Types of Welding Flames (Neutral, Carburizing & Oxidizing)

In this session, we are examining the various types of welding flames used in gas welding. In a previous discussion, we learned about the fundamental aspects of gas welding, when you combust a mixture of fuel and gases to create high temperature flames appropriate to join together metals.

The welding flame type is the predominant factor in determining the quality of the welded joint. Essentially, there are three main types of flames produced in gas welding: neutral flame, carburizing flame, and oxidizing flame.

A neutral flame is produced when a balanced mixture of fuel and oxygen are gassed off, therefore the neutral flame burns clean and is stable, meaning it should be used for most welding operations.

A carburizing flame consists of an excessive amount of fuel in comparison to oxygen; therefore, it adds carbon to the weld area, which is appropriate for high carbon steels or when a softer weld is preferred.

An oxidizing flame has excessive air with respect to fuel, therefore is useful for welding materials like brass, which requires a hotter flame.

Hence, how materials and conditions dictate the selection of flame type is paramount to the welding operation in hand.

It is important to understand the three different flame types, since they have a profound effect on the overall strength and expected properties of the weld in the finished joint.

What is Welding Flame?

Welding flames are frequently used to heat metals or thermoplastics to a specific temperature for sale, fusion, and solidification. Of the multitude of gas welding processes, oxyfuel welding is the most common and, one of the oldest known processes. Oxyfuel welding was first developed in 1903.

Oxyfuel welding, or acetylene welding, uses a fuel gas most commonly acetylene, with the addition of oxygen, to produce a working flame with sufficient heat to make effective welds.

The welding torch relies on separated gas cylinders with a flexible gas hose attached from each. After opening the gas valve on the torch, the gas comes flowing through the hose which is ignited.They can be adjusted by the corresponding valve to get the desired fuel gas/oxygen ratio.

The flame used for the oxyfuel welding process is characterized by a unique structure made up of multiple sections or zones known as cones.

The inner most cone is where the fuel gas and oxygen react to produce heat, and is also the hottest zone of the flame, while the outer cone is slightly cooler because it absorbs some oxygen from the air. The outer zone is typically called as the outer envelope or flame 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

True to its name, this type of flame has equal volumes of oxygen and fuel gas. Being a mixture, the flame combusts the fuel gas completely without any residues remaining or any chemical change to the metal being welded.

Primarily, this flame can for welding mild steel, stainless steel, or cast iron because it burns cleanly, with very little smoke as compared to an adjustable flame.

The flame consists of two zones. The inner zone has a white appearance and reaches a temperature of approximately 3100°C.

The outer zone has a bluish color and is cooler, around 1275°C. Each zone plays important roles in welding, mostly around the concentration of heat and stability.

2. Carburizing Flame

This type of flame produces an excessive quantity of fuel gas, which causes a chemical reaction with the metals that form metal carbides, therefore this flame is unsuitable for welding metals that readily absorb carbon. By appearance, the flame looks calm and smoky, and has three distinct zones.

The first zone is the inner cone which is bright white, then a red intermediate zone and lastly, a blue outer cone. The inner cone can achieve up to approximately 2900°C. Typically, it’s used to weld or braze medium carbon steel and nickel when we do not care about controlled carburization.

3. Oxidizing Flame

If the supply of acetylene is lower or the oxygen intake is increased, the inner cone of the flame begins to fade and loses definition. This type of flame is then considered an oxidizing flame. An oxidizing flame has a higher burning temperature than a natural flame, and it has two very directions zones.

The inner zone is a clear bright white cone with clearly defined edges and can be about 3300°C. The outer zone normally displays a bluish tint.

The oxidizing flame can be useful for welding the non ferrous metals and is particularly good at welding certain non ferrous metals, especially oxygen free copper alloys, such as brass and bronze, because of its hotter heat and characteristics.

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

Although acetylene is still the most common fuel gas for the generation of the basic types of gas welding flames, it is certainly not the only one.

For example, MAPP (methylacetylene-propadiene propane) and hydrogen are sometimes seen as acceptable substitutes for acetylene, and each can offer certain advantages depending on the application.

Acetylene’s biggest distinction is that it is comprised of a carbon carbon triple bond. While most other gases gain energy (take in energy) when their bonds are broken for ignition, acetylene does not.

In fact, when the bond(s) break, it liberates energy, facilitating much higher flame temperatures, which is particularly advantageous for welding metals that need intense heat for welding.

