turkey_oct_conf.pdf

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

Technology of flash-butt weldingof reinforcement bars in constructionof structures of monolithic reinforced

concrete

Ye. Chvertko*, I. Skachkov (National Technical University of Ukraine Kyiv PolytechnicInstitute, Kyiv, Ukraine)

P. Chvertko (E.O. Paton Electric Welding Institute, NASU, Kyiv, Ukraine)


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Technology of flash-butt weldingof reinforcement bars in construction

of structures of monolithic reinforced concrete

Type of material:

Carbon steel (the standard states limitations to chemical composition only) hot-rolledand 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


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Technology of flash-butt weldingof reinforcement bars in construction

of structures of monolithic reinforced concrete

Tasks:

to define the technological features of the welding process for hot-rolled andheat-treated bars

to define requirements to the specialized welding equipment

to develop additional means of non-destructive control or process monitoringallowing to increase its effectiveness without increasing percentage ofdestructive tests


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Study of parent metal characteristics

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


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Development of technology of weldingfor hot-rolled bars

Parameters of conditions of welding of hot-rolled concrete reinforcementof 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

a b


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Mechanical testing of welded bars

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

of steel 35GS

D, mm y, MPa t, 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

28

455,1 513,0

490,0

674,8 754,4

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 ofmechanical tests of concrete reinforcement of

A600S class steel 25G2S

Ladj, mmLflash,mm

tupsetI, s t, MPa w.j/ p.m, %Place offracture

25 11 0,1830850

84796,198,2

97,3HAZ

11 4 0,1825825

82595,595,5

95,5In butt

7 4 0,1870875

87393,7102,0

97,9Same

12 8 0,04825835

83195,596,7

96,3In butt

15 10 0,04835845

83696,197,0

96,4Same

17 12 0,04820850

83895,098,2

96,7Parentmetal


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a

b

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

after tensile tests

Microstructure of welded joint (a), HAZ (b) andparent metal (c) of reinforcement bar of St3Gps

a b

c


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1300

1350

1400

1450

1500

1550

1600

1650

1700

1750

1800

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

L,

H,

a

b

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

of steel St3Gps

a

b

2150

2200

2250

2300

2350

2400

2450

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

L,

HV,

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

of steel 25G2S


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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


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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


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Defects in welded joints

Fracture of reinforcement bar after tensile tests

Overheating inwelding

Burnt spot

Macrosection study of reinforcement bar

Low heating inwelding

Defect in parentmetal


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Real time monitoring of welding parameters

Macrosection of the joints produced under different welding conditions

optimal conditions decreased speed

decreased voltage increased extension


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Typical voltage oscillograms at different welding parameters


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


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100

8792

100

9296

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


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Conclusions

1. Application of flash-butt welding is challenging for butt welding of concretereinforcement of high-strength steels. It is performed in automatic conditionand does not require the application of auxiliary consumables. Moreover, thequalification of welders does not influence the quality of welded joints. Theefficiency of the process is rather high, the time of welding of one joint doesnot exceed 1 min.

2. In the presence of a large amount of welded joints (for example, fromhundreds of thousands up to millions of welded joints are performed on onebridge passage) the application of the flash-butt joints under site and semi-stationary conditions provide the high economic efficiency, increases theproductivity 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 themost favorable conditions for plastic deformation during upsetting.


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Conclusions

4. The application of method of flash-butt welding with a pulsed flashing providesa stable quality of welded joints of high-strength and heat-hardenedreinforcing 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 ofcontinuous flashing causes the hazard of formation of burnt spots thatinfluences negatively the quality of welded joints.

6. Analysis of high-frequency component of the welding voltage allows toefficiently provide 100% monitoring of the welding process. It is reasonable touse artificial neural networks for automatic monitoring based on analysis of theintensity of flashing.


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Thank you for your attention

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turkey_oct_conf.pdf