Achieving Cost Savings With Inovative Welding and Examination Techniques

8/12/2019 Achieving Cost Savings With Inovative Welding and Examination Techniques

1/8

Achieving cost savings with innovative welding and examination techniques

B. Messer a, * , C. Patrick b , S. Seitz a

a Fluor Corporation, 55 Sunpark Plaza SE, Calgary, Alta., Canada T2X 3R4b Fluor Corporation, P.O. Box 5014, Sugar Land, TX, USA

Abstract

In an effort to maximize quality and production in both eld and shop fabrication while minimizing cost, Fluor has assessed new and innovativewelding and examination technologies. Welding applications, power sources, arc transfer modes, and shielding gas mixtures using various semi-

automatic processes were compared. The strategies for the selection of these welding variables, as well as the successful application of a modiedshort-circuiting gas metal arc welding process to carbon, alloy, and stainless steels are discussed. The optimized process combination can achievethree times the deposition rate and one-tenth of the repair rate commonly obtained with traditional processes. In weldment examinations, amodied phased array ultrasonic technique is presented. As an alternate for radiography, phased array technology allows for 10 times lowerinspection times and lends itself to an ideal weld-and-check methodology. Additional cost savings and further, productivity improvements havebeen achieved by combining these novel welding and examination techniques. This technology has demonstrated itself with a supporting track record on several major construction projects in North America.q 2006 Elsevier Ltd. All rights reserved.

Keywords: Short-circuiting gas metal arc welding; No backing gas procedures; Software driven power sources; Phased array ultrasonic testing

1. Introduction

Welding plays a critical role in the fabrication of modernfacilities for the pressure equipment industry. Equallyimportant during construction is the inspection and vericationof nished welds using non-destructive techniques. Anycomplications or delays associated with welding or inspectingcan negatively impact the cost and schedule of a project. Overthe past 50 years, several improvements have been made towelding machines but the basic welding practices haveremained unchanged. For example, the only reliable methodfor welding austenitic stainless steel piping while preventingoxidation of the root bead was through the use of a gas tungstenarc welding process and an inert backing gas. However, thisprocess requires a substantial amount of time, effort, andmaterials to implement. Similarly, during the previous 40years, radiography has been the primary means of inspectingwelds in geometrically complex or high-noise materials.

Fluor has been a leader in the development of innovativewelding and examination techniques. The present work outlines the developments and implementations that have

emerged over the past decade. Details of the unique modiedshort-circuiting semi-automatic no-backing gas weldingprocesses are provided, as well as their respective applicationto carbon, alloy, and stainless steels. Weldments weresubsequently examined with a novel phased array ultrasonictechnique that considerably reduces measured defect ambi-guity and required inspection time. A brief assessment of themethods traditionally used for welding and inspection is alsoincluded for comparison.

2. Background

The welding and examination processes implemented aremajor contributors to the project schedule and budget. Skilledlabor shortages coupled with dated technologies provided theimpetus for the development and implementation of improvedwelding and examination processes. A historical account of traditionally employed welding and inspection methods servesto substantiate the need for improved processes.

2.1. History

One of the most crucial weld deposits to make is the rootpass, which is typically in direct contact with the processcommodity. Proper fusion and internal bead prole of the rootpass are critical to the completion and acceptance of the nalweld. Furthermore, the combination of root, hot, ll, and cap

International Journal of Pressure Vessels and Piping 83 (2006) 365372www.elsevier.com/locate/ijpvp

0308-0161/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijpvp.2006.02.027

* Corresponding author.E-mail address: [email protected] (B. Messer).
http://www.elsevier.com/locate/ijpvpmailto:[email protected]:[email protected]://www.elsevier.com/locate/ijpvp


2/8

pass process optimization dictates the deposition rates, whichultimately affects the completion rate of the project.

Historically, open root welding on carbon, low alloy, andstainless steels was performed with one of three main methods,each of which has related advantages and disadvantages:

1. Shielded metal arc welding (SMAW) is a very commonprocess, which involves the use of low cost equipment.However, this manual process requires the welder tocontinually manipulate the arc while compensating for theburn-off rate and thus, is dependent on operator skill.Frequent stops and starts associated with the discontinuousller metal rods further introduce areas for possible welddefects.

2. Gas tungsten arc welding (GTAW) produces high qualitylow-hydrogen welds at the cost of slower travel speeds andhigher heat inputs. This manual process requires the welderto demonstrate increased agility and skill with both handsand therefore, is highly operator dependent.

