CHALLENGES WELDING DUPLEX AND SUPER DUPLEX STAINLESS STEEL
American Fuels & Petrochemical Manufacturers (AFPM)2014 Reliability & Maintenance ConferenceMay 20-23, 2014, San Antonio, Texas, USARMC-14-56CHALLENGES WELDING DUPLEX AND SUPER DUPLEX STAINLESS STEELCharles W. PatrickEuroweld, Ltd.La Porte, Texas, USAABSTRACTWelding duplex and super duplex stainless steels is similarto welding austenitic stainless steels; however, critical stepsmust be taken to maximize both corrosion resistance andmechanical properties. Where maximum results are necessary,such as in corrosive service applications, selecting the properbase material and weld filler metal alone will not guaranteesuccess. Attention to welding process, welder technique, beadshape, preheat/interpass temperatures, heat input on a per beadbasis, and corrosion sample preparation are all essential toachieving satisfactory results. All of these factors will bediscussed and their importance defined. Target parameters andapproaches will also be presented to assist the user in obtainingsuccessful results.INTRODUCTIONInherent metallurgical characteristics have at times plaguedboth Duplex (DSS) and Super Duplex Stainless Steel (SDSS)1in applications where welding is involved. Improper weldingtechniques and procedures can introduce detrimental effectssuch as unbalanced ferrite (α) to austenite (γ) ratios and theformation of intermetallic phases. This often leads toaccelerated corrosion or mechanical failure in the weld zone.Fortunately, these problems can be resolved by implementingwelding procedures and techniques that optimize the ferrite andaustenite ratios and suppress the undesirable metallurgicalphases detrimental to corrosion. Generally all fusion weldingprocesses can be used for welding DSS provided suitablewelding procedures and welding filler metals are used. Ifproperly implemented, DSS and welds will provide a reliablelevel of fitness-for-service.1For the purpose of this paper DSS applies to both DSS and SDSSunless specially noted otherwise.Matthew A. CoxEuroweld, Ltd.Mooresville, North Carolina, USANOMENCLATUREα – ferriteγ – austeniteHISTORY OF DUPLEX STAINLESS STEELDSS is not a new material and spans a history of 84 years.The first wrought duplex stainless steels were produced inSweden in 1930 and were used in the sulfite paper industry.These grades were developed to primarily reduce intergranularcorrosion problems in the early high-carbon austenitic stainlesssteels. That same year duplex castings were produced inFinland; followed by a patent in France in 1936 for theforerunner of what would eventually be known as Uranus 50.Beyond World War II, further alloying developments broughtAISI Type 329/3RE60, which was used extensively in theconstruction of heat exchanger tubing for nitric acid services.3RE60 was the first DSS alloyed specifically to resist chloridestress corrosion cracking, which DSS is still noted for to date.These alloys served well in specific applications up to the late1960’s; however, they were noted for poor performance in theas-welded condition. This was due to a high ferriteconcentration in the heat-affected zone (HAZ) which loweredtoughness and corrosion resistance significantly when comparedwith the base material [1].During the 1970’s, advancements in steel processing and ashortage of raw materials breathed new life into DSS. A costeffective alternative to higher alloyed stainless and nickel alloyswas needed. New technology made the control of residualelements tighter and production was more cost effective. Thissecond generation of alloys improved on previous formulations,however there was still something to be desired when it came toapplications involving welding. Into the 80’s and 90’s, researchand development showed that nitrogen alloying was an effective1Copyright 2014 by AFPM
