ARTICLE IN PRESS

0042-207X/$ - s

doi:10.1016/j.va

�CorrespondE-mail addr

Vacuum 80 (2006) 1331–1335

www.elsevier.com/locate/vacuum

Flash butt resistance welding for duplex stainless steels

Toshio Kuroda�, Kenji Ikeuchi, Hyuma Ikeda

Joining and Welding Research Institute of Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Abstract

Duplex stainless steels were welded using flash butt resistance welding with temperature controlling system. Flash butt resistance

welding is consisting of two stage processes of flash action and contact resistance. First stage is flashing action. The specimen produced

flashing or arcing across the interface of the two butting ends of the specimens. Fine particles of metals near the surface were burned out

towards the opposing surface of the specimen irregularity and then the melted particles were deposited on the surface. The second stage is

resistance welding. The solid state bonding was performed in the region around the deposited particles. The cross-sectional

microstructure of the weld bond region was observed using microscopy. The microstructure showed two types of a deposited fine

particles region and a solid state bonding region. The grain growth was hardly observed in the weld region and the heat-affected zone.

The tensile strength and the impact energy increased with increasing heating time up to 1373K because of increasing fine grained

deposited metal.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Duplex stainless steel; Flash butt welding; Deposited particle; Solid state bonding; Resistance welding

1. Introduction

Duplex stainless steels are consisting of ferritic-austeniticmicrostructure at room temperature and exhibit greatertoughness and better weldability than ferritic stainless steel[1]. They have higher strength and better corrosionresistance than austenitic stainless steel [2]. Their goodengineering performance has led to an increasing numberof applications, mainly in corrosive environments such assour gas pipelines and chemical reaction vessels.

When joining duplex stainless steels, the microstructureof the heat-affected zone (HAZ) is determined only byapplied thermal cycles and is very sensitive to weldingconditions [3]. The HAZ generally develops from a platestructure which is originally rolled and annealed, and inmany duplex stainless steels consists of an approximately50:50 phase distribution of austenite within a ferritic matrix[3]. Especially in the bond region, the phase balance offerrite and austenite is varied significantly, which causesextreme low toughness of the HAZ compared to the base

ee front matter r 2006 Elsevier Ltd. All rights reserved.

cuum.2006.01.068

ing author. Fax: +816 6879 8689.

ess: kuroda@jwri.osaka-u.ac.jp (T. Kuroda).

metal [3]. Therefore, controlling microstructure of the bondregion is important to obtain good weldment.Generally, flash welding is a method of joining in which

no filler metal is used, as in arc welding, and in which nocast nugget is formed, as in spot welding [5–7]. Since theheat in a flash weld is localized between the dies, and thegreatest amount of heat is generated at the face to bewelded by virtue of the flashing action, there are successivemetallurgical changes in the finished flash weld from thehighly heated structure at the center of the weld throughthe HAZ to the undisturbed parent base metal [8,9]. In thisstudy, micro flash phenomena of duplex stainless steelswere investigated using a new flash butt welding apparatus.

2. Experimental

The base metals used in this study were the super duplexstainless steel containing 25%Cr–7%Ni–3%Mo–0.2%N(329J4L) and the conventional duplex stainless steelcontaining 22%Cr–6%Ni–3%Mo–0.17%N (329J3L). Theplates were 12mm thick. The specimens were cut from thebase metal plate along the rolling direction. The sampleswere mounted in the dies using a Gleeble thermal

ARTICLE IN PRESS

Fig. 1. Schematic illustration of flash butt welding apparatus.

Fig. 2. Thermal cycles of flash butt welding at 1373k for super duplex

stainless steel (329J4L).

T. Kuroda et al. / Vacuum 80 (2006) 1331–13351332

simulator, then and flash butt welding was done. Thespecimens were heated up to 1373K for 10, 20 and 30 s.

Charpy impact test was carried out in the temperaturerange from 77 to 373K. The fracture surfaces of thespecimens after Charpy impact test were examined using ascanning electron microscope (SEM). For the SEMobservation, stereoscopic photographs were taken toreconstruct three-dimensional topography of the fracturesurfaces using the image processing technique describedpreviously [4].

