Apparatus (30) for laying pipelines in which adjacent pipe (17, 28) are joined by the technique of magnetically impelled arc butt welding (MIAB). A magnetically impelled arc butt (MIAB) welding device heats facing ends of a pair of wellbore tubulars and a force application device compressively engages the facing ends to form a welded joint. The device may be positioned on the rig floor. The MIAB welding device may include a device configured to hold facing ends of the two wellbore tubulars at a specified distance away from one another.
Also found in: Dictionary, Thesaurus, Medical, Wikipedia. Gastroscopy
examination of the gastric cavity by means of a special instrument, the gastroscope, which is inserted into the stomach through the mouth and the esophagus; a form of endoscopy. The gastroscope is a flexible tube with an optic system of numerous short-focus lenses and an electric bulb inside it. The gastroscope can easily and safely be inserted into the stomach. It is used when a stomach tumor or an ulcer is suspected and, in gastritis, for a detailed examination of the mucous membrane.
REFERENCES
Mnogotomnoe rukovodstvo po vnutrennim bolezniam, vol. 4. Moscow, 1965. Pages 36–39.Want to thank TFD for its existence? Tell a friend about us, add a link to this page, or visit the webmaster's page for free fun content.
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CHARACTERISTICS OF MIAB welding process and joints
CHARACTERISTICS OF MIAB welding process and joints
ABSTRACT
D. Iordachescu, B. Georgescu, M. Iordachescu, R. Lopez, R.M. Miranda and A. García-Beltrán
The paper presents the MIAB welding of thin walled tubes achieved with an original longitudinal magnetization system, designed to assure the magnetic flux concentration on the tube wall. Based on images resulting from process monitoring, the main stages of the process are presented, starting with arc initiation, to apparition of molten metal, upsetting and ending with the weld achievement. Infrared thermography was also used for temperature measurements and visualization of the process. Macro and microstructural analyses accompany the hardness tests made on correspondingly welded samples, demonstrating the process capability in producing sound joints when guided by the operational windows developed through this investigation. IIW-Thesaurus keywords: Hardness; MIAB welding; Microstructure.
1 Introduction Tubes provide light weight, excellent rigidity and low cost for a structure. This is why, nowadays, tubes continue to become more and more important in the automotive and other industries. In this view, MIAB welding is a unique thermomechanical process for joining closed structural tubular shapes. Present industrial applications of this welding process are fully integrated to flexible automated lines [1]. This combined arc and pressure welding technique, known also as MAGNETARC Welding [2], was referred to in previous works as ROTARC Welding by Georgescu and Iordachescu D. [3], since early ‘80s. The process relies on complex interactions between an electric arc and both an applied and an induced magnetic field; it becomes more complex due to the changes that occur during the heating of the parts/tubes which are finally welded by axial upsetting [4]. The joint interface consists of a solid-state bond region, the previously molten material being squeezed outward the tubes contact zone into a flash [5]. Although some reports are of interest for MIAB development and the industrial application of the process involved in-depth knowledge, some limits on approaching MIAB welding still exist, proving less understanding of the whole process and of the relationship between its parameters [6-8]. This investigation used an original longitudinal magnetization system that helps emphasizing the interaction between the arc and the induced magnetic field in the case of MIAB welding of low carbon steel thin-walled tubes. This paper presents an overall approach of the
process, trying to contribute to its better understanding. Based on images resulting from process monitoring, the main stages of the process are presented, starting with arc initiation, to apparition of molten metal, upsetting and ending with the weld achievement. Infrared thermography was also used for temperature measurements and visualization of the process. Macro and microstructural analyses accompany the hardness tests made on correspondingly welded samples, demonstrating the process capability in producing sound joints when guided by the operational windows developed through this investigation.
2 Principle and fundamentals of MIAB welding
Considering the reports of Taneko et al. [9], on arc rotation velocity changes during MIAB welding, and of Iordachescu D. et al. [4], on MIAB equipment peculiarities and joint characteristics, the MIAB process main stages are illustrated in Figure 1. Accordingly, a succession of six regions of parametric changes characterizes the process achievement. At first stage, I - arc initiation, the tubes 1, 2 are coaxially fixed in clamps 6, 7, with their abutting surfaces in contact [Figures 1 a), b)]; a DC arc welding source is connected to tubes and a magnetization field is applied through the solenoids 3. The arc initiation occurs mainly on the inner diameter of the tubes due to their withdrawal at the gap g. During the second stage, II, the beginning of the arc rotation is due to the interaction between the welding current, Iw, and the radial magnetic field, B, induced into the
Doc. IIW-2039-09, recommended for publication by Commission III “Resistance Welding, Solid State Welding and Allied Joining Processes.”
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a) Arc velocity vs. time diagram Process phases, I – VI: I – arc initiation (flash); II – beginning of arc rotation; III – arc transitory rotation; IV – arc stable rotation; V – arc instable rotation (“breaking”); VI – tubes upsetting; v1 – arc rotation of low velocity; v2 – stable arc rotation velocity.
b) MIAB welding process sketch g – tubes gap; m – distance between the solenoids; Fc – clamping force; B – magnetic field; Isc – short circuit current; Fp – upsetting force; 1, 2 – thin-walled tubes; 3 – magnetization system/solenoids; 4 – rotating arc.