3. Gas metal arc welding (GMAW) is a fast semi-automaticprocess with high deposition rates. However, due to thedifculty in controlling heat input with GMAW appli-cations, lack of fusion commonly results.

The SMAW and GMAW processes, as implemented overthe previous decades, are now viewed as slow and inefcientdue to the level of welder dexterity and coordination requiredwhen compared to semi-automatic processes. Since, GTAWand SMAW are manual processes, a signicant number of man-hours are invested in weld deposition and grinding, thusdecreasing efciency of production rates. Furthermore, these

welding processes typically yield repair rates that exceed 5%.Understandably, during the last 2030 years, there has been aworldwide paradigm shift from manual processes to those thatlend themselves to automation and higher deposition rates.

Commonly, when welding with 5% or higher chrome alloyand stainless steels an inert backing gas purge is required toprevent sugaring oxidation on the process side of the weld. Theproper selection of a backing gas and the subsequent setup of agas-tight seal for the weld areas are typically performed at theexpense of higher costs and extended schedules. Other methodssuch as using ux paste, ux coated, and ux cored ller metalswith the GTAW process have been previously investigated as ameans to inhibit oxidation, but all exhibited limited andvarying results. The development of a reliable substitute for thegas purging process had not been presented until recent years[13] .

Another important aspect of construction which impacts aprojects production rate is the methodology applied for non-destructive examination (NDE). NDE testing historicallyrequired visual checks or costly examination methods thatsubjected the users to various hazards. Radiography (RT) andliquid penetrant testing (PT), for example, put the inspector indirect contact with harmful radiation or chemicals. Although,relatively safe, magnetic particle testing (MT) and eddy currentinspections are limited to detecting surface or near surfacewelding defects. Ultrasonic testing (UT) with contact or

immersion probes has been used for weld inspection, asreferred to herein as conventional UT; however, severallimitations occur with this method. Conventional UT, forexample, is limited to use on homogeneous parts with simplegeometries and smooth surfaces. The accuracy of this methodis also limited by the experience and knowledge of the

operator.Until recently, inspection of irregular or complex partgeometries, often associated with welded connections, waslimited to RT. This method is generally limited to materialthickness less than 51 mm and requires qualied operators.Although, RT does provide accurate and permanent records, itrequires substantial set-up and lm development time. Over thepast decade, developments in digital radiography or radioscopyhave allowed its use for detecting aws in the pressureequipment industry. Unlike conventional RT, which uses aphosphorus lm, radioscopy uses an imaging plate compro-mised of photo-stimulable phosphors. Exposed plates can beelectronically scanned allowing for immediate digital proces-sing of the recorded image. This eliminates the tedious lmdevelopment time associated to conventional RT, but does notreduce the cost and time for set-up and scheduling. Because, aradiation source is still required with radioscopy methods,special precautions must be employed and examinations arelimited to times when the work site is free from workers.

Coupling slower NDE methods, such as RT, with datedwelding processes of SMAW and GTAW can causeconsiderable delays in project completions. To avoid thesedelays and their associated costs, substantial steps have beentaken in the development of improved welding and examin-ation techniques.

2.2. Development

The technological advancement of semi-automatic weldingprocesses afforded a signicant decrease in required welderdexterity and coordination. Combined with a subsequentdecrease in repair rates this advancement also contributed toan increase in productivity and efciency. The continualfeeding of the wire negated the need to adjust for burn-off rateof the electrode and further liberated the welder to direct thegun with one hand and steady with the other. Fluor has beenproactively involved in the development of several semi-automatic processes. One such process was the rst generationShort-circuit gas metal arc welding (GMAW-S) process. Usingtransformer-rectier machines, the arc produced by GMAW-Swas violent and unstable which had a strong tendency towardslack of fusion; therefore, the t-ups and position welds stillrequired an exact placement. This limitation prohibited the useof the GMAW-S process on eld t-ups that exhibited even thesmallest degree of weld joint mismatch or out of roundness.

The introduction of the inverter power source provided thetechnology for a second generation of GMAW machines. Theinverter GMAW machines demonstrated a signicant improve-ment in arc optimization and control of GMAW-S whencompared to those of the transformer-rectier. With the addedcontrol, a reduction in operating expenditures resulted due to

B. Messer et al. / International Journal of Pressure Vessels and Piping 83 (2006) 365372366


3/8

increased efciency and reduced energy losses in the powerconversion process. However, the inverter failed to compensatefor weld joint mismatch or out of round piping, therebynegating the desired reduction of specialized welders.

