JULY 200726

All over the world, adoption of gasmetal arc welding (GMAW) andflux cored arc welding (FCAW)processes continues to grow for low-costfabrication of various grades of structuralsteels, including high-strength steels. Thegrowth of GMAW/FCAW is driven pri-marily by the increased availability of nu-merous consumables, including solid,fluxcored, and metalcored wire elec-trodes. But, how does one select an elec-trode for joining a particular grade ofhigh-strength steel? Will a simple relianceon relevant AWS/ANSI electrode specifi-cations be adequate? How does one eval-uate data from a multitude of electrode

manufacturers? This article offers to pro-vide technical insight into those questions.

Factors to Consider

Selection of an electrode for a partic-ular application is based on severalfactors. Chief among them is a fun-damental understanding of the relation-ships among chemical composition, pro-cessing, microstructure, and mechanicalproperties of the steel being welded. Also,specific design requirements for mechan-ical properties of the welded componentor structure should be known.

The things to-do list is long while

underlying issues are complex. However,such an understanding is a prerequisitefor achieving quality, productivity, and improved performance of welded constructions, while controlling overall fabrication cost.

Basic Principles ofElectrode Selection

Electrode selection is based on anelectrodes ability to provide weldmetal that is chemically compati-ble with the base metal. Electrodes thatoffer a similar (not same but matching)

How to ChooseElectrodes forJoining High-

Strength Steels

K. SAMPATH (rs127@yahoo.com) is a technology/business consultant, Johnstown, Pa.

Technical insight isprovided for evaluatingthe variety of GMAW and FCAW electrodes

available for joining high-strength steel

BY K. SAMPATH

Sampath 7/ 07:Layout 1 6/6/07 11:06 AM Page 26

27WELDING JOURNAL

chemical composition as the base metalminimize potential adverse effects of basemetal dilution, which can include local-ized corrosion.

Welding electrodes are also selectedto enhance weldability. A major aspect ofweldability is the ability to obtain crack-free weldments. In the case of high-strength steels, the primary concern isachieving resistance to hydrogen-assistedcracking (HAC) in both the weld metaland the heat-affected zone (HAZ). Re-sistance to solidification cracking is sel-dom a concern. Most often, solidificationcracking in weld metal is attributed to seg-regation of impurities such as sulfur andphosphorus along the weld centerline.Control of impurities (sulfur and phos-phorus, each at 0.01 wt-% maximum) andtrace elements in the welding electrode,and control of weld solidification condi-tions through manipulation of travelspeed, most often avoid solidificationcracking in weld metal.

Microstructure

Microstructure underpins mechan-ical properties. The term mi-crostructure includes type, sizedistribution, morphology, and volumefraction of various microstructural con-stituents. Microstructure, in turn, is de-pendent on chemical composition andprocessing conditions, especially coolingrate. Based on a need to achieve desiredmechanical properties, weldability maybe looked upon as the ability to recreateand/or retain microstructures similar tothe base metal.

Various carbon equivalent formulasallow one to relate chemical compositionwith weldability of steel. In particular, Yu-riokas carbon equivalent number (CEN),as shown in Equation 1, offers a viablemeans to assess relative effects of variousalloy elements on weldability.

(1)

where A(C) = 0.75 + 0.25 tanh [20 (C 0.12)], and concentrations of all ele-ments are expressed in wt-%.

Although the CEN equation was orig-inally developed to assess hydrogen crack-ing sensitivity of structural steels, theequation is also relevant to weld metal.The higher the CEN, the lower is the re-sistance to HAC. Carbon has by far thegreatest impact on weldability. So, it is es-sential to select welding electrodes with acarbon content lower than that of the steelbeing welded. Considering possible car-bon pick-up from CO2 in the weld shield-ing gas, and base metal dilution, it is pru-

dent to select welding electrodes withabout 0.02 to 0.04 wt-% lower carbon thanthe base metal. Lowering carbon contentmust be compensated for by using otheralloy elements to maintain or further in-crease CEN. A 0.12 wt-% for carbon isconsidered an appropriate upper limit inhigh-strength steel welding electrodes, astwinned martensite, which has an ex-tremely poor resistance to HAC, is likelyto form above this limit.

The CEN equation is helpful in select-ing various principal alloy elements in thewelding electrode. Alloy elements with alower coefficient (nickel, copper, andmanganese) are preferable to those witha higher coefficient (chromium andmolybdenum). Yet, weld metal must re-main chemically compatible with the basemetal. A prior knowledge of the chemicalcomposition of the base metal and theroles of various alloy elements is valuable.

Overmatching Strength andOverall Alloy Content

Welding electrodes must provideweld metals with a minimum re-quired weld tensile strength andacceptable impact toughness properties,either in the as-welded or postweld heattreated condition. Use of a welding con-sumable that offers a deposited weldmetal with higher weld tensile strengththan the tensile strength of steel beingwelded is called overmatching. Over-matching is used primarily to protectthe weld deposit from the presence of fabrication-related weld flaws. Theseflaws when subjected to occasional exces-sive service loads can potentially lead tocatastrophic consequences.

