ABSTRACT. This study is part of a pro- gram to obtain fundamental knowledge about rutile electrodes, of which infor- mation is scarce in international welding literature. In this investigation, three ru- tile-coated electrodes of the ANSI/AWS A5.1-91 E6013 type were prepared by in- creasing calcite (natural calcium carbon- ate — CaCO 3 ), at the expense of cellu- lose and Si-bearing components, in their coatings. This modification produced an increase in the slag basicity, which caused a marked increment in all-weld-metal toughness and slight modifications in op- erational behavior with a decrease in pen- etration and width of the weld bead, while maintaining the typical excellent opera- tional characteristics of rutile electrodes. Arc stability studies were also performed. All-weld-metal hardness and tensile properties were measured and metallo- graphic studies undertaken. Introduction During the decade of the 1980s, an im- portant decrease in the use of coated electrodes took place in developed coun- tries (Ref. 1). On the other hand, in Latin America, almost 80% of deposited weld metal is produced from this type of weld- ing consumable (Ref. 2). In China and India, accompanying the noticeable growth of steel production, a marked in- crease of covered electrode use has been observed (Ref. 3). Everything seems to indicate the use of covered electrodes will stabilize in around 30% of the deposited weld metal (Ref. 1). Rutile-coated electrodes of the types ANSI/AWS A5.1-91 E6013 and E7024 (Ref. 4) continue to be required. Large manufacturers have replaced covered electrodes with solid and tubular contin- uous wires, but smaller ones still use cov- ered electrodes for the following reasons (Ref. 2): ■ Simplicity, durability, and low cost of the equipment required ■ Possibility of being used in open and closed locations ■ The relative ease of finding welders with the required skill ■ Wide range of consumables for most applications, which is a function of the quick setup fabrication ■ Availability in small units at rela- tively low cost. (It is generally accepted welding consumables represent 1–2 % of the final cost in overall fabrication). As for covered electrodes for the de- position of C-Mn steels, a lot of research conducted during the last 20 years has been to increase knowledge of basic cov- ered electrodes of types E7018 (Refs. 5, 6) and E7016 (Refs. 7, 8). This is not the case with rutile electrodes. At present, few papers about this type of consumable have been published (Refs. 9–23). Since welding consumable manufac- turers produce larger quantities of rutile- coated electrodes than basic ones and the rutile type is technically more important than the basic, more complete informa- tion about rutile electrodes should be generated for the following reasons: 1) The requirements of international classification societies (ABS, BV, DnV, LRS) establish three grades for the clas- sification of rutile electrodes (Ref. 24) ac- cording to the temperature at which 47 J minimum, on average, is obtained from the Charpy V-notch impacts. They are Grade 1 +20°C (68°F), Grade 2 0°C (32°F), and Grade 3 –20°C (–4°F) de- manding 33 J minimum for each individ- ual value. Grade 3 is normally required for the naval industry, so it is technically important. Not all covered electrode manufacturers have adequate knowledge to consistently satisfy the requirements of the grades mentioned. ANSI/AWS A5.1-91 E6013 Rutile Electrodes: The Effect of Calcite BY N. M. R. DE RISSONE, J. P. FARIAS, I. DE SOUZA BOTT, AND E. S. SURIAN All-weld-metal mechanical properties and microstructure, arc stability, and operational characteristics were investigated KEY WORDS Covered Electrodes Rutile Coating Basic Slag SMAW Calcite Additions Weldability N. M. R. DE RISSONE and E. S. SURIAN are with the Center for Development and Materials Technology, Facultad Regional San Nicolás, National Technology University, Buenos Aires, Argentina. J. P. FARIAS is with the Department of Mechanical Engineering and Production, Universidade Federal de Ceará, Fortaleza, Brazil. I. DE SOUZA BOTT is with the Depart- ment of Science, Materials and Metallurgy, Pontifícia Universidade Católica do Rio de Janeiro, Brazil. 113-S WELDING JOURNAL WELDING RESEARCH SUPPLEMENT TO THE WELDING JOURNAL, JULY 2002 Sponsored by the American Welding Society and the Welding Research Council
Transcript
ABSTRACT. This study is part of a pro-gram to obtain fundamental knowledgeabout rutile electrodes, of which infor-mation is scarce in international weldingliterature. In this investigation, three ru-tile-coated electrodes of the ANSI/AWSA5.1-91 E6013 type were prepared by in-creasing calcite (natural calcium carbon-ate — CaCO3), at the expense of cellu-lose and Si-bearing components, in theircoatings. This modification produced anincrease in the slag basicity, which causeda marked increment in all-weld-metaltoughness and slight modifications in op-erational behavior with a decrease in pen-etration and width of the weld bead, whilemaintaining the typical excellent opera-tional characteristics of rutile electrodes.Arc stability studies were also performed.All-weld-metal hardness and tensileproperties were measured and metallo-graphic studies undertaken.
Introduction
During the decade of the 1980s, an im-portant decrease in the use of coatedelectrodes took place in developed coun-
tries (Ref. 1). On the other hand, in LatinAmerica, almost 80% of deposited weldmetal is produced from this type of weld-ing consumable (Ref. 2). In China andIndia, accompanying the noticeablegrowth of steel production, a marked in-crease of covered electrode use has beenobserved (Ref. 3). Everything seems toindicate the use of covered electrodes willstabilize in around 30% of the depositedweld metal (Ref. 1).
