Welding Digest

A fundamental rule of welding engineering is that, in general, the strength of a weld should be greater than the strength of the base materials being joined. This is sometimes taken to mean that the strength of the weld should be greater than the specified minimum strength of the base materials being joined. There are often no requirements for the strength of the weld to be greater than the actual strength of the base materials being joined. Because there are no requirements for overmatching the actual strength of base materials in the majority of industry codes and standards used around the world for pipeline construction, good engineering judgment must be used to choose appropriate acceptance criteria for tensile testing during procedure qualification. For many applications, the acceptance criteria may be above the minimum requirements in the applicable code or standard. 

Recent industry trends that have contributed to pipeline girth weld failures include strength levels for as-received line pipe toward the upper end of the acceptable range in API Specification 5L:2018, Line Pipe, the use of an alloying strategy for the line pipe that results in a very lean chemical composition that increases the probability for softening in the heat- affected zone (HAZ), the use of undermatching strength electrodes for root pass welding, and the use of cellulosic-coated electrodes in general for intermediate and cover pass welding. 

Areas of Improvement 

The pipeline industry in North America, and in particular, pipeline operator Enbridge, Houston, Tex., has implemented measures in a number of areas to mitigate these low global strain girth weld failures. These areas include line pipe procurement and girth welding practices. These measures are intended to control or limit pipe material strength, increase weld metal strength, and minimize HAZ softening.  

Line Pipe Procurement 

In terms of line pipe procurement practices, Enbridge now requires pipe material tensile testing in the longitudinal direction in its pipe material purchase specifications, whereas API 5L currently requires tensile testing only in the circumferential direction for larger-diameter line pipe. The pipeline operator also specifies maximum allowable yield and ultimate tensile strength levels in the longitudinal direction that are no more than 17 and 22 ksi (117 and 152 MPa), respectively, over the specified minimum values for both Grades X70 and X65 line pipe material. 

In addition, the pipeline operator specifies pipe material chemical composition limits in its pipe material purchase specifications to control and/or limit softening in the HAZ. These limits include a carbon content of no less than 0.040% and a Pcm carbon equivalent of no less than 0.140%. The intent of specifying minimum chemical composition limits is to prevent the loss of strength that occurs because the HAZ of a girth weld is not subjected to accelerated cooling or control rolling, both of which contribute to strength in modern line pipe material. 

Girth Welding Practices 

Measures implemented by Enbridge intended to increase weld metal strength for higher-strength line pipe materials (Grade X65 and higher) include the use of higher-strength electrodes for root pass welding and restricting and/or eliminating the use of cellulosic-coated electrodes for intermediate and cover pass welding. Now that modern high-strength line pipe has a much leaner chemical composition, and a subsequently high resistance to hydrogen cracking in the HAZ, matching-strength electrodes are now appropriate for root pass welding. Recent trials have demonstrated that E8010 electrodes are acceptable for root pass welding in Grade X70 line pipe in terms of operability (e.g., root pass quality), resistance to hydrogen cracking, and an improvement to overall girth weld strength level (Ref. 4). Some welder training may be required because E8010 electrodes tend to produce a slightly softer arc than E6010 electrodes. From a weld strength perspective, the use of higher-strength electrodes for root pass welding is particularly useful for thinner-wall materials (e.g., 0.500 in. and less) where the root pass represents a greater portion of the weld thickness. 

For intermediate and cover pass welding, E8010 electrodes have difficulty matching the longitudinal strength of modern Grades X65 and X70 line pipe material, and the use of cellulosic-coated electrodes with a strength level greater than that of E8010 has been known to produce a significant risk of hydrogen cracking in the weld metal for all but relatively thin-wall (0.250 in. and less) pipelines constructed in relatively flat terrain in warm climates. For these reasons, the pipeline operator has moved toward the use of low-hydrogen welding consumables and/or processes with higher weld metal strength for intermediate and cover pass welding. The preferred option in this area for manual welding is the use of low-hydrogen downhill electrodes (e.g., E9045). For applications where mechanization is appropriate, the use of gas-shielded flux-cored arc welding (FCAW-G) with a minimum of an E91T1 consumable strength level is the preferred option. Both welding procedure options typically specify the use of E8010 electrodes for root pass welding.

Welder Training 

Welder training in the use of low-hydrogen downhill electrodes tends to be necessary for welders who are accustomed to welding downhill using cellulosic-coated electrodes. Differences in technique between the two include the need for higher current levels and faster travel speeds. Other differences include arc initiation techniques, required electrode angles, arc length limitations, techniques for breaking the arc, grinding of starts/stops, and low-hydrogen electrode storage practices. Increased travel speeds and tighter control over welding parameters tend to result in reduced heat input levels compared to the use of cellulosic-coated electrodes, an effect that aids in avoiding HAZ softening. 

Conclusion 

The use of matching-strength girth welds prevents longitudinal strains from accumulating in the weld region, which is a natural stress concentration and is more likely to contain imperfections than the pipe material. Matching strength in this context means deposited weld metal strength that matches or overmatches the actual strength (yield and tensile) of the line pipe material and no significant HAZ softening. This is most important for large-diameter pipelines constructed using modern high-strength line pipe materials, particularly those in hilly terrain or subject to subsidence or other forms of ground deformation.  

This article was written by William A. Bruce (senior principal consultant, welding technology, at DNV) and Russell Scoles (senior specialist welding engineer, pipeline integrity engineering, at Enbridge) for the American Welding Society.