Types of cold crack among ERW steel pipe

Cold cracking, or cold cracking (also known as hydrogen-induced cracking or HIC), is a potential defect that can occur in ERW (Electric Resistance Welded) steel pipes, especially during the welding process or while the pipes are in use under certain conditions. Cold cracking typically happens when the weld metal or the heat-affected zone (HAZ) experiences brittleness, which leads to cracks forming at low temperatures or under mechanical stress. The presence of hydrogen or residual stresses is often a key factor in these types of cracks. Cold cracking can negatively affect the mechanical properties and structural integrity of the pipe, particularly when used in demanding applications like drilling, oil and gas pipelines, and other critical infrastructure.

Types of Cold Cracks in ERW Steel Pipe:

- Hydrogen-Induced Cracking (HIC)

Cause: HIC occurs when hydrogen is absorbed into the weld during the welding process. The presence of hydrogen can cause embrittlement of the weld metal or the heat-affected zone (HAZ), which in turn can lead to cracking under tensile stress.

Appearance: HIC cracks often appear as long, narrow fissures that develop within or near the heat-affected zone. These cracks typically run perpendicular to the weld and may propagate deeper into the steel.

Prevention: Controlling the level of hydrogen in the welding environment, using low-hydrogen electrodes, and proper preheat and post-weld heat treatment can help prevent HIC.

- Heat-Affected Zone Cracking (HAZ Cracking)

Cause: The heat-affected zone (HAZ) is the area of the steel pipe that is heated during welding but not melted. If the cooling rate in this zone is too rapid or if the material is high-carbon or has high hardness, cracking may occur in the HAZ due to the buildup of residual stresses.

Appearance: Cracks in the HAZ tend to be transverse or diagonal and may appear as small surface fissures or cracks that run along the weld.

Prevention: Controlled cooling rates, proper selection of welding parameters, and preheating of the workpieces can reduce the risk of HAZ cracking.

- Weld Metal Cracking (Cold Cracking of Welds)

Cause: This type of cold cracking occurs directly in the weld metal itself, often due to high carbon content, excessive residual stresses, or the presence of hydrogen. It typically happens immediately after welding or during the early stages of cooling, especially when the pipe is subjected to mechanical stresses.

Appearance: Weld metal cracking is often longitudinal or transverse, and cracks may be shallow or deep, depending on the severity of the stresses and material properties.

Prevention: Using low-carbon steel and controlled welding techniques (such as preheating, controlled cooling, and post-weld heat treatment) can mitigate the risk of weld metal cracking.

- Transverse Cracking

Cause: Transverse cracking occurs when the weld bead or the heat-affected zone of the pipe is subjected to tensile stresses while the steel is still in a relatively brittle state due to rapid cooling. The cracks are usually associated with high-strength steels or those that have been improperly welded or treated.

Appearance: These cracks typically appear at right angles to the axis of the pipe and are often visible on the surface of the weld or the HAZ.

Prevention: Preheating, controlled cooling, and the use of low-hydrogen welding rods can reduce the likelihood of transverse cracking.

- Longitudinal Cracking

Cause: Longitudinal cracks can form due to high residual stresses along the length of the ERW steel pipe, often resulting from welding defects or improper heat treatment. This type of crack is generally aligned with the long axis of the pipe.

Appearance: Longitudinal cracks are typically more serious than transverse cracks because they can propagate along the length of the pipe, compromising its strength and structural integrity.

Prevention: This type of cracking can be minimized by controlling the welding process, heat treatment, and ensuring proper material selection that resists cracking under stress.

- Lamellar Tearing

Cause: Lamellar tearing occurs in steel with anisotropic properties, such as deformed or elongated grains in the rolling direction. It happens when the steel is under tensile stress, typically during welding, and is more likely in low-quality steel with low ductility.

Appearance: These cracks appear as slit-like tears along the grain boundaries in the base metal or weld metal. Lamellar tearing is often seen at the surface of the weld and can be hidden below the surface, requiring careful inspection.

Prevention: Controlled cooling, proper material selection, and using a weld procedure that reduces the risk of stress concentration in the material can reduce the likelihood of lamellar tearing.

Factors Contributing to Cold Cracking in ERW Steel Pipes:

Welding Parameters: Incorrect heat input, high welding speed, or low preheating temperature can contribute to cold cracking. Proper control over the welding process is critical.

Material Composition: High carbon content, alloying elements, or materials that are more prone to brittle behavior at low temperatures increase the likelihood of cold cracking.

Residual Stress: Welding-induced residual stresses from improper cooling, excessive heat input, or thermal cycling can promote cracking in the weld or HAZ.

Hydrogen: Hydrogen is a major cause of cold cracking. It can be absorbed into the weld or pipe during welding and diffuse into the heat-affected zone, increasing the risk of cracks.

Pipe Geometry and Thickness: The thicker the pipe, the more prone it is to cold cracking due to the increased heat-affected zone and difficulty in controlling cooling rates.

Prevention and Mitigation Measures:

Weld Procedure Control: Proper welding parameters, including heat input, welding speed, and technique, should be closely controlled.

Preheating and Post-Weld Heat Treatment: These steps help reduce the risk of cracking by controlling the cooling rates and relieving residual stresses.

Material Selection: Low-carbon steels and alloys that are resistant to hydrogen embrittlement should be used, especially for critical applications like drilling pipes or pipeline construction.

Use of Low-Hydrogen Electrode: To reduce the risk of hydrogen-induced cracking, low-hydrogen welding electrodes should be used.

Inspection and Testing: Non-destructive testing (NDT) methods like ultrasonic testing, radiography, and eddy current inspection can help detect cold cracks early and avoid failures during service.