Types of Welding Flames (Neutral, Carburizing & Oxidizing)

In this session, we’ll explore the various types of welding flames used in gas welding. In a previous discussion, we covered the fundamentals of gas welding a process in which a mixture of fuel and gases is combusted to produce high-temperature flames suitable for joining metals.

The type of flame produced during this process plays a crucial role in determining the quality and characteristics of the weld. Essentially, there are three primary types of flames: neutral flame, carburizing flame, and oxidizing flame.

A neutral flame results from a balanced mix of fuel and oxygen, providing a stable and clean burn ideal for most welding tasks.

A carburizing flame, on the other hand, contains an excess of fuel, which introduces carbon into the weld area suitable for applications involving high-carbon steels or when a softer weld is desired.

Lastly, an oxidizing flame is rich in oxygen and is typically used when welding materials like brass, where a hotter flame is required.

The selection of the appropriate flame type depends on the material being welded and the specific requirements of the welding operation. Understanding these flame types is essential for achieving strong, reliable welds and maintaining the desired properties in the finished joint.

What is Welding Flame?

Welding flames are commonly used to heat metals or thermoplastics, enabling them to fuse together as they cool and solidify. Among the various gas welding techniques, oxyfuel welding is the most widely used and also one of the oldest, dating back to its development in 1903.

Often referred to as oxyacetylene welding, this method relies on a fuel gas typically acetylene combined with oxygen to produce a flame hot enough for effective welding.

The welding torch is connected to separate gas cylinders through flexible hoses. To begin the process, the welder opens the valves on the torch, allowing the gases to flow. Once ignited, the flame can be fine-tuned by adjusting these valves to control the ratio between the fuel gas and oxygen.

A notable characteristic of the flame produced during oxyfuel welding is its distinct structure. It consists of multiple zones, commonly known as cones. The innermost cone is where the fuel gas and oxygen react most intensely, making it the hottest part of the flame.

Surrounding this is the outer cone, which is somewhat cooler due to additional oxygen drawn in from the ambient air. This outer region is often referred to 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

As the name suggests, this type of flame contains equal volumes of oxygen and fuel gas. Because of this balanced mixture, the flame burns the fuel completely, leaving no residue and causing no chemical changes to the metal being welded.

It’s commonly used in welding applications involving mild steel, stainless steel, and cast iron, thanks to its clean burn and minimal smoke production.

This flame is made up of two distinct zones. The inner zone appears white and reaches a temperature of approximately 3100°C, while the outer zone has a bluish color and is significantly cooler, around 1275°C. Each zone plays a specific role in the welding process, contributing to both heat concentration and stability.

2. Carburizing Flame

This type of flame contains an excess amount of fuel gas, which leads to a chemical reaction with metals, often resulting in the formation of metal carbides. Because of this, it’s generally not suitable for welding metals that readily absorb carbon. Visually, the flame appears quiet and smoky, and it’s made up of three distinct zones.

The innermost zone is bright white, surrounded by a red-colored intermediate zone, and finally, an outer cone that appears blue. The temperature at the inner cone can reach up to approximately 2900°C. This flame is typically used for welding materials like medium carbon steel and nickel, where controlled carburization isn’t an issue.

3. Oxidizing Flame

When the supply of acetylene is decreased or the oxygen input is increased in a flame, the inner cone begins to fade, resulting in what’s known as an oxidizing flame. This type of flame burns at a higher temperature than a natural flame and is characterized by two distinctly visible zones.

The inner zone appears as a sharply defined, bright white cone and can reach temperatures around 3300°C. Surrounding this is the outer zone, which typically shows a bluish hue.

Due to its higher heat intensity and specific characteristics, the oxidizing flame is particularly well-suited for welding non-ferrous metals, especially oxygen-free copper alloys such as brass and bronze.

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

While acetylene remains the most widely used fuel gas for producing the primary types of gas welding flames, it is not the only option available.

Gases such as MAPP (methylacetylene-propadiene propane) and hydrogen are frequently considered viable alternatives, each offering distinct advantages depending on the application.

One of the key characteristics that sets acetylene apart is its molecular structure a carbon-carbon triple bond. Unlike many other gases that absorb energy when their bonds are broken upon ignition, acetylene does the opposite.

