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Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert Toshio Kuroda * , Masahiro Shimada Joining and Welding Research Institute, Osaka University,11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Keywords: Super duplex stainless steel Micro flash butt welding Intermetallic compound Zr metal insert Zr-based metallic glass Fractography Supercooled liquid abstract Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert was carried out. Zr 55 Cu 30 Ni 5 Al 10 of Zr-based metallic glass with thickness of 50 mm and Zr metal with thickness of 500 mm were used as the insert materials. After welding, Zr-based metallic glass insert became much thinner than that of Zr metal insert. The supercooled liquid of Zr-based metallic glass insert at the interface was protruded outside of the specimen during welding. The formation of the protrusion dis- charged the oxide films on the butting surfaces and contact surface, resulting in metallurgical bonding through the fresh surfaces. The Fe–Zr metallic compounds were observed at the bonding interface for the Zr metal insert, but the metallic compound for Zr-based metallic glass insert was hardly observed. The micro flash butt welding of stainless steel with Zr-based metallic glass insert was successfully welded. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Duplex stainless steels are consisting of ferrite–austenitic mi- crostructure and exhibit greater toughness and better weldability than those of ferritic stainless steels [1]. They have higher strength and better corrosion resistance than austenitic stainless steels [2]. Their good engineering performance has led to an increasing number of applications, mainly in corrosive environments such as sour gas pipelines and chemical reaction vessels. As the welds were heated above 1573 K during GTA welding, the phase balance of ferrite and austenite was varied significantly and the toughness was much lower than that of the base metal [3,4]. In order to improve the weldability, micro flash butt welding with insert materials such as Zr metal and Zr-based metallic glass will be needed. Zr metal and Zr-based metallic glasses are superior to corrosion than that of the super duplex stainless steel. The metallic glasses have many potential applications due to their unique properties, such as superior strength and excellent corrosion resistance. Zr–Cu–Ni–Al metallic glass has high glass formability and wide supercooled liquid region [5]. But if the heating rate and cooling rate during heat treatment were slow, metallic glasses were crystallized and the superior properties were disappeared. There are only a few reports on the joining of metallic glasses by laser welding [6] and the friction bonding [7]. However, there is no report by the micro flash butt welding, because of the difficulty of the temperature controlling at the butting surface. In this study, the joining of the super duplex stainless steel with Zr metal insert and Zr-based metallic glass insert was carried out by the micro flash butt welding with the temperature controlling system. 2. Experimental procedure The base metal was a super duplex stainless steel (329J4L) with chemical composition (mass%) of 25%Cr–7%Ni–3%Mo–0.2%N. The specimens were cut out from the base material plate with 12 mm thickness along rolling direction. The specimens were mounted in the dies using a Gleeble thermal simulator and the flash butt welding was made. Zr 55 Cu 30 Ni 5 Al 10 of 50 mm in thickness as Zr- based metallic glass and Zr metal of 500 mm in thickness were used as the insert materials. The glass transition temperature (T g ) and crystallization temperature (T x ) of Zr 55 Cu 30 Ni 5 Al 10 are 683 K and 767 K, respectively [8]. Fig. 1 indicates a schematic illustration of the flash butt welding using a Gleeble simulator. The specimens were fixed. The weld thermal cycles at the butting surface of the specimen was con- trolled at 1373 K by the attached thermocouple. The specimens were heated up to 1373 K for 30 s under the pressure of 10 MPa. For welding, specimens were mounted, aligned and clamped in the dies. The ends of the insert materials and stainless steel con- tacted each other under the constraint pressure. When the current was turned on, heating begun. This heating consists of bringing the ends of the materials together and separating them several times in succession, each time causing a short circuit. The insert materials were heated during this passage of current, particularly at the butting surfaces. As the current passed through the specimen, * Corresponding author. Fax: þ81 06 6879 8694. E-mail address: [email protected] (T. Kuroda). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.089 Vacuum 83 (2009) 153–156

Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert

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Vacuum 83 (2009) 153–156

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Vacuum

journal homepage: www.elsevier .com/locate/vacuum

Micro flash butt welding of super duplex stainless steel with Zr-basedmetallic glass insert

