IMPROVED CORROSION PERFORMANCE IN SUPER-DUPLEX WELDS
IMPROVED CORROSIONPERFORMANCE INSUPER-DUPLEX WELDSPRACTICAL ASPECTS OF WELDING FOR DUPLEX AND SUPER-DUPLEX STAINLESS STEELSAND OVERCOMING CHALLENGES IN CORROSION TESTING
CONTENTS2THE CHALLENGE3ABSTRACT4INTRODUCTION5WELDING CHALLENGES6EXPERIMENTAL PROCEDURE9RESULTS11DISCUSSION OF IX 1 – WELDING CHECKLIST17Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
THE CHALLENGEIn the offshore oil and gas industry, achieving good corrosion resistancein the welds of duplex and super-duplex stainless steel pipes, tubeand other infrastructure can be challenging. This is especially true forlocalized repair welding of parts exposed to harsh, chloride-containingenvironments. The challenge is to maintain the favorable mechanicalstrength and corrosion-resistant properties of the duplex material whileavoiding intermetallic microstructural changes that can arise in the heataffected zone. Not only does this require a skilled welder, but a deepknowledge of joint preparations, filler metals, proper shielding and rootgases, welding techniques and even post-weld treatment.This white paper is part of a series from the Research and DevelopmentDepartment (R&D) within the Sandvik group. It is based on publishedscientific papers with NACE International and other independentinstitutes. The content has been slightly modified in agreement with theauthor to make it more accessible for a broader range of professionals.It is part of our ongoing efforts to open up new possibilities for the oil andgas industry, reinforcing our commitment: WE HELP YOU GET THERE.Sukamal NaskarWelding & Technical ExpertSandvik Materials TechnologyEmail: sukamal.naskar@sandvik.com3Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
ABSTRACTFor demanding offshore oil and gas applications,duplex (UNS S31803) and super duplex (UNS S32750and UNS S32760) stainless steels provide excellentcorrosion resistance and high mechanical strength.Duplex steels have heterogeneous microstructureswith roughly 50% austenite and 50% ferrite, a microstructural balance that is achieved by controlling thechemical composition and using special heat treatments1,2. The high corrosion resistance of duplex steelsensures significantly more uptime than carbon steelsand conventional stainless steels, while the mechanicalstrength allows for lighter constructions, more compact system design and thus reduced welding.FINDING THE OPTIMAL BALANCEWhen welding duplex and super-duplex steels, thebiggest challenge is to maintain an optimized balancebetween the austenite-ferrite microstructure in thefinal weld metal as well as the desired mechanicalproperties. The purpose of this paper is to share ourknowledge regarding the various practical fabricationaspects of duplex and super-duplex stainless steelwelding. The paper provides practical advice on how tobring out the best of these complex materials as well asrecommendations on how to retain the optimal balanceof the duplex / super-duplex microstructure during andafter welding.CORROSION TEST ON 10 WELDED SAMPLESTen samples of super-duplex stainless steel pipe in twodifferent sizes were welded and then tested by liquidpenetrant test (LPT), radiographic test (RT) and finallyASTM G48 Method A corrosion bath test at 40 C for24 hours. Based on these tests, this paper provides an4overview of which temperatures, shielding and root gasmixes and other parameters are optimal to avoid theformation of any detrimental intermetallic phases orwelding defects. The results of these tests are highlighted in the results section and summary.Key words: Duplex stainless steel, super-duplexstainless steel, welding, corrosion test, ferrite,austenite”THE PURPOSE OF THIS PAPERIS TO SHARE OUR KNOWLEDGEREGARDING THE VARIOUS PRACTICAL FABRICATION ASPECTSOF DUPLEX AND SUPER DUPLEXSTAINLESS STEEL WELDING.”SUKAMAL NASKARSandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
INTRODUCTIONOver the past decades, duplex stainless steels havebuilt a strong reputation in the offshore oil and gasindustry due to a number of attractive features:mechanical strength, corrosion resistance and lighterconstructions than those built using carbon steels.Their two-phase microstructure, with approximatelyequal amounts of austenite and ferrite, imparts a higherstrength than corresponding austenitic grades and provides good resistance to stress corrosion cracking. Thelower nickel contents of duplex grades, typically in therange of 5-10% compared with 8-25% in the austeniticgrades, gives cost advantages. However, the welding ofduplex stainless steels must be handled with care to avoidembrittlement or the formation of impurities in the weld.BALANCED TWO-PHASE MICROSTRUCTUREDuplex and super-duplex stainless steels arecharacterized by a two-phase microstructure thatcontains approximately 45-65% austenite, which isembedded as islands in a matrix of 35-55% ferrite.Since both phases prevent grain growth, it means that”DUPLEX STAINLESS STEELS HAVEBUILT