Rutile-coated electrodes of the typesANSI/AWS A5.1-91 E6013 and E7024(Ref. 4) continue to be required. Largemanufacturers have replaced coveredelectrodes with solid and tubular contin-uous wires, but smaller ones still use cov-ered electrodes for the following reasons(Ref. 2):
Simplicity, durability, and low cost ofthe equipment required Possibility of being used in open andclosed locations The relative ease of finding welderswith the required skill
Wide range of consumables for mostapplications, which is a function of thequick setup fabrication Availability in small units at rela-tively low cost. (It is generally acceptedwelding consumables represent 1–2 %of the final cost in overall fabrication).As for covered electrodes for the de-
position of C-Mn steels, a lot of researchconducted during the last 20 years hasbeen to increase knowledge of basic cov-ered electrodes of types E7018 (Refs. 5,6) and E7016 (Refs. 7, 8). This is not thecase with rutile electrodes. At present,few papers about this type of consumablehave been published (Refs. 9–23).
Since welding consumable manufac-turers produce larger quantities of rutile-coated electrodes than basic ones and therutile type is technically more importantthan the basic, more complete informa-tion about rutile electrodes should begenerated for the following reasons:
1) The requirements of internationalclassification societies (ABS, BV, DnV,LRS) establish three grades for the clas-sification of rutile electrodes (Ref. 24) ac-cording to the temperature at which 47 Jminimum, on average, is obtained fromthe Charpy V-notch impacts. They areGrade 1 +20°C (68°F), Grade 2 0°C(32°F), and Grade 3 –20°C (–4°F) de-manding 33 J minimum for each individ-ual value. Grade 3 is normally requiredfor the naval industry, so it is technicallyimportant. Not all covered electrodemanufacturers have adequate knowledgeto consistently satisfy the requirements ofthe grades mentioned.
ANSI/AWS A5.1-91 E6013 Rutile Electrodes: The Effect of Calcite
BY N. M. R. DE RISSONE, J. P. FARIAS, I. DE SOUZA BOTT, AND E. S. SURIAN
All-weld-metal mechanical properties and microstructure, arc stability, and operational characteristics were investigated
N. M. R. DE RISSONE and E. S. SURIAN arewith the Center for Development and MaterialsTechnology, Facultad Regional San Nicolás,National Technology University, Buenos Aires,Argentina. J. P. FARIAS is with the Departmentof Mechanical Engineering and Production,Universidade Federal de Ceará, Fortaleza,Brazil. I. DE SOUZA BOTT is with the Depart-ment of Science, Materials and Metallurgy,Pontifícia Universidade Católica do Rio deJaneiro, Brazil.
113-SWELDING JOURNAL
WELDING RESEARCHSUPPLEMENT TO THE WELDING JOURNAL, JULY 2002
Sponsored by the American Welding Society and the Welding Research Council
2) Underwater wet welding has re-ceived great interest. Because rutile elec-trodes are used for this purpose (Refs.25–26), it is necessary to generate knowl-edge about them.
3) All-position, flux cored welding
wires are rutile (Ref. 27), as is their slag(it is possible to achieve diffusible hydro-gen content under 5 mL/100 g of de-posited metal). Basic metallurgicalknowledge about rutile electrodes couldbe obtained in a cheaper and simpler waywith covered electrodes, then transferredto tubular wires, as has been done withE7018 basic-coated electrodes.
For these reasons, a research programwith covered electrodes of the E6013 typehas been developed. This program stud-ied the effect that slag basicity variation,through increasing the calcite coatingcontent, has on the operational charac-teristics, arc stability, and all-weld-metalproperties.
Experimental Procedure
Electrodes
Three electrodes with a 4-mm diame-ter and coating factor (coating diameter:wire diameter) of 1.5 were designed by in-creasing calcite (natural calcium carbon-ate-CaCO3) from 5 to 15 wt-% at the ex-pense of cellulose and Si-bearing rawmaterials (quartz, kaolin, mica, andfeldspar) in the dry mix. This replacementwas undertaken to obtain an increase inbasicity of the slag without varying TiO2content to maintain the operational char-acteristics of the rutile electrodes as far aspossible. All the electrodes were pro-duced with the same quantity of potas-sium silicate and the same wire and pow-der raw material batches. The coating drymix composition and the slag chemicalanalyses, with the corresponding basicityindexes (BI) calculated according to Bon-iszewski (Ref. 28) are shown in Table 1.
In all tables and/or figures, the elec-trodes will be identified as 5-calcite, 10-calcite, or 15-calcite, depending on thepercentage of calcite in the electrodecoating (Table 1).
Operational Properties: Manual Welding
The operational behavior of the threeelectrodes was studied using an AC-DC350-A power supply set to AC, alternat-ing current; DC(+), direct current, posi-tive pole to the electrode; and DC(–), di-rect current, negative pole to theelectrode; in flat (F), horizontal fillet(HF), and vertical uphill fillet (VUF) po-sitions; and in vertical downhill (VD) po-sition only on AC.
It was noted as coating calcite in-creased, it was necessary to increase thecurrent to achieve appropriate opera-tional properties in the flat position. Volt-age decreased with increased calcite.Table 2 presents the welding parametersfor the flat welding position.
