Inclusion Formation in Self-Shielded Flux Cored Arc Welds
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Inclusion Formation in Self-Shielded Flux Cored Arc Welds 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 solidifica- tion 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 sig- nificantly 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 pro- duced 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 elec- trodes 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 dif- ferent heat inputs. Transverse macrosections were taken from each weld for the experimental work. Bulk weld metal chemical compo- sitions 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.