A STRONG REPUTATION IN THEOFFSHORE OIL AND GAS INDUSTRYDUE TO A NUMBER OF ATTRACTIVEFEATURES: MECHANICAL STRENGTH,CORROSION RESISTANCE AND LIGHTERCONSTRUCTIONS THAN THOSE BUILTUSING CARBON STEELS.”SUKAMAL NASKAR5duplex stainless steels have a fine grain structure.What’s more, due to a higher concentration of grainboundaries per unit surface area, super duplex stainless steels offer very high strength.CHROMIUM & MOLYBDENUM STABILIZERSFor the most part, such corrosion resistant alloys(CRAs) are produced using chromium (Cr), nickel (Ni),molybdenum (Mo) and nitrogen (N). Chromium andmolybdenum serve mainly as ferrite stabilizers whilenickel and nitrogen are used as austenite stabilizers.To obtain the desired ferrite-austenite mix, all thesealloying elements need to be in balance in the basematerials as well as suitable welding consumables.PRE MEASUREMENT – A GOOD BENCHMARKOne key benchmark for assessing localized corrosion and checking weld quality is the pitting corrosionresistance equivalent number (PRE), as defined below:PRE %Cr 3.3 (%Mo) 16 (%N)In addition to determining the susceptibility of an alloyto pitting, this formula is widely used as a guide for theusability of corresponding welding fillers. Typically UNSS31803 has a PRE value of 35 whereas UNS S32750and UNS S32760 have PRE values of 42. Such valuesprovide an indication of the pitting resistance of analloy in relation to the alloying content. The very highmechanical properties of duplex and super-duplexstainless steels are achieved as a result of the ferriteand austenite structure along with fine grain structure.The tensile properties are governed by ferrite phasewhile toughness properties are governed by austenitephase in conjunction with fine grain structure.Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
WELDINGCHALLENGESThe welding of duplex and super duplex stainlesssteel for offshore and subsea operations can present a number of challenges due to the sudden introduction of extreme heat followed by rapid cooling.During welding or other types of heat treatment,whether isothermal or non-isothermal, several structural changes may occur in the microstructure ofduplex stainless steels.FERRITE PHASE – MORE SUSCEPTIBLEFor the most part, these changes affect the ferritephase with its richer concentrations of chromium (Cr)and molybdenum (Mo). Another reason the ferrite”SLOW COOLING IN THE TEMPERATURE RANGES OF 550OCTO 900OC CAN LEAD TO THEFORMATION OF DIFFERENT TYPES OF INTERMETALLIC PHASESTHAT ARE DETRIMENTAL TO THEMECHANICAL AS WELL AS CORROSION PROPERTIES OF DUPLEXAND SUPER DUPLEX MATERIALS.”SUKAMAL NASKAR6phase is more prone to structural change is the morecompact lattice in the crystal structure, so-called BodyCentered Cubic (BCC), which leads to diffusion rates ofalloying elements that are almost 100 times faster thanthose in the austenite phase with its Face CenteredCubic (FCC) crystal structure.UNDESIRABLE PHASE CHANGESSlow cooling in the temperature ranges of 550 C to900 C can lead to the formation of different types of intermetallic phases that are detrimental to the mechanical as well as corrosion properties of duplex and superduplex materials. Essentially, the higher the degree ofalloying elements, the greater the possibility to formintermetallic phases and thus the more challenges inthe fabrication process.INTERMETALLIC PHASES – MORE LIKELY IN ALLOYSDue to the higher alloy content in super duplexessuch as UNS S32760 and UNS S32750, they are moreprone to the precipitation of intermetallic phases likethe sigma phase, chi phase and R phase than a duplexlike UNS S31803. Since both duplex and super-duplexstainless steels are rich in Cr and Mo, they are alsomore likely to form sigma phase than a lean duplex.However, by increasing the concentrations of Cr andMo, it is possible to shorten the Time-temperaturetransformation (TTT) curves of the sigma phase,thereby increasing the stability region of sigma phase.A quantitative chemical analysis using electronmicroprobe analysis (EPMA) shows that Cr, Mo and Siwere enriched in sigma3. The addition of nitrogen helpsto suppress the formation of sigma by reducing thedifference between Cr and Mo content in the ferriteand austenite phases.Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