All-Weld-Metal Test Assemblies
All-weld-metal test assemblies withthree passes per layer (a total of nine) ac-cording to ISO 2560-73 standard (Ref.29) (Fig. 1) were manually welded in theflat position, applying DC(–) using theequipment previously mentioned. Thebase material was ASTM A36. Table 2shows the welding parameters employed.
Chemical Composition
Chemical analyses were obtainedfrom both the transversal cut samples ex-tracted from the all-weld-metal coupons,welded on DC(–), and from the weldpads, welded on both AC and DC(+), ac-cording to ANSI/AWS A5.1-91 (Ref. 4).The base material was ASTM A36.
Metallographic Study
The metallographic study was carriedout on transverse cross sections of the all-weld-metal test assemblies — Fig. 2. Thepercentages of columnar and reheated
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Fig. 1 — All-weld-metal assembly according to ISO 2560 standard.Fig. 2 — Cross section of the all-weld-metal test assembly.
Table 1 — Coating Composition and SlagChemical Analysis (wt-%)
zones were measured at 500X at theCharpy V-notch location. The averagewidth of the columnar grain size (theprior austenite grains) was measured inthe top bead of the samples at 100X.
To quantify the microstructural con-stituents of the columnar zones in eachweld, 30 fields of 100 points were mea-sured in the top bead at 500X by light op-tical microscopy, according to Ref. 30.The reheated fine grain size was mea-sured in the heat-affected zone of the topbead, according to the linear interceptmethod (ASTM E112 standard).
Inclusion analysis was carried outusing scanning electron microscopy(SEM). The inclusion chemical composi-tion was determined using energy disper-sive spectrometry (EDS) through semi-quantitative measurements. Inclusionswith diameters higher than 2 micronswere selected. The percentage of eachoxide was calculated by stoichiometryfrom the EDS measurements taking therelative quantities of each element foundwith the system used (Ti, Al, Si, and M),considering their sum was 100%.
Mechanical Testing
Microhardness was determined on thetransverse cross section of the all-weld-metal test assemblies at the Charpy V-notch location (Fig. 2) using the Vickers1000-g scale.
From each all-weld-metal test assem-bly, a Minitrac (Ref. 31) test specimen(total length = 55 mm, gauge length = 25mm, reduced section diameter = 5 mm,ratio of gauge length to diameter = 5:1)was extracted (Fig. 2), as well as sufficient
Charpy V-notch impact specimens to con-struct the absorbed energy vs. test tem-perature curve between 20°C (68°F) and–40°C (–40°F). Tensile property test spec-imens were tested at room temperature.
Arc Stability Study
With the electrodes described in Table1, bead-on-plate welds in the flat positionwith AC, DC(+), and DC(–) on 50 x 150x 8 mm ASTM A-36 steel plates were de-posited using an automated test benchwith computerized data acquisition asshown schematically in Fig. 3. The auto-mated test bench consisted of a 350-A in-
verter, constant power supply with 65 Vof open voltage (item 9 of Fig. 3), and anautomated welding system developedthrough a cooperative research projectbetween the Engineering Welding Labo-ratory (ENGESOLDA) and the Controland Instrumentation Laboratory (LIC)of the Federal University of Ceará,Brazil. This system has a card control(item 3 of Fig. 3) that uses arc voltage tocontrol arc length. Once a voltage varia-tion occurs in the arc, the card controlchanges the voltage signals sent to themotor (item 1 of Fig. 3), which in turn al-ters its rotation and consequently thefeed velocity of the electrode (item 4 of
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Fig. 3 — Automatic welding equipment. Fig. 4 — The cup, the arc length, and the visible arc length.
Fig. 5 — Beads obtained in the flat position welding with AC.
Fig. 3). The welding speed is selected di-rectly by a travel carriage (item 2 of Fig. 3) and kept constant.
The components of the data acquisi-tion system were a sensor hall of 300 A(item 5 of Fig. 3), a signal processing cir-cuit, an A/D converter (item 6 of Fig. 3),a microcomputer (item 7 of Fig. 3), and aprinter (item 8 of Fig. 3). This acquisitiondata system allowed current and voltagereading at programmable frequencies upto 10 Hz at 12 bits per channel.
The simulator controlled the arc volt-age in order to weld with the same visiblearc length in all experiments. The arclength in turn was controlled via the volt-age. The simulator does not measure thearc length but the voltage. The arc lengthwas established for each electrode priorto welding using a calibration curve (volt-age vs. arc length). In order to obtain thiscalibration curve, different current con-ditions and arc lengths were used. The arc
was projected by means of lenses on ascreen with a measurement scale, allow-ing the arc length to be determined. Oncethe arc length was determined, the cali-bration curve indicated the arc voltagevalue, which in turn was adjusted via thecontrol card of the simulator and keptconstant during the whole procedure.