Figure 1 – MIAB welding principle
26
arc gap; the electric arc moves at low arc rotating velocity from the tubes inner diameter to their outer surface. During this phase, the arc quenching may occur if inadequate or insufficient magnetization is applied. Once the arc surpasses the critical velocity, v1, [Figure 1 a)], then starts the third process phase, III, the arc transitory rotation. This phase is characterized by an abrupt increase of arc velocity. After that, the process stabilizes around v2 arc velocity [Figure 1 a)]; in this phase the visual perception of the electric arc is of a continuous ring of fire, due to its high rotating velocity. A thin uniform layer of molten metal on tubes abutting surfaces forms in this arc stable rotation, IV, phase; it ends when the molten metal bridges form into the tubes gap. Thus, the arc instable rotation/breaking phase, V, initiates, and the molten metal waving is produced due to the abrupt fluctuations of the arc velocity [Figure 1 a)]. The combined action of waving and arc rotating tendency is to break and expel the previously formed bridges of molten metal. The tubes upsetting phase, VI, with the force Fp, ends the MIAB welding process. Formation of molten metal bridges indicates the right moment of upsetting. By upsetting, the molten metal flows, as well as the plasticized material from the tubes contact area, producing a transversal burr of barrel shape and the weld seam. A qualitative joint formation results after tubes forging; the dynamic effect of the mechanical action also assures the weld strength. Consequently, the clamping forces, Fc, have to prevent the bars axial sliding during upsetting. The process achievement depends also on the surface contamination of the tubes abutting surfaces with rust, scale or any type of grease. Therefore, to obtain a defectfree weld, normally a simple saw or machined cut is a sufficient preparation of the abutting surfaces.
3 Original magnetization system and experimental results
Knowing that MIAB welding is a process sensitive on tubes local magnetization, the paper presents first the performances of an original longitudinal magnetizing system; its design is intended to ensure the magnetic flux concentration on tubes surface. Different welding experiments were made using low carbon steel tubes of 25.4 mm outer diameter (OD), 3 mm thickness, grade ST 37 tube (DIN 1626-84/ASTM A 536).
3.1 Magnetization system with peripheral solenoids The classical longitudinal magnetization solution of MIAB welding uses a single solenoid coil concentric with the tube. The new system aims at concentrating the magnetic flux distribution into the tube wall, improving the arc rotation stability. Figure 2 a) draws the principle of the developed system made of two independent half-shells. The magnetization system for each tube is made of 8 solenoids [4 coils on each half-shell, Figure 2 b)], which are connected in series, and are working positioned in parallel with the tube longitudinal axis. The use of two half-shells is of high practical importance, allowing the easy positioning and removal of the magnetization systems on/from tubes, irrespective of their length. The image of the MIAB welding equipment used during experiments is presented in Figure 2 b); a hydraulic system ensures the tubes safe clamping and their longitudinal movement during initial and upsetting phases. A KEMPPI Pro5200 power source provides the necessary DC welding current.
CHARACTERISTICS OF MIAB welding process and joints
a) Original longitudinal magnetization system with multiple solenoids mounted in two independent shells
b) MIAB welding equipment
B – magnetic field; 1 – tube peripheral solenoid; 2 – air gap; 3 – thin walled tube; 4, 5 – half-shells with peripheral solenoids; 6 – rotating arc; 7 – clamping system.
Figure 2 – MIAB welding
3.2 MIAB welding – process parameters and operational windows In MIAB welding the gap between pipe edges, g, has a major influence on process stability, as has the arc length in conventional fusion welding processes. Moreover, in this case the magnetic field value in the tubes gap also influences the arc rotation, but its magnitude depends on the distance between the magnetization shells. During experiments, the magnetization current domain was Im = 0.1A-1.0 A, whereas for the welding current it was Iw = 100 A–400 A. Qualitative joints were sought to be obtained after the final upsetting phase. Thus, a set of specific operational windows for MIAB welding was developed for achieving sound joints when using the peripheral solenoids magnetization system. Figure 3 a) shows the dependency of the magnetic field induction on the magnetization current at different gap
a) Magnetic field vs. magnetization current at different gap values B – magnetic field, [T]; Im – magnetization current, [A]; g – gap between the tubes, [mm].
size values; the distance between the magnetization shells, m, was kept constant (m = 10 mm). The magnetic field increases with the magnetization current increase, and the highest values of B were recorded at g = 0.5 mm, found as being the minimum gap size (arc length) that assures the arc stable rotation in this particular case of 3 mm tube wall thickness. The gap size increase over the maximum value of 1.5 mm may cause the decrease of B value until the arc stops. Consequently, the stability of the welding process depends on gap size, the optimum values pertaining to the interval g = (0.5-1.5) mm. Figure 3 b) illustrates the welding process stability diagram as depending on welding current intensity and tubes gap value, the most sensitive process parameters. The diagram is valid for a constant distance between the magnetization shells m = 10 mm. According to the welding tests results, three different regions characterize the process stability.
b) Welding stability diagram – welding current intensity, Iw [A], vs. gap value C – area without arc movement or with unstable rotation; A – area of appropriate arc initiation and stable rotation; D – area of short lifetime arc, with unstable rotation or without movement.
Figure 3 – Magnetic field diagrams
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Region A characterizes the process stability – the arc forms and stably rotates between the tubes ends. In region C the arc does not move for low current values, whereas for higher currents it rotates very unstably – although highenough values of welding currents and magnetic field are provided, the gap/arc length is too small to assure a sufficient value of the magnetic force. In region D the arc initiates but has short lifetime; it is not starting to rotate (it quenches immediately after initiation), or its rotation is unstable, due to a big gap value and/or to a too small current value. If the tubes upsetting occurs after an unstable welding process (regions C or D), the joint does not form or the weld has inadequate strength.