WELDING CHALLENGESSIGMA PHASE – RISKS OF EMBRITTLEMENTSigma phase is a tetragonally close-packed structurethat is very brittle in nature and thus has a negative effect on the mechanical properties of the duplex. Sincesigma phase is brittle in nature, hardness levels canbe used to determine its presence in the base metalas well as weld metal. However, up to 4% presence ofsigma phase has no significant impact on hardness4;hence hardness measurement is not appropriate toconclude the presence of sigma phase.CORROSION-RESISTANCE CHALLENGESApart from reducing mechanical strength, sigma phasealso has a negative impact on the corrosion-resistantproperties of the duplex. Corrosion properties arereduced at lower temperature due to the formation ofsecondary austenite. While thermodynamically stable,the austenite formed at low temperature also containslower amounts of Cr, Mo and, most likely, nitrogen thanprimary austenite. This altered chemical compositionresults in lower PRE values at some localized areas onthe average matrix, making secondary austenite moresusceptible to the formation of pitting.AVOIDING CHI AND R PHASESIn addition to the detrimental effects of sigma phaseand secondary austenite, duplex steels can precipitatechi phase, R phase and chromium nitrides. Tungstencan be used as an alloying element to stabilize the chiphase and help move the TTT curves towards a shortertime period. The R phase is molybdenum rich intermetallic compound and usually forms at 550-700 C, butits practical importance is less since it takes longer tonucleate.CHROMIUM NITRIDE ISSUESThe biggest challenge when welding duplex steels isthe presence of chromium nitrides and sigma phasetogether in the weld metal as well as base metal of theduplex family. Chromium nitride is formed during isothermal heat treatment in the temperature range of 700- 900 C following too fast cooling from high temperature. While the solubility of nitrogen in ferrite is lower,it increases at higher temperatures. However, it hasa significantly higher solubility in austenite. At highertemperature, due to increasing temperature gradientfor cooling, nitrogen does not have enough time todiffuse in austenite and has limited amount of austenite formation. As a result, when ferrite formations are7”SIGMA PHASE IS A TETRAGONALLY CLOSE-PACKEDSTRUCTURE THAT IS VERYBRITTLE IN NATURE AND THUSHAS A NEGATIVE EFFECT ON THEMECHANICAL PROPERTIES OFTHE DUPLEX.”SUKAMAL NASKARsupersaturated with nitrogen, chromium nitrides format the ferrite grains or at grain boundaries of δ/δ or δ/γ.ADVANTAGES OF NITROGENWith duplex stainless steels, nitrogen is beneficial in therapid heating and slow cooling occuring in welding operations. Higher nitrogen content increases the reformation rate of austenite in the weld metal and HAZ (heataffected zone), giving shorter distance for nitrogen todiffuse from austenite to ferrite. Nitrogen has 16 timesinfluence on PRE value and hence can significantlycontribute to pitting corrosion resistance on weld metalas well as base metal.BENEFITS OF SOLUTION ANNEALINGWhen manufacturing duplex and super-duplex basetubes and pipes, solution annealing is part of the finaloperations to prevent intermetallic phases or defects.However, due to structural integrity and economicfeasibility, welded joints do not go through the solutionannealing process. As a result, the biggest challengesfor duplex and super duplex welding are to avoidintermetallic phases, obtain the desired microstructureand maintain the proper ferrite-austenite phasebalance throughout the matrix to achieve the desiredmechanical and corrosion properties.Naturally, tube and materials fabricators in the oil andgas industry must carry out welding in careful accordance with strict guidelines. Repair welding is even morecomplex since one is not starting with fresh joints. Thiscan present tougher challenges in terms of passingthe 24-hour corrosion test for ASTM G48 method A.The fabrication of duplex and super duplex stainlesssteel tubes and pipes with welding processes andcorresponding consumable selection depends on theseveral project technical requirements.Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
WELDING CHALLENGESSELECTING THE RIGHT CONSUMABLESThe selection of suitable welding consumables depends on a number of key criteria: corrosion testing,impact testing, the ferrite number or ferrite percentagerequirement on welding metal and HAZ (heat affectedzone). The right welding filler material is selected forduplex steels UNS S32205 or UNS S31803, dependingon the ASTM G48 Method A corrosion testing requirement at 20 C; 22 C and 25 C. The most suitable consumables for super duplexes like UNS S32750 or UNSS32760 are selected on the basis of corrosion testingtemperatures at 40 C, 42 C and 50 C.Welding consumables are selected on the basis ofthree welding options:SUKAMAL NASKARGas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding processthat uses a non-consumable tungsten electrode toproduce the weld.Other welding options are as follows:1. Welding of root pass, second pass (cold pass), fillpass and cap pass by matching weld metal chemistry to base metal.1. Shielded metal arc welding (SMAW)2. Welding of root pass by over alloying welding consumable followed by cold pass, fill pass and cap passby matching welding consumables to base material.3. Gas metal arc welding (GMAW)3. W elding of root pass, second pass (cold pass), fillpass and cap pass by over matching welding metalchemistry to base metal.Among the above options, option two is most commonly used for duplex steels to achieve safe weldingwith optimal results. Since the pipe walls of duplexand super duplex materials are generally