WELDING OF CARBON STEEL

WELDING OF CARBON STEEL

Welding of carbon steel is widely used in pressure vessels, pipelines, structures, fabrication industries, and power plants.

During welding, rapid heating and cooling significantly change the microstructure and mechanical properties of steel.

Understanding the Heat Affected Zone (HAZ), Fusion Zone, Fe–C Phase Diagram, and welding thermal cycles is essential for:

Controlling weld quality

Preventing defects

Improving structural reliability

Key Regions in Weld Joint

Fusion Zone (FZ)

• Completely melted weld area

• Highest temperature region

• Determines weld strength and fusion

Partially Melted Zone (PMZ)

• Base metal partially melted

• Located between FZ and HAZ

• Highly sensitive to cracking

Heat Affected Zone (HAZ)

• Heated but not melted

• Microstructure changes occur

• Affects hardness and toughness

Base Metal (BM)

• Unaffected parent material

• Original properties retained

• Provides structural support

Important Metallurgical Concepts

Fe–C Phase Diagram

• Iron-carbon transformation diagram

• Shows phase changes with temperature

• Crucial for welding metallurgy

A1 Temperature (~723°C)

• Lower critical temperature

• Austenite starts forming

• Controls phase transformation

A3 Temperature

• Upper critical temperature

• Complete austenite formation occurs

• Important for heat treatment

Liquidus Temperature (TL)

• Metal becomes liquid

• Forms weld pool

• Controls penetration

Welding Thermal Cycle

• Rapid heating and cooling occur during welding

• Short time at high temperature creates localized changes

• Cooling rate directly affects hardness and microstructure

Important Phases in Carbon Steel

Austenite (γ)

• High-temperature steel phase

• FCC crystal structure

• Influences hardenability

Ferrite (α)

• Soft and ductile phase

• BCC crystal structure

• Improves toughness

Cementite (Fe₃C)

• Hard iron carbide phase

• Increases hardness

• May increase brittleness

Residual Stresses & PWHT

Residual Stresses

• Caused by uneven cooling

• May lead to distortion and cracking

• Requires stress control measures

PWHT (Post Weld Heat Treatment)

• Controlled heating and cooling process

• Reduces residual stress and hardness

• Improves weld reliability and service life

Typical Welding Defects

Cracking

Porosity

Lack of Fusion

Slag Inclusion

Undercut

Distortion

Key Welding Takeaways

Control heat input and travel speed

Use proper preheat and interpass temperature

Follow approved WPS and PQR

Select suitable electrodes and consumables

Apply PWHT where required

Maintain cleanliness and joint preparation

Understanding welding metallurgy is not just about making a weld — it is about ensuring safety, reliability, and long-term performance of engineering structures.