3.3 Monitoring of the MIAB process MIAB welding lifetime was studied together with arc rotation velocity. The processing parameters were Iw = 250 A, B = 0.08 T, and Fp = 12 kN. Figure 4 a) shows the arc velocity variation during the process lifetime. Representative images captured with a special featured HRDC camera at low (10 fps) and higher shooting speed (100 fps) are presented in Figure 4 b), and Figure 4 c), respectively. Image no. 1 corresponds to the process first phase, I, arc initiation (see Figure 1); its red colour indicates the arc flash formation. 28
Image no. 2 is an intermediate arc capture taken during the process second phase, II; its duration is of about 0.5 s. In this phase the arc rotating velocity increases up to the critical velocity, v1 = 10 m/s (image no. 3), where the arc transitory phase, III, starts. Low spattering also characterizes this arc rotating phase. The arc velocity continues to increase up to the second critical velocity, v2 = 50 m/s (image no. 4). Starting from
this point, corresponding to the process phase IV, the arc velocity relative stable rotation was found (images no. 5–6); a continuous arc covers the tubes abutting surface, heating and superficially melting their surfaces. After 2.7 s from the arc initiation, starts the process phase V, characterized by arc instable rotation/breaking. This is due to the molten metal bridges previously formed during the arc stable rotation. Image no. 7 characterizes this stage of high spattering and unstable arc velocity. The last process phase, VI, starts after 4 s from the process beginning and the tubes upsetting produces the quenching of the already instable electric arc. Maintaining for about 1.5 s the upsetting (image no. 8) assures the achievement of a qualitative joint. Finally, to characterize a qualitative joint achievement, the tubes deformation, δ, was chosen as relevant indicator. It represents the ratio of variation of the total standoff, Lf, (after upsetting) and the initial total standoff value, Li, as previously defined by Iordachescu M. et al. [10]. The experiments showed that a qualitative joint achievement may be obtained by a deformation bigger than δ = 0.5.
3.4 MIAB welding temperature Nowadays, infrared thermography (IRT) is a reliable technique to measure the surface temperature of metallic bodies, although conventional measurements are still made using thermocouples [11]. Heat flux investigation is one of the major topics in welding processes using heat sources. In this regard, the surface temperature of the starting and ending phases of MIAB welding was achieved by using the infrared camera, AGEMA Thermovision 900, that has a wide temperature range (from –30 to 2 000 °C). After camera calibration with a black body source MIKRON
a) Diagram of arc velocity vs. process phases duration; I–VI MIAB welding phases (see Figure 1) b) Process images captured at low shooting speed (10 fps) c) Process images captured at higher shooting speed (100 fps); 1-8) relevant images taken during process development Figure 4 – MIAB welding process lifetime
CHARACTERISTICS OF MIAB welding process and joints
a) Infrared image captured during the process starting phase when applying the short-circuit current
c) Infrared image captured during the process ending phase when applying the upsetting force
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b) Temperature distribution along the LI01 line, the short-circuiting phase
d) Temperature distribution along the LI01 line, the upsetting phase
Figure 5 – Infrared images and temperature distribution of different MIAB welding phases of ST 37 tubes
M335, and black painting of the tubes outer surface to ensure an emissivity average of 0.75, the infrared images (IR) shown in Figures 5 a), c) were acquired. Figure 5 a) illustrates the MIAB process initiation, with the tubes heating when the short-circuit current is applied; this phenomenon occurs before the arc ignition, during the tubes withdrawal at the gap g. Figure 5 b) shows the temperature distribution along the tubes centreline (axis), LI01; it was found that the tubes temperature sharply increases along 4 mm length (half of the distance between the magnetizing coils, m/2), from an average temperature of 45 °C up to 90 °C close to their contact area. Figure 5 c) shows the IR image captured during the MIAB ending phase, during upsetting; to obtain consistent data
with the emissivity calibration, the camera was set to capture the tubes temperature distribution between 300 °C and 1 200 °C. The temperature distribution on the tubes axis is presented in Figure 5 d). The graph resulted by processing the acquired data with the AGEMA dedicated software shows that the weld peak temperature slightly surpasses the maximum temperature of 1 200 °C set on the IR camera [Figure 5 d)]. A 2 mm total length of the weld zone plus thermomechanically-affected zone (WZ +TMAZ) for each tube was identified. Further correlation of the data with the joint macro and microstructural analyses is needed. Capturing IR images of other intermediate MIAB processing phases is a difficult task due to the IR camera limitations in acquiring information from high speed processes. N° 01 02 2011 Vol. 55 WELDING IN THE WORLD
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3.5 Joint characterization Axial longitudinal sections of tubes joints were considered for microstructural and hardness analyses. Characterization of tubes joints made by MIAB welding was performed on specimens previously polished and etched with nital 5 % solution. Figure 6 illustrates the joint macrostructure of typical barrel-shape, the main proof of upsetting force influence in case of MIAB joint formation. The weld occurs on previously heated contact area of tubes together with the transversal burr formation; its dimension indicates the range of deformation after upsetting. The recrystallization process is influenced by pressing which decreases within 2 mm length from the weld centreline to the base metal (BM), along the thermomechanically-affected zone (TMAZ). These data are consistent with the IR thermography results presented in Figures 5 c), d).