However, overmatching of high-strength steels using welding electrodeswith high-carbon content requires expen-sive preheat, interpass, and occasionallypost-soak temperature controls duringwelding to ensure against HAC, thus hurt-ing productivity and overall economics offabrication. Therefore, overmatching isan option only when the overmatchedweld metal offers adequate toughness,particularly acceptable low-temperatureimpact toughness, and overmatching al-lows cost-effective fabrication.

Other aspects of strength considera-tion are heat input and cooling rates. It iswell known that high weld energy inputand associated slow weld cooling ratesproduce a lower strength weld metal, andvice versa. Depending on the electrode di-ameter, the weld energy input commonlyranges between 20 and 80 kJ/in. A high-performance welding electrode is ex-pected to overmatch at the highest usableweld energy input while meeting or ex-ceeding weld metal toughness require-ments. This invariably means that at the

lowest usable weld energy input, the samewelding electrode may overmatch theminimum specified tensile strength of thebase metal, possibly in excess of 10%. Inother words, an electrode that providesmarginal overmatching at the highest us-able energy input is likely to offer exces-sive overmatching at the lowest usableweld energy input. Fortuitously, the high-strength weld metal simultaneously offershigher toughness, primarily due to thepresence of refined grains and mi-crostructural constituents. Expectedly,CEN of the corresponding welding elec-trode would be higher than the base metal,in excess of 10%.

The strength and other mechanicalproperties of a clean, defect-free weldmetal depend primarily on chemical com-position, and secondarily on weld coolingrate. As shown in Equation 1, a higheralloy content results in a higher CEN, andthus a higher tensile strength. As a higherCEN progressively impairs weldability,control of alloy content of the selectedelectrode to a desirable range of CEN iscrucial. The inherent conflict requiresbalancing or optimization of competingcriteria. When there is an inability to re-solve this underlying conflict, as in the caseof certain very high-strength steels suchas HY-130, overmatching may no longerbe a viable option.

Toughness andTransformationTemperature

How does one select a welding elec-trode to improve weld metaltoughness? Besides chemicalcomposition, welding conditions (partic-ularly weld cooling rate) contribute to mi-crostructure development.

The following on-cooling transforma-tion temperatures are important with re-gard to microstructural development inhigh-strength steels: 1) austenite-to-ferrite (Ar3), 2) austenite-to-pearlite (i.e.,eutectoid transformation), 3) austenite-to-bainite (i.e., BS, bainite-start and BF,bainite-finish), and 4) austenite-to-martensite (i.e., MS, martensite-start andMF, martensite-finish) temperatures.

Controlled lowering of the relevanttransformation temperatures allows oneto refine grains and microstructural con-stituents in weld metal, and thus simulta-neously improve both strength and over-all toughness. Here again, several consti-tutive equations allow one to relate chem-ical composition with transformation tem-peratures, thus further allowing selectionand manipulation of various microstruc-tural constituents.

The Ar3 temperature is approximatelyrelated to chemical composition as shown

CEN C A C

Si Mn Cu Ni

Cr Mo V Nb= + ( )

+ + +

++ + +

+

24 6 15 20

55BB

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JULY 200728

in Equation 2. Likewise, BS, BF, and MStemperatures are statistically related tochemical composition of low-alloy steelsas shown in Equations 35.

Ar3 (C) ~ 910 (310 C)(80 Mn) (80 Mo)(55 Ni) (20 Cu) (15 Cr) (2)

BS (C) = 830 (270 C)(90 Mn) (37 Ni)(70 Cr) (83 Mo) (3)

BF (C) = 710 (270 C)(90 Mn) (37 Ni)(70 Cr) (83 Mo) (4)

MS (C) = 561 (474 C)(33 Mn) (17 Ni)(17 Cr) (21 Mo) (5)

The above statistically valid relation-ships between chemical composition andtransformation temperatures were origi-nally developed for particular types ofsteels, under specific experimental condi-tions. Nevertheless, these equations areuseful for manipulating alloying elementsin welding electrodes, thus targeting desirable ranges of transformation temperatures.

The objective is to select a weldingelectrode or control its alloy contentwithin a desirable range of CEN, whileachieving a 30 to 50C lowering of the rel-evant transformation temperatures com-pared to the characteristics of the high-strength steel being welded. Thus, a com-plete understanding of chemical compo-sition and microstructures of the basemetal is a prerequisite to selecting a high-performance welding electrode.

Besides alloy content, increasing(weld) cooling rate is known to suppress(undercool) transformation tempera-tures. The welding operational envelopecontrols weld cooling rate. As mentionedpreviously, increasing the weld coolingrate contributes to a further refining ofboth grain size and various microstruc-tural constituents, thus strengthening theweld metal while simultaneously increas-ing its toughness.