Over 10 years ago, a third generation of software-drivenpower sources was introduced with controlled waveform

technology that enabled optimization of arc characteristics.This technology allowed for modications to the short-circuiting transfer mode by remotely monitoring and control-ling the electrode current output via computers through allphases of weld metal transfer. This facilitated the developmentof the modied Short-circuit gas metal arc welding (GMAW-Sm) process. This process overcame many limitations of conventional GMAW-S while maintaining comparable weldmetal deposition rates and consistently achieving radiographicquality welds. In addition, GMAW-Sm exhibited an increasedtolerance of less experienced and less skilled welders, thusovercoming the hurdle of misalignment and out of round

piping. The GMAW-Sm process is also tolerant of gaps andcapable of automatically maintaining the optimum wire feedspeed and contact tip to work distance thereby increasingproduction rates and reducing welder fatigue. Table 1 shows atypical optimized welding strategy.

A noteworthy track record with GMAW-Sm on carbon,alloy, and stainless steels has been developed by successfullyimplementing the method on tens of thousands of welds invarious large scale projects. Approximately 5 years ago, theGMAW-Sm process was coupled with a no-backing gas (NBG)technique for use on Types 304/304L and 316/316L stainlesssteels [1]. This new technique completely removes the use of apurging gas while maintaining corrosion integrity of the root.Oxidization of the root is eliminated by directing a smallquantity of the shielding gas ahead of the root weld puddle.Initially established as an in-house development, an extensiveinvestigation was conducted regarding the mechanical,chemical, and corrosion properties of the welds made using

the GMAW-Sm and NBG procedure. Following further eldtesting and procedure approval by the local authorizedinspection agency, the welding method was implemented inseveral renery projects in North America. Currently, theGMAW-Sm with a NBG technique is commonly being usedfor Types 304/304L and 316/316L stainless steel production

welds and has found extended application to heavy walled,chemically stabilized 321 and 347 stainless steels [2]. Byutilizing NBG with the wide operating window of GMAW-Sm,considerable time and costs are saved without compromisingproduction performance or weld quality. The new GMAW-Smprocess totally eliminated the lack of fusion and penetrationconcerns typically linked with GMAW-S.

To extend the benets of GMAW-Sm and NBG, a new NDEmethod for welded connections has been investigated [4]. Thenew technique is a modied version of UT, known as ultrasonictesting-phased array (UT-PA), which has an arrangement of multiple piezoelectric elements that are independently con-trolled for developing synchronized and manageable sonicwaves. The technique is applicable to carbon, alloy, andaustenitic stainless steels; requires less time than conventionalUT; is not hazardous as compared to RT; and allows for 100%volumetric inspection. Other advantages of UT-PA include itsease of use, increased accuracy, and development of instantaneous digital inspection records for tracking defectpropagations in the future.

Combining UT-PA with GMAW-Sm allows for rapid weldproduction and integrity verication and therefore, establishesa more efcient weld-and-check methodology in comparison tothe dated methods previously mentioned.

3. Welding techniques

The development of more productive and cost efcientwelding processes has been done in conjunction withsignicant in-house, laboratory, and eld investigations.Currently, the presented welding processes are beingimplemented on several projects. This section summarizessome of the results obtained over the past decade in regards toimproving welding methodologies for carbon, alloy, andstainless steels.

The ease of process mastery, high deposition rate, andincreased gap bridging characteristics of modied short-circuiting GMAW makes this technique very desirable forwelding carbon, alloy, and stainless steels. Before GMAW-Smwas used in shop or eld environments several mechanical,chemical, and corrosion tests were conducted on nishedwelds. Further NDE, microscopy, and spectrocopy testsindicated positive results.

