THE INFLUENCE OF SHIELDING GASES WITH HIGH PERCENTAGE OF CO 2 ON THE MICROSTRUCTURE OF FERRITIC STAINLESS STEEL WELDS

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1 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil THE INFLUENCE OF SHIELDING GASES WITH HIGH PERCENTAGE OF CO 2 ON THE MICROSTRUCTURE OF FERRITIC STAINLESS STEEL WELDS Demostenes Ferreir Filho, demostenesferreir@furg.r Cleiton Rodrigues Teixeir, cleiton@gefmt.eng.r Federl University of Rio Grnde - FURG - EE, Ru Alfredo Huch, 475. CEP , Rio Grnde-RS. Vltir Antonio Ferrresi, vltir@mecnic.ufu.r Federl University of Uerlândi - UFU - FEMEC, Cmpus Snt Mônic, Bloco M, Cix Postl 593. CEP , Uerlândi-MG. Astrct. Ferritic stinless steels re not chrcterized y good weldility. Wires mde of ustenitic stinless steels re usully pplied to join ferritic mterils ecuse of the good mechnicl resistnce, tencity nd ductility reched y ustenitic weld metl. However, novl ferritic stinless steel wires, stilized with N nd Ti, hve een developed to tckle the trget of mtching the required mechnicl properties nd keeping the costs lower y using ferritic stinless steels. These ferritics wires re een used in the exhust systems mnufcturing using shielding gses with high percentge of CO 2 nd there ren t studies to show wht cn hppen in the microstructure. The im of this work ws to study the influence of the shielding gs composition (pure rgon nd mixtures with O 2 or CO 2 ) on the microstructure of weld deposits otined with GMAW using stilized ferritic wires (ER430Ti nd ER430LN) in ferritic se metl (UNS nd AISI 441) used in the exhust system industries. The study ws mde comprtively to n ustenitic wire (ER308LSi). A specil experimentl pproch ws pplied to permit more relile comprison mongst different comintions wire-shielding gs. The results showed tht the shielding gs composition doesn't ply importnt role on the finl microstructure of the deposits, ut to one wire it ws showed tht the grin ws increse. Keywords: ferritic stinless steel, shielding gs, GMAW, short-circuit. 1. INTRODUCTION The mrket for stinless steels hs experienced constnt growth ecuse of the ove mentioned excellent mteril properties nd continuous mnufcturing improvement, especilly when items like process productivity increse nd cost reduction re tken in considertion. Recently, producers hve fced shrp increse in the interntionl prices of the lloying elements, minly, nickel nd molydenum. As result, the most trditionl clss (ustenitic) went through severe price rise worldwide. An lterntive for some pplictions is the use of ferritic stinless steel, contining no nickel. Ferritic stinless steels re widely used in utomotive exhust system prts ecuse they possess excellent corrosion resistnce nd high temperture properties. They re more economicl thn the ustenitic stinless steels, which contin lrge mounts of expensive Ni, nd hve smll therml expnsion coefficients. In generl the ferritics stinless steel present low weldility (comprtive with the ustenitic ones), therefore their welds re chrcterized y low ductility nd tencity eyond sensitivity to the intergrnulr corrosion. Moreover, the rough grins formtion exists. The ddition of elements s titnium, copper nd luminum in the welding process crries through grin refining in the welded metl (Reddy; Mohnds, 2001). Blmforth; Lippold (2000) cited tht ferritic stinless steel weld re relted to the gotten microstructure (presence of mrtensite structure), d control of this microstructure cn limit its ppliction. In stinless steel MIG/MAG welding the pure rgon or mixed with smll percentges of oxygen or cron dioxide is recommended s shielding gs (Lyttle; Stpon, 1990). But now ones there re some compnies tht re using lrge quntities of cron dioxide in mixture with rgon, nd there ren t studies to know wht hppened in the weld if these kind of shielding gses. The Argon (Ar) is n inert gs with low ioniztion potentil, low oxidtion potentil nd low therml conductivity. In ccordnce with Dilleneck; Cstgno (1987), the rgon high density in comprison with the other gses (1.38 in reltion to ir) promotes igger efficiency of protection, ecuse the rgon esily sustitutes ir round the weld. For eing n inert gs the protection to the rgon se promotes retention of legue elements in the weld fillet, leving the weld fillet free of inclusions, improving the mechnicl properties. Moreover, it fcilittes to open the rc, improves the stility in current decreses, eyond llowing spry trnsferences. The gs oxygen (O 2 ) is oxidnt tht in the mixture with rgon llevites the profile of the fillet weld, improving the qulity of the fillet, minly the wettility of the fusing puddle, for the reduction of the superficil tension in the contct csting puddle/se metl nd for the stiliztion of the position of the root of the rc (Jönsson et l, 1995). The ddition of smll mounts of O 2 to the rgon (up to 5% of O 2 ) hs influence on the column of the rc reducing the