Toshio Kuroda*, Masahiro ShimadaJoining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Keywords:Super duplex stainless steelMicro flash butt weldingIntermetallic compoundZr metal insertZr-based metallic glassFractographySupercooled liquid

* Corresponding author. Fax: þ81 06 6879 8694.E-mail address: [email protected] (T. Kuro

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.03.089

a b s t r a c t

Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert was carriedout. Zr55Cu30Ni5Al10 of Zr-based metallic glass with thickness of 50 mm and Zr metal with thickness of500 mm were used as the insert materials. After welding, Zr-based metallic glass insert became muchthinner than that of Zr metal insert. The supercooled liquid of Zr-based metallic glass insert at theinterface was protruded outside of the specimen during welding. The formation of the protrusion dis-charged the oxide films on the butting surfaces and contact surface, resulting in metallurgical bondingthrough the fresh surfaces. The Fe–Zr metallic compounds were observed at the bonding interface for theZr metal insert, but the metallic compound for Zr-based metallic glass insert was hardly observed. Themicro flash butt welding of stainless steel with Zr-based metallic glass insert was successfully welded.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Duplex stainless steels are consisting of ferrite–austenitic mi-crostructure and exhibit greater toughness and better weldabilitythan those of ferritic stainless steels [1]. They have higher strengthand better corrosion resistance than austenitic stainless steels [2].Their good engineering performance has led to an increasingnumber of applications, mainly in corrosive environments such assour gas pipelines and chemical reaction vessels. As the welds wereheated above 1573 K during GTA welding, the phase balance offerrite and austenite was varied significantly and the toughness wasmuch lower than that of the base metal [3,4].

In order to improve the weldability, micro flash butt weldingwith insert materials such as Zr metal and Zr-based metallic glasswill be needed. Zr metal and Zr-based metallic glasses are superiorto corrosion than that of the super duplex stainless steel.

The metallic glasses have many potential applications due totheir unique properties, such as superior strength and excellentcorrosion resistance. Zr–Cu–Ni–Al metallic glass has high glassformability and wide supercooled liquid region [5]. But if theheating rate and cooling rate during heat treatment were slow,metallic glasses were crystallized and the superior properties weredisappeared. There are only a few reports on the joining of metallicglasses by laser welding [6] and the friction bonding [7]. However,there is no report by the micro flash butt welding, because of thedifficulty of the temperature controlling at the butting surface.

da).

All rights reserved.

In this study, the joining of the super duplex stainless steel withZr metal insert and Zr-based metallic glass insert was carried out bythe micro flash butt welding with the temperature controllingsystem.

2. Experimental procedure

The base metal was a super duplex stainless steel (329J4L) withchemical composition (mass%) of 25%Cr–7%Ni–3%Mo–0.2%N. Thespecimens were cut out from the base material plate with 12 mmthickness along rolling direction. The specimens were mounted inthe dies using a Gleeble thermal simulator and the flash buttwelding was made. Zr55Cu30Ni5Al10 of 50 mm in thickness as Zr-based metallic glass and Zr metal of 500 mm in thickness were usedas the insert materials. The glass transition temperature (Tg) andcrystallization temperature (Tx) of Zr55Cu30Ni5Al10 are 683 K and767 K, respectively [8].

Fig. 1 indicates a schematic illustration of the flash butt weldingusing a Gleeble simulator. The specimens were fixed. The weldthermal cycles at the butting surface of the specimen was con-trolled at 1373 K by the attached thermocouple. The specimenswere heated up to 1373 K for 30 s under the pressure of 10 MPa.

For welding, specimens were mounted, aligned and clamped inthe dies. The ends of the insert materials and stainless steel con-tacted each other under the constraint pressure. When the currentwas turned on, heating begun. This heating consists of bringing theends of the materials together and separating them several times insuccession, each time causing a short circuit. The insert materialswere heated during this passage of current, particularly at thebutting surfaces. As the current passed through the specimen,

Page 2: Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert

Fig. 1. Schematic illustration of resistance welding apparatus.

Fig. 3. Optical microstructures of the bonding interface with Zr metal of 0.5 mm inthickness insert.

