Inclusion Formation in Self-Shielded Flux Cored Arc Welds

Home , Arc welding, Gas metal arc welding, Metal fabrication

An investigation was made to see whether the same analytical models used to predict oxide inclusion in weld metal can also be used to predict nitride formations

BY M. A. QUINTANA, J. McLANE, S. S. BABU AND S. A. DAVID

A B S T R AC T. Nonmetallic inclusions in erties. If present in sufficient numbers (Refs. 7–10). By contrast, relatively little two weld metals were characterized with and size, inclusions may also influence effort has focused on weld metals in respect to variations in weld aluminum the ductile-to-brittle Charpy transition by which nitride rather than oxide formation c o n c e n t ration. Two self-shielded flux p r oviding initiation sites for cleava g e is dominant [i.e., self-shielded flux cored cored arc welding (FCAW-S) electrodes cracks and reduce upper-shelf energy. a rc welding (FCAW-S)] (Refs. 11–16). were used to produce welds for optical, Most of the documented work on in- This work was undertaken to determine scanning and transmission electron mi- clusions in weld metal has focused on whether the same analytical techniques croscopy. The inclusions in the weld with welding processes that shield the arc and could be used to accurately predict in- high-aluminum concentration were pre- molten metal from atmospheric contam- clusion formation in FCAW-S deposits. dominantly aluminum nitride. In con- ination [i . e . , gas metal arc welding Weld metal produced by FCAW-S is trast, the inclusions in welds with low- ( G M AW), gas-shielded flux cored arc unique in that the welding process and aluminum and high-titanium concentrat i o n s welding (FCAW-G), shielded metal arc consumables do not intentionally pro- were mostly aluminum oxide and titanium welding (SMAW) and submerged arc tect the molten metal from atmospheric carbonitrides. The measurements were welding (SAW)]. In conventional steel contamination. Rather, such contamina- compared with predictions from multi- weld metals produced by these tion is anticipated and necessitates the phase, multicomponent thermodynamic processes, the oxidizing atmosphere pro- use of strong deoxidizers and denitriders equilibrium calculations. The calcula- duced by the consumables and/or sup- to ensure deposition of sound weld de- tions agreed with the experimental mea- plied by the shielding gas is accommo- posits. Inclusion of these elements (e . g. , surements and predicted the formation of dated by excess Mn and Si in the Al, Ti and Zr) results in weld metal ch e m- aluminum nitride in high-aluminum electrode, wh i ch effectively deoxidize ical compositions that are significantly welds and also simultaneous formation the molten weld metal. Much of the de- different from other conventional arc of aluminum oxide and titanium car- oxidation product “floats out,” creating weld deposits in the same strength bonitrides. However, the predicted vol- silicate islands on the surface of GMA range. As illustrated in Table 1, FCAW- S ume fractions were lower than experi- welds and contributing to the slag layer deposits have higher aluminum and ni- mental values. in FCAW-G, SMAW and SAW. Histori- trogen in conjunction with lower oxygen cally, much experimental work was done than other conventional arc weld de- Introduction to ch a racterize the oxide inclusions in posits. The alloy balance in terms of car- these systems in relation to the resulting bon and manganese levels may also dif- It is well known that nonmetallic in- microstructures and properties (Refs. fer in some cases, but the major clusions play an important role in the 1–8). Recent advances in computational differences are in the nitrogen and oxy- evolution of microstructures in steel weld models and analytical tools make it pos- gen contents and the amount of excess metals. They influence the partitioning of sible, in some cases, to predict the deox- deoxidizer/denitrider remaining in the alloying elements between solid solution idation sequence and the oxide inclusion weld metal (Refs. 14, 15). and second phases depending upon the formation with reasonable accura cy temperature of the formation. Also, they Experimental Approach may act as nucleation sites for solidification and solid-state transformations on Two FCAW-S weld metal systems cooling. Inclusions are also known to were selected for investigation, E70T- 4 have a direct effect on mechanical prop- KEY WORDS and E71T-8 (Ref. 17), which represent significantly different Al, O and N levels as M. A. QUINTANA is with The Lincoln Electric Self-Shielded Flux Cored well as alloy balance. Specifically, these Co., Cleveland, Ohio. J. McLANE is with Oxide Inclusions electrodes represent the extremes of the E ve r e a dy Battery Co., Cleveland, Ohio (for- Nitride Inclusions typical aluminum range for FCAW-S de- merly with The Lincoln Electric Co.). S. S. Nonmetallic Inclusions posits. Single V-groove welds were produced over steel backing using the joint BABU and S. A. DAVID are with Oak Ridge Aluminum Oxide geometry illustrated in Fig. 1. National Laboratory, Oak Ridge, Tenn. Aluminum Nitride The welding conditions summarized Paper presented at the AWS 80th Annual in Table 2 are within the manufacturer’s Meeting, April 12–15, 1999, St. Louis, Mo. recommended operating ranges for each

