CMT Process

8/21/2019 CMT Process

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4th Pipeline Technology Conference 2009

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 methodfor thin gauged materials in the automotive industry. The idea of combining thistechnology with a bug and band system for mechanized root pass deposition forpipeline applications was conceived. Initial trials gave favorable results andprovided further justification to integrate the two individual systems to become asingle platform for pipeline girth welding.

CMT was originally packaged for robotic work cell applications. The necessarycomponents that make the system perform would need to be merged with themost current bug and band technology. One of the early decisions regarding thebuild process was driven by the capabilities of the components to allow digitaltechnology between the welding power supply and the bug itself. This digitalcapability 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 tobe taken into consideration.

The need for mounting the push-pull wire feed motor onto a conventional CRCEvans P450 welding system was another necessary component in making theconversion a success. The ability to mount this hardware would providesignificant challenges, however the result would be a system that could be usedin a production environment. Engineers from both CRC-Evans and Fronius hadto 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 thatuses a new method of droplet detachment based on short circuit welding. Themoment the power source detects a short circuit, the welding current drops andthe filler wire starts to retract. Exactly one droplet is detached into the molten




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 definedretraction of the wire facilitates controlled droplet detachment to give a clean,virtually spatter-free material transfer. Figure 3 demonstrates the dropletdetachment sequence.

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 andmobilization for field qualifications and development programs was initiated.

Project Implementation

The first project for deployment of CMT was to be a spool base application wherewelding of 345 mm diameter x 18.3 mm wall thickness (SAW 415 FPD + 316L3mm layer) was required. The procedures were developed such that the rootpass 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

US

IS

wfs

T

Short circuit phase Arc phaseBoost phase

Plasma phase




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 10degrees.

Procedure welds were made and consistency trials were performed to ensure therobustness of the procedure and the system. The root profile on the projectCorrosion Resistant Alloy (CRA) material exhibited more positive re-enforcementthan observed on carbon steel. This is partly due to the argon back-purge that isused to shield the root from contamination during welding. The fusion area fromthe parent material to weld bead were smooth and even however, and weldsurface chevrons were uniform. Figure 4 demonstrates a completed root passwith internal and external profiles

Figure 4. Internal and External profiles

Mechanical testing was carried out both in the unstrained and strain agedcondition. For both scenarios, the mechanical performance met the acceptancecriteria for the project. A summary of the mechanical test results are listed belowin table 1.

Test Condition Yield Strength0.2% (N/mm

2)

Ultimate TensileStrength (N/mm

2)

CVN ToughnessProperties WCL @

-46oC

Typical HardnessRanges (HV10)

Weld metal

Strained & Aged

527 796 149J 228-252

Unstrained 515 753.5 132 J 193-222

Table 1 – Mechanical Properties

.




In January of 2009 the first field weld production was attempted. As with mostpipeline project kick offs, challenges were present early and the installation ofnew technology only made such challenges greater. There were unknowns abouthow the system would perform in the field, and just how operators would take tothe new system.

For a total of 8 days, 84 welds were completed for an average of 10.5 welds perday.

Although the production achieved was less than the desired target,improvements in the production process and welding procedure could be madeto realize significant gains. Implementation of CMT as a field production weldingmethod was considered a success, but further use in production environmentswould be required to increase the productivity of the system.

Carbon Steel

Additional work has been carried out to qualify the mechanized CMT process forcarbon steel applications. For carbon steel, the root pass welding procedure ismostly transparent, in that the same parameters can be used for differentapplications. The joint design is a closed gap J-Bevel, with a 5 degree bevelangle. The shielding gas is an Argon / CO2 mixture, with travel speeds rangingfrom 355 mm/min to 508 mm/min and wire feed speeds from 4.5 m/min to 6.5m/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 CMTwas the single sided welding of the root pass on a closure weld for a pressurevessel. The project material was 42” x 19.1mm Gr. 550 and the intendedapplication for this project was the transportation of compressed natural gas(CNG in a fatigue sensitive environment). Low service temperature requirementsdictated the need for good toughness properties at -40oC and the use of a solidwire 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 volumeand allows for use of mechanized fill and cap passes. This meant productivitycould be increased and mechanical properties achieved. The second advantagewas the need for the root pass to be flush with the ID surface of the parentmaterial. The specification requirement was to remove all internal excesspenetration upon completion. The root profile with CMT eliminated concern forthis 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




welded using Pulsed GMAW with an ER70S-6 wire. Overmatching of the Gr 550pipe material was achieved and the toughness requirements were met. Table 2demonstrates the mechanical properties of the qualified procedure.

Test Condition Yield Strength0.2% (N/mm

2)

Ultimate TensileStrength (N/mm

2)

CVN ToughnessProperties WCL @

-50oC

CTOD ToughnessProperties WCL @

-40oC (ave)

5G Girth Weld 620 727 106 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 0.250” Gr 485 pipe material. Theproject application was to weld pipes spooled horizontally and made into acoselle arrangement (see figure 5). The fatigue effects exhibited on these weldsrequired careful consideration as the loading would be different than a typicaloffshore Steel Cantenary Riser (SCR). As the coselle is pumped full of CNG,pressure is exerted internally on the weld in the hoop direction. Subsequentdepressurized during offloading created cyclic loading that had to be taken intoconsideration.

Figure 5 Horizontal Spooling of 6” pipe

One of the most important advantages of CMT as a root pass welding technologyfor carbon steel is the finished profile of the internal weld bead. For fatigueapplications CMT provides a root profile that is uniform and has a shallowreentrant angle. The shallow reentrant angle reduces stress concentration andsubsequently increases fatigue life. Preliminary fatigue testing of strip typespecimens’ demonstrated increased fatigue life over welds using traditional rootpass welding technology. This meant that using CMT for root pass would extendthe life of the coselle.




Another carbon steel project using CMT that should be highlighted involvesdouble jointing small diameter flow lines. For double joint applications, CMToffers significant advances in productivity, especially considering that thetechnology does not require the use of an internal alignment tool. The platformversatility allows CMT to be deployed in project locations that traditionalmechanized welding processes may not be ideal, such as offshore tie ins.

SCR qualification requirements for double jointing pipe are also considered amajor target for this technology. Fatigue properties of the root bead have shownthat the weld can be carried out without the need to remove the internalreinforcement, while still meeting the stringent requirements required for suchcritical service. The ability of the system to adapt to small diameter pipelineapplications 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 rootpasses and Pulsed GMAW fill and cap passes for welding high strength pipelinetie-ins

ConclusionThe CMT process is an up and coming mechanized root pass weldingtechnology for pipeline girth welds. The technology is just now entering themarket and has the potential to increase quality, productivity, and expandcapability. 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 developmentprojects have been conducted on carbon steel pipe materials. Pressure vesselswith stringent requirements for fatigue life and low temperature toughnesswelded with CMT root passes have been carried out with proven quality andmechanical performance. Double jointing and welding of SCR quality welds areadditional 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 weldingprocesses can significantly improve productivity.

• Elimination of copper backing saves on consumable costs

• Deposition thicknesses of approximately 5mm




• Accommodation of mismatch up to 3.0mm

Low heat inputs, minimized bevel volume, accommodation of mismatch, fatigueperformance and root profile are the drivers for continued implementation of thistechnology into the pipeline industry.

CMT Process

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