3.1. Carbon steel

Carbon steels are used in a multitude of services includinglow temperature, sour, and other critical services. To preservethe versatility of carbon steel when using GMAW-Sm, anextensive testing program was implemented. This program

Table 1Shop and eld process optimization for welding pipe

Pipe diameter and wallthickness

Joint Recommended process

d Z 1525 mm Socketweld

GTAW (two pass minimum)

d R 40 mm Socketweld

GMAW-Sm (two passminimum)

d Z 1550 mm Buttweld

GTAW (all passes)

d Z 80200 mm t Z 38.2 mm Buttweld

GMAW-Sm (all passes) orGMAW-Sm (root) andGMAW-P (balance)

d Z 100200 mm t R 8.6 mm Buttweld

GMAW-Sm (root) andGMAW-P (balance)

d R 150 mm t Z 18.2 mm Buttweld

GMAW-Sm (root) andGMAW-P (balance) or GMAW-Sm (root) and FCAW (balance)

d R 250 mm t Z 4.115.1 mm Buttweld


d R 250 mm t R 15.1 mm Buttweld

GMAW-Sm (root), GMAW-P(hot pass) and FCAW (balance)

B. Messer et al. / International Journal of Pressure Vessels and Piping 83 (2006) 365372 367


4/8

involved a full range of mechanical properties with an in-depthhardness evaluation and chemistry analysis.

Various consumables and shielding gases were used withthe GMAW-Sm process. Excellent results were obtained whenan ER70S-6 electrode with controlled chemistry was used inconjunction with a SG-AC-25 shielding gas, as shown in

Table 3 . Although, a SG-AC-25 works well, a SG-AC-50 isrecommended where tack welds are to be incorporated into theweld joint.

3.2. Alloy steel

Following the successful outcomes with carbon steel, themodied short-circuit welding process was employed on PNo.4 (1Cr0.5Mo and 1.25Cr0.5Mo) and P-No. 5B (5Cr0.5Mo)alloy steels. Similar to carbon steel, positive results wereobtained in all testing areas. Extending the application to alloyswith relatively higher levels of chromium such as P-No. 5B, P9(9Cr1Mo) and P91 (9Cr1Mo1V) materials, could not bepossible without rst considering the oxidation of the root pass.Traditionally, a GTAW process is performed with a backinggas for welding P9 and P91 piping at the expense of additionalmaterials and higher set-up costs. In an effort to decrease thesecosts, a wire and gas combination that supports the use of aNBG procedure was developed [3]. Combining this noveltechnique with semiautomatic GMAW-Sm eliminated all costsassociated with the use of a backing gas (i.e. installation of purge dams, backing gas materials, man-hours required for set-up and application, etc.). The research and experimentationstages of NBG evolved over a several year period.

Test results for welding P9 and P91 materials are

summarized in Tables 2 and 3 . An extensive eld testingprogram was established on P91 piping with diameters up to610 mm and thickness up to 45 mm Inspection by 100% RTfound no rejectable indications [3].

3.3. Austenitic stainless steel

Austenitic stainless steels are commonly used in pressureequipment and piping systems due to their high heat andcorrosive resistant properties. However, the high susceptibilityto oxidation that occurs when welding stainless steel requiresthe use of special preventative measures similar to those used

with P9 and P91 alloy steels. To counteract the drawbacks of traditionally used processes, the user friendliness and econ-omic benets of the NBG procedures were applied.

3.3.1. Types 304/304L and 316/316L The combined GMAW-Sm and NBG processes as

established with carbon and alloy steels were initiated onTypes 304/304L and 316/316L materials at two major reneryprojects in North America. To prepare the existing welders forthe GMAW-Sm and NBG procedures, a 3-day training coursewas conducted on the job site. Welders with little or noexperience in short-circuit GMAW were able to make codequality welds by the end of the training session. Fieldwork withshielding gases, combined with exhaustive laboratory testing,revealed that a higher than normal ow of a tri-mix shieldinggas, containing SG-HeAC-7.5/2.5, resulted in a reduction of oxidation of the root. Optimized shielding gas rates were usedto ood the weld root area during welding. The consumablesthat proved to be the most effective were ER308L-Si andER316L-Si. Although, these electrodes are the same classi-cation as the commonly used ER308L and ER316L materials,ER308L-Si and ER316L-Si possess a higher silicon contentthat improves the oxidation immunity and uidity of the weldmetal. The chemical combination of the electrodes furtherprotects the backside of the weld bead from the damagingoxidation that traditionally required the use of an inert backinggas on stainless steel pipe welds. Recorded welding variablesand testing results on 304/304L and 316/316L materials aredetailed in Tables 2 and 3 .