Each experiment, conducted accord-ing to the welding parameters listed inTable 3, was undertaken in triplicate. Theinstantaneous values of arc voltage andcurrent were registered for periods of 6 s,totalling 18 s of acquisition for each com-bination of electrode/current type. To un-dertake a behavior analysis, a dedicatedsoftware (ANALYSER) was used fortreating the instantaneous voltage andcurrent signals acquired (Ref. 32).ANALYSER software determines all theparameters used to evaluate the arc be-havior. These parameters, as well as thecriteria used to make these measure-
ments, are described in the Appendix.The study of arc behavior was based
on a methodology that evaluated arc sta-bility, assessed by considering both theelectric charge transfer and metal trans-fer mechanisms (Refs. 32, 33). All the re-sults were submitted to a variance analy-sis (Ref. 34), with a confidence level of95%, for determination of the signifi-cance level (α) of the analyzed effects. Inthis study, α = 5.0% was considered forcomparing average values.
Electric Charge Transfer Mechanism
The ease of electric charge transferwas evaluated by considering the indexFE1 with DC, and the index B+ with AC.
(1)
FE1 is the inverse of the restriking meanenergy after the short circuit occurrencewith DC welding. (E1, P1, P0, and t1 aredefined in the Appendix.)
(2)
B+ is the mean rate of increase in electri-cal conductivity of the interelectrodespace during the positive prearc period.(I1
+, U1+ and t1
+ are defined in the Ap-pendix.)
The uniformity of the electric chargetransfer was also evaluated by means ofthe indices RE1 with DC and RB+ withAC. They represent the inverse of the rel-ative root-mean-square deviations of theindices E1 and B+.
(3)
(4)
Metal Transfer Mechanism
In this case, the ease of the metaltransfer was evaluated considering the in-dices Fcc and Fmt. These represent theease of short circuit occurrence and theease of drop transfer in the short circuittransition mode, respectively.
(5)
(6)F
tmt
sc
=
1 s-1
FT
sc =
1 s-1
RBB
B
++
=+σ
REE
E1
1
1
=σ
BI
U ts+
+
+ +−=
⋅⋅
1
1 1
11000 -1Ω
FEE P P t
s11 1 0 1
11 2000= =−( ) ⋅
− W -1
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Fig. 6 — Beads obtained in vertical uphill position welding with AC.
Table 3 — Welding Parameters for the Arc Stability Study
Type of CurrentElectrode Parameters DC(-) DC(+) AC
5-Calcite Mean 160 164 —Root-mean-square 161 164 160
10-Calcite Current (A) Mean 160 166 —Root-mean-square 161 167 160
15-Calcite Mean 164 167 —Root-mean-square 164 168 160
5-Calcite Mean 20 20 —Root-mean-square 20 21 20
10-Calcite Voltage(V) Mean 19 19 —Root-mean-square 19 19 18
15-Calcite Mean 16 16 —Root-mean-square 16 17 17
Welding Speed 2.9 2.9 2.9(mm/s)
The consistency of metal transfer wasevaluated considering the indices Rcc andRmt. These indices represent the inverseof the relative root-mean-square devia-tion of the short-circuit period (T) and ofthe short-circuit time (tsc), respectively.
(7)
(8)
With the exception of the figures re-lated to the electric charge transfer withAC, the rest of the analyses refer to theprocess involving metallic transfer, that isto say, only short circuits longer than 2.0ms were considered, because they actu-ally transfer the metallic drop (Ref. 32).
Cup Depth Measurement
The cup is formed at the arc end of theelectrode by the core wire of the elec-trode melting back inside the still un-melted coating, as indicated in Fig. 4. Itwas measured using a vernier callipergauge with 0.01-mm sensitivity.
Geometry of the Weld Bead
The weld beads obtained from the arcstability study were utilized to measurebead geometry. The transverse cut sam-ples obtained from the weldments wereetched with 5% Nital for 5 s to examinethe bead geometry. The vernier callipergauge mentioned previously was em-ployed to measure bead width (BW), pen-etration (P), and weld reinforcement(WR) of the bead.
Results and Discussion
Operational Properties
As the properties studied varied withthe changes in the coating calcite content,only 5 calcite and 15 calcite will be men-tioned, with the understanding that 10calcite results lie between the two.
Arc Characteristics
All electrodes presented a stable arc foreach type of current. The 15-calcite arc wassofter and visually showed lower penetra-tion than 5 calcite. This is probably due tothe presence of cellulose in the 5-calcitecoating. The differences mentioned weremore notable on DC than on AC. This ef-fect was confirmed through the weld beadgeometry determination, as stated previ-ously. In all cases, the arc was well directed.
Transfer Characteristics
“Spray” transfer was dominant withthe three electrodes. In F position, 5 cal-cite was faster than 15 calcite. However,in VUF, 5-calcite transfer was lower: ittook longer to deposit a bead with 5 cal-cite than with 15 calcite in VUF. In allcases, transfer was faster on AC than onDC.
Spatte r
In general, spatter was moderate,medium sized, and cold (it was possible toremove by simple brushing). For DC(+),15 calcite presented higher spatter than 5calcite. This difference is less notable forAC.
Slag Characteristics
As expected, all the slags were of therutile type but with slight variations. Theyall completely covered the beads and,once removed, the bead borders re-mained clean. The 15-calcite slag wasthicker and more abundant than the onedeposited by 5 calcite. In VD position, 5-calcite slag did not interfere with the weldpool; the bead was well shaped and de-posited quickly. On the other hand, 15-calcite slag was too abundant and tendedto interfere with the weld pool. In the lastcase, the weld was deposited more slowlyand bead conformation was irregular. InVUF, the best electrode was 15-calcite.Its slag was the most adequate for thiswelding position. In this position, the
Rt
mtsc
tsc
=σ
RT
scT
=σ
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Fig. 7 — Beads obtained welding on DC(+). A — Flat position; B — vertical uphill position.
