Flash butt resistance welding for duplex stainless steels

Flash butt resistance welding for duplex stainless steels

ARTICLE IN PRESS Vacuum 80 (2006) 1331–1335 www.elsevier.com/locate/vacuum Flash butt resistance welding for duplex stainless steels Toshio Kuroda,...

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ARTICLE IN PRESS

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 1373 K 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-austenitic microstructure at room temperature and exhibit greater toughness and better weldability than ferritic stainless steel [1]. They have higher strength and better corrosion resistance than austenitic stainless steel [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. When joining duplex stainless steels, the microstructure of the heat-affected zone (HAZ) is determined only by applied thermal cycles and is very sensitive to welding conditions [3]. The HAZ generally develops from a plate structure which is originally rolled and annealed, and in many duplex stainless steels consists of an approximately 50:50 phase distribution of austenite within a ferritic matrix [3]. Especially in the bond region, the phase balance of ferrite and austenite is varied significantly, which causes extreme low toughness of the HAZ compared to the base

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E-mail address: [email protected] (T. Kuroda). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.068

metal [3]. Therefore, controlling microstructure of the bond region 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 no cast nugget is formed, as in spot welding [5–7]. Since the heat in a flash weld is localized between the dies, and the greatest amount of heat is generated at the face to be welded by virtue of the flashing action, there are successive metallurgical changes in the finished flash weld from the highly heated structure at the center of the weld through the HAZ to the undisturbed parent base metal [8,9]. In this study, micro flash phenomena of duplex stainless steels were investigated using a new flash butt welding apparatus.

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

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

simulator, then and flash butt welding was done. The specimens were heated up to 1373 K for 10, 20 and 30 s. Charpy impact test was carried out in the temperature range from 77 to 373 K. The fracture surfaces of the specimens after Charpy impact test were examined using a scanning electron microscope (SEM). For the SEM observation, stereoscopic photographs were taken to reconstruct three-dimensional topography of the fracture surfaces using the image processing technique described previously [4]. Fig. 1 indicates schematic illustration of a new flash butt welding using Gleeble simulator. Samples were fixed to electrode. The welding temperature at the butting surface of the specimen was set at 1373 K by the attached thermo couple. For welding, specimens were mounted, aligned and clamped in the dies. The ends of the specimens contacted each other under the constraint pressure. When the current was turned on, heating begun. This heating consists of bringing the end of the specimens together and separating them several times in succession, each time causing a short circuit. The specimens were heated during this passage of current, particularly at the butting surfaces. As the current passes through the specimen, and intense localized heating occurs between the contact faces. During the flashing period, the heat generated is intensified by the inadequate contact between the faces to be welded, which rapidly brings the flashing surfaces to a high temperature. On both sides of the flashing surfaces the temperature falls rapidly off, resulting in a narrow heated zone.

application of pressure after heating is substantially completed. Flashing and upsetting are accompanied by expulsion of metal from the joint. During the welding operation there is an intense flashing arc and heating, two samples are forced together and coalescent occurs at the interface, flow of current is possible because of the light contact between the two part being flash welded. The heating is generated by the flashing and localized in the area between the two parts. The surfaces are brought to the melting point and expelled through the abutting area. As soon as the material is flashed away another small arc is formed which continues until the entire abutting surfaces reaches the melting temperature. Pressure is then applied and arcs are extinguished and upset occurs. Fig. 2 indicates the temperature time curves during flash butt welding for super duplex stainless steel (329J4L). The heating up to 1373 K was carried out for 10, 20, and 30 s, in order to change the contact area condition of the abutting samples. In case of heating time of 10 s, the temperature at the weld joining increased with increasing heating time. The rise and drop of the temperature, so called zig-zag phenomena in the linear curve up to 800 K was fairly observed. In case of heating time for 20 and 30 s, the temperature indicates the zig-zag phenomena, which was arise and drop of the temperature during heating process. The machine used in the present investigation was Gleeble 1500 thermal cycle apparatus, which was controlled by the computer-aided system. As the samples without joining were heated, the temperature–time curves during heating process was almost linear and hardly showed such a zig-zag phenomena. Consequently, the zig-zag phenomena suggested 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 1373 K, the temperature reveals rise and drop phenomena. In case of the heating time of 30 s, the temperature drop amount is large. This means that the short circuit of the electric current breaks and the resistance heating ceases, resulting

3. Results and discussions 3.1. Micro flashing phenomena during heating process Flash welding is a resistance welding process which simultaneously produces coalescence over the entire area abutting surfaces, by the heat obtained from resistance to electric current between the two surfaces, and by the

Fig. 2. Thermal cycles of flash butt welding at 1373 k for super duplex stainless steel (329J4L).

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3.2. Microstructure of bonding interface for flash butt welding

Fig. 3. Thermal cycles of flash butt welding at 1373 k for conventional duplex stainless steel (329J3L).

in decrease in temperature. In another contact area, the current began to flow and the resistance heating starts and the current cease. The flash phenomena is considered to be as followed. The heating and flashing in the flash-welding process are closely related to short circuit heating. A small portion of the contact surfaces come together during the heating. Therefore, the actual contact area of the surfaces through which the electric current flows is considerably smaller than the cross-section of the entire sample. Through the bar, current flows uniformly which converges at the contact points to form a high localization of current, which results in an intensive generation of heat. At the first instant, the temperature rise linearly with time since there is no effective heat loss. As the first heating short circuit continues, the local increase in temperature is dissipated by absorption of the heat in the comparatively large mass of material at the butting surfaces, which is still cold. However, when the heating continues far enough softening of the contact points takes place. This causes an increase in the actual contact surface, due to the thermal expansion, thus producing better current condition. The conductivity and specific heat, which vary with temperature, produce an accelerating increase in electrical resistance in those section which are most highly heated. The temperature distribution in the specimen and the way these temperatures increase as welding proceeded and during heating. Therefore, heating is generated at the contact points corresponding to the high current density. After the contact surfaces are separated, the circuit is broken and heat generation ceases. The temperature of the contact surfaces drops because of the heat absorption by the body of the specimen. During the current-off period, temperature equalization tends to take place, and by repeating these short circuit events, the ends of the specimen are reheated. During heating, the actual contact surfaces become gradually larger.