The main concern with welding stainless steels without a

backing gas was the nished welds resistance to corrosiveeld environments. To resolve this issue, extensive corrosionand oxide testing programs were conducted, as summarized for316L stainless steel in Table 4 . A series of welding couponswere prepared and welded with no backing gas in the shop withproduction welders using Types 304/304L and 316/316Lsteels. Matched control samples were also prepared with thesame welding parameters, but with the use of a backing gas forcomparison. An analysis of the sample and matched controlwelds exhibited no signicant difference, as all samples passedthe subjected tests. A typical root bead in Types 316/316Lmaterial welded with ER308L-Si is pictured in Fig. 1.

Table 2Qualied welding parameters using GMAW-Sm with a NBG technique

Material Shielding gas Filler metal Volts (V) Amperes (A) Max heat input (kJ/mm)

Carbon steel SG-AC-25 ER70S-6 15.6 a , 16.3b 280 Peak C 50background

1.2a , 4.8b

1.25Cr SG-AC-25 ER80S-B2 13.7 a 156a 1.1a

5Cr SG-AC-25 ER80S-B6 15.5 a 145a 1.2a

9Cr SG-HeAC-7.5/2.5 ER80S-B8 19.5 a , 19.520 b 137a , 129b 1.6a , 1.5b

P91 SG-HeC-14 ER90S-B9 ER90S-G 11.814.7 a 83.3128.2 a 1.6a

304/304L SG-HeAC-7.5/2.5 ER308L-Si 1617 a , 1618 b 110120 a , 90130 b 0.6a , 2.4b

316/316L SG-HeAC-7.5/2.5 ER316L-Si 1617 a , 1618 b 110120 a , 90130 b 0.6a , 2.4b

321/347 SG-ACO-3/1 ER347Si 13.515.2 a 142165 a 0.7a

a Root pass.b

Fill and cap passes.



5/8

Furthermore, RT and PT of the welds were determined to beacceptable to piping and pressure vessel code requirements.

In the eld, increased productivity and welding efciencyhave resulted from the reduced set-up times previouslyrequired to install and remove inert gas purge dams withinthe piping. The welding repair rate was reduced to less than1%. To date, tens of thousands of welds have been performedon 304/304L and 316/316L stainless steel piping with variousthickness and diameters up to 762 mm at typical depositionrates of 1.593.4 kg/h.

3.3.2. Chemically stabilized 321/347 An expansion of the GMAW-Sm NBG process, previously

implemented and characterized for welding all thickness of Types 304/304L and 316/316L stainless steels was evaluatedfor chemically stabilized heavy wall 321/347 materials. Thecombination of GMAW-Sm with other semi-automaticprocesses such as Pulsed gas metal arc welding (GMAW-P),and ux cored arc welding (FCAW) were evaluated for shopand eld welding [2]. The results of these evaluations werebased on visual appearance, welder appeal, metallurgical testresults, and the potential for increased productivity. Variouslaboratory results from these evaluations are shown in Table 5 .

The selection of process and transfer modes for the rootpass, hot pass layers, and ll and cap passes was directed by therequirement for prompt and reproducible code acceptablewelds by moderately skilled craftsmen. Downhill GMAW-Sm

was selected for the root pass producing a smooth, convex topsurface which required supercial clean-up prior to depositingsubsequent weld layers. Higher heat transfer modes such asuphill GMAW-Sm, uphill and downhill GMAW-P, and FCAWwere discounted due to their higher degree of difculty.Succeeding hot passes were completed with GMAW-P therebyproviding the advantages of a higher deposition rate and moremanageable weld puddle in comparison to conventional spraytransfer. Additional, hot passes provided adequate backingthickness for the absorption of the heat generated by the llpasses without causing reheat damage to the non-purged root.A combination of GMAW-P and FCAW was chosen as a viableprocess for depositing the ll and cap passes based on theunique attributes afforded by individual processes. FCAW alsoprovides higher deposition rates that attribute to larger weldbeads and higher heat inputs. This, in turn, provides additionaladvantages since, larger weld beads can be obtained in fewer

Table 3Laboratory results of welds detailed in Table 2 for carbon, alloy, and stainless steel materials

Material Guided bend test,ASME Section IX

Tensile test, ASMESection IX

Charpy V notch test Hardness test (max.weld hardness)

Carbon steel 13 mm without PWHT a Acceptable 549 MPa acceptable 46 J ( K 40 8 Cb ), 53 J(K 40 8 Cc)

86 HRB

13 mm with PWHT Acceptable 536 MPa acceptable 130 J ( K 40 8 Cb), 47 J(K 40 8 Cc)

