What are the pipe welding processes

Pipe Welding refers to the process of joining two or more sections of steel pipes together through various welding techniques to form a continuous, sealed and strong pipe. Welded pipes are one of the important types in the steel Pipe industry and are on par with Seamless pipes. The difference lies in that: seamless steel pipes are formed by extrusion and do not require weld seams. Welded Steel pipes, on the other hand, rely on welding processes to shape and weld steel plates or coils. The welding quality directly determines the strength, durability and application range of Welded pipes.

Common Pipe Welding Methods

In the process of steel pipe manufacturing, the commonly used pipe welding methods mainly include:

Electric Resistance Welding (ERW)
Resistance welding refers to a method that utilizes the resistance thermal effect generated when current flows through the contact surface and adjacent areas of workpieces to heat metals to a molten or plastic state, thereby forming a bond. Resistance welding is widely used in the manufacturing of steel pipes, especially in the production of ERW straight seam welded pipes. According to different process characteristics, resistance welding is mainly divided into four types:

Spot Welding: It is suitable for local spot welding and is often used for the connection of thin-walled components or pipe fittings.
Seam Welding: A sealed weld seam is formed by continuous spot welding and is suitable for the production of pipes for liquid or gas transportation.
Projection Welding: It utilizes the concentrated heating of the raised parts of the workpiece and is often used for welding pipe fittings, accessories or support components.
Butt Welding: It involves heating and pressurizing the end faces of two pipe sections to join them together, and is suitable for the butt welding of straight pipes.

Submerged Arc Welding (SAW, Submerged arc welding)
Submerged arc welding includes two forms: straight seam submerged arc welding (LSAW) and spiral submerged arc welding (SSAW). During welding, the arc is buried under the flux layer, resulting in high weld quality and deep penetration. It is often used in high-pressure oil pipelines and large-diameter water supply pipes.

LSAW Longitudinal Submerged Arc Welded Pipe: after the steel plate is rolled into a cylindrical shape and welded along the longitudinal seam, it is widely used in oil and gas pipelines.
SSAW Spiral Submerged Arc Welded Pipe (spiral Submerged arc welded pipe) : Steel coils are formed into spiral shapes and then welded. It is commonly seen in large-diameter water supply projects and long-distance pipelines.

Gas shielded welding (GMAW/MIG)
Welding is carried out by shielding gas (including inert gas and active gas) and continuous welding wire, usually with semi-automatic or fully automatic equipment. The welding speed is fast and it is suitable for construction projects and pipeline maintenance.

Argon arc welding (TIG/GTAW)
Argon arc welding is a welding technique based on the principle of ordinary arc welding. It uses argon gas to protect the metal welding material and melts the welding material into a liquid state on the base material to be welded through a high current to form a molten pool, achieving a metallurgical bond between the metal to be welded and the welding material. It is suitable for high-quality welding of stainless steel pipes and thin-walled pipes, with beautiful weld seams.

Manual arc Welding (SMAW)
It is a method of welding that uses a manually operated electrode and the workpiece to be welded as two electrodes, and melts the metal by the heat of the electric arc between the electrode and the workpiece. It is widely used in field pipeline installation and maintenance projects.

Flux-cored wire arc welding (FCAW)
The basic working principle of flux-cored wire gas shielded welding is the same as that of common gas shielded welding with a consumable electrode. It uses a consumable flux-cored wire as one electrode (usually connected to the positive electrode, that is, reverse DC connection), and the base material as the other electrode. The main difference from the common gas shielded metal arc welding lies in the fact that the welding wire is filled with flux mixture inside. During welding, under the effect of arc heat, the molten flux material, wire metal, base metal and shielding gas undergo metallurgical reactions with each other, simultaneously forming a thin layer of liquid slag to coat the droplet and cover the molten pool, providing another layer of protection for the molten metal. In essence, this welding method is a combined protection method of gas and slag.


The general steps of pipe welding

The details of different processes vary, but welded steel pipes typically involve the following steps:

Preparation - Steel plate or coil blanking, Pipe Beveling, surface cleaning, alignment.
Forming - Straight seam welded pipes are made by rolling steel plates into a circle, while spiral welded pipes are formed by rolling steel plates into a spiral shape.
Spot welding (positioning welding) - Pre-welding a small area to fix the shape.
Main welding - Select ERW welding, LSAW welding or SSAW welding according to the process.
Inspection - including non-destructive testing (NDT), X-ray testing, ultrasonic testing (UT) and hydrostatic testing.
Post-treatment - straightening, port processing, surface anti-corrosion (such as hot-dip galvanizing treatment, epoxy coating).

The differences between ERW, LSAW and SSAW

ERW welded pipe: Made from steel coils and welded longitudinally through resistance heat, it is suitable for medium and low pressure pipelines, scaffolding, and steel structure pipes.
LSAW straight seam welded pipe: Made of steel plate as raw material, it is welded by straight seam submerged arc welding, with high strength, and is mostly used for high-pressure oil and gas transportation.
SSAW spiral welded pipe: Made from steel coils, it is spirally formed and welded by submerged arc welding. It has a wide range of diameters and is commonly used in water conservancy projects and large oil pipelines.

The advantages of welded steel pipes (compared with seamless steel pipes)

Lower cost - Welded pipes are usually more economical than seamless pipes and are suitable for large-scale projects.
Larger Diameter - The diameter of seamless steel pipes is limited, while Large diameter Welded pipes can reach several meters.
Uniform wall thickness - It is easier to ensure consistent wall thickness during the production process of welded pipes.
Flexible length - Welded pipes can be produced in longer lengths, reducing interfaces and lowering installation costs.

Common welding problems

Welding precautions for galvanized pipes - During welding, the zinc layer will produce toxic gases, so ventilation should be enhanced. At the same time, the zinc coating may cause Porosity (Porosity) and cracks (Cracking).
Incomplete Penetration - Due to insufficient heat input or poor alignment, the strength of the weld seam decreases.
Crack (Weld Cracking) - caused by rapid cooling, stress concentration or substandard material.
Porosity (Porosity) - caused by moisture, oil stains or insufficient protective gas.
Misalignment - Inaccurate alignment of pipes, which can easily lead to stress concentration after welding.

Summary

Pipe welding is the core process in steel pipe manufacturing, directly affecting the quality and service life of welded pipes. ERW welded pipes, LSAW straight seam welded pipes and SSAW spiral welded pipes each have their own advantages in application scope. Compared with seamless steel pipes, welded pipes have more obvious advantages in terms of cost, size and wall thickness consistency. However, in the actual welding process, the process parameters should be strictly controlled, especially when welding galvanized pipes, effective protective measures need to be taken.