30
Figure 6 b) shows the coarsened grains of TMAZ that ends at 2 mm from the weld centreline, indicating the austenite decomposition into ferrite and pearlite between Ac1 and Ac3. While approaching the weld zone, Widmanstatten ferrite (WF) and ferrite are found, but different grain sizes characterize the right (R) and left (L) joint areas. This might be due to different values of heat input received by each tube during the arc rotating phase; the tube wall from the joint R side was playing the anode role. This might explain the presence of higher refined grains in the TMAZ on this side, when comparing it with the L side. Although the microstructure of the weld zone contains mostly fine grains of WF and bainite, some microporosities can be seen in this area, explained by the short lifetime of the rotating arc (4 s), and by the low heat input, respectively. The microhardness analysis of MIAB welds revealed an increase in hardness along the weld interface and in the TMAZ. According to Figure 7, a peak value average of 240HV0.5 corresponding to the weld centreline was found. These regions of elevated hardness are most likely
Figure 7 – Microhardness measurements on ST 37 MIAB welded joint
due to a high concentration of dislocations as a consequence of the process final forging action.
4 Conclusions Several conclusions emerge: – An original longitudinal magnetizing system with multiple solenoids, designed to assure the magnetic flux concentration on tube wall, was used to investigate the MIAB welding of thin-walled tubes. – The authors have identified, defined and characterized six different process phases: arc initiation, beginning of arc rotation, arc transitory rotation, arc stable rotation, arc instable rotation (“breaking”), and tubes upsetting. – The experiments show that a qualitative joint may be obtained by assuring a deformation bigger than δ = 0.5. – MIAB welding critical parameters were identified as being the gap between the tubes, the magnetic field, the arc welding current, the arc rotation duration, and the upsetting force and rate. – Capturing infrared (IR) images of all MIAB processing phases is a difficult task due to the IR camera limitations in acquiring information from high speed processes.
a) Joint macrostructure indicating the microstructures position; b) Joint optical microstructure; c) Base metal (BM) optical microstructure TMAZ - thermomechanically-affected zone ; L - left side of the joint, R – right side of the joint.
Figure 6 – MIAB joint features in case of ST 37 grade tube steel
CHARACTERISTICS OF MIAB welding process and joints
– The investigation provided two main operational windows: the influence of the gap between the tubes on arc stability and the influence of the magnetic induction on the arc rotation/heating duration. – The recrystallization process is influenced by pressing which decreases from the weld centreline to the base metal (BM) along the thermomechanically-affected zone (TMAZ).
References [1] Haman R.: Welding process meets modern production line demands, Welding Journal, 2001, vol. 80, no. 5, pp. 37-40. [2] http://www.kuka-systems.com/en/branches/ technologies/magnetarc/ [3] Georgescu V. and Iordachescu D.: Magnetization systems for ROTARC welding, Proceedings of EUROJOIN3 Third European Conference on Joining Technology, Bern, Switzerland, March 1998, pp. 751-758.
[7] Arungalai Vendan S., Manoharan S., Buvanashekaran G. and Nagamani C.: Investigation of weld parameters in MIAB welding process by developing a module validation using finite element analysis, Journal of Manufacturing Engineering, 2008, no. 4, pp. 24-29. [8] Satoh T., Katayama J. and Otani M.: an experimental study of rotational behaviour of the arc during magnetically impelled arc butt welding, Welding International, 1991, vol. 5, no. 1, pp. 5-10. [9] Taneko A., Arakida F. and Takagi K.: Analysis of arc rotation velocity in magnetically impelled arc butt welding, Welding International, 1986, vol. 4, no. 3, pp. 570-576. [10] Iordachescu M., Iordachescu D., Planas J., Scutelnicu E. and Ocaña J.L.: Material flow and hardening at butt cold welding of aluminium, Journal of Materials Processing Technology, 1 May 2009, vol. 209, no. 9, pp. 4255-4263. [11] Scutelnicu E., Iordachescu M. and Iordachescu D.: Arc welding of dissimilar metals: FEA and experiments in case of copper – carbon steel joints, Metalurgia International, 2009, vol. XIV, no. 7, pp. 33-37.
[4] Iordachescu D., Georgescu B., Iordachescu M., Scutelnicu E. and Blasco M.: MAGNETARC welding equipment peculiarities and joint characteristics, Welding in the World, 2007, vol. 51, Special issue, pp. 367-376. [5] Kachinskiy V.S., Krivenko V.G. and Ignatenko V.Yu.: Magnetically impelled arc butt welding of hollow and solid parts, Doc. IIW-1564-02, Welding in the World, 2002, vol. 46, no. 7/8, pp. 49-56. [6] Kim J.W. and Choi D.H.: A study on the numerical analysis of magnetic flux density by a solenoid for magnetically impelled arc butt welding, Proceedings of the Institution of Mechanical Engineers, Part B, Journal of Engineering Manufacture, 2003, vol. 217, no. 10, pp. 1401-1407.
