Pipe Welding Procedure

Pipe Welding Procedure: A Complete Guide to Downward Vertical Welding Technology

In the construction of long-distance pipelines, municipal pipe networks, and industrial pipeline projects, welding quality directly affects the safe operation and service life of the pipelines. Different welding processes have a significant impact on construction efficiency, weld quality, and overall cost. Downward vertical welding technology, as the mainstream process for large-diameter pipeline welding, is widely used in pipeline engineering construction both domestically and internationally due to its high efficiency, high quality, and economy. Its technical characteristics and application points deserve a systematic review.

The downward vertical welding process described in this article is applicable to carbon steel and low-alloy steel pipelines (such as ASTM A53 Gr.B ERW pipes, API 5L X42/X52 line pipes, etc.), and is most widely used in the field installation welding of long-distance oil and gas pipelines, urban gas pipelines, and industrial pipelines.

Process Features and Advantages: 

The downward vertical welding technique using cellulose-based electrodes offers fast welding speed, aesthetically pleasing weld appearance, and high weld quality. Compared to traditional upward vertical welding, it saves approximately 10-20% of welding materials and reduces labor intensity. It is currently widely used in large-diameter, long-distance pipeline welding, and this downward vertical welding technique has significant potential for wider application in thin-walled pipe construction.

Welding Operation Points: 

During welding, appropriate welding current, electrode angle, and welding speed are typically selected. Welding is completed using a straight downward dragging motion or slight oscillation. Ordinary electrodes are prone to dripping, molten iron, and slag problems, while using pipeline-specific electrodes (such as cellulose-based electrodes like E6010 and E7010-P1) and strictly adhering to welding specifications can solve these problems. Cellulose-based electrodes produce very little slag, have strong arc force, and sufficient rigidity to prevent slag dripping. They also have high arc penetration, making them particularly suitable for pipe root welding (root pass welding), eliminating the need for root removal work, thereby improving work efficiency and working conditions. However, due to the high hydrogen content in the weld (approximately 20-30 ml/100g), high-pressure pipeline welding in China typically employs a composite process of root pass welding with cellulose electrodes and filler/cap weld with low-hydrogen electrodes.

Proper pipe assembly and tack welding are crucial for ensuring weld quality, with good weld back formation being a significant factor. Tack welding is an integral part of the final weld, requiring not only proper weld formation but also ensuring overall weld quality. The length and thickness of the tack weld should be determined based on the pipe wall thickness (typically, the tack weld length is 10-30 mm, and the thickness is approximately 1/2 to 2/3 of the pipe wall thickness, and ≤6 mm). The ends of the tack weld should be ground into a gentle slope of 30°-45° to facilitate a smooth transition during the final weld.

The electrode placement angle is also very important. The electrode angles for root pass welding, filler pass welding, and cap weld welding are basically the same, but the arc length and electrode manipulation differ. During welding, the arc length and oscillation width should be strictly controlled. Pay attention to the arc heating position. If an error or defect is found in the tack weld position, the original tack weld should be removed immediately and a new tack weld should be performed.

Typical welding parameter reference: 

Taking a Φ219×6mm carbon steel pipe as an example, the recommended welding parameters are as follows:

Electrode type: E6010 (Φ3.2mm) for root pass welding, E7018 (Φ4.0mm) for fill/cover weld
Welding current: 90~110A for root pass welding, 110~130A for fill/cover weld, 100~120A for cover weld
Welding speed: 80~120mm/min for root pass welding, 100~150mm/min for fill/cover weld
Arc length: ≤3mm for root pass welding, 3~5mm for fill/cover weld

Specific parameters should be determined according to the Welding Procedure Qualification Report (PQR) and Welding Procedure Specification (WPS). The above parameters are for reference only.

Pre-welding preparation and post-welding treatment:

Pre-welding preparation: Before welding, remove oil, rust, moisture, and other impurities from within a 20mm radius on both sides of the bevel. Preheating may be necessary. When the ambient temperature is below 5℃ or the pipe wall thickness is large, preheating should be performed according to the Welding Procedure Specification (WPS), with a preheating temperature typically between 50~100℃.

Post-welding treatment: After welding, promptly clean the weld slag from the weld surface and perform a visual inspection of the weld. Perform non-destructive testing (radiological testing RT or ultrasonic testing UT) on the weld according to design requirements to ensure the internal quality of the weld meets standards. For pipes prone to delayed cracking, post-weld heat treatment should be performed as specified.

Key Quality Control Points:

● Welders should hold corresponding welding operation certificates.
● Strictly adhere to the approved Welding Procedure Specification (WPS).
● Each weld should be marked with the welder's code to ensure quality traceability.
● When the ambient wind speed is ≥8m/s or the relative humidity is ≥80%, protective measures should be taken (such as erecting windbreaks, heating and dehumidification, etc.).

Conclusion: 

Downward vertical welding technology, with its high efficiency, high quality, and economic advantages, has become one of the mainstream processes for welding carbon steel and low alloy steel pipes. The key to successful application lies in: appropriate electrode selection + strict parameter control + standardized pre-weld preparation and post-weld treatment + a complete quality control system.

Read more: Design and Operation Guidelines for Long-Distance Steam Pipelines or SAW vs ERW and EFW Welded Steel Pipe