The University of Adelaide, Adelaide, South Australia 5005
Relationship Between the Formation of Hollow Bead Defects and Cold Cracking
I.H.Brown, G.L.F.Powell, V.M.LintonUniversity of Adelaide
A.KufnerUniversity of Applied Science, Konstanz,
Germany
The University of Adelaide, Adelaide, South Australia 5005
Introduction
• What is Cold Cracking?• Effect of Segregation on Cold Cracking• What is a Hollow Bead Defect?• Experimental investigation of the formation of
Hollow Bead Defects• Model of the formation of Hollow Bead Defect• Relationship between Hollow Bead Defect
and Cold Cracking
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Hydrogen Assisted Cold Cracking
• Contributing factors- Hydrogen- Stress: applied or residual- Susceptible microstructure- Susceptibility expressed in terms of carbon
equivalent calculated from nominal weld metal composition
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Segregation during Solidification of Weld Metal
LIQUID
SOLID
Micro segregation of Ni, Mn, Mo, Cr
START
FINISH
Macro segregation
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Summary of Work Presented Previously (Trends 2002)
• Micro-segregation occurs in the cellular dendritic regions.
• Micro-segregation of all elements appears to be in the same ratio of 1.4:1
• The micro-segregated region is harder than the matrix by the order of 100Hv
• The crack path is through the intercellular dendritic micro-segregated harder regions
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What is Hollow Bead Defect?
• A tubular void running in the direction of the weld bead
• When present it is usually found in the root run of a multi-pass weld
• Serious problem particularly during the laying of line-pipe
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Appearance of Hollow Bead Defect
Hollow Bead
Welded Plate
X-ray of Welded Plate
showing Hollow Bead Defect
(white)
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Scanning Electron Micrograph of a section through a Hollow Bead Defect (After Cantin 1998)
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Experimental Investigation• Consumable – Lincoln Fleetweld 5P+
cellulosic electrode (AWS E6010/AS E4110)
C P Mn Si S Ni Cr Mo Cu Al0.08 0.012 0.32 0.075 0.007 0.018 0.019 0.002 0.01 0.008
Parent Plate API 5L X80C Mn Si P S Al Nb Mo Ti Ca
0.09 1.7 0.38 0.02 0.001 0.05 0.08 0.035 0.025 0.001
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Joint Geometry
Root Gap 1.3 – 1.6mm
Root Face1.6 – 2.1mm
30o
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Automated Welding Machine
Set-up – Electrode at 15o
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Welding Conditions
• Welding Current: 190 amps
• Voltage: 30 volts
• Travel Speed: 500mm/minute
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Welded Sample
Welded Plate
Metallographically prepared sample showing hollow bead
defect (arrowed)
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Collage of micrographs showing the crack path through the weld metal
crack
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Crack along the weld centreline following the intercellular dendritic segregation
(Etchant LePera’s)
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SEM Results
Collage of scanning electron micrographs of the crack. Note that the path of the crack runs between
the inclusions.
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Microprobe x-ray maps100m
100m
Fe
100m
Mn
100m
Si
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X-ray line scans across
the crack
Distance in mDistance in mDistance in m
Weight% Manganese
0
1
2
3
4
5
6
0 100 200 300 400 500 600 700
distance in um
wei
gh
t% M
n
Weight% Silicon
0
0.1
0.2
0.3
0.4
0.5
0.6
0 100 200 300 400 500 600 700
distance in um
wei
gh
t% S
i
Weight% Iron
95
96
97
98
99
100
101
102
0 100 200 300 400 500 600 700
distance in umw
eig
ht%
Fe
The University of Adelaide, Adelaide, South Australia 5005
Result
• The cold crack followed the intercellular dendritic segregation from the hollow bead pore near the bottom surface of the weld to the top surface of the weld.
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Development of Hollow Bead Defect
Longitudinal section of the pore
Transverse section of the pore
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Transverse Section
Hollow Bead Pore
Dark lines are intercellular
dendrite regions of segregation
Red arrows indicate cellular dendrite growth
directions
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Triangular region indicates change in growth direction to normal to the page
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Longitudinal Section
Hollow Bead Pore
Growth direction of
pore
Red arrow indicates cellular growth direction
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Scanning electron micrograph of the inside of a pore. The arrow indicates the welding direction.
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Hollow Bead Defect
parent metalparent metal
bottom
top
weld centre-line
Schematic diagram of transverse section through the Hollow Bead Defect
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liquid
Hollow Bead Defectthin layer of solidified metal on
surface
rejected hydrogen
last region to solidify
segregation
solid-liquid interface
welding direction
top
bottom
Schematic diagram of longitudinal section of Hollow Bead Defect
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Summary• The weld metal solidified as delta ferrite and the
segregation was revealed using LePera’s reagent• Existance of segregation confirmed with microprobe X-ray
analysis• Solidification was from the parent material to the weld
centreline except in the region around the Hollow Bead Defect
• The cellular dendrites grew in the direction of welding in a triangular region adjacent to the Defect but at approximately 90o to the welding direction further away from the Defect
• Samples produced with a slow welding travel speed had no growth parallel to the welding direction
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• The location of the Hollow Bead Defect corresponded with the centreline segregation in the weld metal
• Ahead of the solid/liquid interface hydrogen is rejected from the liquid and forms bubbles (Cantin)
• The gas pore is encapsulated by a solidified thin layer before it can escape from the surface and so it forms more frequently under conditions of high welding travel speed
• The cellular dendrites around the pore are larger than in the other areas of the weld due to heterogeneous nucleation on the pore surface and because it is the last metal to solidify, a slow rate of solidification
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• High current and fast welding speed produced centreline cracking due to:– Centreline segregation– The presence of hydrogen both diffusible
and molecular– Residual stresses resulting from
solidification