It is common knowledge in welding and thermal cutting practice that when a hydrocarbon gas is burnt with oxygen or air, the major flame types produced are carburising flame, neutral flame, or oxidising flame. Correct flame selection is important for steel preheating, weld preparation, and defect prevention.

This article briefly explains neutral flame, carburising flame behaviour, their effects on steel surfaces, and the welding standards that guide safe preheating practice.

Neutral Flame in Welding and Preheating

For a neutral flame, the fuel and oxygen are in correct balance. This flame is chemically neutral to the steel work surface.

Neutral flame characteristics

• Minimal soot
• Minimal oxidation
• Stable inner cone
• Clean combustion

Neutral flame applications

• Preferred for most heating work
• Preferred for most welding work
• Suitable for controlled preheating of steel
• Helps control the preheating process

Because the gas and oxygen are balanced, the neutral flame does not significantly change the surface chemistry of the steel.

Carburising Flame or Reducing Flame in Steel Preheat

When the fuel gas is excessive relative to the oxygen, a reducing flame or carburising flame is produced.

A carburising flame is identified by:

• A visible feathery extension beyond the inner cone
• A yellowish flame appearance
• Incomplete combustion characteristics

The video below shows the use of carburising flame for preheating steel structures. This practice introduces several technical problems.

Preheating with reducing flame

Consequences of using a Carburising Flame for Steel Preheating

Using a reducing or carburising flame to preheat steel will result in the following problems.

Less Temperature Efficiency

• Lower heating efficiency
• Slower temperature rise

Less controlled heat input

Carbon Pickup and Surface Carburisation

Carbon pickup occurs where the steel surface absorbs extra carbon atoms during heating.

Effects of carburisation

• Increased surface hardness after cooling
• Higher risk of cracking in and near the heated zone
• Greater risk for higher carbon steels and alloy steels

Reduced weldability because the surface chemistry has been altered.

Heavy Soot Formation and Surface Contamination

Yellow flames typically generate soot due to incomplete combustion. Unburnt gas can settle on the prepared joint surface.

Effects of soot contamination

• Soot deposition on the prepared joint surface before welding
• Increased risk of weld defects such as porosity
• Increased risk of lack of fusion
• Extra cleaning required before welding

Standards That Address Flame Preheating Practice

The following standards address flame preheating practice, including the need to avoid contamination, overheating, and uncontrolled heating.

BS EN 1011 2 Welding Recommendations for Welding of Metallic Materials

Part 2 Arc welding of ferritic steels

This guidance standard covers:

• Purpose of preheating and reduction of hydrogen cracking risk
• Practical preheating methods including flame heating
• Control of preheat temperature
• Avoidance of local overheating and uneven heating
• Surface condition requirements before welding

ISO 13916 Welding Measurement of Preheat Temperature Interpass Temperature and Preheat Maintenance Temperature

This standard governs:

• How preheat temperature is checked and measured
• Where temperature should be measured
• How to ensure temperature control regardless of heating method such as flame, electrical pads, or induction

AWS D1.1 Structural Welding Code Steel

AWS D1.1 includes requirements related to:

• Minimum preheat temperature selection
• Approved methods of applying preheat
• Workmanship requirements to prevent welding defects

It does not focus on flame color, but it supports the principle that preheating must be controlled and effective and must not introduce conditions that increase cracking risk.

ISO 17663 Welding Quality Requirements for Heat Treatment in Connection with Welding and Allied Processes

This standard reinforces:

• Control of heating method and temperature application
• Documentation and traceability of heating cycles where required
• Process control for thermal operations connected with welding

Practical Note from the Standards

Most standards will not literally state do not use a yellow flame, but they strongly enforce technical outcomes that a yellow carburising flame can violate, including:

• Clean surfaces before welding
• No contamination by soot or hydrocarbons
• Avoid overheating and thermal shock
• Apply uniform heating
• Measure and record preheat temperature

Best Practice Recommendation for Gas Flame Preheating

If gas flame preheat must be used for steel:

Use a neutral flame

• Blue flame
• No feather beyond the inner cone
• Balanced fuel and oxygen
• Clean heating effect

This is the preferred and safest flame condition for steel preheating before welding.

FAQ: Flame Type and Steel Preheating in Welding

1. What are the main flame types produced when burning fuel gas with oxygen or air?

The three main flame types are neutral flame, carburising flame, and oxidising flame. Each flame type has a different gas balance and produces different chemical effects on the steel surface during heating and welding.

2. Which flame is best for preheating steel before welding?

Neutral flame is best for steel preheating. It has a correct balance of fuel and oxygen, produces clean combustion, creates minimal soot, and does not significantly change the steel surface chemistry.

3. What is a neutral flame in welding practice?

A neutral flame occurs when oxygen and fuel gas are correctly balanced. It is chemically neutral to the steel surface, produces minimal soot and oxidation, and is preferred for most heating and welding operations.

4. What is a carburising flame?

A carburising flame occurs when there is excess fuel gas compared to oxygen. It shows a feathery extension beyond the inner cone and often appears yellowish. It is also called a reducing flame.

5. Why is a carburising flame risky for steel preheating?

A carburising flame can reduce heating efficiency, introduce excess carbon into the steel surface, and generate soot contamination. These effects can reduce weldability and increase the risk of weld defects and cracking.

6. What is carbon pickup during flame preheating?

Carbon pickup is when the heated steel surface absorbs extra carbon atoms from a fuel rich flame. This surface carburisation can increase hardness after cooling and raise cracking risk near the heated zone.

7. How does soot from a yellow flame affect welding quality?

Soot from incomplete combustion can settle on the prepared joint surface. This contamination can lead to porosity, lack of fusion, and other weld defects. It also creates extra cleaning work before welding can proceed.

8. Does flame type affect preheat temperature control?

Yes. Neutral flame gives more stable and efficient heating, which supports better temperature control. Carburising flame gives less efficient and less predictable heating, which makes temperature control more difficult.

9. Do welding standards specify flame color for preheating?

Most standards do not specify flame color directly. However, they require clean surfaces, controlled heating, correct preheat temperature, and avoidance of contamination and overheating. These requirements support the use of neutral flame instead of carburising flame.

10. Which standards guide proper preheating practice in welding?

Commonly referenced standards include:

BS EN 1011 2 for welding recommendations on ferritic steels
ISO 13916 for measurement of preheat and interpass temperature
AWS D1.1 for structural steel welding requirements
ISO 17663 for quality requirements in heat treatment related to welding

11. Can flame preheating be used if temperature is properly measured?

Yes. Flame preheating can be used if it is controlled, uniform, and temperature is properly measured and recorded where required. The flame should be neutral and should not contaminate the joint surface.

12. What is the best practice for gas flame preheating before welding?

Best practice is to use a neutral flame with balanced oxygen and fuel, clean combustion, no feather beyond the inner cone, and verified preheat temperature before welding starts.

Read also:

Heat Treatment of Welded Joints  – part 1

Heat Treatment of Welded Joints  – part 2: Tempering and PWHT

Heat Treatment of Welded Joints  – part 3: Precipitation or Age Hardening