98-s | APRIL 2001 Table 1 — Weld Metal Chemical Composition Table 2 — Welding Conditions Used in This Investigation Comparison (wt-%) High-Aluminum Low-Aluminum Element SMAW FCAW-G FCAW-S E70T-4 E71T-8 E7018 E70T-1 E7XT-X Electrode diameter 0.120 (3) 0.078 (2.0) C <0.08 0.03–0.08 <0.4 [in. (mm)] Mn 1.2–1.5 1.3–1.7 0.5–1.2 Tip-to work distance 2.75 (70) 0.75 (19) Si 0.2–0.5 0.6–0.9 0.2–0.5 [in (mm)] Al 0.01 <0.2 0.5–1.8 Voltage 31–36 19–20 N <0.01 <0.01 ~0.05 Amperage 525–590 270–280 O ~0.040 ~0.070 0.005–0.040 Wire feed rate 225–250 (95–106) 120 (51) [in./min (mm/s)] Heat input 86–98 (3.4–3.8) 54–58 (2.1–2.3) [kJ/in. (kJ/mm)] of the electrodes. All welding was ac- Preheat ambient ambient complished in the flat (1G) position. The Interpass (see note) 163 (325) max. 163 (325) max. two welds were made with significantly [°C (°F)] different welding heat inputs necessi- Note: Test welds heat quickly, reaching maximum interpass temperature in 1–2 passes. Thereafter, interpass temperature was tated by the respective electrode diame- maintained at 150–163 °C (300–325°F) through completion of the test welds. ters and are representative of actual usage. It was not possible to obtain electrodes representing extremely high and extremely low aluminum levels in sizes that would permit welding with the same heat input. Although differences in the thermal cycle due to heat input influence inclusion formation (Ref. 18), the large differences in chemical composition are expected to overshadow the effect of different heat inputs. Transverse macrosections were taken from each weld for the experimental work. Bulk weld metal chemical compositions were determined using a BAIRD Model DV 4 emission spectrometer and Fig. 1 — Weld joint geometry. LECO analysis equipment. Samples for carbon, sulfur and aluminum analyses were taken by collecting chips after equipped with light element energy- countered in the two FCAW-S systems drilling at the same locations. Total alu- di s p e r s i ve X-ray (EDS) analysis capability. considered here, the maximum dimen- minum content was determined by Scanning electron microscopy was ac- sions and equivalent diameters were atomic absorption spectroscopy follow- complished using a JEOL 5800 and an considered more relevant. “Equiva l e n t ing dissolution in aqua regia/hy d r o g e n Am r ay 1645 with light element EDS. diameter” is the diameter of a circle of a fluoride and fuming in perchloric acid. Chemical compositions of inclusions size equivalent to the area of complex Solid cylinders were removed from were determined semiquantitative l y shape. Consequently, inclusion vo l u m e equivalent locations in adjacent sections using EDS in both SEM and TEM. Inclu- was determined by calculating the vol- for oxygen and nitrogen determinations. sion number density, size distribution ume of an equivalent sphere. The total Me t a l l o g r aphic specimens were pol- and volume fractions were determined area sampled for each weld multiplied by ished with the final step with a 1-µm dia- using SEM images from the Amray at the respective mean equivalent inclusion mond and were examined without 5000X. Data from ten randomly selected diameter was considered a reasonable et c hing in a light microscope at magnifi- frames for each weld deposit were col- estimate of the volume of material sam- cations up to 1000X. Subsequently, lected, resulting in a sample size of about pled. Volume fractions were estimated by ca r b o n - e x t r action replicas and thin foils 4300 square µm, 265 particles for the dividing total inclusion volume by sam- were prepared for examination at higher high-aluminum case and 492 particles ple volume. magnifications. All metallographic sam- for the low-aluminum case. The SEM im- ple preparation utilized standard tech- ages from extraction replicas made it Thermodynamic Calculations niques with one exception. Because the possible to determine the inclusion size likelihood of aluminum nitride formation and shape more accurately than would Th e r m o dynamic equilibrium among was considered high and aluminum ni- have been possible with just the polished various oxides, nitrides and liquid steel tride is known to be soluble in water and cross sections. Image analysis softwa r e was calculated using version L of The r - mild alkali, it was necessary to eliminate was used to collect statistics on wh a t moCalc™ software (Ref. 19). The calcu- the use of all soap and water in the sam- amounted to two-dimensional projec- lations considered the elements Fe, Al, C, ple preparation. Accordingly, polished tions of three-dimensional particles with Mn, Si, Al, Ti, O and N and the followi n g samples were rinsed in either reagent- relatively complex geometric configura- phases: liquid, d-ferrite, austenite, ce- gr ade methanol or toluene, and carbon- tions. In conventional C-Mn weld metal mentite, Al2O3, MnOA l2O3, Ti O, Ti O2, ex t r action replicas were rinsed in alcohol systems that produce generally spherical Ti2O3, Ti3O5, MnOTi O2, SiO2, MnO- rather than wat e r . Transmission electron particles, it is customary to compile in- Si O 2, MnO, AlN and Ti(CN). In order to m i c r o s c o py (TEM) was accomplished clusion statistics based on diameter. Be- estimate chemical compositions, vol u m e using Philips CM-12 and Philips CM200T cause of the more complex shapes en- fr actions and initial temperatures of for-

