4 th Pipeline Technology Conference 2009

Transcription

1 In 2006, CRC-Evans was first introduced to the Cold Metal Transfer (CMT) process. At the time, CMT was a technology intended for use as a joining method for thin gauged materials in the automotive industry. The idea of combining this technology with a bug and band system for mechanized root pass deposition for pipeline applications was conceived. Initial trials gave favorable results and provided further justification to integrate the two individual systems to become a single platform for pipeline girth welding. CMT was originally packaged for robotic work cell applications. The necessary components that make the system perform would need to be merged with the most current bug and band technology. One of the early decisions regarding the build process was driven by the capabilities of the components to allow digital technology between the welding power supply and the bug itself. This digital capability allows the system to react quickly and make adjustments to compensate for changes during the deposition phase. As critical welds are made, changes is joint geometry, variations in fit up and band misalignment, all have to be taken into consideration. The need for mounting the push-pull wire feed motor onto a conventional CRC Evans P450 welding system was another necessary component in making the conversion a success. The ability to mount this hardware would provide significant challenges, however the result would be a system that could be used in a production environment. Engineers from both CRC-Evans and Fronius had to create a platform that would satisfy the need for robustness, ease of service, mobility, and performance, especially in harsh environments. Figures 1 and 2 demonstrate the original configuration and the final modified platform. Figure 1 Original Configuration Figure 2 Modified configuration for Bug and Band System The Process Cold Metal Transfer is a modified Gas Metal Arc Welding (GMAW) process that uses a new method of droplet detachment based on short circuit welding. The moment the power source detects a short circuit, the welding current drops and the filler wire starts to retract. Exactly one droplet is detached into the molten

2 weld puddle. The filler wire then moves forwards again and the cycle is repeated. The filler wire is constantly retracted at very short intervals. The precisely defined retraction of the wire facilitates controlled droplet detachment to give a clean, virtually spatter-free material transfer. Figure 3 demonstrates the droplet detachment sequence. wfs Plasma phase I S Short circuit phase Boost phase Arc phase U S T Figure 3 Droplet detachment sequence When the combined platform had been developed laboratory trials commenced. Through several months of development, a workable system was achieved and mobilization for field qualifications and development programs was initiated. Project Implementation The first project for deployment of CMT was to be a spool base application where welding of 345 mm diameter x 18.3 mm wall thickness (SAW 415 FPD + 316L 3mm layer) was required. The procedures were developed such that the root pass deposition was completed by using a 309L Mo (ER309L Mo modified, AWS A5.9-93) filler material, 1.2 mm diameter. The remaining procedure, consisting of

3 the hot, fill, and cap passes were welded with Thermanit 625 (ERNiCrMo-3, AWS A5.14) 1.0 mm diameter. The shielding gas was an Argon / CO2 balance and the joint configuration was a closed gap J-bevel. The bevel angle measured 10 degrees. Procedure welds were made and consistency trials were performed to ensure the robustness of the procedure and the system. The root profile on the project Corrosion Resistant Alloy (CRA) material exhibited more positive re-enforcement than observed on carbon steel. This is partly due to the argon back-purge that is used to shield the root from contamination during welding. The fusion area from the parent material to weld bead were smooth and even however, and weld surface chevrons were uniform. Figure 4 demonstrates a completed root pass with internal and external profiles Figure 4. Internal and External profiles Mechanical testing was carried out both in the unstrained and strain aged condition. For both scenarios, the mechanical performance met the acceptance criteria for the project. A summary of the mechanical test results are listed below in table 1. Test Condition Yield Strength 0.2% (N/mm 2 ) Ultimate Tensile Strength (N/mm 2 ) CVN Toughness Properties -46 o C Typical Hardness Ranges (HV10) Weld metal Strained & Aged J Unstrained J Table 1 Mechanical Properties.

