PUBLISHED BY THE AMERICAN WELDING SOCIETY TO ADVANCE THE SCIENCE, TECHNOLOGY, AND APPLICATION OF WELDINGAND ALLIED JOINING AND CUTTING PROCESSES WORLDWIDE, INCLUDING BRAZING, SOLDERING, AND THERMAL SPRAYING

JUNE 2016

59010.qxp_layout 1 8/2/16 11:49 aM Page 1

California is entering its fourth year of unprecedenteddrought. And with an ocean of water off its coast, finding away to turn a drought-proof source into a potable one is seen

as the best insurance policy for rapidly diminishing rivers andreservoirs. San Diego County, in particular, is especially vulnerablein this drought; the region imports over 90% of the water it uses.With sources like the Colorado River being stretched thin,compounded by the drought in the west and lack of snowpack, aninnovative solution is being brought to the forefront. To ease thecrisis, California looks to other countries like Israel, Saudi Arabia,and Australia, which have embraced the technology of desalinationand constructed numerous plants over the past few decades. The Carlsbad Desalination Plant, located about 35 miles northof San Diego, is a reverse osmosis (RO) seawater plant that willturn roughly 100 million gal/day of seawater into 50 million

How welders at the CarlsbadDesalination Plant aresuccessfully weldingcorrosion-resistantalloys

BY MATTHEW STERNISHA

Welding Austenitic SMO 254Stainless Steel for theDesalination Industry

An aerial view of the CarlsbadDesalination Plant taken in Sep-tember 2015.

59010.qxp_layout 1 8/2/16 11:49 aM Page 60

gal/day of potable water. Thisrepresents about 7% of San DiegoCounty’s usage, and diversifies theregion’s water supply to a drought-proof and locally controlled supply. Reverse osmosis desalination has beenwidely used in other countries to solvewater shortages. This design, however,requires a pipe material that canwithstand the roughly 1000 lb/in.2

required to remove the salt with osmoticpressure through the semipermeablemembrane but still allows watermolecules to pass through. In addition,this pipe must be able to resist thecorrosive properties of seawater and ahighly concentrated brine at 7% salinity,all while maintaining a minimum designlife of 30 years. In the past, super duplexwas the piping material of choice;however, super duplex has its drawbacks,being much more unforgiving inweldability. It also poses a potential riskof lessening its mechanical properties,toughness, and corrosion resistance if thebalance of austenite and ferrite in thewelded joint is not maintained. The new choice of material is SMO254 (UNS 31254), chosen for itsability to provide excellent resistanceto corrosion (PREN>40) and pitting, for its high impact toughness,resistance to chloride stress corrosioncracking, and excellent workabilityand weldability.

Joint Design and Welding Procedure

The process engineer dictated thewelding parameters of the SMO 254piping, with much of the procedureincorporating past lessons learned fromyears of desalination plants in operation

as well as new research. The gas tungstenarc welding (GTAW) process was chosenfor this welding. Although this requiredthe most skill, it also provided the mostcontrol of heat input and of the weldingvariables. Typical joint design was asingle V-groove butt joint, withparameters shown in Fig. 1. Pipe thicknesses ranged fromSchedule 40S (0.300 in.) to 22 mm(0.866 in.). Figures 2, 3 show atypical fitup of the piping joint withbridge tacks used to hold the fitup atthe root opening. Welding of fully austenitic stainlesssteels carries the potential risk of hot-cracking in the weld metal, in particularif the weldment is under constraint. Inaddition, an SMO 254 weldment haslow thermal conductivity and highthermal expansion. Thus, heat input intothe weld and welding sequence mustmitigate potential distortion during thewelding process. The welding procedurespecification (WPS) limited the heatinput to 1.5 kJ/mm at the root pass, and0.9 kJ/mm on the cold and fill passes.This required constant monitoring oftemperature and speed of the welder,certainly a tedious task for a qualitycontrol program, but necessaryconsidering the sensitivity of the materialto heat input and the risk of weldfailures. The amperage range was80–125 A on the root, with 70–100 Aon the cold and fill passes. Another element that affects heatinput is the welding technique. The pipemanufacturer as well as the processengineer specified the use of a stringerbead on the welds. The stringer bead isnot common to many pipefitter weldersand required practice and training toensure the correct technique was beingapplied within the required parametersspecified by the WPS. Figure 10 shows acompleted weld, which demonstrates thestringer technique used. SMO 254 does not usually requireany preheat, provided ambienttemperature is above 50°F. However,maximum interpass temperature waslimited to 95°C (203°F) in the WPS,again to address the material’s sensitivityto heat input and cracking. Gas purging is critical when weldingall types of stainless steel alloys tofacilitate good weld pool fusioncharacteristics and protect the weld andheat-affected zone (HAZ) surfaces fromatmospheric contamination andoxidation. Backing gas utilized was a98% argon/2% nitrogen blend to a

Joints (QW-402)Joint Design: ButtBacking or Backing Material: See Para Gas (QW-408)Root Spacing: See below

Fig. 1 — Typical single V-groove butt joint used for SMO 254 piping at the Carlsbad Desalination Project.