When its bond breaks, it releases energy, allowing it to reach significantly higher flame temperatures. This makes it particularly effective for welding metals that require intense heat.

Another benefit of acetylene is that it tends to produce fewer oxidizing effects compared to some other gases. However, this comes with a trade-off: acetylene is highly flammable and can ignite easily, which brings safety concerns, especially during storage and transportation.

To address some of these safety issues, MAPP gas was developed. It’s a blend of liquefied petroleum gas with propane and acetylene components, designed to be a safer alternative.

MAPP can be stored in smaller cylinders, tolerates higher working pressures, and has a higher ignition temperature than acetylene, making it more convenient in certain contexts.

However, MAPP doesn’t quite match acetylene when it comes to flame temperature. Its lower heat output makes it less suitable for welding most types of steel, although it can be a decent choice for metals like aluminum.

Hydrogen shares similar limitations. While it can be used to weld aluminum effectively and also operates at higher pressures, the temperature of its flame is not high enough for working with harder metals.

For most welding tasks especially those involving steel acetylene remains the most effective option. MAPP and hydrogen simply do not generate the necessary heat or withdrawal rates to ensure strong welds in harder materials.

That said, both MAPP and hydrogen have found their niches in other areas. In gas cutting applications, where precision and clean cuts are prioritized over high flame temperature, these gases perform quite well when used with high-pressure torches.

Additionally, because of their relatively lower temperatures, MAPP and hydrogen are commonly used for heating, metal bending, and brazing tasks. In these contexts, the risk of overheating the material is reduced, making them reliable choices for more delicate thermal work.

Common Ratios for Producing Oxyacetylene Flames

In gas welding, understanding the type of flame you’re working with starts by recognizing how oxygen and fuel gas interact. A neutral flame is achieved when there’s a balanced mix—equal parts oxygen and acetylene. This is the standard starting point for most welding tasks.

From there, the flame can be adjusted based on the requirements of the job. If you decrease the oxygen slightly, you’ll get a carburizing flame, which has excess acetylene.

On the other hand, increasing the oxygen beyond the neutral point gives you an oxidizing flame. Each variation serves a purpose depending on the material and the desired weld characteristics.

To simplify things, here’s 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 gradually increased, a noticeable feather begins to form, extending outward from the inner cone of the flame.

Ideally, this feather should reach approximately two to three times the length of the inner cone. This adjustment limits complete combustion, which in turn reduces the overall flame temperature.

On the other hand, when an oxidizing flame is required, it’s the oxygen flow that needs to be increased not the acetylene. The added oxygen promotes more complete combustion, producing a hotter, more intense flame. This shift toward an oxidizing environment is essential when higher temperatures are needed for specific applications.

How Do You Create a Neutral Flame for Gas Welding?

When I first started learning about flame types, the neutral flame was my foundation it’s the starting point for producing all other flame variations, so it made sense to master it first.

The process begins with adjusting the regulators on both the oxygen and acetylene cylinders. Each of these regulators includes two gauges: one shows the remaining pressure inside the cylinder, and the other displays the working pressure that you’ll actually be using.

To control that working pressure, you use the screw on the regulator. Turning this screw lets you increase or decrease the flow of either oxygen or acetylene, depending on which cylinder you’re adjusting.

Before you go ahead and light the torch, it’s important to stand to the side never directly in front of the regulators.

Once you’re in position, open the oxygen cylinder slowly, followed by the acetylene cylinder. Then, use the regulator screws to set your desired working pressures.

With the pressures dialed in, you’re ready to light the torch. Start by opening the acetylene valve just a quarter turn no need to overdo it and ignite the gas. Once the flame is lit, slowly begin opening the oxygen valve.

As you do, you’ll notice three zones in the flame: the bright inner cone, a feather-like acetylene cone, and the outer envelope. Keep adjusting the oxygen until that feather-shaped cone blends into the inner cone. That’s when you know you’ve achieved a neutral flame.

If you need a carburizing flame, which has more acetylene than oxygen, gradually open the acetylene valve until the feather extends to about two to three times the length of the inner cone. On the other hand, if you’re aiming for an oxidizing flame, increase the oxygen flow instead.

You’ll see the inner cone shrink to roughly a quarter of its original size, and you’ll hear a clear hissing sound that’s your cue that you’ve got it right.

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