Fig. 4 presents the cross-sectional microstructures near bonding interface after flash butt welding. In case of 10 s heating up to welding temperature of 1373 K, as shown in Fig. 4(a), black line near the bond line is observed, and it means that the bonding is not perfect. However, some of austenite grains were bonded each other. The bonding of ferrite to ferrite is barely observed. According to the microstructure appearance, the bonding seems to be proceeded by the solid state bonding mechanism. In case of 20 s heating up to 1373 K as shown in Fig. 4(b), austenite seen as white elongated phase proceeded to another specimen and bonding is perfect. However, joining of ferrite to ferrite, and joining of ferrite to austenite are hardly completed perfectly. Solid state bonding occurred mainly for the short heating time to the welding temperature. As shown in Fig. 4(c), clusters consisting of numerous fine grains are observed along the bonding interface. This is a flash action phenomena. In case of super duplex stainless steel, flashing is considered to be generated because that heating time is slow. As shown in Fig. 2, The rise and drop phenomena of temperature suggested the flashing phenomena occurred. The welding process takes place both flash action and solid bonding. Fig. 5 presents the microstructure of the bonding interface for flash butt welding of conventional duplex stainless steel (329J3L). In every sample, clusters of numerous ultra fine grains and white bands along the bonding interface are observed. These micro fine grains and white elongated bands are austenite. The formation mechanism of austenite has not been clarified yet. This means that a lot of micro flashing actions took place during heating, and the flashing is effective for the bonding characteristics shown in Fig. 5(a). Flash butt welding consists of flashing action and resistance welding. At the first stage, flashing action occurred mainly and then the resistance welding becomes

Fig. 4. Microstructure of bonding interface for flash butt welding of 329J4L super duplex stainless steel. (a) Welding time up to 1373 K: 10 s, (b) 20 s, (c) 30 s.

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Fig. 5. Microstructure of bonding interface for flash butt welding of 329J3L conventional duplex stainless steel. (a) Welding time up to 1373 K: 10 s, (b) 20 s, (c) 30 s.

the important factor. Because the samples were heated at the contact area and then the thermal expansion occurs, the plastic deformation occurs easily near the contact area as shown in Fig. 5(b). The diffusion bonding will mainly operate at the higher temperature of the flash butt welding process. The contact surfaces have been brought to a specific temperature level by heating, flashing takes place immediately after the last short circuit event. During this part of the process the ends to be welded are slowly brought together without any perceptible pressure. At the slightest contact of the sample passage of current takes place which, because of the very low contact pressure, causes an intense heat generation at the contact points as shown in Fig. 5(c). At these points the metal becomes liquid almost instantaneously. This liquid metal forms a bridge which conducts the current. The bridge is broken in a very short time by vaporization of the molten metal, which results in ejection of a part of the remainder. After the first explosion of the current bridge, the surface is again brought into contact by the continuous advance of the sample by thermal expansion, and the cycle repeats at new contact points. 3.3. Charpy impact energy of flash butt weld Fig. 6 indicates the effect of heating time up to 1373 K on Charpy impact energy for flash butt welding of super duplex stainless steel (329J4L). The energy increases with increasing heating time up to the welding temperature of 1373 K. According to the microstructure shown in Figs. 4(a) and (b), joining is carried out mainly as solid state 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 1373 K on Charpy impact energy for flash butt welding of conventional duplex stainless steel (329J3L). The energy increases with increasing heating time. According to the

Fig. 6. Effect of heating time up to 1373 K on Charpy impact energy for flash butt welding of super duplex stainless steel (329J4L).

Fig. 7. Effect of heating time up to 1373 K on Charpy impact energy for flash butt welding of conventional duplex stainless steel (329J3L).

microstructure shown in Fig. 5, joining is carried out with flash action and the energy is high. For the desired kind of flashing, rough contact surfaces are required since the passage of current must be restricted to small cross-sectional areas to produce the melting and evaporation of the metal. By continuously changing the location of the contact points, the loss of material is equalized over the entire cross-section. The individual explosions occur in such rapid succession that the impression of a continuous process is given, which is called flashing. The rapid sequence of the individual explosions not only ceases uniform heating of the contact surfaces but also causes metal evaporation, serving as a protective factor against oxidation. If the flashing is interrupted, the contact points of the current bridge appear as craters on the abutting surfaces. Since both of the abutting ends are

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similarly heated, the effect of the metal vapors on the flashing surfaces should be same. 4. Conclusion Duplex stainless steels were welded using new flash butt welding technology of temperature controlling. Flash butt welding rises two stage process flash action and a contact resistance welding. First stage is a flashing action. The specimen produced a 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 HAZ. The impact energy increased with increasing of time of heating up to 1373 K because of the increasing fine grained deposited metal.

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Acknowledgements This work was supported by Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institute on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from the Ministry of Education, Culture. Sports, Science and Technology, 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.