WELDING RESEARCH SUPPLEMENT | 99-s A B

Fig. 2 — Optical micrographs of the as-welded mirostructure. A — High-aluminum weld deposit (E70T-4); B — low-aluminum weld deposit (E 7 1 T- 8 ) . were predicted to form. Table 3 — Chemical Test Summary To evaluate the relative stability of oxides and nitrides in this weld metal sys- Weld C S P Mn Si Al Ni Ti O N tem, the stability diagrams for liquid steel High-aluminum 0.234 <0.003 0.011 0.50 0.28 1.70 0.02 0.003 0.006 0.064 in equilibrium with Al2O3, AlN or Ti(CN) E70T-4 were calculated at 1527°C and are Low-aluminum 0.149 <0.003 0.005 0.64 0.30 0.53 0.01 0.058 0.030 0.033 shown in Fig. 3B. This calculation is con- E71T-8 sistent with multicomponent, but wa s limited to two phases at a time. The nominal concentrations of Al, O, Ti and N of mation of the nonmetallic phases in each in these two weld metals is discussed in the weld metal are also shown in the di- of the two weld metal systems were cal- detail elsewhere (Ref. 21), the optical a g ram. The calculations clearly show culated at a temperature just before solid- m i c r o g raphs of as-welded microstruc- that for this weld metal only AlN will ification. It was assumed the ch e m i c a l ture from both weld deposits are pre- form. However, the stability diagram also composition of the molten pool at this sented in Fig. 2 to supplement the inclu- shows some interesting features. The for- t e m p e rature could be approximated by sion comparisons. It is apparent that the mation of Al2O3 is predicted to occur the final weld deposit composition. differences in chemical composition af- only below certain Al and O concentra- While calculating the volume fraction of fect more than the inclusion formation. tions, and an inadvertent increase in Al oxides and nitrides, the compounds that The micrographs show a large fraction of will not promote Al O formation. This formed during cooling from initial tem- columnar skeletal d-ferrite morphology 2 3 complex stability of Al2O3 is attributed to pe r ature of formation to 1527°C (~ melt- in high-aluminum welds. The presence the interaction energies of Al and O in ing point of the welds) were added to- of d-ferrite is attributed to the sluggish liquid steel. The stability diagrams also ge t h e r . Moreover , the changes in liquid austenite formation in these welds dur- show minimum concentrations of Al, Ti composition due to the formation of these ing weld cooling. The sluggish austenite and N are needed for the initiation of AlN compounds were also considered using formation is related to the increase in the and/or Ti(CN) phases in the liquid. Sc heil assumptions. Further details about stability of d-ferrite, owing to a large con- The above calculations considered su c h calculations can be found in Ref. 20. c e n t ration of aluminum in solid solution. each phase in isolation. However, it is de- In contrast, the low-aluminum weld s i rable to evaluate the competition be- Results and Discussion a s h owed classic -ferrite morphology, tween each phase as a function of one or wh i ch formed from decomposition of more alloying element concentra t i o n s . The chemical test results for the two austenite. These observations are consis- Therefore, the calculations were ex- F CAW-S deposits are summarized in tent with earlier work by Ko t e cki (Refs. tended to predict the volume fraction of Table 3. In the