solution to counter act low toughness and low corrosionresistance that plagued the HAZ. This advancement broughtweldability of modern DSS/SDSS to an acceptable and realisticlevel for most fabricators – when proper care is taken. Due totheir higher strength to weight ratio, good toughness strengthand greater resistance to corrosion, erosion and stress corrosioncracking (SCC) DSS/SDSS have become the material of choicein many industries. The list within these industries and theirapplications includes pipelines, pressure vessels, tanks,digesters, manifolds, risers, rotors, impellers and shafts are afew of the applications; certainly, many more haven’t beenaddressed. One example of this is illustrated in Fig. 1.properties of the welded joint. At times, some users will desireslightly more austenite because of higher ductility andformability. However, experience has shown that desirableproperties still exist where phases ranges from 30 – 70% ferrite[1].These qualities combine to make DSS very appealingto designers requiring a strong, yet ductile material whencompared to popular 300 series austenitic stainless steels. Otheradvantages include excellent pitting, corrosion resistance andlow thermal expansion. Having a relatively “lean” chemicalcomposition when compared to competitive alloys with highernickel (Ni) and molybdenum (Mo) content DSS offer additionallong term savings.Austenite (γ)Ferrite (α)FIG. 1 ALLOY 2205 DSS ABSORBER TOWER AT COALFIRED POWER PLANTCHARACTERISTICS OF DUPLEX STAINLESSCorrosion performance and cost effectiveness havemade the various types of DSS desirable for corrosive serviceapplications in several industries. Having a dual phasemicrostructure that consist of both austenite and ferrite give thishybrid of stainless steels a unique microstructure which exhibitsa high strength and sufficient ductility. Because of the presenceof relatively strong ferrite, DSS has greater strength thanstandard austenitic stainless steel. The increase in strengthtranslates to thinner sections resulting in lighter fabricatedcomponents. In addition, ferrite provides a significant resistanceto chloride stress corrosion cracking. On the other hand, theaustenitic phase content allows DSS to maintain sufficientductility and toughness. In general, a 50/50 phase balance offerrite and austenite, as shown in Fig. 2, is targeted inproduction of DSS to ensure a good austenite reformation in theHAZ and thereby ensuring good mechanical and corrosionFIG. 2 - DUPLEX STRUCTURE SHOWING THE DUALPHASE STRUCTURE (LIGHT BANDS AUSTENITE ANDDARK BANDS FERRITE)Source: Maverick Testing Laboratories, Inc.As previously noted, weldability of modern DSS isconsidered to be good. The weld puddle characteristics aresimilar to austenitic stainless steels, but slightly sluggish.Machining and forming is also considered good, however dueto its strength, it can be more difficult to form than othermaterials.DSS, like austenitic stainless steel are a family ofgrades that range in corrosion performance depending on theiralloy content. Currently, this family can be divided into fivegroups: Lean Duplex – Contains no deliberate addition ofMo (2304), with assumed PREN 26; Standard Duplex (2205), with *PREN 32 - 33; 25 Cr Duplex – *PREN 40 - (Alloy 255); Super Duplex – addition of Mo and nitrogen (N) *PREN 40 – 45; Hyper Duplex – Highly alloyed DSS - *PREN 45.2Copyright 2014 by AFPM
*PREN Pitting Resistance Equivalent Number %Cr 3.3Mo 16N %Cr 3.3(Mo 0.5W) 16N (for W bearing,e.g., SDSS) [1]Key1)3)Cr – ChromiumN - Nitrogen2)4)Mo - MolybdenumW - TungstenThe grade selected for a certain application should beconsidered on a case by case basis since there is no single gradefor a given service environment. The grade should be selectedcarefully based on PREN, manufacturers’ recommendations andexperience. Price should always be a consideration, whenappropriate; however the selection of material should not bebased on cost alone.Although there are several advantages to using DSS,there are some inherent disadvantages. The unique metallurgicalstructure of DSS makes it somewhat less stable at elevatedtemperatures (i.e. welding) compared to other alloys. Thisinstability can lead to the formation of detrimental intermetallicphases, which ultimately reduce corrosion resistance andtoughness. In most cases the HAZ adjacent to the weld will bemost problematic. It is not uncommon to find DSS applicationswhere improper welding techniques and procedures havecaused welds to become severely corroded to the point ofthrough wall leaks, as depicted in Fig. 3, or to find welds thathave mechanically failed due to the lack of ductility. It isimperative that the phase balance be controlled as the weldsolidifies. The transformation during the cooling phase allowssome of the ferrite to transform to austenite; however, theamount of transformation