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About the authors Prof. Dr. Eng. Danut IORDACHESCU ([email protected]), Eng. Raul LOPEZ ([email protected]) and Prof. Dr. Eng. Angel GARCÍA-BELTRÁN ([email protected] es) are all with UPM Laser Centre, Universidad Politécnica de Madrid (Spain). Lecturer Dr. Eng. Bogdan GEORGESCU ([email protected]) is with Robotics and Welding Department, Dunarea de Jos University of Galati (Romania). Dr. Eng. Mihaela IORDACHESCU ([email protected]) is with Materials Science Department, ETSI Caminos, Canales y Puertos, Universidad Politécnica de Madrid (Spain). Prof. Dr. Eng. Rosa Maria MIRANDA ([email protected]) is with Mechanical and Industrial Department, FCT, Universidad Nova de Lisboa (Portugal).
N° 01 02 2011 Vol. 55 WELDING IN THE WORLD
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CHARACTERISTICS OF MIAB welding process and joints
ABSTRACT
D. Iordachescu, B. Georgescu, M. Iordachescu, R. Lopez, R.M. Miranda and A. García-Beltrán
The paper presents the MIAB welding of thin walled tubes achieved with an original longitudinal magnetization system, designed to assure the magnetic flux concentration on the tube wall. Based on images resulting from process monitoring, the main stages of the process are presented, starting with arc initiation, to apparition of molten metal, upsetting and ending with the weld achievement. Infrared thermography was also used for temperature measurements and visualization of the process. Macro and microstructural analyses accompany the hardness tests made on correspondingly welded samples, demonstrating the process capability in producing sound joints when guided by the operational windows developed through this investigation. IIW-Thesaurus keywords: Hardness; MIAB welding; Microstructure.
1 Introduction Tubes provide light weight, excellent rigidity and low cost for a structure. This is why, nowadays, tubes continue to become more and more important in the automotive and other industries. In this view, MIAB welding is a unique thermomechanical process for joining closed structural tubular shapes. Present industrial applications of this welding process are fully integrated to flexible automated lines [1]. This combined arc and pressure welding technique, known also as MAGNETARC Welding [2], was referred to in previous works as ROTARC Welding by Georgescu and Iordachescu D. [3], since early ‘80s. The process relies on complex interactions between an electric arc and both an applied and an induced magnetic field; it becomes more complex due to the changes that occur during the heating of the parts/tubes which are finally welded by axial upsetting [4]. The joint interface consists of a solid-state bond region, the previously molten material being squeezed outward the tubes contact zone into a flash [5]. Although some reports are of interest for MIAB development and the industrial application of the process involved in-depth knowledge, some limits on approaching MIAB welding still exist, proving less understanding of the whole process and of the relationship between its parameters [6-8]. This investigation used an original longitudinal magnetization system that helps emphasizing the interaction between the arc and the induced magnetic field in the case of MIAB welding of low carbon steel thin-walled tubes. This paper presents an overall approach of the
process, trying to contribute to its better understanding. Based on images resulting from process monitoring, the main stages of the process are presented, starting with arc initiation, to apparition of molten metal, upsetting and ending with the weld achievement. Infrared thermography was also used for temperature measurements and visualization of the process. Macro and microstructural analyses accompany the hardness tests made on correspondingly welded samples, demonstrating the process capability in producing sound joints when guided by the operational windows developed through this investigation.
2 Principle and fundamentals of MIAB welding
Considering the reports of Taneko et al. [9], on arc rotation velocity changes during MIAB welding, and of Iordachescu D. et al. [4], on MIAB equipment peculiarities and joint characteristics, the MIAB process main stages are illustrated in Figure 1. Accordingly, a succession of six regions of parametric changes characterizes the process achievement. At first stage, I - arc initiation, the tubes 1, 2 are coaxially fixed in clamps 6, 7, with their abutting surfaces in contact [Figures 1 a), b)]; a DC arc welding source is connected to tubes and a magnetization field is applied through the solenoids 3. The arc initiation occurs mainly on the inner diameter of the tubes due to their withdrawal at the gap g. During the second stage, II, the beginning of the arc rotation is due to the interaction between the welding current, Iw, and the radial magnetic field, B, induced into the
Doc. IIW-2039-09, recommended for publication by Commission III “Resistance Welding, Solid State Welding and Allied Joining Processes.”
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Peer-reviewed Section
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CHARACTERISTICS OF MIAB welding process and joints
a) Arc velocity vs. time diagram Process phases, I – VI: I – arc initiation (flash); II – beginning of arc rotation; III – arc transitory rotation; IV – arc stable rotation; V – arc instable rotation (“breaking”); VI – tubes upsetting; v1 – arc rotation of low velocity; v2 – stable arc rotation velocity.
b) MIAB welding process sketch g – tubes gap; m – distance between the solenoids; Fc – clamping force; B – magnetic field; Isc – short circuit current; Fp – upsetting force; 1, 2 – thin-walled tubes; 3 – magnetization system/solenoids; 4 – rotating arc.