MAPP is typically seen as having fewer oxidizing effects than some other gases. The drawback is that acetylene is very volatile and easily combusts, especially during transportation and storage.

Solution to address some of these safety issues, MAPP was created. MAPP is a blend of liquefied petroleum gas with components of propane and acetylene, and is intended to be a nontoxic, less hazardous alternative.

MAPP is available in a smaller cylinder form, takes tolerable working pressures higher than acetylene, and exceeds the ignition temperature of acetylene, particularly with respect to the application context.

However, MAPP will not achieve the results that acetylene will with flame temperature. MAPP’s heat will not be sufficient for welding most steel alloys, but it can be good for aluminum.

Like MAPP, hydrogen has similar limitations. Hydrogen compares well to aluminum and is still better in that it runs at higher pressures. However, the temperature of the hydrogen flame is again not high enough to work with harder metals.

In general, acetylene is still the best fuel gas for most welding purposes, especially for steel welding. The necessary heat or withdrawal rates to achieve acceptable welds in harder materials are just not achievable with either MAPP or hydrogen.

However, both MAPP and hydrogen are still used in many areas. For example, they perform quite well in gas cutting applications, as gas cutting does not have high flame temperature as its primary concern; in fact, unmet precision is more important than flame temperature with high-pressure torches.

Additionally, MAPP and hydrogen are used quite often for heating, metal bending, and brazing. Often, the temperatures are quite sufficient for the vast majority of tasks in which excessive heating can cause problems for the metal applicant, making them more dependable than acetylene with respect to finer thermal work.

Common Ratios for Producing Oxyacetylene Flames

In gas welding, the process of understanding the flame you are working with requires understanding how oxygen and fuel gas interact. A neutral flame has a balance of oxygen and acetylene. For most welding jobs you will want to start with a neutral flame.

Once you understand a neutral flame, you may easily change how much of each you are using at your job. If you pull the oxy valve back slightly you will have a carburizing flame that has an excessive amount of acetylene compared to oxygen.

By comparison if you push the oxygen past the neutral position you will have an oxidizing flame. Each flame serves a purpose whether you are at the material or whether you are trying to achieve a certain weld quality and appearance.

To simplify the flame types, here is a general guide to the typical oxygen to acetylene ratios for the different flame types:

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

As the flow of acetylene is increased, a very noticeable feather will occur extending outwards from the inner cone of the flame.

It is, in fact, desirable that the feather will extend two to three times the length of the inner cone. The feather indicates that complete combustion is limited and thus, the flame temperature is reduced.

With the oxidizing flame, increasing the flow of oxygen not the acetylene, helps in complete combustion and thus more heat intensity produces a heat and lighter flame. The shift towards an oxidizing environment is necessary when high temperatures are needed for any specific application.

How Do You Create a Neutral Flame for Gas Welding?

When I first began to learn about flame types, the neutral flame was my baseline it’s the baseline for creating all the other flame types, so it was appropriate to learn it first.

The first step is to adjust the regulators on the oxygen and acetylene cylinders. Both of these regulators will have two gauges, one shows the remaining pressure left in the cylinder, and the other indicates the working pressure that you’re going to use.

The working pressure is controlled with the screw found on the regulator by turning that screw you can increase or decrease the flow of oxygen or acetylene depending on which cylinder you are adjusting.

At this point you are ready to light the torch. Before you do, it is important to stand to the side never directly in front of the regulators.

Once you are standing at the side of the path of the torch, you can open the oxygen cylinder slowly, then the acetylene cylinder. From there you can adjust the regulators to the desired working pressures.

Now that the regulators are adjusted to the pressures you want, you can light the torch. You will first open the acetylene valve approximately a quarter turn for now don’t over open the valve, and you can ignite it. Once you have the flame, you can begin to open the oxygen valve slowly.

When you do this you will see three zones of flame: the bright inner cone, a feather-like cone of acetylene, and the outer envelope. Continue to adjust the oxygen until the feather cone blends into the inner cone. That is your neutral flame.

If you need a carburizing flame, which has excess acetylene, slowly open the acetylene valve until the feather extends to approximately two to three times the length of the inner cone. If, on the other hand, you want an oxidizing flame, increase the flow of oxygen.

You will notice that the inner cone will get small approximately one quarter of the size, and you will hear a distinct hissing sound which is your sign to stop as you have successfully made the appropriate flame.

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