A
B
5-calcite slag was too fluid and spilled out.Once again, the behavior mentioned wasless marked for AC than for DC.
The slag detachment was better forAC than DC. For AC, all electrodesshowed slag self-detachment in F posi-tion. In horizontal fillet position, 5 calciteand 10 calcite only presented this effect;15 calcite did not self-detach but crackedwithout lifting up. For DC(+) the slagwas more difficult to take off, especiallywith 15 calcite. For this type of weldingcurrent, in F position, the slag crackedbut did not self-detach.
Bead Characteristics
All beads deposited for all types ofcurrent were well shaped with fine rip-ples. In F position, the best bead wasachieved with 5 calcite, but, on the con-trary, in VU position, the best result wasobtained with 15 calcite for all currenttypes. Figures 5, 6, and 7 show these re-sults. In VD position, only 5 calcite couldbe used, probably due to the elimination
of cellulose from the coatings of 10-cal-cite and 15-calcite electrodes.
Arc Stability
The average values of current, voltage,and welding velocity obtained from thetests for each type of current are shown inTable 3. Since all experiments were madewith the same visible arc length, all the dif-ferences observed in the arc voltage valuesas a function of the increase in slag basicityshould be due to the variation of both thephysical characteristics of the arc atmos-phere and the cup depth. The variation ofcellulose and SiO2 contents (Table1) musthave been the major influence in the in-crease of the ionization potential on the arcatmosphere.
According to Pokhodnya, et al. (Ref.35), SiO2 increases the electron workfunction, thereby reducing the amount ofelectrons emitted from the cathode. Ad-ditionally, with increasing amounts ofgases evolving from the cellulose, theconcentration percentage of ionized par-
ticles of potassium, sodium, and theircompounds is reduced (Ref. 36). Themean ionization potential of the gaseousatmosphere of the arc is increased be-cause the ionization potential of gasessuch as CO, CO2, H2O, and their dissoci-ated products is high, sometimes as muchas four times greater than the ionizationpotential of potassium or sodium (Ref.36). As a result of these effects, the arcvoltage decreases with the decrease in thecellulose and SiO2 contents. The impor-tance of the basic oxides, of low workfunction, regarding the good operationalbehavior of the arc should also be men-tioned, especially when welding with al-ternating current, when the currentreaches very low values during the polar-ity inversion, due to the fact the emissionproperties of the slag improve with an in-crease in the basicity and degree of deox-idation (Ref. 35).
Table 4 shows the average values ofcup depth (in mm) and its related vari-ance analysis. There was a decrease oftwo thirds in the values of cup depth from
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Fig. 8 — Columnar zones (500X). Fig. 9 — Fine reheated zones (740X).
the most acid to the most basic electrode.This reduction can be seen as an advan-tage since it makes the arc reopening byshort circuit easier in welding of smallbeads or spot welds, or it can be seen as adisadvantage because it decreases metal-lic transfer guiding, exposes the arc, andreduces the possibility of drag welding.This effect can give rise to higher spatterand a larger variation of the arc length, aswell as a higher contamination of theweld metal.
Tables 5 and 6 show the results for thequantitative indices and the associatedanalysis of variance (α) for arc stability inDC and AC conditions. Only with directpolarity, DC(–), was the type of electrodeobserved to have a significant effect onarc stability, as shown by the indices FE1and Fmt, where the variance (α) was<5%. The increase of calcite content in-creased the facility of charge transfer FE1in DC(–). However, this increase in cal-cite content also increased the size of thedroplets, reducing the ease of metaltransfer given by Fmt. This tendency to in-crease the droplet size was observed forall three types of current, being more sig-nificant for DC(–) (α <5%), as shown inTables 5 and 6. These results show the in-crease in calcite content in electrodestested under DC current tends to im-prove the electric charge transfer throughthe arc, although the metal transfer isworsened. It may be noted, the frequencyof metal transfer by short circuit (Fsc) isnot affected in DC (α >5%). Althoughcoating chemical composition variationchanges the short-circuit times, mainly inDC(–), these variations do not affect thefrequency, as shown in Table 5. The samedid not occur in AC, where metal transfertended to be more difficult due to the in-crease in calcite content. On AC, the fre-quencies of short circuit (Fsc) exhibited adrastic decrease with the increase of cal-cite, as shown in Table 6.
Arc stability, for alternating current,evaluated by means of the B+ and RB+
indices increased with calcite content, asit did for continuous current (index FE1),proving the increase in slag basicity alsoincreases the charge transfer through theelectric arc.
The increase in calcite content associ-
ated with the decrease of cellulose andSiO2 in the electrodes analyzed reducedcup depth and arc voltage and improvedthe conditions of arc reopening in AC(higher indices of B+ and RB+) and aftershort circuit in DC(–) (higher indices ofFE1), contributing to an improvement incharge transfer through the arc. Also, itimpaired the metal transfer due to the in-crease in short-circuit time (small indicesFmt) having a more significant effect inDC(–) and AC due to the reduction of thefrequency of short circuit (Fsc).