T. Kuroda, M. Shimada / Vacuum 83 (2009) 153–156154

intense localized heating occurred between the contact faces.During the flashing period, the heat generated was intensified bythe inadequate contact between the faces to be welded, whichrapidly brought the flashing surfaces to a high temperature. Onboth sides of the flashing surfaces the temperature falls rapidly off,resulting in a narrow heated zone.

The welded specimens were examined by an optical microscopy,a scanning electron microscopy (SEM), and an energy dispersiveX-ray spectroscopy (EDX). Intermetallic compounds were identifiedby X-ray diffraction technique using Cu radiation. Charpy impacttest was carried out at room temperature after welding. The frac-ture surfaces were observed using scanning electron microscopy.

3. Results and discussion

Fig. 2 shows the temperature–time curve during the flash buttwelding for super duplex stainless steel (329J4L) with Zr-basedmetallic glass insert. The temperature curve indicates the fluctua-tion, and extremely fluctuated at 683 K, 1100 K and 1373 K [9],because of the superplasticity at the supercooled liquid state, the

Fig. 2. Thermal cycle of resistance welding at 1373 K for super duplex stainless steelwith Zr-based metallic glass insert.

liquid state of Zr-based metallic glasses at the melting point, andthe liquid state by eutectic of Ferrous and Zr metal, respectively.

As the samples without joining was heated, the temperature–time curves during the heating process were almost linear andhardly showed fluctuation such as a zigzag phenomena [6]. Con-sequently, the fluctuation suggests that flashing action took place[6], and no electrical interference occurred.

On the basis of the continuous cooling transformation (CCT)curves from a supercooled liquid to the amorphous or crystallinephase for Zr55Cu30Ni5Al10 and CCT curve of Zr55Al10Ni10Cu25 [10,11],the cooling rate shown in Fig. 2 is faster and the cooling time wasshorter than the nose-point of CCT curve of Zr55Al10Ni10Cu25.Consequently, it is considered that the Zr-based metallic glass wasalmost amorphous after welding.

Fig. 3 shows the cross-sectional microstructure of the bondinginterface for the specimen with Zr metal insert after flash buttwelding. Elongated white region in the super duplex steel showsaustenite phase, and gray region in the super duplex steel showsferrite phase. Ferrite grain growth of the steel is hardly observed[3]. The inserted Zr metal indicated Widmanstatten alpha micro-structure, and the point analysis at mark A indicated only Zr peak asshown in Fig. 4A.

The intermetallic compound of 2 mm in thickness is observed asa narrow layer of the dark contrast with uniform thickness betweenthe super duplex stainless steel and the Zr metal as shown in Fig. 3.According to the X-ray diffraction results, ZrCr2 and Zr–Fe of theintermetallic compounds were detected as shown in Fig. 4B. Theresult is similar to a report of the hot pressure bonding result be-tween AISI 304L and Zircaloy-2.

The Vickers hardness was measured. As per the results, thehardness of the duplex stainless steel was Hv350, the hardness ofthe Zr insert metal was Hv200, and the hardness of the in-termetallic compounds was Hv500. The fracture is expected tooccur at the intermetallic compounds for Charpy impact test. WhenZr metal was heated above 1143 K, alpha phase (alpha-Zr) wastransformed to beta phase (beta-Zr), and then beta phase (beta-Zr)was transformed to alpha-Zr phase by rapid cooling (Fig. 3).

Page 3: Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert

Fig. 4. EDX analysis of Zr metal of 0.5 mm in thickness insert after micro flash welding.

Fig. 5. SEM microstructure of the bonding interface with Zr-based metallic glass insert.

Fig. 6. EDX analysis of Zr-based metallic glass insert after micro flash welding.

T. Kuroda, M. Shimada / Vacuum 83 (2009) 153–156 155

Fig. 5 indicates the cross-sectional microstructure at the bond-ing interface after flash butt welding for 329J4L with Zr-basedmetallic glass insert by SEM. The thickness became much thinnerthan that of Zr metal insert. The thickness of Zr-based metallic glasschanged from 50 mm to 0.5 mm after welding. The Fe–Zr metalliccompound was hardly observed. The welding of ferrite phase andZr-based metallic glass and austenite phase and Zr-based metallicglass for the super duplex stainless steel was completed. The in-termetallic compound was hardly observed.