4 In January of 2009 the first field weld production was attempted. As with most pipeline project kick offs, challenges were present early and the installation of new technology only made such challenges greater. There were unknowns about how the system would perform in the field, and just how operators would take to the new system. For a total of 8 days, 84 welds were completed for an average of 10.5 welds per day. Although the production achieved was less than the desired target, improvements in the production process and welding procedure could be made to realize significant gains. Implementation of CMT as a field production welding method was considered a success, but further use in production environments would be required to increase the productivity of the system. Carbon Steel Additional work has been carried out to qualify the mechanized CMT process for carbon steel applications. For carbon steel, the root pass welding procedure is mostly transparent, in that the same parameters can be used for different applications. The joint design is a closed gap J-Bevel, with a 5 degree bevel angle. The shielding gas is an Argon / CO 2 mixture, with travel speeds ranging from 355 mm/min to 508 mm/min and wire feed speeds from 4.5 m/min to 6.5 m/min. The welding consumable diameter is 1.0 mm and trials with ER90S-G, ER80S-G, and ER70S-6 wire variations have all yielded good results. A technically significant carbon steel application that involved the use of CMT was the single sided welding of the root pass on a closure weld for a pressure vessel. The project material was 42 x 19.1mm Gr. 550 and the intended application for this project was the transportation of compressed natural gas (CNG in a fatigue sensitive environment). Low service temperature requirements dictated the need for good toughness properties at -40 o C and the use of a solid wire procedure could deliver the required properties. As internal clamping was not an option, mechanized CMT was a natural choice. The advantages of using CMT for the root pass were two fold. The first was maintaining a narrow bevel. As the narrow bevel reduces weld deposit volume and allows for use of mechanized fill and cap passes. This meant productivity could be increased and mechanical properties achieved. The second advantage was the need for the root pass to be flush with the ID surface of the parent material. The specification requirement was to remove all internal excess penetration upon completion. The root profile with CMT eliminated concern for this major requirement because of its flat profile. By using the mechanized CMT process with the narrow groove J-Bevel, mechanized fill pass welding could be implemented. The fill and cap passes were

5 welded using Pulsed GMAW with an ER70S-6 wire. Overmatching of the Gr 550 pipe material was achieved and the toughness requirements were met. Table 2 demonstrates the mechanical properties of the qualified procedure. Test Condition Yield Strength 0.2% (N/mm 2 ) Ultimate Tensile Strength (N/mm 2 ) CVN Toughness Properties -50 o C CTOD Toughness Properties -40 o C (ave) 5G Girth Weld J 0.34mm Table 2 Mechanical Properties Additional procedures for a separate project that involved the transportation of compressed natural gas were qualified on 6 x Gr 485 pipe material. The project application was to weld pipes spooled horizontally and made into a coselle arrangement (see figure 5). The fatigue effects exhibited on these welds required careful consideration as the loading would be different than a typical offshore Steel Cantenary Riser (SCR). As the coselle is pumped full of CNG, pressure is exerted internally on the weld in the hoop direction. Subsequent depressurized during offloading created cyclic loading that had to be taken into consideration. Figure 5 Horizontal Spooling of 6 pipe One of the most important advantages of CMT as a root pass welding technology for carbon steel is the finished profile of the internal weld bead. For fatigue applications CMT provides a root profile that is uniform and has a shallow reentrant angle. The shallow reentrant angle reduces stress concentration and subsequently increases fatigue life. Preliminary fatigue testing of strip type specimens demonstrated increased fatigue life over welds using traditional root pass welding technology. This meant that using CMT for root pass would extend the life of the coselle.

6 Another carbon steel project using CMT that should be highlighted involves double jointing small diameter flow lines. For double joint applications, CMT offers significant advances in productivity, especially considering that the technology does not require the use of an internal alignment tool. The platform versatility allows CMT to be deployed in project locations that traditional mechanized welding processes may not be ideal, such as offshore tie ins. SCR qualification requirements for double jointing pipe are also considered a major target for this technology. Fatigue properties of the root bead have shown that the weld can be carried out without the need to remove the internal reinforcement, while still meeting the stringent requirements required for such critical service. The ability of the system to adapt to small diameter pipeline applications offers significant benefits for welding of fatigue critical pipelines, which is worthy of note. Upcoming work is also scheduled for examining the use of mechanized CMT root passes and Pulsed GMAW fill and cap passes for welding high strength pipeline tie-ins Conclusion The CMT process is an up and coming mechanized root pass welding technology for pipeline girth welds. The technology is just now entering the market and has the potential to increase quality, productivity, and expand capability. Field trials on Inconel clad material in a production environment have demonstrated the potential capabilities of the system. In addition to welding CRA materials, welding qualifications and development projects have been conducted on carbon steel pipe materials. Pressure vessels with stringent requirements for fatigue life and low temperature toughness welded with CMT root passes have been carried out with proven quality and mechanical performance. Double jointing and welding of SCR quality welds are additional applications where CMT may be applied. For welding cross country pipelines with HSLA steels grades X-80 and above, mechanized CMT and GMAW-P tie-in welding provides the low hydrogen solution that meets the need for elevated mechanical properties requirements. The CMT Process can also provide the following benefits: Faster travel speeds when compared to conventional root pass welding processes can significantly improve productivity. Elimination of copper backing saves on consumable costs Deposition thicknesses of approximately 5mm

7 Accommodation of mismatch up to 3.0mm Low heat inputs, minimized bevel volume, accommodation of mismatch, fatigue performance and root profile are the drivers for continued implementation of this technology into the pipeline industry.