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

The University of Adelaide, Adelaide, South Australia 5005

Appearance of a Cold Crack in Weld Metal

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

Segregation during Solidification of Weld Metal

LIQUID

SOLID

Micro segregation of Ni, Mn, Mo, Cr

START

FINISH

Macro segregation

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

Effect of Segregation

Cold Crack

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

Appearance of Hollow Bead Defect

Hollow Bead

Welded Plate

X-ray of Welded Plate

showing Hollow Bead Defect

(white)

The University of Adelaide, Adelaide, South Australia 5005

Scanning Electron Micrograph of a section through a Hollow Bead Defect (After Cantin 1998)

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

Joint Geometry

Root Gap 1.3 – 1.6mm

Root Face1.6 – 2.1mm

30o

The University of Adelaide, Adelaide, South Australia 5005

Automated Welding Machine

Set-up – Electrode at 15o

The University of Adelaide, Adelaide, South Australia 5005

Welding Conditions

• Welding Current: 190 amps

• Voltage: 30 volts

• Travel Speed: 500mm/minute

The University of Adelaide, Adelaide, South Australia 5005

Welded Sample

Welded Plate

Metallographically prepared sample showing hollow bead

defect (arrowed)

The University of Adelaide, Adelaide, South Australia 5005

Collage of micrographs showing the crack path through the weld metal

crack

The University of Adelaide, Adelaide, South Australia 5005

Crack along the weld centreline following the intercellular dendritic segregation

(Etchant LePera’s)

The University of Adelaide, Adelaide, South Australia 5005

SEM Results

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

the inclusions.

The University of Adelaide, Adelaide, South Australia 5005

Cold Crack from Hollow Bead Defect

100m

The University of Adelaide, Adelaide, South Australia 5005

Microprobe x-ray maps100m

100m

Fe

100m

Mn

100m

Si

The University of Adelaide, Adelaide, South Australia 5005

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.

The University of Adelaide, Adelaide, South Australia 5005

Development of Hollow Bead Defect

Longitudinal section of the pore

Transverse section of the pore

The University of Adelaide, Adelaide, South Australia 5005

Transverse Section

Hollow Bead Pore

Dark lines are intercellular

dendrite regions of segregation

Red arrows indicate cellular dendrite growth

directions

The University of Adelaide, Adelaide, South Australia 5005

Triangular region indicates change in growth direction to normal to the page

The University of Adelaide, Adelaide, South Australia 5005

Longitudinal Section

Hollow Bead Pore

Growth direction of

pore

Red arrow indicates cellular growth direction

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

Hollow Bead Defect

parent metalparent metal

bottom

top

weld centre-line

Schematic diagram of transverse section through the Hollow Bead Defect

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

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

The University of Adelaide, Adelaide, South Australia 5005

• 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

The University of Adelaide, Adelaide, South Australia 5005

• 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

The University of Adelaide, Adelaide, South Australia 5005

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.