high-aluminum case, the 15, 16). E 7 0 T-4 electrode produced weld metal various phases as a function of aluminum aluminum at 1.70 wt-% with oxygen and Thermodynamic Predictions concentration only in Fig. 3C at 1527°C. nitrogen levels of 60- and 640-wt ppm, The calculations are consistent with mul- r e s p e c t ive l y. The low-aluminum weld High-Aluminum Deposit (E70T-4) ticomponent, multiphase equilibrium. metal produced with the E71T-8 elec- The calculations indicated that below trode had far less aluminum at 0.53% by In the case of the E70T-4 deposit, the ~0.5 wt-% Al concentration, the forma- weight with oxygen and nitrogen leve l s ThermoCalc™ predictions favored the tion of AlN will cease and Al2O3 forma- of 300- and 330-wt ppm, respective l y. formation of AlN over Al2O3 or Ti(CN). tion will be favored while other elemen- Also, the presence of 0.058 wt-% tita- Fig. 3A shows the volume fraction of AlN tal concentrations remain constant. nium in the low-aluminum deposit as a function of temperature while cool- indicates the use of a second deoxi- ing. The initiation of AlN formation from Low-Aluminum Deposit (E71T-8) dizer/denitrider in the E71T-8 electrode. liquid steel was predicted to occur at While the evolution of microstructures 1666°C. No other nonmetallic phases Similar calculations were performed

100-s | APRIL 2001 A A

B B

C C

Fig. 3 — Equilibrium thermodynamic calculations for high-aluminum Fig. 4 — Equilibrium thermodynamic calculations for low-aluminum weld (E70T-4). A — Variation of volume fraction of AlN with temper- weld (E71T-8). A — Variation of volume fraction of Al2O3 and Ti(CN) ature while weld cooling; B — stability diagrams for AlN, Al2O3 and with temperature while weld cooling; B — stability diagrams for AlN, Ti(CN) phases as a function of aluminum, titanium, oxygen and nitro- Al2O3 and Ti(CN) phases as a function of aluminum, titanium, oxygen gen concentrations at 1527°C; C — variation of volume fraction of and nitrogen concentrations at 1527°C; C — variation of volume frac- Al2O3 and AlN as a function of aluminum concentration at 1527°C. tion of Al2O3, Ti(CN), AlN and Ti2O3 as a function of aluminum concentration at 1527°C. for low-aluminum weld deposits. In the Stabilities of oxides and nitrides in this Al 2O3 and Ti(CN) can form at 1527°C. case of the E71T-8 deposit, The r m o C a l c ™ weld metal system were evaluated at How e ve r , the formation of AlN is not fa- fa vored the formation of aluminum oxide 1527°C and are compared in Fig. 4B. The vored due to low concentrations of alu- and titanium carbonitride over the forma- nominal concentrations of Al, O, Ti and N minum and nitrogen. Because both Al2O3 tion of aluminum nitride. Figure 4A of the weld metal are also shown in the and Ti(CN) form at 1527°C, the probabil- sh o ws the fraction of AlN and Ti(CN) as a di a g r am. The stability diagrams for Al2O3 ity of each acting as a heterogeneous site function of temperature. The Al2O3 fo r - and AlN are more or less similar to that of for the other is high. The calculations also mation was predicted to start at 1809°C the high-aluminum weld. The calcula- sh o wed the stability of Ti(CN) in this weld with Ti(CN) beginning at 1654°C. tions show that, for this weld metal, both metal is quite different from that shown in

WELDING RESEARCH SUPPLEMENT | 101-s Fig. 6 — SEM micrograph of high-aluminum weld (E70T-4).