84 HRB

25 mm with PWHT Acceptable 498 MPa acceptable 107J ( K 40 8 Cb), 96 J(K 40 8 Cc)

77 HRB

1.25Cr with PWHT Acceptable 471 MPa acceptable 240 HV5Cr with PWHT Acceptable 556 MPa acceptable 238 HV9Cr with PWHT Acceptable 574 MPa 98 HRBP91 10 mm with PWHT Acceptable 718 MPa acceptable 242 HV

38 mm with PWHT Acceptable 690 MPa acceptable 109 J (20 8 Cb ), 195 J(20 8 Cc)

253 HV

304/304L Acceptable 364 MPa, acceptable 156 J ( K 46 8 C) 202 HV316/316L Acceptable 527 MPa, acceptable 128 J ( K 46 8 C) 209 HV321/347 Acceptable 620 MPa, acceptable 53 J ( K 196 8 Cb), 128 J

(K 196 8 Cc)

a PWHT refers to post weld heat treatment.b Charpy impact test in weld metal.c Charpy impact test in HAZ.

Table 4Type 316L corrosion test summary

Test Result

ASTM A 262-02a, practice E (as welded) PassedASTM G 48-00 method A PassedASTM D 1141, pH 8.2 PassedElectrolytic test, pH 5.0 (100 ppm chloride solution) Passed

Fig. 1. Cross sectional macrograph of a root weld in Type 316/316L stainlesssteel without backing gas (note: polymer added above root to enhance contrastof macro cross-section).



6/8

passes with fewer stops and starts, thereby reducing theopportunities to produce weld defects. A comparison of typicaldeposition and repair rates for traditional and the presentedmethods is shown in Table 6 .

The optimized ller metals for the root pass and balancingpasses were determined to be ER347Si and ER16.8.2,respectively. ER347Si is a high silicon niobium stabilizedller metal formulated to specically weld stabilized 321 and

347 type stainless steels. The molybdenum content found inER347Si also provides an additional benet regarding creeprupture ductility of chromium-nickel austenitic steels, thusafrming its use in high temperature plants. Since, ER347 llermetals have a tendency to experience hot cracking, low stressrupture ductility, and in service heat affected zone (HAZ)relaxation cracking at elevated service temperatures, amodied ER16.8.2 ller metal was selected for the balancingpasses. This modied version had a specied minimum carboncontent of 0.04%, maximum molybdenum content of 1.30%,and ferrite range of 16.

Of the several shielding gases investigated a SG-ACO-3/1

mixture was determined to be most suitable for the GMAW-Smroot and GMAW-P passes when welding 321/347 stainlesssteels. Although, efciency and favorable economics favoredthe use of a single gas for all processes, this ternary gas mixtureproduced unacceptable dirty welds when used with FCAWER16.8.2 ller metal. Therefore, it is recommended to use aSG-AC-25 mixture since, it has historically performed wellwith ER16.8.2 FCAW. Sampled data of a weld completed in321/347 stainless steel is presented in Tables 2 and 3 .

In production, the combinations of a GMAW-Sm root andGMAW-P hot pass with GMAW-P and FCAW ll and capswere implemented. Numerous welds were completed on321/347 piping with diameters up to 457 mm and maximum

thickness of 40 mm. All welds were fully inspected with RTper American society of mechanical engineers (ASME) B31.3with no rejections. Current eld production maintains similarweld results. A survey of the foreman and the welders furtherdemonstrated a high degree of preference of the three weldingprocesses implemented in tandem over the traditional GTAW

and SMAW commonly utilized in industry.

4. Examination techniques

Time and cost saving benets of welding technologicaladvancements can only be realized if the restrictive weldinspection process can be improved. The UT-PA methodprovides a solution to this setback through its fast applicationafter welding and instantaneous output. This novel techniquecan be used to inspect all types of steels in both thin and thick wall applications.

4.1. Ultrasonic testingphased array

Ultrasonic inspection of complex geometry components andaustenitic stainless steels with conventional UT methods canlead to limited performances. Similarly, other NDE methodshave several drawbacks when applied to irregular parts withanisotropic grain structure. Phased-array technology allows forthe UT inspection of these materials without sacricing thebenets of conventional UT.