2 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil current trnsition of gloulr/spry. When the oxygen level increses in the mixture, lso increses the losses of legue elements, eing le to spoil the mechnicl properties. The cron dioxide (CO 2 ) is chepest enters the types of shielding gses nd more used in MIG/MAG welding in steel with short circuit trnsference. The CO 2 dissocition in the rc forms CO nd O nd the glol effect is to generte n oxidnt protection (Lyttle; Stpon, 1990). Its high therml conductivity is responsile for one high het trnsference for the se metl wider stndrd of penetrtion nd rounded off it is gotten when it is compred with the rgon. The cron dioxide, in the shielding gs, results in cron inclusions, s well s, n oxidtion in the deposited metl. A disdvntge of the cron inclusions is tht the ferrite content in the deposited metl cn decrese, ecuse the cron is strong ustenite former (Lundqvist, 1980). Lio; Chen (1998) hd crried through study evluting the mechnicl properties s well s the microstructures with the chnge of the shielding gs (pure rgon nd mixtures of rgon with oxygen nd/or cron dioxide) for ustenitic stinless steel, where they hd oserved significnt chnges, s much in the microstructure, how much in the mechnicl properties. Notwithstnding, the volume of informtion ville in the literture on GMAW welding with ferritic stinless steel wire is still very scrce, minly in reltion to shielding gses, specilly ecuse of the recent use of lrge quntities of cron dioxide in mixture with rgon. Thus, this pper seeks to nlyze the metllurgicl chrcteristic of ferritic stinless steel welding y studying the influence of the shielding gs compositions (rgon nd mixtures with O 2 or CO 2 ) on the microstructure of the weld ed of ferritic stinless steels. 2. EXPERIMENTAL PROCEDURE For the experimentl procedure ccomplishment the following equipments hd een used: multiprocess welding source; co-ordinted tle with utomtic movement of the torch; current nd voltge of welding cquisition system nd determintion system for the microstructures. Three different types of wires for GMAW (AWS clsses ER308LSi, ER430Ti nd ER430LN, ll of them with 1.0 mm) were used to study the influence of stilizers on the deposit microstructure. The ER308LSi were used (s reference) ecuse it is the most used in the industry. Ech wire ws comined with different shielding gs to depict the influence of the rc environment of the microstructure of the fusion zone. The chemicl composition of the wires cn e seen in the Tle 1. Tle 1. Chemicl composition mesured of the wires (weight, %) Elem. C N Cr Mn N Ni S Si Ti ER308LSi 0,018 0,056 19,620 1,930 0,001 10,290 0,012 0,750 0,056 ER430Ti 0,108 0,014 17,450 0,650 0,001 0,400 0,002 1,040 0,350 ER430LN 0,027 0,014 17,660 0,425 0,440 0,440 0,004 0,430 0,004 2-mm-thick test pltes with 50 x 300 mm were used s se metl. They were mde of two kinds of i-stilized ferritic stinless steel (UNS43932 nd AWS 441). The chemicl composition of the se metls cn e seen in the Tle 2. Tle 2. Chemicl composition mesured of the wires (weight, %) MB Cr C N Ti S N Ti N ,05 0,011 0,013 0,2 0,0015 0,19 0,14 0, ,01 0,014 0,009 0,13 0,0012 0,56 0,08 0,49 It ws used only Ar+2%O 2 s shielding gs to the ER308LSi, ecuse this is the comintion tht is the most used in the in industry, thn this condition ws used s reference. For the others wires it ws used five kinds of shielding gs, Ar, Ar+2%O 2, Ar+4%CO 2, Ar+8%CO 2 nd Ar+25%CO 2. The lst one it ws used ecuse some compnies re welding ferritic stinless steel with tht kind of shielding gs, ut it doesn t hve studies to know wht hppened in the microstructure nd properties of the weld. To compre the influence of the shielding gs in the qulity of the weld ed it is necessry to find welding condition tht is est possile for ll the types of shielding gs. The serch of these prmeters ecomes little complex s function of the involved mount of vriles in the welding process, eing necessry to count on some considertions. It is importnt to lwys hve the sme welding current, sme deposition rte (to hve constnt vlue enters the wire feeding speed nd the welding speed) nd if possile to lwys hve the sme energy deposited in the weld fillet for ll the shielding gses used. It is importnt lso to get lwys stedy metllic trnsference for ll conditions.