Fig. 1 indicates schematic illustration of a new flash buttwelding using Gleeble simulator. Samples were fixed toelectrode. The welding temperature at the butting surfaceof the specimen was set at 1373K by the attached thermocouple.

For welding, specimens were mounted, aligned andclamped in the dies. The ends of the specimens contactedeach other under the constraint pressure. When the currentwas turned on, heating begun. This heating consists ofbringing the end of the specimens together and separatingthem several times in succession, each time causing a shortcircuit. The specimens were heated during this passage ofcurrent, particularly at the butting surfaces. As the currentpasses through the specimen, and intense localized heatingoccurs between the contact faces. During the flashingperiod, the heat generated is intensified by the inadequatecontact between the faces to be welded, which rapidlybrings the flashing surfaces to a high temperature. On bothsides of the flashing surfaces the temperature falls rapidlyoff, resulting in a narrow heated zone.

3. Results and discussions

3.1. Micro flashing phenomena during heating process

Flash welding is a resistance welding process whichsimultaneously produces coalescence over the entire areaabutting surfaces, by the heat obtained from resistance toelectric current between the two surfaces, and by the

application of pressure after heating is substantiallycompleted. Flashing and upsetting are accompanied byexpulsion of metal from the joint. During the weldingoperation there is an intense flashing arc and heating, twosamples are forced together and coalescent occurs at theinterface, flow of current is possible because of the lightcontact between the two part being flash welded. Theheating is generated by the flashing and localized in thearea between the two parts. The surfaces are brought to themelting point and expelled through the abutting area. Assoon as the material is flashed away another small arc isformed which continues until the entire abutting surfacesreaches the melting temperature. Pressure is then appliedand arcs are extinguished and upset occurs.Fig. 2 indicates the temperature time curves during flash

butt welding for super duplex stainless steel (329J4L). Theheating up to 1373K was carried out for 10, 20, and 30 s, inorder to change the contact area condition of the abuttingsamples. In case of heating time of 10 s, the temperature atthe weld joining increased with increasing heating time.The rise and drop of the temperature, so called zig-zagphenomena in the linear curve up to 800K was fairlyobserved. In case of heating time for 20 and 30 s, thetemperature indicates the zig-zag phenomena, which wasarise and drop of the temperature during heating process.The machine used in the present investigation was Gleeble1500 thermal cycle apparatus, which was controlled by thecomputer-aided system. As the samples without joiningwere heated, the temperature–time curves during heatingprocess was almost linear and hardly showed such a zig-zagphenomena. Consequently, the zig-zag phenomena sug-gested that flashing action takes place.Fig. 3 shows the temperature–time curves during flash

butt welding for conventional duplex stainless steel(329J3L). In every samples during heating up to 1373K,the temperature reveals rise and drop phenomena. In caseof the heating time of 30 s, the temperature drop amount islarge. This means that the short circuit of the electriccurrent breaks and the resistance heating ceases, resulting

ARTICLE IN PRESS

Fig. 3. Thermal cycles of flash butt welding at 1373 k for conventional

duplex stainless steel (329J3L).

Fig. 4. Microstructure of bonding interface for flash butt welding of

329J4L super duplex stainless steel. (a) Welding time up to 1373K: 10 s,

(b) 20 s, (c) 30 s.

T. Kuroda et al. / Vacuum 80 (2006) 1331–1335 1333

in decrease in temperature. In another contact area, thecurrent began to flow and the resistance heating starts andthe current cease.

The flash phenomena is considered to be as followed.The heating and flashing in the flash-welding process areclosely related to short circuit heating. A small portion ofthe contact surfaces come together during the heating.Therefore, the actual contact area of the surfaces throughwhich the electric current flows is considerably smaller thanthe cross-section of the entire sample. Through the bar,current flows uniformly which converges at the contactpoints to form a high localization of current, which resultsin an intensive generation of heat. At the first instant, thetemperature rise linearly with time since there is noeffective heat loss. As the first heating short circuitcontinues, the local increase in temperature is dissipatedby absorption of the heat in the comparatively large massof material at the butting surfaces, which is still cold.