is highly dependent on the chemistry.Relatively small changes in Ni, Cr, Mo and N can have asignificant effect on the phase transformation rates andequilibrium.WELDING DUPLEX STAINLESS STEELA necessity of any successful operation is a good plan. Thenext step is to make sure everyone involved knows the plan.The same is true when working with DSS. Good intentions areoften foiled by a lack of communication. When DSS is selectedto be used for an application, everyone from the top down mustbe involved, i.e., upper management, engineering, supervision,quality control and most importantly, the welders/weldingoperators. There are many DSS fabricated components inservice that are literally falling apart because no time or budgetwas given to properly qualify welding procedures or implementgood practices in the field. Simply using the correct fillermaterials and performing NDE is not enough and will notguarantee a DSS weld joint is fit-for-service. The cost savingsand high performance potential of DSS can only be realizedwhen the necessary commitment is given to the effort.Generally all fusion welding processes can be used forjoining DSS provided suitable welding procedures and weldingconsumables are used. However, the root pass is typicallywelded with gas tungsten-arc (GTAW), Fig. 4, plasma-arc(PAW) or pulsed gas metal-arc (GMAW-P) welding processwith GTAW the most widely used. The fill and cap passes canbe accomplished with any of the fusion welding processes orcombination of processes since the goal is to fill the weld jointas quickly as possible while maintaining the procedural controlnecessary to obtain weld metal and HAZ properties that matchthe corrosion resistance and impact strength properties of thebase material.Fig. 4 MANUAL GTAW ROOT PASS IN SDSSSource: Maverick Testing Laboratories, Inc.FILLER MATERIAL SELECTIONFig. 3 CORRODED 2205 WELD DUE TO UNDERALLOYED WELD METALIn general, DSS materials should be welded with a fillermetal composition that closely matches the base materialcomposition. In addition, it is recommended that the fillermetal meet the chemical composition requirements shown inTable 1.Filler metals commonly used to weld DSS aresummarized in Table 2; however, depending on the applicationother combinations may be more appropriate. In most cases thefiller materials are found to be slightly over alloyed, typicallywith about 3% Ni, to help promote austenite formation in thecompleted weld.3Copyright 2014 by AFPM
face and a more open groove angle are preferred, as shown inFig. 6.TABLE 1RECOMMENDED ADDITIONAL CHEMICALCOMPOSITION ercent (%)(minimum)0.148.03.0Super DuplexPercent (%)(minimum)0.29.03.5Often times during fabrication there is a need for weldedjoints between DSS and other alloys, e.g., carbon steel,austenitic stainless steel, etc. these “dissimilar” welds alwaysrequire careful attention to achieve acceptable mechanical andcorrosion properties. Common practice is to use either an overor under matching filler metal based on the composition of thebase material and service requirements. A summarization of themost common dissimilar combinations filler metals/basematerials is listed in Table 3; however, as previously noteddepending on application other combinations may be moreappropriate. Just as with matching welds, a filler material with aslight increase in nickel content relative to the DSS basematerial should be considered.Fig. 5 PITFALLS TO AVOID [2]Prior to any welding, the joint shall be thoroughly cleaned.Heavy oxides and rough grinding burs shall be removed bygrinding with dedicated grinding wheel, flapper disc and whenrequired brushed with stainless steel wire brushes. All paint, oil,grease, dirt or other foreign materials shall be removed from theinside diameter/surface (I.D.) and outside diameter/surface(O.D.) at least a minimum of two (2) inches beyond the edge ofthe groove opening. Isopropyl alcohol or other solventsapproved for use on stainless steel, i.e., controlled fluorides,chlorides and sulphides, which will not leave a residue, may beused. Implement good austenitic stainless practices such as: Caution should be used when selecting filler metalscontaining columbium / niobium (Nb), such as NiCrMo-3classificationshould not be used. The Nb has an affinity fornitrogen; therefore, depleting the nitrogen from the DSS. Thisdepletion