3 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil To find the weld conditions to ll the comintions, tht give similr energy, tests hd een done vrying the contct tip-to-work distnce (CTWD), nd keeping the others prmeters constnt. It ws used voltge of 16 V; inductnce of scent nd descending of the mchine in n verge vlue of the nd of vrition of the welding source; wire feeding speed of 4 m/min nd welding speed of 20 cm/min, in utt joints s cn e seen in the Figure 2. The im ws to hve to ll the conditions one current of 90 A. Microstructurl nlysis on the fusion zone were done. Figure 1. Formt of the test specimens The Tles 3 to 5 present the weld conditions to the wires ER308LSi, ER430Ti nd ER430LN respectively. It is importnt to emphsize tht to ll the conditions of this work the current used ws lmost 90 A, s cn e seen in these three Tles. Tle 3. Welding condition to ER308LSi Shielding Gs CTWD [mm] V FEEDING [m/min] V WELD [cm/min] U [V] I [A] Ar+2%O Tle 4. Welding condition to ER430Ti Shielding Gs CTWD [mm] V FEEDING [m/min] V WELD [cm/min] U [V] I [A] Ar Ar+2%O Ar+4%O Ar+8%CO Ar+25%CO Tle 5. Welding condition to ER430LN Shielding Gs CTWD [mm] V FEEDING [m/min] V WELD [cm/min] U [V] I [A] 3. RESULTS AND DISCUSSION 3.1. Weld Bed Aspect Ar Ar+2%O Ar+4%O Ar+8%CO Ar+25%CO The Figures 2 to 6 present the spects for ll the conditions tht cn e seen in the Tles 3 to 5. With these Figures is possile to see tht we hd lmost the sme energy for ll the conditions, ecuse the weld eds hd lmost the sme dimensions. It is importnt to see tht for ll the conditions there were lmost nothing of oxide in the weld ed, it is proly ecuse of the wire nd se metl stiliztion.

