Principles for selecting welding methods for welded pipes

Principles for selecting welding methods for welded pipes

1. Priority principle for arc welding with electrodes

For the installation and welding of pipelines whose diameter is not too large (such as less than 610mm) and the pipeline length is not very long (such as less than 100km), electrode arc welding should be considered as the first choice. In this case, electrode arc welding is the most economical Welding method.Compared with automatic welding, it requires less equipment and labor, has low maintenance costs, and the construction team has relatively mature technology.

Electrode arc welding has been used for installation welding for more than 50 years. Various electrodes and various operating methods are relatively mature technically. APISPEC5L-2004 "Pipeline Steel Pipe Specification" X70 and below have accumulated experience in pipeline welding of various steel grades Plenty of data and easy quality assessment.Of course, for the welding of high-strength steel pipes, attention should also be paid to the selection and control of welding rods and process measures. When welding follows the standard pipeline specification AP1STD1104-2005 (Welding of Pipelines and Related Equipment), use welders who have passed the training and examination and perform With 100% radiographic inspection, it is possible to control the repair rate of all welds below 3%.Due to lower cost and maintenance. Coupled with the quality assurance, arc welding with electrodes has been the first choice of most project contractors in the past.

2. Priority principle for submerged arc automatic welding

As mentioned before, automatic submerged arc welding of pipes is performed in pipe welding stations specially designed for pipes. If the two pipes are welded close to the site (double pipe welding), the number of welds on the main line can be reduced by 40% to 50%, greatly shortening the laying operation cycle.
The high efficiency and high quality of automatic submerged arc welding for installation and welding are obvious, especially for pipelines with larger diameters (above 406mm) and wall thicknesses exceeding 9.5mm. When the laying distance is very long, for economic reasons, Usually the submerged arc automatic welding method is first considered.
However, what has a veto power is whether the road for transporting double pipes is feasible, whether the road conditions allow it, and whether there are conditions for transporting double pipes longer than 25m. Otherwise, the use of automatic arc welding will be meaningless.
Therefore, for long-distance pipelines with a diameter of 406mm or more and large wall thickness, when there are no problems with transportation and road conditions, submerged arc automatic welding for double or triple pipe welding is the best choice for the project contractor.

3. Flux-cored wire semi-automatic welding priority principle

Combined with electrode arc welding, flux-cored wire semi-automatic welding is used for filler welding and cover welding of large-diameter, thick-walled steel pipes. It is a good welding process.
The main purpose is to change the intermittent welding process into a continuous production method, and the welding current density is larger than that of electrode arc welding, the wire melts quickly, and the production efficiency can be 3 to 5 times that of electrode arc welding, so the production efficiency is high.
Currently, self-shielded flux-cored wire semi-automatic welding is widely used in field pipeline welding due to its strong wind resistance, low hydrogen content in the weld, and high efficiency. It is the preferred method for pipeline construction in my country.

4. Priority principles for gas metal arc automatic welding

For long-distance pipelines with a diameter greater than 710mm and a large wall thickness, in order to obtain high efficiency and high quality of construction, gas-shielded automatic welding with melting arc is often first considered.
This method has been used for 25 years. It has been widely recognized for large-diameter pipelines in the world, including onshore and underwater pipeline groups, and has received widespread attention in Canada, Europe, the Middle East and other countries and regions.
The important reason why this method is widely used is that the installation and welding quality can be guaranteed, especially when welding high-strength pipelines.
Due to the low hydrogen content of this welding method and the strict composition and manufacturing requirements of the welding wire, if the toughness requirements are high or the pipeline is used to transport acidic media, stable welding quality can be obtained by welding high-grade steel pipes using this method.
It is worth noting that compared with electrode arc welding, the gas shielded automatic welding system requires a large investment and has high requirements on equipment and personnel. The required advanced maintenance must be considered, as well as accessories and mixed gases that meet hygienic requirements supply.