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Relationship Between the Formation of Hollow Bead Defects and Cold Cracking. I.H.Brown, G.L.F.Powell, V.M.Linton University of Adelaide A.Kufner University of Applied Science, Konstanz, Germany. Introduction. What is Cold Cracking? Effect of Segregation on Cold Cracking

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relationship between the formation of hollow bead defects and cold cracking

Relationship Between the Formation of Hollow Bead Defects and Cold Cracking

I.H.Brown, G.L.F.Powell, V.M.Linton

University of Adelaide

A.Kufner

University of Applied Science, Konstanz,

Germany

introduction
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
hydrogen assisted cold cracking
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
summary of work presented previously trends 2002
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
what is hollow bead defect
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
appearance of hollow bead defect

Hollow Bead

Appearance of Hollow Bead Defect

Welded Plate

X-ray of Welded Plate showing Hollow Bead Defect (white)

experimental investigation
Experimental Investigation
  • Consumable – Lincoln Fleetweld 5P+ cellulosic electrode (AWS E6010/AS E4110)

Parent Plate API 5L X80

joint geometry

30o

Root Face1.6 – 2.1mm

Root Gap 1.3 – 1.6mm

Joint Geometry
automated welding machine
Automated Welding Machine

Set-up – Electrode at 15o

welding conditions
Welding Conditions
  • Welding Current: 190 amps
  • Voltage: 30 volts
  • Travel Speed: 500mm/minute
welded sample
Welded Sample

Welded Plate

Metallographically prepared sample showing hollow bead defect (arrowed)

slide17

Crack along the weld centreline following the intercellular dendritic segregation (Etchant LePera’s)

sem results
SEM Results

Collage of scanning electron micrographs of the crack. Note that the path of the crack runs between the inclusions.

microprobe x ray maps

100m

100m

100m

Mn

Fe

100m

Si

Microprobe x-ray maps
x ray line scans across the crack
X-ray line scans across the crack

Distance in m

Distance in m

Distance in m

result
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.
development of hollow bead defect
Development of Hollow Bead Defect

Transverse section of the pore

Longitudinal section of the pore

transverse section

Red arrows indicate cellular dendrite growth directions

Dark lines are intercellular dendrite regions of segregation

Hollow Bead Pore

Transverse Section
longitudinal section

Growth direction of pore

Hollow Bead Pore

Longitudinal Section

Red arrow indicates cellular growth direction

slide27

Scanning electron micrograph of the inside of a pore. The arrow indicates the welding direction.

slide28

weld centre-line

top

parent metal

parent metal

bottom

Hollow Bead Defect

Schematic diagram of transverse section through the Hollow Bead Defect

slide29

solid-liquid interface

welding direction

liquid

rejected hydrogen

top

bottom

last region to solidify

segregation

thin layer of solidified metal on surface

Hollow Bead Defect

Schematic diagram of longitudinal section of Hollow Bead Defect

summary
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
slide31
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
slide32
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
conclusion
Conclusion

It appears likely that cold cracking occurs in welds containing Hollow Bead Defect, and it is therefore likely that failures of line-pipe welds can be related to cold cracking if a Hollow Bead Defect is present.