Phased array ultrasonic inspection is an extension of conventional ultrasonic inspection and utilizes an array of piezoelectric elements rather than just one. The elements arecontained in a signal probe; however, with computer controls

the individual elements can be manipulated and coordinated toproduce a focused ultrasonic beam with steering capabilities asshown in Fig. 2. With electronically controlled ultrasonic wavebeams it is possible to detect defects in joints with complexgeometry, thereby allowing complete volumetric inspection.Focusing of the beam also enhances space resolution withbetter sizing and mapping characteristics. These expandedcapabilities of UT-PA can be performed in a fraction of thetime required than when using conventional UT methods.

Another plausible aspect of UT-PA is its application to highnoise materials such as stainless steel castings and weld areas.Stainless steels are normally very difcult to inspect withconventional UT because of the beam distortion and scatteringeffects of large anisotropic grains typically found in thesemetals. These grains are generally characterized by coarsecolumnar shapes and provide numerous interfaces with a rangeof orientations. A variation in the sound velocities amongst thecrystals can lead to signal scattering, mode conversion, andbeam attenuation. UT-PA combats the adverse effects in highnoise materials by focusing the ultrasonic beam on these partsand obtaining improved signal-to-noise ratios.

4.2. Field application

A UT-PA technique was used on a large scale project inNorth America. For this case study, accurate and fast

Table 5Test summary for 321 stainless steel welds

Test Result

ASTM A 370-03a, elevated tensile test (400 8 C) 256 MPa yieldASTM A 370-03a, elevated tensile test (500 8 C) 244 MPa yieldASTM A 262-02a, practice A (as welded) PassedASTM A 262-02a, practice A (PWHT) PassedASTM A 262-02a, practice E (as welded) PassedASTM A 262-02a, practice E (PWHT) PassedASTM G 48-00 method A Passed

Table 6Comparison of deposition and repair rates for traditional and new weldingpractices when using a NBG technique

Process Deposition rate(kg/h)

Shop repairrate (%)

Field repairrate (%)

GTAW (all passes) 0.50.7 35 510GTAW (root) andSMAW (balance)

0.51.2 45 610


1.63.4 ! 1 ! 1

GMAW-Sm (root),GMAW-P (hot) andFCAW (balance)

1.84.3 ! 1 ! 1



7/8

verication results for the reinforced ttings and outlets werecritical to the success of the project. At site, there wereconsiderable challenges in examining weld integrity for thebranch connections as the welds were in locations where pipecongurations made examination by traditional RT methodsvery difcult and time consuming. In addition, to thesechallenges, poor results from sample radiograph tests motiv-ated a search for an alternative examination method that couldbe applied to the welded connections. The welds inspectedconsisted of a cast modied chromium-iron-nickel alloy withthickness up to 38 mm; traditionally not suitable for traditionalultrasonic examination.

To ensure that the UT-PA method was compliant to Code,several standards and guidelines were referenced. The ASMEboiler and pressure vessel Section VIII, code case 2235-6allows use of ultrasonic examination in lieu of radiographyfor welds in material 12.7 mm or greater in thickness [5]. Thiscase restricts UT use to the conformance of ASME boiler andpressure vessel Section V, which references UT-PA inparagraphs T-452 and E-474. Furthermore, in cases whereweldment geometries or materials prohibit conventional UT,the statements of API-560 and ASME B31.3 support UT-PAexamination. API-560, Paragraph 14.2.2.7 states In cases,where weld or material conguration makes radiographicexamination difcult to interpret or impossible to perform, such

as nozzles welds, ultrasonic examination may be substituted

[6]. With these supporting provisions, the issue of UT-PAcompliance to code is satised.

Application of UT-PA on this case study with 624 complexconnection examinations revealed that 128 (20.5%) failed tomeet ASME B31.3 [7] acceptance criteria. Field inspectiontimes were one-tenth of the time demanded by conventional

RT methods and as such, presented UT-PA as a plausibletechnique a quick weld-and-check eld or shop application.UT-PA also provided the client with a method of tracking over400 non-rejected welds for future preventative inspections. Atrial correlation of RT versus UT-PA established a 100%correlation of defect detection or, in some cases, an indicationproviding evidence that UT-PA located more aws than RT[4]. Output data samples of the correlation tests for the UT-PAand RT methods are shown in Fig. 3.