However, when the heating continues far enough soft-ening of the contact points takes place. This causes anincrease in the actual contact surface, due to the thermalexpansion, thus producing better current condition. Theconductivity and specific heat, which vary with tempera-ture, produce an accelerating increase in electrical resis-tance in those section which are most highly heated. Thetemperature distribution in the specimen and the way thesetemperatures increase as welding proceeded and duringheating. Therefore, heating is generated at the contactpoints corresponding to the high current density. After thecontact surfaces are separated, the circuit is broken andheat generation ceases. The temperature of the contactsurfaces drops because of the heat absorption by the bodyof the specimen. During the current-off period, tempera-ture equalization tends to take place, and by repeatingthese short circuit events, the ends of the specimen arereheated. During heating, the actual contact surfacesbecome gradually larger.

3.2. Microstructure of bonding interface for flash butt

welding

Fig. 4 presents the cross-sectional microstructures nearbonding interface after flash butt welding. In case of 10 sheating up to welding temperature of 1373K, as shown inFig. 4(a), black line near the bond line is observed, and itmeans that the bonding is not perfect. However, some ofaustenite grains were bonded each other. The bonding offerrite to ferrite is barely observed. According to themicrostructure appearance, the bonding seems to beproceeded by the solid state bonding mechanism. In caseof 20 s heating up to 1373K as shown in Fig. 4(b), austeniteseen as white elongated phase proceeded to anotherspecimen and bonding is perfect. However, joining offerrite to ferrite, and joining of ferrite to austenite arehardly completed perfectly. Solid state bonding occurredmainly for the short heating time to the weldingtemperature. As shown in Fig. 4(c), clusters consisting ofnumerous fine grains are observed along the bondinginterface. This is a flash action phenomena. In case of superduplex stainless steel, flashing is considered to be generatedbecause that heating time is slow. As shown in Fig. 2, Therise and drop phenomena of temperature suggested theflashing phenomena occurred. The welding process takesplace both flash action and solid bonding.Fig. 5 presents the microstructure of the bonding

interface for flash butt welding of conventional duplexstainless steel (329J3L). In every sample, clusters ofnumerous ultra fine grains and white bands along thebonding interface are observed.These micro fine grains and white elongated bands are

austenite. The formation mechanism of austenite has notbeen clarified yet.This means that a lot of micro flashing actions took

place during heating, and the flashing is effective for thebonding characteristics shown in Fig. 5(a).Flash butt welding consists of flashing action and

resistance welding. At the first stage, flashing actionoccurred mainly and then the resistance welding becomes

ARTICLE IN PRESS

Fig. 5. Microstructure of bonding interface for flash butt welding of

329J3L conventional duplex stainless steel. (a) Welding time up to 1373K:

10 s, (b) 20 s, (c) 30 s.Fig. 6. Effect of heating time up to 1373K on Charpy impact energy for

flash butt welding of super duplex stainless steel (329J4L).

Fig. 7. Effect of heating time up to 1373K on Charpy impact energy for

flash butt welding of conventional duplex stainless steel (329J3L).

T. Kuroda et al. / Vacuum 80 (2006) 1331–13351334

the important factor. Because the samples were heated atthe contact area and then the thermal expansion occurs, theplastic deformation occurs easily near the contact area asshown in Fig. 5(b). The diffusion bonding will mainlyoperate at the higher temperature of the flash butt weldingprocess. The contact surfaces have been brought to aspecific temperature level by heating, flashing takes placeimmediately after the last short circuit event. During thispart of the process the ends to be welded are slowlybrought together without any perceptible pressure. At theslightest contact of the sample passage of current takesplace which, because of the very low contact pressure,causes an intense heat generation at the contact points asshown in Fig. 5(c). At these points the metal becomesliquid almost instantaneously. This liquid metal forms abridge which conducts the current. The bridge is broken ina very short time by vaporization of the molten metal,which results in ejection of a part of the remainder. Afterthe first explosion of the current bridge, the surface is againbrought into contact by the continuous advance of thesample by thermal expansion, and the cycle repeats at newcontact points.