thinner thancarbon or low alloy steels, the most commonly usedwelding process is gas tungsten arc welding (GTAW).8”REPAIR WELDING IS EVEN MORECOMPLEX SINCE ONE IS NOTSTARTING WITH FRESH JOINTS.”2. Submerged arc welding (SAW)When using the GTAW process, duplex grade materialsare welded with AWS A5.9 ER2209 filler while superduplexes are welded with AWS A5.9 ER2594 filler(without Cu & T / with Cu & T). Apart from suitablewelding consumables, other important factors inpassing the ASTM G48 Method A corrosion testinginclude the following: shielding gas composition andflow rate; purging gas composition and flow rate;welding technique; oxygen level; heat input; inter-passtemperature; welding position; tube and pipe diameter;tube and pipe thickness; corrosion test samplepreparation; and welder skill.Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel
EXPERIMENTALPROCEDURE9Our corrosion testing of multi-pass welds waslimited to two different sizes of super-duplexstainless steel pipe UNS S32750: 1 inch/ Schedule10 and 2 inch/ Schedule 160.of failure in the super-duplex UNS S32750 pipe, whichhad been welded with filler ER2594, following corrosiontested in accordance with the ASTM G48 method A for24 hours at 40 C.The only welding process used was gas tungsten arcwelding (GTAW), with welding consumables matchedto the base material: AWS A5.9 ER2594 welding wire.Our main objective was to eliminate possible causesThe main variables considered as parameters were:root pass heat input, second pass (cold pass) heatinput, interpass temperature, shielding gas flow rate,purging gas flow rate and sample preparation.1. 6G pipe weldingcertification testThe welding was done with6G pipe fixed at a 45 angleto the base position.3. Calibrated oxygen monitorWith the help of a calibratedoxygen monitor, the oxygenlevel was kept below 100ppm on the root side duringwelding.2. Clean environmentThe welding was carried out ina clean, air conditioned workshop with wooden floors by asingle welder using new handgloves. Both the 1 inch and 2inch pipes were cleaned withacetone to remove all foreignparticles, paints, oil and otherimpurities.Sandvik white paper Practical aspects of welding for duplex and super-duplex stainless steel4. Premixed gas cylinderThe shielding and purging gasused was Argon 2% nitrogen,supplied from a premixed gascylinder.
EXPERIMENTAL PROCEDURE5. Calibratedwelding machinesCalibrated welding machineswere also used during thewelding. Both shielding gasand purging gas was usedfrom root pass to cap pass ofthe welding. A calibrated digital instrument was also used tocheck actual welding currentand voltage during welding.Inter-pass temperature waskept below 100 0C duringwelding.6. Cleaning TIG rodswith acetoneAn iron-free grinding disc andpower brush were used for theinter-pass cleaning operationwhile an ASME-qualified 6Gwelder was used during welding. Proper earth connectionto the tubes and the weldingmachine was checked beforeand during welding. Weldingcurrent and the voltage weremeasured close to the weldingarea to reduce amps loss dueto resistance from the powersource. TIG rods were alsocleaned properly with acetoneto clean any foreign particleson the surface.Following welding, a liquid penetrant test (LPT) andradiographic test (RT) were performed on all test coupons to check for any surface and subsurface weldingdefects prior to sending the samples to the laboratoryfor the ASTM G48 Method A corrosion testing. Thefiller metals in all the test coupons had a root-passdiameter of 1.6mm followed by 2.4mm in diameter untilthe cap pass. The welding trials were conducted in afabrication yard to simulate the actual fabrication environment for mass production instead of doing them inan R&D facility.7. Cutting pipes with band sawAll the pipes were cut with aband saw machine.8. Fit-up procedureThe pipes were carefully calibrated to fit up precisely.After welding, all ten samples according to the different parameters, we sent them for liquid (dye) penetranttesting (LPT) and radiographic testing (RT). For allthe samples, LPT and RT are regarded as acceptableforms of testing according to the American Society ofNondestructive Testing ASNT NDT Level-II certification program. Finally, all the samples were sent to aSAC-SINGLAS accredited laboratory in Singapore forthe corrosion testing as per ASTM G48 method A forperiod of 24 hours.TABLE 1: SUMMARY OF TEST SAMPLESSamplenumberDimensionHeat I/P RootHeat I/P 2nd passH/I Fill & capShielding gasflow rateSample 11 Inch pipe2.12.01.8-1.9822 LPM25 LPMSample 22 Inch pipe2.22.11.8-2.022 LPM10 LPMSample 31 Inch pipe2.12.01.8-2.022 LPM25 LPMSample 42 Inch pipe1.00.58
improved corrosion performance in super-duplex welds practical aspects of welding for duplex and super-duplex stainless steels and overcoming challenges in corrosion testing . contents the challenge 3 abstract 4 introduction 5 welding challenges 6 experimental procedure 9 results 11
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