Figure 1 – MIAB welding principle
26
arc gap; the electric arc moves at low arc rotating velocity from the tubes inner diameter to their outer surface. During this phase, the arc quenching may occur if inadequate or insufficient magnetization is applied. Once the arc surpasses the critical velocity, v1, [Figure 1 a)], then starts the third process phase, III, the arc transitory rotation. This phase is characterized by an abrupt increase of arc velocity. After that, the process stabilizes around v2 arc velocity [Figure 1 a)]; in this phase the visual perception of the electric arc is of a continuous ring of fire, due to its high rotating velocity. A thin uniform layer of molten metal on tubes abutting surfaces forms in this arc stable rotation, IV, phase; it ends when the molten metal bridges form into the tubes gap. Thus, the arc instable rotation/breaking phase, V, initiates, and the molten metal waving is produced due to the abrupt fluctuations of the arc velocity [Figure 1 a)]. The combined action of waving and arc rotating tendency is to break and expel the previously formed bridges of molten metal. The tubes upsetting phase, VI, with the force Fp, ends the MIAB welding process. Formation of molten metal bridges indicates the right moment of upsetting. By upsetting, the molten metal flows, as well as the plasticized material from the tubes contact area, producing a transversal burr of barrel shape and the weld seam. A qualitative joint formation results after tubes forging; the dynamic effect of the mechanical action also assures the weld strength. Consequently, the clamping forces, Fc, have to prevent the bars axial sliding during upsetting. The process achievement depends also on the surface contamination of the tubes abutting surfaces with rust, scale or any type of grease. Therefore, to obtain a defectfree weld, normally a simple saw or machined cut is a sufficient preparation of the abutting surfaces.
3 Original magnetization system and experimental results
Knowing that MIAB welding is a process sensitive on tubes local magnetization, the paper presents first the performances of an original longitudinal magnetizing system; its design is intended to ensure the magnetic flux concentration on tubes surface. Different welding experiments were made using low carbon steel tubes of 25.4 mm outer diameter (OD), 3 mm thickness, grade ST 37 tube (DIN 1626-84/ASTM A 536).
3.1 Magnetization system with peripheral solenoids The classical longitudinal magnetization solution of MIAB welding uses a single solenoid coil concentric with the tube. The new system aims at concentrating the magnetic flux distribution into the tube wall, improving the arc rotation stability. Figure 2 a) draws the principle of the developed system made of two independent half-shells. The magnetization system for each tube is made of 8 solenoids [4 coils on each half-shell, Figure 2 b)], which are connected in series, and are working positioned in parallel with the tube longitudinal axis. The use of two half-shells is of high practical importance, allowing the easy positioning and removal of the magnetization systems on/from tubes, irrespective of their length. The image of the MIAB welding equipment used during experiments is presented in Figure 2 b); a hydraulic system ensures the tubes safe clamping and their longitudinal movement during initial and upsetting phases. A KEMPPI Pro5200 power source provides the necessary DC welding current.
CHARACTERISTICS OF MIAB welding process and joints
a) Original longitudinal magnetization system with multiple solenoids mounted in two independent shells
b) MIAB welding equipment
B – magnetic field; 1 – tube peripheral solenoid; 2 – air gap; 3 – thin walled tube; 4, 5 – half-shells with peripheral solenoids; 6 – rotating arc; 7 – clamping system.
Figure 2 – MIAB welding
3.2 MIAB welding – process parameters and operational windows In MIAB welding the gap between pipe edges, g, has a major influence on process stability, as has the arc length in conventional fusion welding processes. Moreover, in this case the magnetic field value in the tubes gap also influences the arc rotation, but its magnitude depends on the distance between the magnetization shells. During experiments, the magnetization current domain was Im = 0.1A-1.0 A, whereas for the welding current it was Iw = 100 A–400 A. Qualitative joints were sought to be obtained after the final upsetting phase. Thus, a set of specific operational windows for MIAB welding was developed for achieving sound joints when using the peripheral solenoids magnetization system. Figure 3 a) shows the dependency of the magnetic field induction on the magnetization current at different gap
a) Magnetic field vs. magnetization current at different gap values B – magnetic field, [T]; Im – magnetization current, [A]; g – gap between the tubes, [mm].
size values; the distance between the magnetization shells, m, was kept constant (m = 10 mm). The magnetic field increases with the magnetization current increase, and the highest values of B were recorded at g = 0.5 mm, found as being the minimum gap size (arc length) that assures the arc stable rotation in this particular case of 3 mm tube wall thickness. The gap size increase over the maximum value of 1.5 mm may cause the decrease of B value until the arc stops. Consequently, the stability of the welding process depends on gap size, the optimum values pertaining to the interval g = (0.5-1.5) mm. Figure 3 b) illustrates the welding process stability diagram as depending on welding current intensity and tubes gap value, the most sensitive process parameters. The diagram is valid for a constant distance between the magnetization shells m = 10 mm. According to the welding tests results, three different regions characterize the process stability.
b) Welding stability diagram – welding current intensity, Iw [A], vs. gap value C – area without arc movement or with unstable rotation; A – area of appropriate arc initiation and stable rotation; D – area of short lifetime arc, with unstable rotation or without movement.
Figure 3 – Magnetic field diagrams
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CHARACTERISTICS OF MIAB welding process and joints
Region A characterizes the process stability – the arc forms and stably rotates between the tubes ends. In region C the arc does not move for low current values, whereas for higher currents it rotates very unstably – although highenough values of welding currents and magnetic field are provided, the gap/arc length is too small to assure a sufficient value of the magnetic force. In region D the arc initiates but has short lifetime; it is not starting to rotate (it quenches immediately after initiation), or its rotation is unstable, due to a big gap value and/or to a too small current value. If the tubes upsetting occurs after an unstable welding process (regions C or D), the joint does not form or the weld has inadequate strength.