Weld Bead Geometry
Table 7 shows results of the weld beadmeasurements. It can be observed in-creasing slag basicity produced a reduc-tion in both penetration and bead widthfor the three types of current used. In allcases, joint penetration and bead widthwere lower with AC than with DC weld-ing. The slag basicity increase did notseem to have a clear effect on bead rein-forcement.
All-Weld Metal Properties
Chemical Composition
Table 8 presents the all-weld-metalchemical composition. In the case of ACand DC(+), the analysis samples wereobtained from weld pads and for DC(–)
from mechanical property test speci-mens.
The highest transfer of C, Mn, and Sito deposited metal was achieved forDC(–), being slightly lower for DC(+)but markedly lower for AC. This factagreed with previous results obtainedwith different types of covered elec-trodes, E7016 (Ref. 8) and E7024 (Ref.21). This effect is probably due to thehigher oxygen contents found in ACwelding with respect to those for DC(+)and DC(–) (Ref. 8).
On the other hand, it was observedthat as both slag CaO content and slag ba-sicity increased the deposited metal Sicontent decreased. This was probably dueto two effects: the decrease of coatingSiO2 content as it was replaced with
119-SWELDING JOURNAL
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Fmt (s–1) — the ease of metal transfer in the short circuit.Rmt — the regularity of metal transfer in the short circuit.Rsc — the regularity of short-circuit occurrence (short-circuit frequency).Fsc (s–1) — the ease of short-circuit occurrence.
Table 6 — Results of Arc Stability of AC
Charge Transfer Metal TransferFacility Regularity Facility Regularity
CaCO3 and the increase of slag calciumoxide (as calcite increased in the coating,Table 1), which decreased the activity of Sithat was transferred to the slag as oxide.
The O values did not present signifi-cant variations as had been found in otherrutile electrode studies when slag basicityincreased (Refs. 10, 22). The N contentswere similar to those for DC(–) weldingwith E6013-type electrodes (Ref. 10) andhigher than those achieved with E7024-type electrodes (Refs. 21, 22) (it shouldbe noted the E7024-type electrodes had athicker coating, and therefore more pro-tection from the atmosphere) and evenhigher than the N values obtained frombasic coated electrodes (Refs. 5–8). Thisresult is probably due to the presence ofrutile assisting the transfer of nitrogen by
the formation ofTiN, which does nothappen in basiccoated electrodes(Ref. 37).
The Mn contentswere approximatelythe same for eachtype of current inspite of the increasein slag basicity, be-cause the Mn powder content of the coat-ings was adjusted to obtain these resultsto avoid introducing another variable tothe system.
Cr, Ni, Mo, and Ti values were notmarkedly affected by changes in slag ba-sicity. Nb and V (probably coming fromrutile) increased their values as slag ba-
sicity increased. Al levels were so low itwas not possible to observe any effect.The actual chemical composition of thebase metal used for both the all-weld-metal test coupons and the weld pads wasnot taken into account because the sam-ples to be analyzed were extracted froman undiluted area.
Fig. 10 — Inclusion size distribution.
Fig. 11 — Semiquantitative inclusion analysis of 15-calcite deposit.
a) Chemical analysis from the mechanical property determination coupon.b) Chemical analysis from the weld pad.
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Metallographic Study
General
The percentage values of columnarand reheated zones at the Charpy V-notch location are shown in Table 9. Thethree electrodes were used with the sameheat input. The 15-calcite electrode pre-sented lower penetration resulting fromthe elimination of cellulose from thecoating. This caused an increase of thecolumnar zone size at the expense of thereheated one.
Columnar Zone
Table 10 shows the microstructuralcomposition of the columnar zone inwhich the components were quantifiedaccording to Ref. 31. There seems to be aslight increase in the acicular ferrite con-tent in the 15 calcite, but the differencewas not significant.
The average width of the columnargrains (the prior austenite grains) in theas-deposited weld metal of the finalpasses is shown in Table 10. The valuesobtained were very similar. Figure 8shows typical microphotographs of thecolumnar zones of the three electrodes.
Reheated Zone
Values achieved from measurement ofthe reheated grain size (ASTM E112Method) of the fine reheated zone arepresented in Table 11. They did not showany significant difference, as can be seenin micrographs of these regions in Fig. 9.
Inclusions
In Figure 10, it can be seen the aver-
age size of inclusions is 0.7 mm for thethree weld metals. Inclusions larger than1 mm were not present in significantquantities. The 15-calcite electrode pre-sented less scatter in the inclusion size.Table 12 shows the inclusion volume frac-tions measured with the SEM. The elec-trode with the highest slag basicity, 15-calcite, presented the lower volumefraction of inclusions, but there were noimportant differences among the valuesobtained with the three electrodes.
Table 12 also presents the approximateinclusion chemical compositions. It wasobserved that as the coating calcite in-creased, SiO2, TiO2 and Al2O3 inclusioncontents decreased and MnO increased.CaO was not found in the inclusions. Fig-ure 11 shows, as an example, the semi-quantitative inclusion chemical analysisresults from 15-calcite, all weld metal.
