National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O....

National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding DepartmentE.O. Paton Electric Welding Institute, Department of Pressure Welding

Technology of flash-butt weldingTechnology of flash-butt weldingof reinforcement bars in of reinforcement bars in

constructionconstructionof structures of monolithic of structures of monolithic

reinforced concretereinforced 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)


Technology of flash-butt weldingTechnology of flash-butt weldingof reinforcement bars in constructionof reinforcement bars in construction

of structures of monolithic reinforced concreteof structures of monolithic reinforced concrete

Type of material:Type of material:

Carbon steel (the standard states limitations to chemical composition only) – hot-Carbon steel (the standard states limitations to chemical composition only) – hot-rolledrolledand heat-treatedand heat-treated

Welding methods used for concrete reinforcement bars:Welding methods used for concrete reinforcement bars:

► Arc welding (MMA, MAG, bath-arc welding)Arc welding (MMA, MAG, bath-arc welding)

► Resistance welding (flash-butt welding)Resistance welding (flash-butt welding)

Typical welding conditions:Typical welding conditions:

► Shop conditionsShop conditions

► Semi-stationarySemi-stationary

► On-siteOn-site

Methods of quality control:Methods of quality control:

► Tensile testTensile test

► Ultrasonic methodUltrasonic method


Technology of flash-butt weldingTechnology of flash-butt weldingof reinforcement bars in constructionof reinforcement bars in construction

of structures of monolithic reinforced concreteof 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


Study of parent metal characteristics

Chemical composition of bar reinforcing steels, wt.%Chemical composition of bar reinforcing steels, wt.%

Steel grade

C Mn Si Cr Ni S P Cu

35GS 0.20 – 0.29

1.20 – 1.60

0.60 – 0.90

<0.30 <0.30 <0.045 <0.040 <0.30

25G2S 0.30 – 0.37

0.80 – 1.20

0.60 – 0.90

<0.30 <0.30 <0.045 <0.040 <0.30

St3Gsp 0.14 – 0.22

0.80 – 1.10

<0.15 <0.30 <0.30 <0.050 <0.040 <0.30

Mechanical characteristics of bar reinforcing steels

Mechanical characteristics of bar reinforcing steels

Steel grade

σ0.2, MPa σt, MPa δ, %

35GS 370 – 500 610 – 670 18 – 30

25G2S 380 – 400 590 – 620 23 – 31

St3Gsp 235 370 - 490 25


Development of technology of weldingfor hot-rolled bars

Parameters of conditions of welding of hot-rolled concrete reinforcement of A240S – A500S classes

Parameters of conditions of welding of hot-rolled concrete reinforcement of A240S – A500S classes

D, mm U2o.-c, V Lflash, mm Lupset, mm tw, s

12 – 18 5.5 – 6.0 10 – 12 3 8 – 10

20 – 28 5.5 – 6.0 11 – 15 4 – 5 15 – 20

32 - 40 6.0 – 6.5 13 - 17 5 - 6 Up to 30

Welded specimens of hot-rolled (a) and heat-hardened (b)concrete reinforcement

Welded specimens of hot-rolled (a) and heat-hardened (b)concrete reinforcement

aa bb


Mechanical testing of welded bars

Results of mechanical tests of welded joints of hot-rolled concrete reinforcement of A400S

classof steel 35GS

Results of mechanical tests of welded joints of hot-rolled concrete reinforcement of A400S

classof steel 35GS

D, mm σy, MPa σt, MPa

16432,7 … 472,4

457,9721,2 …731,1

726,1

18361,4 … 402,6

387,9628,8 … 660,2

647,6

20394,7 … 451,2

418,0652,5 … 719,4

699,4

22356,6 … 394,4

372,2639,3 … 684,0

657,7

25399,2 … 482,8

432,9652,5 … 754,4

678,7

28455,1 … 513,0

490,0674,8 … 754,4

724,8

32456,2 … 565,8

505,9684,4 … 719,4

697,1

36469,4 … 517,3

498,1646,2 … 757,6

708,8

Welding conditions parameters and results of mechanical tests of concrete

reinforcement of A600S class steel 25G2S

Welding conditions parameters and results of mechanical tests of concrete

reinforcement of A600S class steel 25G2S

Ladj, mmLflash, mm

tupsetI, s σt, MPa σw.j/ σp.m, % Place of fracture

25 11 0,1830…850847

96,1…98,297,3

HAZ

11 4 0,1825…825825

95,5…95,595,5

In butt

7 4 0,1870…875873

93,7…102,097,9

Same

12 8 0,04825…835831

95,5…96,796,3

In butt

15 10 0,04835…845836

96,1…97,096,4

Same

17 12 0,04820…850838

95,0…98,296,7

Parent metal



aa

bb

Welded specimens of hot-rolled (a) and heat-hardened (b) concrete reinforcement after tensile tests

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 St3GpsMicrostructure of welded joint (a), HAZ (b) and parent metal (c) of reinforcement bar of St3Gps

aa bb

cc



1300

1350

1400

1450

1500

1550

1600

1650

1700

1750

1800

-10 -8 -6 -4 -2 0 2 4 6 8 10

L, мм

H, М

Па

aa

bb

Macrosection (a) and distribution of microhardness (b) in the zone of welded joint

of reinforcement bar of steel St3Gps


of reinforcement bar of steel St3Gps

aa

bb

2150

2200

2250

2300

2350

2400

2450

-40 -30 -20 -10 0 10 20 30

L, мм

HV

, М

Па


of reinforcement bar of steel 25G2S


of reinforcement bar of steel 25G2S


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


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


Defects in welded joints

Fracture of reinforcement bar after tensile tests Fracture of reinforcement bar after tensile tests

Overheating in weldingOverheating in welding Burnt spotBurnt spot

Macrosection study of reinforcement barMacrosection study of reinforcement bar

Low heating in weldingLow heating in welding Defect in parent

metalDefect in parent metal


Real time monitoring of welding parameters

Macrosection of the joints produced under different welding conditions

Macrosection of the joints produced under different welding conditions

optimal conditionsoptimal conditions decreased speeddecreased speed

decreased voltagedecreased voltage increased extensionincreased extension





Typical voltage oscillograms at different welding parameters Typical voltage oscillograms at different welding parameters





Typical data arrays reflecting the intensity of flashing for groups of joints Typical data arrays reflecting the intensity of flashing for groups of joints



100

8792

10092

96

0

20

40

60

80

100

Optimal conditions(no deviations)

Increased extension +decreased voltage

Decreased speed

PNN LVQ

Application of artificial neural networksfor monitoring of flash-butt welding

Application of artificial neural networksfor monitoring of flash-butt 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.


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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National Technical University of Ukraine “Kyiv Polytechnic Institute”, Welding Department E.O....

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