of nitrogen can potentially lead to acceleratedformation of harmful intermetallic inclusions in the HAZ. JOINT DESIGN Segregation of materialsCareful storage and handling e.g. cover steelracks, forks on fork truck, chains, roller,fabrication tables, etc.Avoid using contaminated toolsAvoid carbon contamination e.g. oil, grease, shopdirt, grinding sparks, etc.Use clean airFiller metal controlFollow qualified welding procedure specificationrequirementsTraining of “all personnel”Joints should be designed based on the thickness of thematerials, access and the welding process. As with mostwelding applications, the joint should be designed so that it willhave the smallest cross section possible but still allow for fullpenetration and manipulation of the weld puddle for good sidewall fusion. Joints that are too narrow, have a thick root face orwide root gaps must be avoided. These conditions will causeexcessive root melting, high dilution, and slower travel speedsresulting in longer exposure time in the harmful temperaturerange, is illustrated in Fig. 5.Groove welds may be prepared by grinding, machining orplasma arc cutting/beveling; however, if plasma arc is used theHAZ should be removed prior to fitting and welding. Toachieve better consistency in fit-ups and ensure balanced heatinput around the circumference of the weld joint machining ishighly recommended. Joint preparations for DSS are basicallythe same as those used for austenitic stainless steel. However,for one-sided groove welds a slightly wider gap, thinner rootFig. 6 DSS VERSUS AUSTENITIC STAINLESS STEELJOINT PREP FOR SINGLE-SIDED GROOVE WELD4Copyright 2014 by AFPM
PREHEAT AND INTERPASS TEMPERATUREAn elevated preheat is generally unnecessary and usuallynot recommended. In some cases depending on temperatureand humidity, a light preheat, 100 F (38 C), may be used toremove moisture or condensation that may be on the surface ofthe weld joint. If preheat is used it should be applied after theweld joint is cleaned with an approved solvent and be applieduniformly around the weld joint. Also, be aware that preheatwith an oxy-fuel torch or air-fuel torch to a peak temperaturelower than 212 F (100 C) can allow the combustion products(water vapor) from the flame to condense on the base metal, andmay actually do more harm than good by promoting porosity.So if preheating is required it should be performed using eitherinfrared heaters or electrical resistance strip heaters [3]. On theother hand, the interpass temperature must be monitoredclosely; since it has a significant effect on time at transitiontemperatures where phase transformations and intergranularmigrations take place. Too high of an interpass temperature willincrease the cooling rate which has a key effect on the phasebalance in the HAZ and weld. It also significantly affects theformation of intermetallic phases and corrosion resistance.Table 4 shows maximum interpass temperatures based on basematerial at the weld joint base on the current consensus of thewelding industry.Preheat, when required and interpasstemperatures can be checked using thermocouples, temperatureindicating crayons, pyrometers or other suitable means.However, if temperature crayons are used they should beapproved for use on stainless steel, i.e., controlled fluorides,chlorides and sulphides. For critical applications the use ofeither contact thermocouples or pyrometers is recommendedand the temperature should be checked precisely at the point ofthe arc start-up just prior to welding.GMAW, SMAW, FCAW and Submerged-Arc Welding (SAW).Mechanization of those suitable processes is advantageousbecause of consistent heat input and precise control ofparameters offered, as illustrated in Fig. 7.FIG. 7 MACHINE HOT WIRE GTAW WELDINGOF HEAVY WALL DSS USING NARROWGROOVE JOINT DESIGNSources: Liburdi Dimetrics, Corporation andMaverick Testing Laboratories, Inc.TABLE 4GAS SHIELDING AND PURGEMAXIMUM RECOMMENDED INTERPASSTEMPERATURE FOR DSS & SDSSThicknessinches (mm) 1/8 (3) 1/4 (6) 3/8 (10) 3/8 (10)Maximum Interpass TemperatureDuplex StainlessSuper DuplexSteelStainless Steel(e.g. UNS S32205) (e.g. UNS S32750) F ( C) F ( C)120 (50)120 (50)160 (70)160 (70)210 (100)210 (100)300 (150)250 (120)Recommendations for shielding and purging gascompositions for those welding processes requiring these gasesare