Technology of flash-butt welding of reinforcement bars in construction of structures of monolithic reinforced concrete

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25 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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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 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

Steel grade

σ
0.2
, MPa

σ
t
, MPa

δ, %

35GS

370


500

610


670

18


30

25G2S

380


400

590


620

23


31

St3Gsp

235

370
-

490

25

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
, mm

U
2o.
-
c
, V

L
flash
, mm

L
upset
, mm

t
w
, 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

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
, mm

σ
y
, MPa

σ
t
, MPa

16

432,7 … 472,4

457,9

721,2 …731,1

726,1

18

361,4 … 402,6

387,9

628,8 … 660,2

647,6

20

394,7 … 451,2

418,0

652,5 … 719,4

699,4

22

356,6 … 394,4

372,2

639,3 … 684,0

657,7

25

399,2 … 482,8

432,9

652,5 … 754,4

678,7

28

455,1 … 513,0

490,0

674,8 … 754,4

724,8

32

456,2 … 565,8

505,9

684,4 … 719,4

697,1

36

469,4 … 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
upset
I
, s

σ
t
, MPa

σ
w.j
/ σ
p.m
, %

Place of
fracture

25

11

0,1

830…850

847

96,1…98,2

97,3

HAZ

11

4

0,1

825…825

825

95,5…95,5

95,5

In butt

7

4

0,1

870…875

873

93,7…102,0

97,9

Same

12

8

0,04

825…835

831

95,5…96,7

96,3

In butt

15

10

0,04

835…845

836

96,1…97,0

96,4

Same

17

12

0,04

820…850

838

95,0…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

a

b

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

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, МПа
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, мм
HV, МПа
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 conditions

decreased speed

decreased voltage

increased 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 conditions

decreased speed

decreased voltage

increased 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 conditions

decreased speed

decreased voltage

increased 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

100
87
92
100
92
96
0
20
40
60
80
100
Optimal conditions
(no deviations)
Increased extension +
decreased voltage
Decreased speed
PNN
LVQ
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.


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

E.O. Paton Electric Welding Institute, Department of Pressure Welding


Thank you for your attention




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