4 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil Figure 2. Weld ed spect welded with ER308LSi nd Ar+2%O 2 to the se metl: -439 nd -441 c d e Figure 3. Weld ed spect welded with ER430Ti, se metl 439 nd shielding gs: -Ar, -Ar+2%O 2, c-ar+4%co 2, d-ar+8%co 2 nd e-ar+25%co 2 c d e Figure 4. Weld ed spect welded with ER430Ti, se metl 441 nd shielding gs: -Ar, -Ar+2%O 2, c-ar+4%co 2, d-ar+8%co 2 nd e-ar+25%co 2 c d e Figure 5. Weld ed spect welded with ER430LN, se metl 439 nd shielding gs: -Ar, -Ar+2%O 2, c- Ar+4%CO 2, d-ar+8%co 2 nd e-ar+25%co 2 c d e Figure 6. Weld ed spect welded with ER430LN, se metl 441 nd shielding gs: -Ar, -Ar+2%O 2, c- Ar+4%CO 2, d-ar+8%co 2 nd e-ar+25%co 2

to 439 nd 441 respectively. The imges hd een crried through y optic microscopy where () possess n increse of 10x nd () n increse of 45x. It cn e seen n ustenitic mtrix with smll grins. Figure 7.

Microstructure of the fusion zone of the 441 welded with ER308LSi nd Ar+2%O 2 The Figures 9 nd 11 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER430Ti nd

5 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil 3.2. Microestruturl Anlysis The Figures 7 nd 8 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER308LSi nd the shielding gs Ar+2%O 2, to 439 nd 441 respectively. The imges hd een crried through y optic microscopy where () possess n increse of 10x nd () n increse of 45x. It cn e seen n ustenitic mtrix with smll grins. Figure 7. Microstructure of the fusion zone of the 439 welded with ER308LSi nd Ar+2%O 2 Figure 8. Microstructure of the fusion zone of the 441 welded with ER308LSi nd Ar+2%O 2 The Figures 9 nd 11 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER430Ti nd the shielding gs Ar, to 439 nd 441 respectively. The Figures 10 nd 12 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER430Ti nd the shielding gs Ar+25%CO 2, to 439 nd 441 respectively. The imges hd een crried through y optic microscopy where () possess n increse of 10x nd () n increse of 45x. It cn e seen n ustenitic mtrix with smll grins. To the oth se metls, using ll the five shielding gses used, s cn e seen in the Figures 9 to 12 ll the microstructure ws composed y ferritic mtrix. At lest with the shielding gs Ar+25%CO 2 there wsn t ny formtion of mrtensite, it cn e explined y the stiliztion of the wire nd the se metl. To chieve steel stiliztion it is necessry to dd correct mount of stilizing elements. A short mount of them llows formtion of Cr precipittes (corrosion relted prolems). Stilizers in excess worsen the mechnicl properties of the joint (intermetllic compound precipittion). DeArdo et l (1996) proposed Equtions 1 nd 2 to estimte the pproprite mount of Ti nd N in ferritic stinless steel sed metl, ccording to the interstitil element (C + N) contents. Ti = Ti 4 C - 3,42 N (for steels stilized with Ti) (1)

15-20, 2009, Grmdo, RS, Brzil N = N - 7,74

The worse stiliztion of the ER430Ti cn e

439 welded with ER430Ti nd Ar Figure 10.

Figure 11. 441 welded with ER430Ti nd Ar

6 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil N = N - 7,74 (C + N) (for steels stilized with N) (2) The worse stiliztion of the ER430Ti cn e seen y the increse of the grin size with the dioxide cron content. Figure 9. Microstructure of the fusion zone of the 439 welded with ER430Ti nd Ar Figure 10. Microstructure of the fusion zone of the 439 welded with ER430Ti nd Ar+25%CO 2 Figure 11. Microstructure of the fusion zone of the 441 welded with ER430Ti nd Ar

Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil Figure 12.

for the ed welded with ER430LN nd the shielding gs Ar, to 439 nd 441 respectively.

respectively.the imges hd een crried through y optic microscopy where () possess n increse of 10x nd () n increse of 45x.