Fig. 5 — Optical micrograph of unetched high-aluminum weld (E 7 0 T- 4 ) .

Fig. 3B. This is attributed to the interac t i o n Table 4 — Inclusion Summary Statistics energies between dissolved titanium, nitrogen and oxygen. It is notewor t h y that Weld Mean Maximum Max. Number Volume Elemental ev en the formation of Al2O3 is very close Equivalent Equivalent Dimension, Density Fraction(a) Composition to the stability line and that a small Diameter, Diameter µm (N), m–3 change in aluminum concentration can dmean, µm dmax, µm eliminate the Al2O3 formation, favor i n g 16 only Ti(CN) formation. High-aluminum 0.93 5.0 8.5 6.627 x 10 0.0279 Al, N E70T-4 some Fe Calculations were extended to pre- Low-aluminum 0.37 2.5 4.0 3.092 x 1017 0.0080 Al, O dict the volume fractions of va r i o u s E71T-8 Ti, N phases as functions of aluminum con- (a) 3 c e n t ration, considering all the phases at Estimated volume fraction = N*(4/3)*Õ*(dmean/2) 1527°C — Fig. 4C. Some interesting features can be observed in this calculation. smaller particles (Fig. 7). EDS verified the centers of the hexagonal inclusions — At very low levels of aluminum, the cal- presence of Al and N in the inclusions Fig. 8A. EDS indicated strong Fe peaks culations predicted the formation of (Fig. 7C), indicating that they are, indeed, associated with these particles, wh i ch Ti O . With an increase in aluminum the 2 3 aluminum nitride and supported the pre- were absent at the inclusion perimeters volume fraction of Al O i n c r e a s e s . 2 3 dictions from thermodynamic calcula- — Fig. 8B. EDS in the SEM indicated the H ow e ve r, at 0.7 wt-% aluminum the for- tions. It is important to note that no ex- presence only of aluminum and nitrogen. mation of Al O ceases. The formation 2 3 perimental evidence was found for No Fe-rich phases were found on the ex- of aluminum nitride starts only above 2 aluminum oxide formation in this weld. terior surfaces of the inclusions, suggest- wt-%. This interval of the absence of alu- The large agglomerations and the rela- ing the Fe-rich particles observed by the minum reaction is an interesting obser- tively uniform distribution of smaller par- TEM were internal, serving as heteroge- vation, and further work is needed to ticles suggest initial formation of these in- neous nucleation sites. Although the e valuate this composition experimen- clusions in the liquid prior to the start of presence of Fe in the nitride inclusions t a l l y. Interestingly, the formation of solidification. This is consistent with a was not predicted, it is consistent with Ti(CN) was found to occur in all alu- predicted inclusion-formation start tem- other reported results (Ref. 11). Because minum concentrations, wh i ch needs to perature above the liquidus — Fig. 3A. In the formation of iron compounds was not be evaluated with future experiments. addition, the AlN inclusions were highly anticipated for either of the two weld faceted — Figs. 6, 7. metal systems under consideration, it Microstructural Observations It was apparent from detailed TEM ob- was not included in the thermodynamic servations of the Fe-rich cores found in analysis, which considered only the ox- High-Aluminum Deposit (E70T-4) some of the smaller discrete particles that ides and nitrides of aluminum and tita- the aluminum nitrides had nucleated het- nium. Therefore, these Fe - r i ch particles Initial microscopic examination re- erogeneously — Fig. 8. Specific identifi- are assumed to be the unmelted Fe-rich vealed a relatively uniform distribution of cation of these Fe-rich particles proved to compounds. Further work is needed to coarse, faceted particles, many with be rather difficult. They were first ob- understand the formation of these com- complex shapes, as illustrated in Fig. 5. s e r ved in the TEM images of the ve r y pounds during self-shielded flux-cored On closer examination at higher magni- small inclusions in the carbon extraction arc welding. fications, the complex shapes were actu- replicas, appearing as dark spots near the Simple statistical analysis of the in- ally large agglomerations (Fig. 6) of

