oxidizing Flame

Conversely to the carburizing flame, the oxidizing welding flame occurs when there is slightly more oxygen than acetylene. As with the carburizing option, the flame should first be adjusted to a neutral flame, then the amount of oxygen is increased until the inner core is shortened by about 1/10 of its original length. You know that you have adjusted the flame properly when the inner core is pointed and has a slightly purplish color. You will also notice that there will be a distinct hissing sound that is made. It is quite recognizable, and you will know it once you initially identify it. The oxidizing flame reaches a temperature of 6300°F at its inner core tip. That is the highest of any of the flame types we have talked about here. Oxidizing welding flames are commonly used with metals such as:

• manganese steel
• copper
• zinc
• cast iron

When the oxidizing flame is used with a metal, it causes the metal to foam and give off sparks. This tells the welder than excess amount of oxygen is combining with the steel, causing it to burn. In most cases, you will find that in oxidizing flame should not be used for welding steel. This is because the metal that is deposited becomes porous, brittle, and oxidized. The flame ultimately ruins most metals, and is why it is not used in the vast majority of welds where steel is used. Where you will find the oxidizing flame use most of is in the torch brazing of steel and cast iron. It is also used in the welding of brass or bronze. A Final Note Now that you know the different types of welding flames, a little about the ratios of oxygen to acetylene is important. For the best carburizing flame, it should be about 0.8 of oxygen for every 1.0 of acetylene. The neutral flame should have the ratios at zero, while the amount of oxygen in the oxidizing flame can be as much as 2.5 times that of the acetylene.