In the sme wy of the ER430Ti, to the oth se metls, using ll the shielding gses, s cn e seen in the Figures 13 to 16 ll the microstructure ws composed y ferritic

7 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil Figure 12. Microstructure of the fusion zone of the 441 welded with ER430Ti nd Ar+25%CO 2 The Figures 13 nd 15 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER430LN nd the shielding gs Ar, to 439 nd 441 respectively. The Figures 14 nd 16 present the microstructure of the fusion zone (center of the ed) for the ed welded with ER430LN nd the shielding gs Ar+25%CO 2, to 439 nd 441 respectively.the imges hd een crried through y optic microscopy where () possess n increse of 10x nd () n increse of 45x. It cn e seen n ustenitic mtrix with smll grins. In the sme wy of the ER430Ti, to the oth se metls, using ll the shielding gses, s cn e seen in the Figures 13 to 16 ll the microstructure ws composed y ferritic mtrix. And with ll the shielding gses, including the Ar+25%CO 2, there wsn t ny formtion of mrtensite, it cn e explined y the stiliztion of the wire nd the se metl. In comprison with ER430Ti, the fusion zone re formed y corse grins, it cn e relted with the dissocition of the N precipittions, tht strts in low tempertures, thn it cn e hppened n increse of the ground oundry. Figure 13. Microstructure of the fusion zone of the 439 welded with ER430LN nd Ar

8 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil Figure 14. Microstructure of the fusion zone of the 439 welded with ER430LN nd Ar+25%CO 2 Figure 15. Microstructure of the fusion zone of the 441 welded with ER430LN nd Ar Figure 16. Microstructure of the fusion zone of the 441 welded with ER430LN nd Ar+25%CO 2

9 Copyright 2009 y ABCM 20th Interntionl Congress of Mechnicl Engineering Novemer 15-20, 2009, Grmdo, RS, Brzil 4. CONCLUSIONS With the conditions of ssys crried through in this work, it cn conclude the following one: - It is possile to weld with ll the shielding gses with one sme welding energy; - The shielding gs didn t ig differences in the weld ed structure formtion; - There is no formtion of mrtensite with ll the shielding gses used in this work; - With the increse of the cron dioxide content in the mixture with rgon it hs n increse in size of the grin when it is welded with ER430Ti; 5 ACKNOWLEDGEMENTS The uthors grtefully cknowledge the institutions tht supported this reserch (CNPq, CAPES, FAPEMIG nd FEME-UFU). 6. REFERENCES BALMFORTH, M. C.; LIPPOLD, J. C. A New Ferritic-Mrtensitic Stinless Steel Constitution Digrm. Welding Journl, V. 79, n. 12, p. 339s-345s, Dec DEARDO, A. J., Grci, C. I.., HUA, M., TITHER, G., HULKA, K., Ferritic Stinless Steel - the metllurgy ckground nd enefits of dul stiliztion, In: Cong. Stinless Steels 96, Dusseldorf, p , DILLENBECK, V. R.; CASTAGNO, L. The Effects of Vrious Shielding Gses nd Associted Mixtures in GMA Welding of Mild Steel, Welding Journl, pp set JÖNSSON, P. G., MURPHY, A. B. nd SZEKELY, J. Oxygen Additions on Argon-Shielded Gs Metl Arc Welding Processes, Welding Reserch Supplement - Welding Journl, V. 74, N. 2, p. 48-s 58-s, fev LIAO, M. T., CHEN, W. J., The effect of shielding-gs compositions on the microstructure nd mechnicl properties of stinless steel weldments, Mterils Chemistry nd Physics, 55, p , LUNDQVIST, B., Aspects of Gs-Metl Arc Welding of Stinless Steels, in Swedish. Sndvik AB, Sndviken, Sweden, LYTTLE, K. A.; STAPON, F. G. Select the Best Shielding Gs Blend for the Appliction, Welding Journl, p 21 28, nov REDDY, G. M.; MOHANDAS, T. Explortive studies on grin refinement of ferrítico stinless steel welds. Journl of Mterils Science Letters, 20, p , 2001.

MACROSCOPIC INVESTIGATIONS OF THE PRIMARY STRUCTURE OF CONTINUOUS CASTING INGOTS

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