102-s | APRIL 2001 A B

clusions from both welds are summa- C rized in Table 4, and the size distributions are plotted in Fig. 9. In the case of high- aluminum deposit E70T-4, average inclusion size is on the order of 1 µm, with the greatest number of inclusions falling in the 1- to 2-µm range. The relatively small f r e q u e n cy of very small inclusions (<1 µm) is consistent with the clustering and agglomeration seen experimentally, giv- ing the impression of larger indiv i d u a l particle size. The maximum equiva l e n t diameter is 5 µm. However, because of the complex shapes caused by the agglomeration of large numbers of individual inclusions, the maximum dimension is greater than 8.5 µm. The volume fraction of inclusions in this high-aluminum deposit was estimated experimentally to Fig. 7 — High-aluminum weld (E70T-4): individual inclusion in carbon extraction replicas. A — be on the order of 0.0279, an order of SEM image showing faceted nature of AlN; B — morphology observed in TEM micrographs; C magnitude higher than the thermody- — EDS spectrum obtained from the AlN inclusion. namic prediction of 0.0037 — Fig. 3A. In this case, the ThermoCalc™ prediction is not a reasonable estimate of the experi- EDS analysis, Ti, Al, N, C and O were ver- tate on the other during cooling. mental result, perhaps because the ified to be present. It was not possible to The statistics summarized in Table 4 analysis did not consider other com- determine whether the C peaks resulted and Fig. 12 are consistent with the ob- pounds or reactions during solidification. e x c l u s ively from the carbon extra c t i o n servation that the low-aluminum (E71T- For instance, the residual concentrations film or if the inclusions were responsible 8) deposit exhibits a finer inclusion dis- of dissolved Al and N at 1527°C after the for some of the variations in the peak in- tribution than the high-aluminum Scheil calculations were found to be 1.61 tensity observed. No other chemical con- ( E 7 0 T-4) deposit — Fig. 9. The ave ra g e and 0.016 wt-%, respective l y. This dis- stituents were associated with the inclu- equivalent diameter, maximum diameter solved aluminum and nitrogen can react sions. The spherical inclusions appeared and maximum dimension are approxi- to form AlN, even after the completion of to be rich in Al and O at the surface with mately a factor of two lower than similar solidification. However, further implica- isolated areas of Ti and N — Fig. 11A. statistics for the high-aluminum deposit. tions of slow diffusivity of elements in The more complex shapes tended to be The frequency plot shows the greatest solid need to be evaluated further. rich in Ti and N with cubic facets and a number of inclusions are less than 1 µm growth habit in three orthogonal direc- compared with the 1 to 2 µm for the high- Low-Aluminum Deposit (E71T-8) tions — Fig. 11B. The experimental evi- aluminum case. The higher number den- dence demonstrates heterogeneous nu- sity in this case is consistent with a uni- Initial microscopy revealed a rela- cleation of one phase on another. Th e form distribution of finer, more discrete tively uniform distribution of finer spher- apparent precipitation of aluminum particles. The estimated volume fraction ical particles — Fig. 10. Subsequent ex- oxide on the titanium nitride is not con- of 0.0082 is higher than the predicted amination of the extraction replicas at sistent with the prediction of Ti(CN) for- value of 0.0026. This value was obtained higher magnifications in the SEM re- mation at a temperature ~150°K low e r by adding the volume fractions of both vealed the presence of more complex than Al2O3 formation. However, the fact Al2O3 and Ti(CN). This underestimation shapes, although not to the same level of that the stable temperature ranges for is again related to the inability of ther- complexity observed previously for the e a ch phase overlap considerably (Fi g . modynamic calculations to consider the high-aluminum (E70T-4) deposit. Fr o m 4A) suggests each phase could precipi- reactions at low temperature in the solid

WELDING RESEARCH SUPPLEMENT | 103-s A B

Fig. 8 — A — High-aluminum weld (E70T-4), AlN with Fe-rich areas; B — EDS spectrum obtained from Fe-rich areas.