Mechanical Properties
Microhardness Measurements
Table 13 shows microhardness valuesobtained from different zones in theCharpy V-notch location. It was observedthat as slag basicity increased, the micro-hardness values decreased in each zone,probably due to the decrease of C and Si.
Weighted averages were calculatedtaking into account the percentages ofcolumnar and recrystallized zones (Table9). The averages obtained showed thesame trend: microhardness decreasedwith increments in slag basicity.
Tensile Properties
Table 14 presents tensile property re-sults of all weld metals. A decrease in ten-
Table 9 — Percentage of Columnar andReheated Zones
sile and yield strengths were observed asSi content of the weld decreased accom-panied by a decrease in hardness. All arewithin the expected ranges for this type ofdeposit and satisfy AWS E6013 require-ments.
Toughness Results
Charpy V-notch values are presented inTable 15. The absorbed energy vs. temper-ature curves of the three electrodes areshown in Fig. 12. Figure 13 presents thesame curve with its scatter band and thecurve resulting from the average valuesachieved at each temperature for the max-
imum slag basicity electrode (15 calcite).In Figs. 12 and 13 and Table 15, an im-
portant improvement in toughness is ob-served as slag basicity increased. With the15-calcite electrode, this improvementwas so marked it was possible to satisfythe minimal average Register Society re-quirements of 47 J at –20°C, as well as the33-J minimum for each individual value.The ANSI/AWS A5.1-91 E7018 tough-ness requirement of 27 J at –29 °C (Ref.4) was also achieved with the 15 calcite.
It is generally accepted that toughnessis related to several factors (Refs. 5, 6,and 38): nitrogen content, hardness level,tensile properties, type and quantity of in-
clusions, and microstructural characteris-tics. In this case, the toughness increasedoes not seem to be accompanied by sig-nificant microstructural variations. Onlythe inclusion chemical compositions var-ied with slag basicity; however, this varia-tion was not reflected in microstructuralchanges, as happens when Ti is varied inE7018 electrodes (Ref. 6). So, this incre-ment of toughness appears to be relatedto the decrease in tensile properties andmicrohardness values associated with thedecrease in deposited metal Si content,which was shown to be detrimental totoughness in this system, as found in Refs.9, 12, 13, 17, and 18.
Conclusions
The coating composition of a standardANSI/AWS A5.1-91 E6013-type elec-trode was modified by increasing calcitecontent to incrementally increase slag ba-sicity. As slag basicity increased, the fol-lowing were observed:
1) A slight deterioration of opera-tional properties in the flat position butan improvement in the vertical-up fillet.This effect is less noticeable for AC thanDC. Joint penetration and bead width de-creased. In general, the rutile electrodeoperational behavior was maintained, butto obtain good operational properties, itwas necessary to increase welding currentas basicity increased.
2) An improvement of arc electrical
Fig. 12 — Charpy V-notch impact results for the three all-weld-metal sam-ples.
Fig. 13 — Scatter band of Charpy V-notch impact results for 15 calcite.
Table 15 — All-Weld-Metal Charpy V-Notch Impact Test Results
5-Calcite 10-Calcite 15-CalciteTest Charpy V Average Charpy V Average Charpy VAverageTemp(°C) (J) (J) (J) (J) (J) (J)
charge transfer and an impoverishmentof metal transfer.
3) A decrease in all-weld-metal Si con-tent.
4) No important microstructuralchanges were observed by optical mi-croscopy.
5) No significant variation in size dis-tribution of inclusions.
6) Modification in the chemical com-position of the inclusions, which were notreflected in microstructural changes ob-served with optical microscopy.
7) A slight decrease in microhardnessvalues.
8) A reduction in tensile property val-ues.
9) A very important increase in tough-ness properties.
Acknowledgments
The authors thank Eng. Celina LealMendes da Silva (from CEFET-PA,Brazil) and Eng. Vinicius Sales Rocha(from the Universidade Federal de Uber-lândia-Brazil) for the weld bead geome-try measurement. The authors Ivani de S.Bott and Jesualdo P. Farias wish to thankCNPq and FINEP (both of Brazil) for thefinancial support.
2. Taylor, D. S. 1990. The role of MMAwelding in 1990. Welding and Metal Fabrication,May.
3. Timerman, R., and de Vedia, L. 1991.Welding in Latin America. International Con-ference of South African Welding Institute: “To-wards Cheaper Welding,” Pretoria, SouthAfrica.
4. ANSI/AWS A5.1-91 Specification for Car-bon Steel Electrodes for Shielded Metal Arc Weld-ing. Miami, Fla.: American Welding Society.
5. Evans, G. 1983. Factors affecting the mi-crostructure and properties of C-Mn all-weld-metal deposits. Weld. Research Abroad. WRC,28.
6. Evans, G. 1990. The effect of alloying el-ements on the microstructure and propertiesof ferritic all-weld-metal deposits. IIS/IIWDoc. II-A-817-90.
7. Taylor, D. S. 1982. The effect of man-ganese on the toughness of E7016 type weldmetal. Welding and Metal Fabrication 50(9):452–460.
8. Surian, E., and Boniszewski, T. 1992. Ef-fect of manganese and type of current on theproperties and microstructure of all-weld-metal deposited with E7016-1 electrodes.Welding Journal 71(1): 348-s to 363-s.