Despite this potential, it must be recog-nized that in fusion welding situations, be-cause of epitaxial growth considerations,the level of undercooling achieved is oftenminimal, not exceeding a few degrees.

Dissolved Gases andToughness

Weld metal toughness can be se-verely impaired by the presenceof dissolved gases such as oxy-gen and nitrogen (in excess of 500 ppm,total), and too many inclusions that con-tribute to a dirty weld. Proper controlof shielding gas during welding, and thepresence of controlled amounts of alu-

minum, titanium, and zirconium (each at0.03 wt-% maximum) in the welding elec-trode are necessary to minimize airingress, and effectively deoxidize, fix ni-trogen in weld metal, allow scavengingand grain refining, and thus enhanceweld metal toughness.

Specifications

Standard setting organizations such asthe American Welding Society (AWS)codify the above rationale and knowl-edge for welding electrode selection intoappropriate welding electrode specifica-tions, such as AWS A5.28/A5.28M:2005,Specification for Low-Alloy Steel Electrodesand Rods for Gas Shielded Arc Welding, andA5.29-05, Specification for Low-Alloy SteelElectrodes for Flux Cored Arc Welding. Un-derlying parameters in a specification aresupported by both historical data and testdata developed by electrode manufactur-ers and researchers, among others. Thespecification parameters allow users to select one or more electrode classifica-tion(s), and corresponding electrodes of-fered by one or more welding electrode manufacturer(s).

Welding electrode specifications sim-plify the above complex electrode selec-tion criteria, and present the recommen-dations, as clearly and concisely as possi-ble. To maintain neutrality or eliminatebias, the recommendations are classifiedinto groups of welding electrodes basedon chemical composition of the electrodeor the as-deposited weld metal (as in thecase of cored electrodes), and appropri-ate and acceptable mechanical property(commonly strength and toughness) testresults of undiluted, buttered, or dilutedweld metal. The relevant electrode classi-fication system also recognizes the fact thatelectrode manufacturers often produceone type of electrode that can be used tojoin a broad range of high-strength steels.

It is instructive to recognize that de-spite a strong attention to detail in reduc-ing various risks inherent to welding elec-trodes while enhancing reliability ofwelded structures, welding electrode spec-ifications do not offer an ability to distin-guish the combined effects of critical ele-ments in electrodes and weld metals. Allthe same, as shown by the effects of CENand calculated transformation tempera-tures on weldability, microstructure de-velopment, and weld mechanical proper-ties, such an ability is essential to achiev-ing desirable combinations of high pro-ductivity and superior performance.

Current welding electrode specifica-tions do not distinguish a high-perform-ance welding electrode composition fromeither a rich or a lean welding electrodecomposition, although all of them meetelectrode specification requirements.

Compared to either a rich or a lean weld-ing electrode composition, a high-performance welding electrode composi-tion is flexible or more forgiving whenit allows welding over a wide welding op-erational envelope while providing weldmetals meeting minimum mechanicalproperty requirements.

Current welding electrode specifica-tions also do not highlight to a potentialuser various fabrication-related cost risksin selecting either a rich or a lean weldingelectrode composition that otherwisemeets electrode specification require-ments. Such limitations could adverselyimpact weld procedure qualification ef-forts, particularly in terms of meetingschedules and cost estimates.

Summary

Selection and use of GMAW/FCAWelectrodes that eliminate a need forexpensive preheat, interpass, andpost-soak temperature controls duringwelding of high-strength steels, yet per-form satisfactorily over a broad weldingoperational envelope, while providingweld metal with an overmatched tensilestrength and acceptable toughness, offerexceptional value to both electrode man-ufacturers and weld fabricators.

To find such high-performanceGMAW/FCAW electrodes, first, know thechemical composition, microstructure, andmechanical properties (strength and tough-ness) of the steel being welded. Know theactual carbon content, and calculate CEN.Based on microstructures of the high-strength steel, identify and calculate rele-vant transformation temperatures.

Second, know the minimum acceptablestructural design requirements forstrength and toughness.

Third, refer to AWS A5.28/A5.28M:205, Specification for Low-AlloySteel Electrodes and Rods for Gas ShieldedArc Welding, and A5.29/A5.29M:205,Specification for Low-Alloy Steel Electrodesfor Flux Cored Arc Welding, and identifyappropriate electrode classificationsbased on minimum acceptable require-ments for transverse-weld tensile strengthand toughness.

Fourth, obtain electrode manufactur-ers data sheets for the relevant electrodeclassification. Identify an electrode thathas 0.02 to 0.04 wt-% less carbon, is chem-ically compatible, and shows a desirableCEN and 30 to 50C lower calculatedtransformation temperatures than thesteel being welded.

Lastly, evaluate the candidate weldingelectrode using previously certified weld-ing procedures, and determine that mini-mum acceptable requirements for weldmetal strength and toughness can be con-sistently achieved.

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