Fig. 2 — Typical fitup on an 8-in.-diameter, 11-mm pipe wall thicknessSMO 254 piping at the Carlsbad Desalination Plant.

Fig. 3 — Picture of a joint design and fit-up on a 24-in.-diameter, 22-mm pipewall thickness SMO 254 pipe at theCarlsbad Desalination Project. Bridgetacks were utilized to hold the rootopening and fitup.

59010.qxp_layout 1 8/2/16 11:49 aM Page 61

purity of 0.001%. Inflatable purge gasbladders were used in the pipes toprovide purge dams for the backing gas,to confine the pipeline inside purgevolume to a more localized andcontrollable level versus purging wholepipeline sections, reducing the amountof backing gas used on the project andreducing overall cost. Shielding gas wasalso of the same composition, and flowrate was 20–35 ft3/min (9.34–16.4L/min). Flow rate was controlled by thewelder, and a calibrated oxygen sensorwas utilized to analyze the oxygencontent (<0.05% per WPS) within theargon purge gas environment. Backinggas flow rate was ranged 5–60 ft3/min(2.3–28.3 L/min). Purge gas was left inplace on the weld until a weld thicknessof 8 mm was achieved, at which timeonly shielding gas was required to finishthe weld. EW-Th-2 thoriated tungsten withDC Straight Polarity (DCSP or DCEN)was used during the GTAW process withAvesta P12 Polarit (1.6–3.2 mmdiameter) ER NiCrMo-3 as the fillermetal. Figure 4 shows a welderperforming the root pass on the SMO 254 pipe.

Training and Union Craft Support

San Diego is certainly home to manyskilled workers, on account of the Navyshipyards, numerous power plants, andother heavy industry. But welding onAlloy SMO 254 was new to the localunion craft force. This presented apotential challenge to the contractor asthe welder qualification is a key part of asuccessful project. But several steps weretaken to overcome this hurdle. Jobmanagers and supervisors met with thelocal union to discuss the jobrequirements, with local union agreeingto utilize their local training center fortraining and testing on the SMO 254coupons. The contractor sent a qualitycontrol manager to the facility to assist,ensure the highest standards were held,and provide instruction during thetraining process. Testing was performedon 3-in.-diameter, Schedule 160(thickness 0.436 in. or 11 mm) SMOpipe coupons in the 6G position to meetthe thickness requirements for the pipebeing welded at the jobsite. Figure 5 shows a weld couponperformed by a welder during training. All weld coupons wereradiographically inspected. In addition,

Fig. 4 — Welder performing a root pass on 36-in.-diameter SMO 254 pipe utilizing theGTAW process at the Carlsbad Desalination Project.

Fig. 5 — A 3-in.-diameter, Schedule 160 SMO 254 pipe coupon that was welded as part of training at the local union training center and used for training and qualification.

59010.qxp_layout 1 8/2/16 11:49 aM Page 62

the weld coupon was sent out for ASTMG41 corrosion testing, which also wasrequired to yield acceptable results priorto the welder being job certified to weldon the plant’s pipe. It took, on average,two weeks for a welder to master the skillprior to taking and passing the welderqualification test. What resulted,however, was a new skillset for welders inSan Diego County and a growingworking relationship between thecontractor and the Local.

Verification and Inspection

The SMO 254 pipes are critical tothe process of the desalination plant. Nowater can be desalted with these pipesout of service. Therefore, these pipes areconsidered process critical, and as such,the engineer was very concerned abouttheir reliability. In addition, the roughly1000 lb/in.2 pressure of the pipescompelled the engineer to designate the

pipes as Category M fluid service pipesper ASME B31.3, which required 100%radiographic inspection on every fieldweld. In addition, as recommended bythe process engineer, dye-penetranttesting of the root pass was performedprior to the cold and fill passes beingplaced, and all welds were visuallyinspected to the applicable ASME code. Figures 6, 7 show dye-penetrant testingof the root pass on typical pipes at theproject. The following parameters wererequired to be recorded during welding:

• Voltage (V)

• Amperage (A)

• Welding length/travel distance (in.)

