11.3.1 Cracks Explained

Key Concepts of Cracks in Welding

1. Definition

Cracks in welding refer to fractures that occur in the weld metal or the heat-affected zone (HAZ) during or after the welding process. These cracks can significantly compromise the structural integrity of the welded joint.

2. Types of Cracks

There are several types of cracks that can occur in welding:

• Hot Cracks: Form during or immediately after solidification due to the contraction of the weld metal.
• Cold Cracks: Occur after the weld has cooled below the critical temperature, often due to hydrogen embrittlement and residual stresses.
• Lamellar Tear: Develop in the base metal parallel to the rolling direction, typically due to high tensile stresses.
• Stress-Oriented Hydrogen-Induced Cracks (SOHIC): Form in the HAZ due to the combined effect of hydrogen and high residual stresses.

3. Causes of Cracks

Cracks can be caused by various factors, including:

• High Residual Stresses: Resulting from the thermal cycle of welding.
• Hydrogen Embrittlement: Hydrogen atoms in the weld metal can cause brittle fractures.
• Inadequate Preheat and Post-Weld Heat Treatment: Insufficient preheat can lead to rapid cooling and cracking.
• Poor Weld Design: Uneven stress distribution can lead to crack formation.

4. Prevention Measures

To prevent cracks in welding, several measures can be taken:

• Proper Preheat and Post-Weld Heat Treatment: To reduce residual stresses and hydrogen content.
• Controlled Cooling Rates: To avoid rapid cooling that can lead to cracking.
• Use of Low-Hydrogen Electrodes: To minimize hydrogen embrittlement.
• Optimized Weld Design: To ensure even stress distribution and reduce the risk of cracks.

5. Inspection and Detection

Cracks can be detected using various inspection methods:

• Visual Inspection: Using magnifying glasses or cameras to detect surface cracks.
• Non-Destructive Testing (NDT): Techniques such as ultrasonic testing, radiography, and magnetic particle inspection to detect internal and surface cracks.

Explanation of Each Concept

Types of Cracks

Hot cracks form during solidification when the weld metal contracts, leading to high tensile stresses. Cold cracks occur after cooling, often due to hydrogen embrittlement and residual stresses. Lamellar tears develop in the base metal due to high tensile stresses, and SOHICs form in the HAZ due to the combined effect of hydrogen and high residual stresses.

Causes of Cracks

High residual stresses result from the thermal cycle of welding, causing the metal to contract and crack. Hydrogen embrittlement occurs when hydrogen atoms in the weld metal cause brittle fractures. Inadequate preheat and post-weld heat treatment can lead to rapid cooling and cracking. Poor weld design can result in uneven stress distribution, leading to crack formation.

Prevention Measures

Proper preheat and post-weld heat treatment reduce residual stresses and hydrogen content. Controlled cooling rates avoid rapid cooling that can lead to cracking. Low-hydrogen electrodes minimize hydrogen embrittlement. Optimized weld design ensures even stress distribution and reduces the risk of cracks.

Inspection and Detection

Visual inspection detects surface cracks using magnifying glasses or cameras. Non-destructive testing techniques such as ultrasonic testing, radiography, and magnetic particle inspection detect internal and surface cracks without damaging the weld.

Examples and Analogies

Imagine hot cracks as the cracks that form in ice cream when it melts and refreezes. Just as ice cream cracks due to contraction, weld metal cracks due to contraction during solidification.

Think of cold cracks as the cracks that form in a dried-out sponge. Just as a sponge cracks due to dehydration, weld metal cracks due to hydrogen embrittlement and residual stresses.

Consider lamellar tears as the cracks that form in a piece of wood when it is bent. Just as wood cracks parallel to the grain when bent, base metal cracks parallel to the rolling direction under high tensile stresses.

Visualize SOHICs as the cracks that form in a rubber band when it is stretched and exposed to moisture. Just as a rubber band cracks due to stretching and moisture, HAZ cracks due to the combined effect of hydrogen and high residual stresses.