3.3. Charpy impact energy of flash butt weld

Fig. 6 indicates the effect of heating time up to 1373K onCharpy impact energy for flash butt welding of superduplex stainless steel (329J4L). The energy increaseswith increasing heating time up to the welding temperatureof 1373K. According to the microstructure shown inFigs. 4(a) and (b), joining is carried out mainly as solidstate bonding, and the energy is also 120 J=cm2 and few.However, for heating time of 30 s, because of flash action,the energy is high.

Fig. 7 indicates the effect of time of heating up to 1373Kon Charpy impact energy for flash butt welding ofconventional duplex stainless steel (329J3L). The energyincreases with increasing heating time. According to the

microstructure shown in Fig. 5, joining is carried out withflash action and the energy is high.For the desired kind of flashing, rough contact surfaces

are required since the passage of current must be restrictedto small cross-sectional areas to produce the melting andevaporation of the metal. By continuously changing thelocation of the contact points, the loss of material isequalized over the entire cross-section. The individualexplosions occur in such rapid succession that theimpression of a continuous process is given, which is calledflashing. The rapid sequence of the individual explosionsnot only ceases uniform heating of the contact surfaces butalso causes metal evaporation, serving as a protectivefactor against oxidation. If the flashing is interrupted, thecontact points of the current bridge appear as craters onthe abutting surfaces. Since both of the abutting ends are

ARTICLE IN PRESST. Kuroda et al. / Vacuum 80 (2006) 1331–1335 1335

similarly heated, the effect of the metal vapors on theflashing surfaces should be same.

4. Conclusion

Duplex stainless steels were welded using new flash buttwelding technology of temperature controlling. Flash buttwelding rises two stage process flash action and a contactresistance welding. First stage is a flashing action. Thespecimen produced a flashing or arcing across the interfaceof the two butting ends of the specimens. Fine particles ofmetals near the surface were burned out towards theopposing surface of the specimen irregularity and then themelted particles were deposited on the surface. The secondstage is resistance welding. The solid state bonding wasperformed in the region around the deposited particles.

The cross-sectional microstructure of the weld bondregion was observed using microscopy. The microstructureshowed two types of a deposited fine particles region and asolid state bonding region. The grain growth was hardlyobserved in the weld region and the HAZ. The impactenergy increased with increasing of time of heating up to1373K because of the increasing fine grained depositedmetal.

Acknowledgements

This work was supported by Grant-in-Aid for Coopera-tive Research Project of Nationwide Joint-Use ResearchInstitute on Development Base of Joining Technology forNew Metallic Glasses and Inorganic Materials from theMinistry of Education, Culture. Sports, Science andTechnology, Japan.

References

[1] Nilsson JO. Mater Sci Technol 1992;8:685.

[2] Atamwert S, King JE. Mater Sci Technol 1992;8:896.

[3] Kuroda T, Ikeuchi K, Takahashi M, Kitagawa Y. Proceedings of the

IIW Asian Pacific international congress, October, Singapore.

Singapore: IIW; 2002. p. 151–61.

[4] Kuroda T, Ikeuchi K, Nakade K, Inoue K, Kitagawa Y. Vacuum

2002;65:541–6.

[5] Ando A, Nakata S, Sugimoto T. J. Japan Welding Society 1970;

39(10):1086 [in Japanese].

[6] Ando A, Nakata S, Sugimoto T. J. Japan Welding Society 1971;

40(1):35 [in Japanese].

[7] Ando A, Nakata S, Fukumoto I. J. Japan Welding Society 1971;

40(2):137 [in Japanese].

[8] Barrett JC. Weld J 1945;24:24s.

[9] Kilger H. Weld J 1945;24:413s.