• Welding time (s)

• Shielding gas flow rate (ft3/min)

• Backing gas flow rate (ft3/min)

• Oxygen content (%)

• Temperature (°F).

The following parameters were thencalculated using the measured data:

• Travel speed (in./min) calculated bywelding length (in.) ÷ welding time (min)

• Heat input (KJ/in.) calculated by

Fig. 6 — Dye-penetrant testing of the root pass on a 12-in.-diameter weld on SMO 254pipe. In this stage, the dye is first applied to the joint and allowed to dry prior to applyingthe developer solution.

Fig. 7 — Dye-penetrant testing on theroot pass of a 36-in.-diameter SMO 254alloy pipe. In this stage, the developer is applied and the joint is inspected for defects.

Fig. 8 — Welder performing a typical weld on SMO 254 pipe. The QC inspector is nearbyto record relevant data and ensure compliance with WPS requirements for each weld.

59010.qxp_layout 1 8/2/16 11:50 aM Page 63

V � A � 60 ÷ travel speed (in./min).

Values were then reviewed to ensureno welding passes were made thatviolated the parameters set forth in theWPS, particularly heat input. Thisrequired close monitoring by a qualitycontrol (QC) representative to collectthe data, accurately perform thecalculations, review the results, andensure compliance. In addition, thesedata were retained as a record document. Figure 9 shows a welder performing aweld with the QC monitor nearbyrecording the required data. Datacollection for the welds and weldingprocess on the SMO 254 alloy was anextensive undertaking by the qualitycontrol and quality assurancedepartments, but it was absolutelycritical to mitigate potential shutdownsin the plant for repairs, as well asensuring a long design life. Due to the SMO 254 alloy’ssensitivity to heat input, commondefects found were cracks, incompletefusion (particularly at the sidewall), andlack of penetration. Radiographicinspection lends itself well to detect thesetypes of defects and was thenondestructive examination of choice.

All welds on the SMO 254 piping were100% radiographic inspected.

Conclusion Desalination has the potential forhuge growth within the United States,and the Carlsbad Desalination Plant is atthe forefront of that progression. Understanding the weldabilty andessential variables in Alloy SMO 254 iscritical if one hopes to withstand thehighly corrosive properties of seawaterand the osmotic pressures that must beovercome during the RO process. SMO254 has the ability to resist corrosion inthe seawater process, as well as handlethe high pressures. It has superiorweldability characteristics compared toother alloy options, such as duplex, butit is imperative that the risk of heat-induced cracking is considered, andmeasures are taken to mitigate this riskthrough careful monitoring andplanning of the welds and weldsequence. In addition, providing trainingand obtaining a skilled workforce is alsoimperative to ensure success whenworking with fully austenitic stainlesssteels like Alloy SMO 254. Last, a carefully controlled and

documented QC program is crucial toproducing sound welds, ensuring longservice life, and avoiding weld failuresand costly repairs in the future.Successfully welding corrosion-resistantalloys like SMO 254, utilizing themeasures mentioned previously, leavesdesalination poised to quench thedrought and spawn a new industrywithin the United States.

Works Consulted

1. smt.sandvik.com/en/materials-center/material-datasheets/tube-and-pipe-seamless/sandvik-254-smo/. 2. carlsbaddesal.com/. 3. weldersuniverse.com/welding_beads.html. 4. lincolnelectric.com/en-us/support/process-and-theory/Pages/understanding-polarity-detail.aspx. 5. rjsales.com/techdata/alloys/254smo.html.

WJ

Fig. 9 — A welder makes an in-place weld on SMO 254 pipe on thereverse osmosis train at the Carlsbad Desalination Plant. Each of the14 RO trains produces roughly 2000 g/min of fresh water.

Fig. 10 — A completed weld on the SMO 254 alloy pipe at theCarlsbad Desalination Plant. Stringer beads were used per thepipe manufacturer’s recommendation and the WPS.

Matthew Sternisha(Matthew.Sternisha@kiewit.com) is aproject manager with Kiewit Corp., Santa FeSprings, Calif.

Electronic permissions to voestalpine Bohler Welding GmbH from WEldinG JournalJune © 2016 american Welding Society.

..

59010.qxp_layout 1 8/2/16 11:50 aM Page 64