Carburizing Flame

In the world of welding, achieving the desired properties in steel is crucial. One important aspect to consider is the type of flame used during the welding process.

In this article, we will learn the concept of the carburizing flame, its definition, and why it should be avoided when welding steel. We will also compare the carburizing flame with the oxidizing flame, discuss their temperatures, and explore the differences between a neutral flame and a carburizing flame. Lastly, we will examine the composition of a slightly carburizing flame. Let’s get started!

Carburizing Flame Definition

A carburizing flame refers to a type of flame used in welding that contains an excess of acetylene gas compared to the amount of oxygen. This flame is characterized by its reducing properties, which means it has a high concentration of carbon monoxide (CO) and a low concentration of oxygen (O2). The carburizing flame is sometimes referred to as a reducing flame.

Carburizing Flame

A carburizing flame has a white feather created by the presence of excess acetylene gas in the flame. This excess acetylene gas leads to incomplete combustion, resulting in a reducing atmosphere that promotes the absorption of carbon into the material being carburized. The white feather appearance is a visual indication of the carburizing process.

What is Carburizing Flame?

The carburizing flame is primarily used in processes such as gas welding and brazing. It is employed to introduce carbon into the surface of the steel, thereby increasing its hardness and wear resistance. This flame is particularly useful when working with low-carbon steels that require additional carbon content.

Why Should a Carburizing Flame be Avoided When Welding Steel?

While the carburizing flame has its applications, it should be avoided when welding steel. The excessive carbon introduced by the carburizing flame can lead to undesirable effects, such as increased brittleness and reduced ductility in the welded joint. This can compromise the structural integrity of the weld and potentially lead to failure under stress.

Carburizing Flame vs. Oxidizing Flame

Carburizing Flame vs. Oxidizing Flame
AspectCarburizing FlameOxidizing Flame
Fuel-to-Oxygen RatioExcess fuelExcess oxygen
Flame ColorYellowBlue
Carbon Monoxide (CO)High concentrationLow concentration
Oxygen (O2)Low concentrationHigh concentration
Effect on SteelIncreases carbon contentOxidizes carbon content
Resulting PropertiesIncreased hardness and wear resistanceReduced hardness and wear resistance
Carburizing Flame vs. Oxidizing Flame

Carburizing Flame Temperature

The temperature of a carburizing flame typically ranges between 3200°F (1760°C) and 3500°F (1925°C). This high temperature is necessary to facilitate the diffusion of carbon into the steel surface.

Difference between a Neutral Flame and a Carburizing Flame

A neutral flame is achieved when the fuel-to-oxygen ratio is balanced, resulting in complete combustion. It has an equal concentration of carbon monoxide and oxygen. In contrast, a carburizing flame has an excess of fuel, leading to a higher concentration of carbon monoxide and a lower concentration of oxygen.

Flame Composition of a Slightly Carburizing Flame

A slightly carburizing flame is achieved by slightly increasing the acetylene flow while maintaining a balanced fuel-to-oxygen ratio. This flame composition contains a slightly higher concentration of carbon monoxide compared to a neutral flame, but it is still within acceptable limits for most welding applications.


Understanding the carburizing flame is essential for welders working with steel. While it has its applications in certain processes, it should be avoided when welding steel due to the potential negative effects on the welded joint. By maintaining a neutral flame or using an oxidizing flame, welders can achieve the desired properties in steel without compromising its structural integrity.

Material Welding is run by highly experienced welding engineers, welding trainers & ASNT NDT Level III bloggers. We strive to provide most accurate and practical knowledge in welding, metallurgy, NDT and Engineering domains.