3.3 Monitoring of the MIAB process MIAB welding lifetime was studied together with arc rotation velocity. The processing parameters were Iw = 250 A, B = 0.08 T, and Fp = 12 kN. Figure 4 a) shows the arc velocity variation during the process lifetime. Representative images captured with a special featured HRDC camera at low (10 fps) and higher shooting speed (100 fps) are presented in Figure 4 b), and Figure 4 c), respectively. Image no. 1 corresponds to the process first phase, I, arc initiation (see Figure 1); its red colour indicates the arc flash formation. 28
Image no. 2 is an intermediate arc capture taken during the process second phase, II; its duration is of about 0.5 s. In this phase the arc rotating velocity increases up to the critical velocity, v1 = 10 m/s (image no. 3), where the arc transitory phase, III, starts. Low spattering also characterizes this arc rotating phase. The arc velocity continues to increase up to the second critical velocity, v2 = 50 m/s (image no. 4). Starting from
this point, corresponding to the process phase IV, the arc velocity relative stable rotation was found (images no. 5–6); a continuous arc covers the tubes abutting surface, heating and superficially melting their surfaces. After 2.7 s from the arc initiation, starts the process phase V, characterized by arc instable rotation/breaking. This is due to the molten metal bridges previously formed during the arc stable rotation. Image no. 7 characterizes this stage of high spattering and unstable arc velocity. The last process phase, VI, starts after 4 s from the process beginning and the tubes upsetting produces the quenching of the already instable electric arc. Maintaining for about 1.5 s the upsetting (image no. 8) assures the achievement of a qualitative joint. Finally, to characterize a qualitative joint achievement, the tubes deformation, δ, was chosen as relevant indicator. It represents the ratio of variation of the total standoff, Lf, (after upsetting) and the initial total standoff value, Li, as previously defined by Iordachescu M. et al. [10]. The experiments showed that a qualitative joint achievement may be obtained by a deformation bigger than δ = 0.5.
3.4 MIAB welding temperature Nowadays, infrared thermography (IRT) is a reliable technique to measure the surface temperature of metallic bodies, although conventional measurements are still made using thermocouples [11]. Heat flux investigation is one of the major topics in welding processes using heat sources. In this regard, the surface temperature of the starting and ending phases of MIAB welding was achieved by using the infrared camera, AGEMA Thermovision 900, that has a wide temperature range (from –30 to 2 000 °C). After camera calibration with a black body source MIKRON
a) Diagram of arc velocity vs. process phases duration; I–VI MIAB welding phases (see Figure 1) b) Process images captured at low shooting speed (10 fps) c) Process images captured at higher shooting speed (100 fps); 1-8) relevant images taken during process development Figure 4 – MIAB welding process lifetime
CHARACTERISTICS OF MIAB welding process and joints
a) Infrared image captured during the process starting phase when applying the short-circuit current
c) Infrared image captured during the process ending phase when applying the upsetting force
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b) Temperature distribution along the LI01 line, the short-circuiting phase
d) Temperature distribution along the LI01 line, the upsetting phase
Figure 5 – Infrared images and temperature distribution of different MIAB welding phases of ST 37 tubes
M335, and black painting of the tubes outer surface to ensure an emissivity average of 0.75, the infrared images (IR) shown in Figures 5 a), c) were acquired. Figure 5 a) illustrates the MIAB process initiation, with the tubes heating when the short-circuit current is applied; this phenomenon occurs before the arc ignition, during the tubes withdrawal at the gap g. Figure 5 b) shows the temperature distribution along the tubes centreline (axis), LI01; it was found that the tubes temperature sharply increases along 4 mm length (half of the distance between the magnetizing coils, m/2), from an average temperature of 45 °C up to 90 °C close to their contact area. Figure 5 c) shows the IR image captured during the MIAB ending phase, during upsetting; to obtain consistent data
with the emissivity calibration, the camera was set to capture the tubes temperature distribution between 300 °C and 1 200 °C. The temperature distribution on the tubes axis is presented in Figure 5 d). The graph resulted by processing the acquired data with the AGEMA dedicated software shows that the weld peak temperature slightly surpasses the maximum temperature of 1 200 °C set on the IR camera [Figure 5 d)]. A 2 mm total length of the weld zone plus thermomechanically-affected zone (WZ +TMAZ) for each tube was identified. Further correlation of the data with the joint macro and microstructural analyses is needed. Capturing IR images of other intermediate MIAB processing phases is a difficult task due to the IR camera limitations in acquiring information from high speed processes. N° 01 02 2011 Vol. 55 WELDING IN THE WORLD
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CHARACTERISTICS OF MIAB welding process and joints
3.5 Joint characterization Axial longitudinal sections of tubes joints were considered for microstructural and hardness analyses. Characterization of tubes joints made by MIAB welding was performed on specimens previously polished and etched with nital 5 % solution. Figure 6 illustrates the joint macrostructure of typical barrel-shape, the main proof of upsetting force influence in case of MIAB joint formation. The weld occurs on previously heated contact area of tubes together with the transversal burr formation; its dimension indicates the range of deformation after upsetting. The recrystallization process is influenced by pressing which decreases within 2 mm length from the weld centreline to the base metal (BM), along the thermomechanically-affected zone (TMAZ). These data are consistent with the IR thermography results presented in Figures 5 c), d).