Fig. 9 — Inclusion size distributions from high-aluminum weld. state. However, thermodynamic predic- errors in prediction arise tions agree with the trends observed in because inclusion forma- the experimental data. tion after solidification was From the results presented above, it is not considered in the apparent thermodynamic calculations analysis. Further, any com- agree reasonably well with the experi- parison of heat input va r i- Fig. 10 — Typical inclusions of low-aluminum weld (E71T-8). mentally observed deoxidation and den- ance between the two weld itriding conditions in self-shielded flux types studied would have cored arc welds. The calculations pre- little relevance considering dict the formation of AlN in high- the inclusions are of totally different centers of the aluminum nitride inclu- aluminum welds and the formation of chemical compositions. sions were not predicted because their A l2O3 and Ti(CN) in low - a l u m i n u m formation was not anticipated and there- welds. The calculations predicted the Summary and Conclusions fore were not included in the analysis. expected trend of decreasing inclusion This oversight and inability to consider volume fraction in the low - a l u m i n u m The chemical constituents identified the inclusion formation at low tempera- weld compared with that of the high-alu- in the inclusions from both the E70T- 4 ture may have contributed to the fact that minum weld. Howe ve r , the calculations and E71T-8 weld metals are consistent accurate thermodynamic estimates of in- do not agree with the magnitudes of vo l- with the thermodynamic calculations. clusion volume fraction were not consis- ume fra c t i o n s . Although it was not possible to experi- tently obtained. Although the prediction The decision to neglect the influence mentally verify temperatures at wh i ch for the low-aluminum weld is in fair of heat input and reaction kinetics does precipitation occurred, the agglomera- agreement with experimental results, the not explain the consistent underpredic- tion of inclusions in the E70T-4 weld prediction for the high-aluminum weld tion of inclusion volume fraction. If any- metal was consistent with the prediction was not; volume fraction was underesti- thing, the nonequilibruim conditions of initial formation in the liquid. mated by an order of magnitude. prevalent during weld solidification are The calculations predicted ch e m i c a l These initial results are by no means expected to lead to over prediction of in- constituents with reasonable accuracy ; complete; however, they suggest thermo- clusion volume fraction when predic- h ow e ve r, the comparisons with experi- dynamic predictions can provide reason- tions are based exclusively on thermody- mental measurements of volume fraction able estimates of inclusion composition, namic equilibria. It is more likely the were only fair. The iron-rich phases at the if not volume fraction, in these FCAW-S

104-s | APRIL 2001 Fig. 11 — Inclusions of low-aluminum weld (E71T-8) observed with SEM.

