Flash butt resistance welding for duplex stainless steels

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  • Vacuum 80 (2006) 13

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    r 2006 Elsevier Ltd. All rights reserved.

    When joining duplex stainless steels, the microstructure

    extreme low toughness of the HAZ compared to the base

    the HAZ to the undisturbed parent base metal [8,9]. In this

    stainless steel containing 25%Cr7%Ni3%Mo0.2%N(329J4L) and the conventional duplex stainless steelcontaining 22%Cr6%Ni3%Mo0.17%N (329J3L). The

    ARTICLE IN PRESSplates 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

    0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.vacuum.2006.01.068

    Corresponding author. Fax: +816 6879 8689.E-mail address: kuroda@jwri.osaka-u.ac.jp (T. Kuroda).of 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 signicantly, which causes

    study, micro ash phenomena of duplex stainless steelswere investigated using a new ash butt welding apparatus.

    2. Experimental

    The base metals used in this study were the super duplexKeywords: 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.

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

    no ller metal is used, as in arc welding, and in which nocast nugget is formed, as in spot welding [57]. Since theheat in a ash weld is localized between the dies, and thegreatest amount of heat is generated at the face to bewelded by virtue of the ashing action, there are successivemetallurgical changes in the nished ash weld from thehighly heated structure at the center of the weld throughFlash butt resistance weldin

    Toshio Kuroda, Kenj

    Joining and Welding Research Institute of Osaka Univ

    Abstract

    Duplex stainless steels were welded using ash butt resistance

    welding is consisting of two stage processes of ash action and con

    ashing or arcing across the interface of the two butting ends of th

    towards the opposing surface of the specimen irregularity and then

    resistance welding. The solid state bonding was performed in

    microstructure of the weld bond region was observed using mic

    particles region and a solid state bonding region. The grain growt

    The tensile strength and the impact energy increased with increa

    deposited metal.ty, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan

    lding with temperature controlling system. Flash butt resistance

    t resistance. First stage is ashing action. The specimen produced

    ecimens. Fine particles of metals near the surface were burned out

    melted particles were deposited on the surface. The second stage is

    he region around the deposited particles. The cross-sectional

    copy. The microstructure showed two types of a deposited ne

    as hardly observed in the weld region and the heat-affected zone.

    g heating time up to 1373K because of increasing ne grained311335

    for duplex stainless steels

    keuchi, Hyuma Ikeda

    www.elsevier.com/locate/vacuum

  • 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 ashing arc and heating, twosamples are forced together and coalescent occurs at theinterface, ow of current is possible because of the lightcontact between the two part being ash welded. Theheating is generated by the ashing 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 ashed 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 ash

    ARTICLE IN PRESSum 80 (2006) 13311335simulator, then and ash butt welding was done. Thespecimens were heated up to 1373K for 10, 20 and 30 s.Charpy impact test was carried out in the temperature

    range 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 ash butt

    welding using Gleeble simulator. Samples were xed toelectrode. The welding temperature at the butting surfaceof the specimen was set at 1373K by the attached thermocouple.For welding, specimens were mounted, aligned and

    clamped in the dies. The ends of the specimens contactedeach other under the constraint pressure. When the currentwas turned on, heating begun. This heating consists of

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

    T. Kuroda et al. / Vacu1332bringing 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 ashingperiod, the heat generated is intensied by the inadequatecontact between the faces to be welded, which rapidlybrings the ashing surfaces to a high temperature. On bothsides of the ashing 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 thebutt 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 temperaturetime curves during heatingprocess was almost linear and hardly showed such a zig-zagphenomena. Consequently, the zig-zag phenomena sug-gested that ashing action takes place.Fig. 3 shows the temperaturetime curves during ash

    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, resultingFig. 2. Thermal cycles of ash butt welding at 1373k for super duplex

    stainless steel (329J4L).

  • 3.2. Microstructure of bonding interface for flash butt

    welding

    Fig. 4 presents the cross-sectional microstructures nearbonding interface after ash 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 another

    ARTICLE IN PRESSum 80 (2006) 13311335 1333in decrease in temperature. In another contact area, thecurrent began to ow and the resistance heating starts andthe current cease.The ash phenomena is considered to be as followed.

    The heating and ashing in the ash-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 ows is considerably smaller thanthe cross-section of the entire sample. Through the bar,current ows uniformly which converges at the contactpoints to form a high localization of current, which resultsin an intensive generation of heat. At the rst instant, thetemperature rise linearly with time since there is noeffective heat loss. As the rst heating short circuit

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

    duplex stainless steel (329J3L).

    T. Kuroda et al. / Vacucontinues, 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 specic 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.specimen 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 ne grains are observed along the bondinginterface. This is a ash action phenomena. In case of superduplex stainless steel, ashing is considered to be generatedbecause that heating time is slow. As shown in Fig. 2, Therise and drop phenomena of temperature suggested theashing phenomena occurred. The welding process takesplace both ash action and solid bonding.Fig. 5 presents the microstructure of the bonding

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

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

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

    resistance welding. At the rst stage, ashing actionoccurred mainly and then the resistance welding becomes

    Fig. 4. Microstructure of bonding interface for ash butt welding of329J4L super duplex stainless steel. (a) Welding time up to 1373K: 10 s,

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

  • ARTICLE IN PRESS

    Fig. 6. Effect of heating time up to 1373K on Charpy impact energy for

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

    umthe 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 ash butt weldingprocess. The contact surfaces have been brought to aspecic temperature level by heating, ashing 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,

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

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

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

    T. Kuroda et al. / Vacu1334which results in ejection of a part of the remainder. Afterthe rst 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 ash 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 ash action,the energy is high.Fig. 7 indicates the effect of time of heating up to 1373K

    on Charpy impact energy for ash butt welding ofconventional duplex stainless steel (329J3L). The energyincreases with increasing heating time. According to the80 (2006) 13311335microstructure shown in Fig. 5, joining is carried out withash action and the energy is high.For the desired kind of ashing, 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, t...

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