9. Boniszewski, T., Evans, G. M., and Hart,P. H. M. 1994. Studies of AWS E6013 rutileelectrodes. part 1: effects of calcium carbon-ate. IIS/IIW-IIS Doc. II-A-931-94 (II-1263-95)
10. Ramini de Rissone, N. M., Bott, I. de S.,Jorge, C. F. J., Corvalan, P., and Surian, E.
1997. ANSI/AWS A5.1-91 E6013 rutile elec-trodes: the effect of Wollastonite. IIW-IISDoc. II-A-951-95. Welding Journal 76(11): 498-s to 507-s.
11. Farias, J. P., Bálsamo de S., P. S., andScotti, A. 1995. AWS E6013 rutile electrodes:the effect of Wollastonite, part 2: on arc stabil-ity and economical characteristics. IIW-IISDoc. II-A-953-95.
12. Boniszewski, T., and Evans, G. M. 1995.Studies of AWS E6013 rutile electrodes, part2: changes in toughness at constant strength.IIW-IIS Doc. II-1263-95.
13. Boniszewski, T. and Evans, G. M. 1995.Studies of AWS E6013 rutile electrodes, part3: deoxidation with silicon. IIW-IIS Doc. II-A-948-95.
14. Boniszewski, T. and Evans, G. M. 1995.Studies of AWS E6013 rutile electrodes, part4: deoxidation with magnesium. IIW-IIS Doc.II-A-949-95.
15. Boniszewski, T., and Malsingh, I. 1995.Studies of AWS E6013 rutile electrodes, part5: deoxidation with titanium and magnesium.IIW-IIS Doc. II-A-963-95.
16. Boniszewski, T., and Malsingh, I. 1996.Studies of AWS E6013 rutile electrodes, part6: generation of acicular ferrite. IIW-IIS Doc.II-A-975-96.
17. Edited by T. Boniszewski. 1996. Roundrobin report electrode 1366 from Doc. II-A-963-95. IIW-IIS Doc. II-A-984-96.
18. Edited by T. Boniszewski. 1996. Modi-fications of electrode 1366 from Doc. II-A-963-95 using FeTi instead of Ti. IIW-IIS Doc.II-A-991-96.
19. Elvander, J. 1996. Results from testing1366 electrode. IIW-IIS Doc. II-A-978-96.
20. Elvander, J. 1997. Results from testing1366 electrode, part two. IIW-IIS Doc. II-A-004-97.
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22. Surian, E. 1997. ANSI/AWS E7024SMAW electrode: The effect of coating mag-nesium additions, part 1: on operational be-havior, diffusible hydrogen and all-weld-metalmechanical properties and microstructure.IIW-IIS Doc. II-A-994-96. Welding Journal76(10): 404-s to 411-s.
23. Mendez da Silva, C. L., Surian, E., andFarias, J. P. 1997. Efeito do magnésio metálicosobre o comportamento do arco de eletrodosAWS E7024. XXIII Encontro de Tecnologiada Soldagem, III Congresso do Mercosul. SãoPaulo, Brasil.
24. Rules and Regulations for the Classifica-tion of Ships. Lloyd’s Register of Shipping, Jan-uary 1994 ed.
25. Ibarra, S., Olson, D. L., and Grubbs, C.E. 1989. Underwater wet welding of higherstrength offshore steels. OTC 5889, pp.277–282, Houston, Tex: 21st OTC.
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Appendix
Definitions of Arc Variables MeasuredUsing ANALYSER Software
Variables Determined with DC
U1: restriking mean voltage (V)I1: restriking mean current (A)t1: restriking mean time (ms)U0: reference voltage (U0 = 10 V)I0: reference current (A), the
correspondent value of currentin the beginning of the arc restriking
These variables are indicated in Fig.A1.
These variables are used to calculatethe indices below:E1 restriking mean energy after
short circuit occurrencewith DC welding (W.s)
123-SWELDING JOURNAL
WELDING RESEARCH
P1 restriking mean power after short circuit (W)
P0 reference power (W)
(A-A)
(A-B)
(A-C)The variable E1 represents the area
over the dynamic behavior (P x t) of thearc power during arc restriking after theshort circuit, which was considered ap-proximately as a triangle.
Variables Determined with AC and DC
Irms root-mean-square current (A)Urms root-mean-square voltage (V)T short-circuit period average
(ms)σT root-mean-square deviation
of Ttsc short-circuit time average (ms).σtsc root-mean-square deviation
of tsc.Figure A2 illustrates the determina-
tion of the short-circuit period and theshort-circuit time with DC.
Variables Determined Only with AC at the Polarity Change for the PositiveHalf Cycle
U1+ positive restriking mean
voltage (V).I1
+ positive restriking mean current (A).
t1+ positive restriking mean
time (ms).These variables are indicated in Fig.
A3.
P U I0 0 0= ⋅
P U I1 1 1= ⋅
EP P t
11 0 1
2000=
−( ) ⋅
( )
Fig. A1 — The variables determined for DC: reference current I0, reference voltage U0, restriking volt-age U1, restriking current I1, and restriking time t1.
Fig. A2 — Determination of the short-circuit time (tsc) and short-circuit period and (T).
Fig. A3 — The variables determined for AC: positive restriking mean current (I1+), positive restrik-ing mean voltage (U1+), and positive restriking mean time (t1+).
