SSAW steel pipe weld defect and detection

SSAW (Spiral Submerged Arc Welding) steel pipes are widely used in laying oil pipelines due to their advantages in production automation, continuous coil forming, and double-sided automatic submerged arc welding. These pipes undergo various testing methods to ensure weld quality. However, defects can still occur during the welding process due to mistakes or operational issues. Here, we will discuss common SSAW steel pipe weld defects and their detection methods.

Lack of Fusion

Lack of fusion refers to the failure of the weld metal to fuse properly with the base metal, resulting in spaces left by the arc melting. This defect typically occurs in the middle of the weld for double-sided spiral welded pipes. Causes include:

Inadequate gap size

Insufficient welding current

Excessive welding speed

Improper wire angle

These factors can lead to partial penetration, where the weld does not fully penetrate the joint.

Porosity and Slag

Porosity occurs when high-temperature gases are absorbed into the weld pool and do not escape before the metal solidifies. This results in gas pockets within the weld metal. Types of porosity include:

Isolated pores

Pore density

Wormholes

Chain porosity

Porosity is primarily caused by hydrogen and carbon monoxide gases. In radiographic images, porosity appears as black holes with greater darkness compared to the background, making them easy to identify.

Cracks

Cracks are serious welding defects that significantly impact weld integrity. They are caused by welding stress and embrittlement, leading to the localized destruction of metal atomic bonds. Types of cracks include:

Hot cracking

Cold cracking

Reheat cracking

Lamellar tearing

Stress corrosion cracking

In SSAW pipes, which are typically made from high-quality carbon steel and low alloy steel, cracks are mostly thermal and cold cracks due to the lack of post-welding heat treatment.

Detection Methods

To ensure weld quality, SSAW steel pipes undergo several detection methods, such as:

- Radiographic Testing (RT): Identifies porosity and slag by capturing radiographic images.

- Ultrasonic Testing (UT): Detects lack of fusion and cracks by sending ultrasonic waves through the weld.

- Magnetic Particle Testing (MPT): Reveals surface and near-surface defects by applying magnetic particles to the weld area.

- Visual Inspection: Checks for surface defects and ensures overall weld quality.

By understanding these common defects and utilizing appropriate detection methods, the integrity and reliability of SSAW steel pipes can be maintained, ensuring their effectiveness in oil pipeline applications.