shown in Table 5. To ensure acceptable corrosionproperties are met nitrogen (N2) is typically added to slowprogression of sigma phase. The flow rates for shielding gasshould be checked at the cup or gas nozzle to ensure adequateflow. Purging gas should be checked with a calibrated oxygenanalyzer and should not have an oxygen content exceeding0.25% oxygen (O2), (2500 ppm). To ensure adequate protectionthe purge should be maintained until a minimum of 1/4 inch(6mm) weld metal has been deposited or until the joint iscomplete, whichever comes first.WELDING PROCESSESAs previously noted DSS is capable of being welded withall the common fusion welding processes including GTAW,5Copyright 2014 by AFPM
TABLE 5RECOMMENDED GASES FOR SHEILDING ANDPURGING DUPLEX STAINLESS STEELShielding ransferFCAWPurging GasAr 2-2.5%N2 max.(Note 1)or100%ArAr 20-30%He 1-2%O2Ar 20-30%He 1-2%O2(DSS)Ar 2-3%CO2(SDSS)Ar 20-30%He 1-2%O2(DSS)100%Ar(SDSS)Ar 18-25%CO2or100% CO2FIG. 8 BALANCED WELD JOINT SEGEMENTS100%N2or90%N2 10%HeorAr N2(not less than5% N2)When welding DSS, the extreme re-heating of the root pass bythe deposit of the “hot pass” will often degrade the corrosionresistance of the root, especially the HAZ, to an unsatisfactorylevel. Since the root pass is a relatively small deposit(volumetrically) with a substantial amount of base metaldilution welding back over it with a very hot and relativelyheavy “hot pass” is simply too much to maintain a stablemetallurgical state, as pointed out in Figs. 9-11.Key1)3)5)Ar - ArgonHe - HeliumCO2 – Carbon Dioxide2)4)N2 - NitrogenO2 - Oxygen1)Strongly recommended that Ar/N2 mix be replacedwith 100%Ar after 2nd pass because excessive use ofAr/N2 shielding gas may lead to micro-porositydefects in multipass welds [4].NoteHEAT INPUTHeat input is critical to the performance of DSS welds forthe same reasons interpass temperature is. When the weld,HAZ and adjacent base material stay at elevated temperatures,they also spend more time passing through temperatures rangewhere the detrimental sigma phase, chi phase and carbideformation occur. Heat input is the most critical when depositingthe root pass in pipe and/or vessel welds made from one side.To help control this and enhance control of root bead quality itis advisable to deposit weld beads as a series of balancedsegments, as illustrated in Fig. 8, yields the followingadvantages: FIG. 9 CROSS SECTION DIAGRAM OF WELD ANDROLES OF EACH REGION [2]Controlling joint gap closure.Reducing overall joint distortion.Maximizing produc
Welding duplex and super duplex stainless steels is similar to welding austenitic stainless steels; however, critical steps must be taken to maximize both corrosion resistance and mechanical properties. Where maximum results are necessary, such as in corrosive service applications, selecting the proper
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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6.3 Mechanised/automatic welding 114 6.4 TIG spot and plug welding 115 7 MIG welding 116 7.1 Introduction 116 7.2 Process principles 116 7.3 Welding consumables 130 7.4 Welding procedures and techniques 135 7.5 Mechanised and robotic welding 141 7.6 Mechanised electro-gas welding 143 7.7 MIG spot welding 144 8 Other welding processes 147 8.1 .
What is the “Family” Lean Duplex SS – lower nickel and no molybdenum – 2101, 2102, 2202, 2304 Duplex SS – higher nickel and molybdenum - 2205, 2003, 2404 Super Duplex – 25Chromium and higher nickel and molybdenum “plus” – 2507, 255 and Z100 Hyper Duplex – More Cr, Ni, Mo and N - 2707
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the welding processes most often used in today's industry including plasma arc cutting, oxyfuel gas cutting and welding, Gas Metal Arc Welding (GMAW), Flux-Cored Arc Welding (FCAW), Shielded Metal Arc Welding (SMAW), and Gas Tungsten Arc Welding (GTAW). Flat welding positions and basic joints will be practiced. Pipe and tube welding
3. Classification of Underwater Welding Underwater welding may be divided into two main types: a) Wet welding b) Dry welding Fig. 3.1 Classification of underwater welding 3.1 Wet welding 3.1.1. Wet welding with coated electrode Wet welding is performed at ambient pressure with the welder-diver in the water and no physical barrier
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