The melting point of Zr-based metallic glass is around 1173 K[12,13]. During welding at 1373 K the liquid phase of the Zr-basedmetallic glass would be extruded outside the specimen and thewelding was done. The thickness of the Zr-based metallic glassinsert extremely decreased from 50 mm to 0.5 mm during welding.This means that the Zr-based metallic glass became solid phase toliquid phase during heating. The supercooled liquid in the interfaceprotruded outside of the specimen [14]. The formation of the pro-trusion discharged the oxide films on the butting surfaces andcontact surface, resulting in metallurgical bonding through thefresh surfaces.

Fig. 6 indicates the EDX analysis of the Zr-based metallic glass atthe bonding interface shown in Fig. 5. The peaks of Zr, Cu, Ni and Alare recognized. The small peaks of Fe, Cr and Mo in the stainlesssteel are slightly recognized, because the analytical beam diameteris generally 1–2 mm. The Zr-based metallic glass insert will

Fig. 7. Fracture morphology after Charpy impact test for micro flash welded specimenof super duplex stainless steel with Zr metal insert.

Page 4: Micro flash butt welding of super duplex stainless steel with Zr-based metallic glass insert

Fig. 8. X-ray diffraction pattern of the fracture surface for micro flash welding of superduplex stainless steel with Zr metal insert.

Fig. 9. Fracture morphology after Charpy impact test for micro flash welded specimenof super duplex stainless steel with Zr-based metallic glass insert.

T. Kuroda, M. Shimada / Vacuum 83 (2009) 153–156156

maintain amorphous state after welding. As per the result, the flashbutt welding with metallic glass insert was successfully welded forduplex stainless steel.

Fig. 7 indicates the fracture surface after Charpy impact test forthe specimen of micro flash butt welding with Zr metal insert. Theimpact value was 10 J. The fracture surface indicated no dimplepattern. This means that the brittle fracture occurred inside theintermetallic compounds of ZrCr2 and ZrFe shown in Fig. 3.

Fig. 8 shows X-ray diffraction pattern of the fracture surface forthe super duplex stainless steel with Zr metal insert. ZrCr2 in-termetallic compound were observed. Consequently, it is con-cluded that the fracture mainly occurred inside the ZrCr2

intermetallic compounds.Fig. 9 indicates the fracture surface of the specimen of micro

flash butt welding with Zr-based metallic glass insert. The Charpyimpact value indicated small value of 50 J. The fracture surface in-dicates the Bain pattern showing the fracture at amorphous region.The fracture morphology is different from that of the intermetalliccompounds. Partially the fracture in the crystallized particle regionis observed. This is considered to have changed from amorphousstate to partially recrystallization during the fracture test.

4. Conclusion

Micro flash butt welding with temperature controlling systemwas carried out for super duplex stainless steel with Zr-basedmetallic glass insert.

(1) When the super duplex stainless steel with Zr-based metallicinsert was welded, the temperature revealed fluctuation dur-ing welding. The phenomena indicated that micro flashing atthe butting surface occurred during welding.

(2) For the flash butt welding of the super duplex stainless steelwith Zr metal insert, the intermetallic compound such as ZrCr2

was observed as a narrow layer with uniform thickness be-tween the super duplex stainless steel and the Zr metal. Thefracture occurred inside the intermetallic compounds after theimpact test.

(3) For the super duplex stainless steel with Zr-based metallic glassinsert, the thickness of the Zr-based metallic glass extremelydecreased from 50 mm to 0.5 mm after flash butt welding. TheZr-based metallic glass became liquid phase during heating,and the supercooled liquid in the interface protruded outside ofthe specimen. The formation of the protrusion discharged

oxide films on the butting surfaces and contact surface,resulting in metallurgical bonding through the fresh surfaces.

(4) The joining of ferrite phase in super duplex stainless steel andZr-based metallic glass, and the joining of austenite phase insuper duplex stainless steel and Zr-based metallic glass weresuccessfully accomplished.

Acknowledgements

This work was supported by Grant-in-Aid for Cooperative Re-search Project of Nationwide Joint-Use Research Institute on De-velopment Base of Joining Technology for New Metallic Glasses andInorganic Materials from The Ministry of Education, Culture, Sports,Science and Technology, Japan.

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