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Figure 6 b) shows the coarsened grains of TMAZ that ends at 2 mm from the weld centreline, indicating the austenite decomposition into ferrite and pearlite between Ac1 and Ac3. While approaching the weld zone, Widmanstatten ferrite (WF) and ferrite are found, but different grain sizes characterize the right (R) and left (L) joint areas. This might be due to different values of heat input received by each tube during the arc rotating phase; the tube wall from the joint R side was playing the anode role. This might explain the presence of higher refined grains in the TMAZ on this side, when comparing it with the L side. Although the microstructure of the weld zone contains mostly fine grains of WF and bainite, some microporosities can be seen in this area, explained by the short lifetime of the rotating arc (4 s), and by the low heat input, respectively. The microhardness analysis of MIAB welds revealed an increase in hardness along the weld interface and in the TMAZ. According to Figure 7, a peak value average of 240HV0.5 corresponding to the weld centreline was found. These regions of elevated hardness are most likely
Figure 7 – Microhardness measurements on ST 37 MIAB welded joint
due to a high concentration of dislocations as a consequence of the process final forging action.
4 Conclusions Several conclusions emerge: – An original longitudinal magnetizing system with multiple solenoids, designed to assure the magnetic flux concentration on tube wall, was used to investigate the MIAB welding of thin-walled tubes. – The authors have identified, defined and characterized six different process phases: arc initiation, beginning of arc rotation, arc transitory rotation, arc stable rotation, arc instable rotation (“breaking”), and tubes upsetting. – The experiments show that a qualitative joint may be obtained by assuring a deformation bigger than δ = 0.5. – MIAB welding critical parameters were identified as being the gap between the tubes, the magnetic field, the arc welding current, the arc rotation duration, and the upsetting force and rate. – Capturing infrared (IR) images of all MIAB processing phases is a difficult task due to the IR camera limitations in acquiring information from high speed processes.
a) Joint macrostructure indicating the microstructures position; b) Joint optical microstructure; c) Base metal (BM) optical microstructure TMAZ - thermomechanically-affected zone ; L - left side of the joint, R – right side of the joint.
Figure 6 – MIAB joint features in case of ST 37 grade tube steel
CHARACTERISTICS OF MIAB welding process and joints
– The investigation provided two main operational windows: the influence of the gap between the tubes on arc stability and the influence of the magnetic induction on the arc rotation/heating duration. – The recrystallization process is influenced by pressing which decreases from the weld centreline to the base metal (BM) along the thermomechanically-affected zone (TMAZ).
References [1] Haman R.: Welding process meets modern production line demands, Welding Journal, 2001, vol. 80, no. 5, pp. 37-40. [2] http://www.kuka-systems.com/en/branches/ technologies/magnetarc/ [3] Georgescu V. and Iordachescu D.: Magnetization systems for ROTARC welding, Proceedings of EUROJOIN3 Third European Conference on Joining Technology, Bern, Switzerland, March 1998, pp. 751-758.
[7] Arungalai Vendan S., Manoharan S., Buvanashekaran G. and Nagamani C.: Investigation of weld parameters in MIAB welding process by developing a module validation using finite element analysis, Journal of Manufacturing Engineering, 2008, no. 4, pp. 24-29. [8] Satoh T., Katayama J. and Otani M.: an experimental study of rotational behaviour of the arc during magnetically impelled arc butt welding, Welding International, 1991, vol. 5, no. 1, pp. 5-10. [9] Taneko A., Arakida F. and Takagi K.: Analysis of arc rotation velocity in magnetically impelled arc butt welding, Welding International, 1986, vol. 4, no. 3, pp. 570-576. [10] Iordachescu M., Iordachescu D., Planas J., Scutelnicu E. and Ocaña J.L.: Material flow and hardening at butt cold welding of aluminium, Journal of Materials Processing Technology, 1 May 2009, vol. 209, no. 9, pp. 4255-4263. [11] Scutelnicu E., Iordachescu M. and Iordachescu D.: Arc welding of dissimilar metals: FEA and experiments in case of copper – carbon steel joints, Metalurgia International, 2009, vol. XIV, no. 7, pp. 33-37.
[4] Iordachescu D., Georgescu B., Iordachescu M., Scutelnicu E. and Blasco M.: MAGNETARC welding equipment peculiarities and joint characteristics, Welding in the World, 2007, vol. 51, Special issue, pp. 367-376. [5] Kachinskiy V.S., Krivenko V.G. and Ignatenko V.Yu.: Magnetically impelled arc butt welding of hollow and solid parts, Doc. IIW-1564-02, Welding in the World, 2002, vol. 46, no. 7/8, pp. 49-56. [6] Kim J.W. and Choi D.H.: A study on the numerical analysis of magnetic flux density by a solenoid for magnetically impelled arc butt welding, Proceedings of the Institution of Mechanical Engineers, Part B, Journal of Engineering Manufacture, 2003, vol. 217, no. 10, pp. 1401-1407.
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About the authors Prof. Dr. Eng. Danut IORDACHESCU ([email protected]), Eng. Raul LOPEZ ([email protected]) and Prof. Dr. Eng. Angel GARCÍA-BELTRÁN ([email protected] es) are all with UPM Laser Centre, Universidad Politécnica de Madrid (Spain). Lecturer Dr. Eng. Bogdan GEORGESCU ([email protected]) is with Robotics and Welding Department, Dunarea de Jos University of Galati (Romania). Dr. Eng. Mihaela IORDACHESCU ([email protected]) is with Materials Science Department, ETSI Caminos, Canales y Puertos, Universidad Politécnica de Madrid (Spain). Prof. Dr. Eng. Rosa Maria MIRANDA ([email protected]) is with Mechanical and Industrial Department, FCT, Universidad Nova de Lisboa (Portugal).
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