Today there are numerous ways to weld high yield pipe in the field. It is necessary to understand these various processes to insure that the process selected with meet the quality and productivity requirements of a pipeline project. Several prcocesses are discussed, with emphasis on shielded metal arc welding with cellulosic electrodes and self-shielded flux cored arc welding.
In today’s world cross country transmission pipelines have to address many issues including higher service pressures, sour products, new high strength steels, more severe operating environments, tighter governing codes, and a host of environmental concerns. These conditions must be balanced by the needs of the pipeline contractor to control costs and complete the project in a timely manner while still meeting more stringent quality requirements. A knowledge of welding processes can help the contractor meet his needs and deliver the required quality. This same knowledge can help the specifying engineer understand that there are numerous ways to meet his quality and design needs without imposing unnecessary costs on the contractor.
Several processes and combinations of processes currently used for the field welding of cross country line pipe. These include shielded metal arc welding (SMAW), self shielded flux cored arc welding (FCAW-S), and gas metal arc welding (GMAW). With GMAW transfer mode must also be consider, short arc, controlled short arc as in Surface Tension Transfer®, spray, and globular. Attention will be placed on those processes which lend themselves to high quality and high productivity field welding with conservative capital investment.
Review of Pipeline Steels
Today’s pipe steels are higher strength than those used previously and are today designed with weldability in mind. The most common steels used for oil and gas cross country pipelines conform to API 5LX or similar such standards.
Strength levels can be achieved by several methods including gross chemistry, micro-alloying, and cold expansion of the pipe when produced at the pipe mill. In higher strength grades the trend is to use cold expansion and micro-alloying so that carbon and manganese can be kept at relatively low levels, thus reducing heat affected zone hardness and helping reduce, though not eliminate concerns about weld metal hydrogen. For example, it is typical to see carbon contents of less than 0.05% in modern X70 and X80 steels with some X80 steels having Pcm values of less than 0.20.
Obviously the first step in the welding of pipe is to run the root pass. This is perhaps the most critical pass on a pipe weld for several reasons. First, this is the most difficult pass to make on a pipe weld, requiring good operator skill for manually applied processes, with good process control combined with good alignment. Automatically applied processes require operators with high degrees of technical skill combined with good alignment and backing systems. The automated process of choice today is gas metal arc welding and is generally used with either an internal copper backup ring, or, if the diameter is large enough, an internal welding system. Both of these approaches add complexity to field welding and impose certain restrictions the use of traditional GMAW transfer modes.
With backup rings there is the possibility of unacceptable copper pick up in the root pass. With internal welding systems there is a minimum pipe diameter below which the systems are not practical. The ideal welding process would allow welding of a root bead without backup rings and internal systems and would have a root bead with sound weld metal and just enough buildup to insure a full thickness weld. This weld would also have no internal undercut, no lack of fusion, no porosity, and good mechanical properties.
Welding speed must also be considered when looking at the welding of the root pass. The pace of pipe laying is determined by how quickly the root pass can be done. While some time can be gained by putting more operators on this pass, there is a practical limit to this approach. Therefore, high travel speeds are essential. Speed is needed to maintain schedules and control equipment leasing costs.
Much of the pipeline welding done today is in the emerging economies of the world, often in remote inhospitable climates and must draw on local labor pools for welders. Tthis means that the process used must cope with adverse conditions of weather including wind, temperature extremes, and moisture. The necessary skills need either to exist in the local labor pool, or be easily learned. The required welding equipment must also be rugged, reliable, and durable.
When all of the above factors are considered, two welding processes emerge as the leading processes, shielded metal arc welding and self shielded flux cored arc welding. In the case of shielded metal arc welding, Figure 1, there are advantages to using cellulosic electrodes run in the vertical down direction instead of using low hydrogen electrodes, even on higher strength steels. Because cellulosic electrodes generate a significant amount of shielding gases in use and have a focused forceful arc, these electrodes tend to have better root pass properties and better root pass control. The high arc force helps to maintain puddle and slag control in vertical down progression, while also having high travel speeds. Low hydrogen electrodes primarily use slag to protect the weld pool and this can lead to contamination of the weld pool from the back side of the bead, reducing weld properties and increasing the chances for porosity. The relatively low penetration of low hydrogen electrodes when compared to cellulosic electrodes also means that wider root gaps must be used which increase welding time and slow down the welding operation. Cellulosic electrodes can put in root passes at speeds that exceed 14 inches per minute (356 mm per minute) and with consistent inside buildups of under 1/16 inch (1.6 mm).
Cracking concerns with cellulosic electrodes are addressed with proper preheat and interpass temperature control, and by using procedures which insure adequate ligament in the root pass. Preheat and interpass temperatures are dictated by steel chemistries which today are more forgiving than previously. Use of the correct electrode size run in the middle to lower portion the range for that electrode helps insure a proper ligament. Root bead cracking can also be minimized by not moving the lineup clamp until the second pass has been completed.
Self shielded flux cored arc welding, Figure 2, has the advantages of shielded metal arc welding with cellulosic electrodes including high arc force, high penetration and excellent puddle control when welding with a vertical down progression. In addition, this process has the advantages of automated processes including high deposition rates, high travel speeds, high arc on times, and controlled hydrogen levels. Frequently self shielded is used over root passes made with shielded metal arc welding. This is one approach to the welding of X80, where hydrogen cracking in the parent steel is not a concern for the root pass, but weld metal hydrogen cracking could be a concern on subsequent passes.
In shielded metal arc welding shielding is generated by the decomposition of the flux at the arc. In self shielded flux cored arc welding a continuous tubular electrode contains are stabilizers and core materials which will generate shielding when they reach the arc. Both processes work outside under severe weather conditions including temperature extremes and high winds. Likewise, both shielded metal arc welding with cellulosic electrodes and self shielded flux cored arc welding are easily learned by welding operators already skilled in other forms of shielded metal arc welding. For example, one instructor was recently able to train and qualify to API 1104 over ninety welding operators unfamiliar with self shielded flux cored arc welding.
Notice in the above table that only the self shielded arc welding process is recommended for the welding of X80 once the root and hot passes are completed.
Both processes are capable of delivering properties which meet or exceed the minimum properties specified for the parent steels, which is all that most governing codes require. Here are a few test results for two pipe grades run with the typical pipe joint detail shown in Figure 3.
The one issue not yet discussed is that of economics. Many things affect the cost of welding including material costs, equipment cost, labor rates and a host of others outside the scope of this paper. For purposes of comparison time to complete a welded joint will be used for a relative indicator of cost. The basic assumption is that if equipment costs and labor rates are similar, the time to complete a weld joint will be indicative of cost, less time translating into lower costs and higher productivity. All comparisons will be done using the typical joint detail used above to simplify the results. In reality compound preparations may reduce the total time on heavier wall pipe. Welding comparisons will use .750 in (19 mm) wall, 48 in (1219 mm) diameter pipe.
These times represent man-minutes of welding. The joint done with all self shielded flux cored arc welding has the lowest total time, but the combination of shielded metal arc welding with self shielded flux cored arc welding will result in the greatest amount of pipe laid on a given day because of the time savings in the root pass. Tthis combination will result in the best overall compromise of reduced total time and maximum pipe laid in a given period of time.
As can be seen, shielded metal arc welding and self shielded flux cored arc welding present cost effective ways to produce quality welds under field conditions. Also, the best solution to field welding of cross country pipelines is often is to use a combination of welding processes.
Welding Handbook, 8th Edition, (1991) American Welding Society, Miami
The Procedure Handbook of Arc Welding, 13th Edition, (1994), Lincoln Electric Company, Cleveland