weld metals. The thermodynamic calcu- M e chanisms of inclu- lations can be a useful tool in under- sion formation in Al-Ti- standing inclusion development in weld Si-Mn deoxidized steel metals, particularly in assessing the rela- weld metals. Me t a l l u r g i - cal Transactions A 20A(8): t ive stability of different phases during 1335–1349. cooling and solidification. 8. Kluken, A. O. , Grøng, Ø., and Rørvik, Acknowledgments G. 1990. Solidification microstructures and R e s e a rch sponsored by The Lincoln ph a s e - t ra n s f o r m a t i o n s Electric Co. and the U.S. Department of in Al-Ti-Si-Mn deoxi- En e r g y , Division of Materials Sciences, and dized steel weld metals. Assistant Secretary for Energy Efficiency Metallurgical Tra n s a c- and Renewable Energy, Office of Industrial t i o n s A 21A(7): Tec hnologies, Metals Processing Labora- 20 4 7 – 2 0 5 8 . tory User Center (MPLUS), Advanced In- 9. Babu, S. S., Fig. 12 — Inclusion size distribution from low-aluminum weld. D avid, S. A., Vitek, J. dustrial Materials Program under contrac t M., Mundra, K., and D E - AC05-96OR22464 with Lock h e e d D e b R oy, T. 1995. De- Martin Energy Research Corp. velopment of macro and microstructures of C–Mn low alloy steel toughness study of steel weld metal from self- References welds — inclusion formation. Materials Sci- shielded flux cored electrodes — part II. Weld- ence and Technology 11(2):186–199. ing Journal 51(3): 138-s to 155-s. 1. Hill, D. C., and Passoja, D. E. 1974. Un- 10. Babu, S. S., David, S. A., and DebRoy, 16. Es-Souni, M., Beaven, P. A., and Evans, derstanding role of inclusions and microstruc- T. 1996. Coarsening of oxide inclusions in low G. M. 1992. Microstructure and AEM studies ture in ductile fracture. Welding Jo u r n a l alloy steel welds. Science and Technology of of self-shielded flux cored arc weldments. 53(11): 481-s to 485-s. Welding and Joining 1(1): 17–27. Welding Journal 71(2): 35-s to 45-s. 2. Widgery, D. J. 1976. Deoxidation prac- 11. Krivenko, L. F. 1967. Research into the 17. American Welding Society. 1995. tice for mild-steel weld metal. Welding Jour- nitrides in the weld metal when steel is welded AWS A5.20, Specification for Carbon Steel nal 55(3): 57-s to 67-s. by open arc process. Avto. Svarka (7):6–9. Electrodes for Flux Cored Arc Welding. 3. Abson, D. J., Dolby, R. E., and Hart, P. 12. Dorling, D. V., Rodriguez, P. E. L. B. 18. Babu, S. S., Reidenbach, F., David, S. H. M. 1978. Investigation into the role of non- and Rogerson, J. H. 1976. A comparison of the A., Böllinghaus, and Hoffmeister, H. 1999. Ef- metallic inclusions on ferrite nucleation in toughness of self-shielded arc and submerged fect of high energy density welding process on carbon steel weld metals. TWI Res. Rep. arc weld metals in C-Mn-Nb steels, part 1: ef- inclusion and microstructure formation in 67/1978/M. fect of consumables and process variables on steel welds. Science and Technology of Weld- 4. Ito, Y., Nakanishi, M., and Komizo, Y. weld metal toughness. Welding and Metal ing and Joining 4(2): 63–73. 1982. Effect of oxygen on low carbon steel Fabrication (7/8): 419–423. 19. Sundman, B., Jansson, B., and Anders- weld metal. Metal Construction (9): 472–478 13. Dorling, D. V., Rodriguez, P. E. L. B., son, J. O. 1985. The thermo-calc databank sys- 5. Abson, D. J. 1989. The influence of alu- and Rogerson, J. H. 1976. A comparison of the tem. Calphad 9(2): 1–153. minum additions to tubular cored welding toughness of self-shielded arc and submerged 20. Hsieh, K. C., Babu, S. S., Vitek, J. M., electrodes on the microstructure and tough- a rc weld metals in C-Mn-Nb steels, part 2: and David, S. A. 1996. Calculation of inclu- ness of as-deposited C-Mn-Ni weld metal. through thickness toughness variation. Weld- sion formation in low alloy steel welds. Mate- TWI Res. Rep. 407/1989. ing and Metal Fabrication (9): 479–481. rials Science and Engineering, A215, 84–91. 6. Liao, F-C., and Liu, S. 1992. Effect of de- 14. Kotecki, D. J., and Moll, R. A. 1970. A 21. Babu, S. S., David, S. A., and Quintana, oxidation sequence on carbon m a n g a n e s e toughness study of steel weld metal from self- M. A. 2001. Modeling microstructure evolu- steel weld metal microstructures. We l d- shielded flux cored electrodes — part 1. Weld- tion in self-shielded flux-cored arc welds. ing Jo u r n a l 71(3): 94-s to 103-s. ing Journal 49(4): 157-s to 165-s. Welding Journal 80(4): 91-s to 97-s. 7. Kluken, A. O., and Grøng, Ø. 1989. 15. Kotecki, D. J., and Moll, R. A. 1972. A

WELDING RESEARCH SUPPLEMENT | 105-s