National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Technology of flash-butt welding of reinforcement bars in construction of structures of monolithic reinforced concrete Ye. Chvertko*, I. Skachkov (National Technical University of Ukraine “Kyiv Polytechnic Institute”, Kyiv, Ukraine) P. Chvertko (E.O. Paton Electric Welding Institute, NASU, Kyiv, Ukraine)

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Technology of flash-butt welding of reinforcement bars in construction of structures of monolithic reinforced concrete Type of material: Carbon steel (the standard states limitations to chemical composition only) – hot-rolled and heat-treated Welding methods used for concrete reinforcement bars: ► Arc welding (MMA, MAG, bath-arc welding) ► Resistance welding (flash-butt welding) Typical welding conditions: ► Shop conditions ► Semi-stationary ► On-site Methods of quality control: ► Tensile test ► Ultrasonic method

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Technology of flash-butt welding of reinforcement bars in construction of structures of monolithic reinforced concrete Tasks: ► ► to define the technological features of the welding process for hot-rolled and heat-treated bars ► ► to define requirements to the specialized welding equipment ► ► to develop additional means of non-destructive control or process monitoring allowing to increase its effectiveness without increasing percentage of destructive tests

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Study of parent metal characteristics Chemical composition of bar reinforcing steels, wt.% Steel gradeCMnSiCrNiSPCu 35GS0.20 – – – 0.90<0.30 <0.045<0.040< G2S0.30 – – – 0.90<0.30 <0.045<0.040<0.30 St3Gsp0.14 – – 1.10<0.15<0.30 <0.050<0.040<0.30 Mechanical characteristics of bar reinforcing steels Steel gradeσ 0.2, MPaσ t, MPaδ, % 35GS370 – – – 30 25G2S380 – – – 31 St3Gsp

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Development of technology of welding for hot-rolled bars Parameters of conditions of welding of hot-rolled concrete reinforcement of A240S – A500S classes D, mmU 2o.-c, VL flash, mmL upset, mmt w, s 12 – – – 1238 – – – – 154 – 515 – – Up to 30 Welded specimens of hot-rolled (a) and heat-hardened (b) concrete reinforcement a b

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Mechanical testing of welded bars Results of mechanical tests of welded joints of hot- rolled concrete reinforcement of A400S class of steel 35GS D, mmσ y, MPaσ t, MPa ,7 … 472,4 457,9 721,2 …731,1 726, ,4 … 402,6 387,9 628,8 … 660,2 647, ,7 … 451,2 418,0 652,5 … 719,4 699, ,6 … 394,4 372,2 639,3 … 684,0 657, ,2 … 482,8 432,9 652,5 … 754,4 678, ,1 … 513,0 490,0 674,8 … 754,4 724, ,2 … 565,8 505,9 684,4 … 719,4 697, ,4 … 517,3 498,1 646,2 … 757,6 708,8 Welding conditions parameters and results of mechanical tests of concrete reinforcement of A600S class steel 25G2S L adj, mm L flash, mm t upsetI, sσ t, MPaσ w.j / σ p.m, % Place of fracture 25110,1 830… ,1…98,2 97,3 HAZ 1140,1 825… ,5…95,5 95,5 In butt 740,1 870… ,7…102,0 97,9 Same 1280,04 825… ,5…96,7 96,3 In butt 15100,04 835… ,1…97,0 96,4 Same 17120,04 820… ,0…98,2 96,7 Parent metal

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Mechanical testing of welded bars a b Welded specimens of hot-rolled (a) and heat-hardened (b) concrete reinforcement after tensile tests Microstructure of welded joint (a), HAZ (b) and parent metal (c) of reinforcement bar of St3Gps ab c

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Mechanical testing of welded bars a b Macrosection (a) and distribution of microhardness (b) in the zone of welded joint of reinforcement bar of steel St3Gps a b Macrosection (a) and distribution of microhardness (b) in the zone of welded joint of reinforcement bar of steel 25G2S

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Requirements to the special welding equipment ► ► method of welding: continuous flashing or pulsed flashing ► ► mobile ► ► compact ► ► minimum possible electric power ► ► maximum protection from the environment effect

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Defects in welded joints Main reasons of defects’ formation in flash-butt welded joints: ► ► Parameters of technological process fault ► ► Operation conditions fault ► ► Parent metal defects

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Defects in welded joints Fracture of reinforcement bar after tensile tests Overheating in welding Burnt spot Macrosection study of reinforcement bar Low heating in welding Defect in parent metal

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Real time monitoring of welding parameters Macrosection of the joints produced under different welding conditions optimal conditionsdecreased speed decreased voltageincreased extension

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Real time monitoring of welding parameters optimal conditionsdecreased speed decreased voltageincreased extension Typical voltage oscillograms at different welding parameters

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Real time monitoring of welding parameters optimal conditionsdecreased speed decreased voltageincreased extension Typical data arrays reflecting the intensity of flashing for groups of joints

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Real time monitoring of welding parameters Application of artificial neural networks for monitoring of flash-butt welding

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Conclusions 1. Application of flash-butt welding is challenging for butt welding of concrete reinforcement of high-strength steels. It is performed in automatic condition and does not require the application of auxiliary consumables. Moreover, the qualification of welders does not influence the quality of welded joints. The efficiency of the process is rather high, the time of welding of one joint does not exceed 1 min. 2. In the presence of a large amount of welded joints (for example, from hundreds of thousands up to millions of welded joints are performed on one bridge passage) the application of the flash-butt joints under site and semi- stationary conditions provide the high economic efficiency, increases the productivity of construction and reduces greatly the amount of rejected joints. 3. Distribution of temperature in a near-contact zone in welding with a continuous flashing at rigid conditions and especially using a pulsed flashing creates the most favorable conditions for plastic deformation during upsetting.

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O. Paton Electric Welding Institute, Department of Pressure Welding Conclusions 4. The application of method of flash-butt welding with a pulsed flashing provides a stable quality of welded joints of high-strength and heat-hardened reinforcing steels. In this case, the value of weakening does not exceed 2-4%. 5. Increase in current density at pulsed flashing as compared with that of continuous flashing causes the hazard of formation of burnt spots that influences negatively the quality of welded joints. 6. Analysis of high-frequency component of the welding voltage allows to efficiently provide 100% monitoring of the welding process. It is reasonable to use artificial neural networks for automatic monitoring based on analysis of the intensity of flashing.

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