5. Results and cost savings

Completion and inspection of pipe welding is a critical stepbefore further actions can be taken on the piping system.Hydrotesting, coating, painting, heat tracing, insulation, andinstrument loop checks are all downstream activities affectedby any delays in welding. On a typical renery or power plantproject, approximately one-half of the total eld hours arerestrained by the completion and inspection of welds.Minimizing this restraint is the goal of using a combinationof the GMAW-Sm and UT-PA technologies. By avoiding theslower welding practices, current welding data suggests apotential savings in welding time of up to 70%. Furthermore, aten times reduction in the weld rejection rate from the industry

Fig. 2. Illustration demonstrating beam steering and focusing capabilities of phased array probes.

Fig. 3. Flaw indication on UT-PA output screen (above) and on a double wallRT shot (bottom).



8/8

average is obtainable when GMAW-Sm is implemented, thusconsiderably diminishing repair time and associated costs. Inaddition, the use of NBG while welding above 5% chromealloy and stainless steels eliminates the costs of sealing andpurging.

When comparing UT-PA to RT, additional benets can be

realized through:1. No downtime while area is being prepared for radiation

hazard.2. No downtime due to radiation source being present.3. Immediate interpretation and results are available.4. An approximate 90% decrease in time required to perform

examination.

The degree of savings is dependent upon the degree of implementation. Cumulative direct savings from employingGMAW-Sm with NBG on a typical project can exceed 14% of the total eld labor cost for the project when used in

conjunction with phased array NDE. An additional 810% insavings is realized from the indirect effects of these newwelding and examination practices on downstream activities.This is accomplished primarily through an improved efciencyof sequential fabrication and reduced construction schedules.Although, other considerations of added equipment costs andrequired training offset some of the savings of GMAW-Sm,NBG, and UT-PA, these expenses are minor in comparison.For example, GMAW-Sm welding machines are nowcommercially available at nominal pricing and the trainingrequirements for the proposed welding and inspectiontechniques should not extend beyond one week. With these

benets, complete industry implementation of the processespresented here is only a matter of time.

6. Summary

The competitive market of facility and pipe fabrication,coupled with skilled labor shortages and a xed industrymindset towards traditional technologies, has necessitated theresearch and development of improved welding and examin-ation techniques. Using the combined processes of GMAW-Smwith NBG, GMAW-P, and FCAW in conjunction with a UT-PA inspection technique has been investigated and proven to be

a viable and code compliant substitute for current datedmethodologies.

A comprehensive track record has been developed usingGMAW-Sm on various materials. The process allows for highdeposition rates and yields high quality welds. Of particularimportance, this welding process is ideal in situations where

backing gas is difcult or uneconomical to use includingrepairs, revamps, furnace applications, module tie-ins, and longcomplex spools. Using NBG procedures with GMAW-Sm forhigher chromium alloy and stainless steels generates similarpositive results without the cost of implementing a backing gas.An optimized practice has been developed for these situationsusing specialized ller metal with suitable shielding gasmixtures and ow parameters. Proven mechanical andchemical weld properties have been achieved withoutsacricing corrosion resistance. Three times greater depositionrates and one-tenth repair rates are the major benets of usingGMAW-Sm with NBG, GMAW-P, and FCAW in comparison

to the traditional SMAW and GTAW processes. Furtherattributes can be achieved by using UT-PA technology forinspections. With a 90% reduction in examination time versusRT, phase array ultrasonics allows for efcient detection andremedy of welding aws.

References

[1] Messer B, Lawrence G, Opera V, Patrick C, Phillips T. Weldingstainless steel piping with no backing gas. Weld J 2002;December:324.

[2] Messer B, Seitz S, Patrick C. A novel technological assessment for weldingheavy wall stainless steel. Proceedings of PVP2005: ASME pressurevessels and piping division conference; July 1721, 2005.

[3] Patrick C, Ferguson T. Pipe welding breakthrough. EPRI fourthinternational conference on advances in materials technology for fossilpower plants; 2004.

[4] Messer B, Fuentes JR, Tarleton B, den Boer P. Novel ultrasonic testing of complex welds. Proceedings of PVP2005: ASME pressure vessels andpiping division conference; July 1721, 2005.

[5] The American Society of Mechanical Engineers. Case 2235-6 supplement#10: cases of ASME boiler and pressure vessel code; May 2003.

[6] Manufacturer Standardization Society of the Valve and Fitting Industry,Inc. MSS SP-97-2001 integrally reinforced forged branch outlet ttingssocket welding, threaded, and butt welding ends.

[7] The American Society of Mechanical Engineers. Process piping ASMEB31.3-2002.


Documents

Achieving Cost Savings With Inovative Welding and Examination Techniques