Relationship Between the Formation of Hollow Bead Defects and Cold Cracking

1. 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

2. 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

3. Appearance of a Cold Crack in Weld Metal

4. 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

5. Micro segregation of Ni, Mn, Mo, Cr START LIQUID Macro segregation SOLID FINISH Segregation during Solidification of Weld Metal

6. 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

7. Effect of Segregation Cold Crack

8. 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

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

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

11. Experimental Investigation • Consumable – Lincoln Fleetweld 5P+ cellulosic electrode (AWS E6010/AS E4110) Parent Plate API 5L X80

12. 30o Root Face1.6 – 2.1mm Root Gap 1.3 – 1.6mm Joint Geometry

13. Automated Welding Machine Set-up – Electrode at 15o

14. Welding Conditions • Welding Current: 190 amps • Voltage: 30 volts • Travel Speed: 500mm/minute

15. Welded Sample Welded Plate Metallographically prepared sample showing hollow bead defect (arrowed)

16. Collage of micrographs showing the crack path through the weld metal crack

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

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

19. 100m Cold Crack from Hollow Bead Defect

20. 100m 100m 100m Mn Fe 100m Si Microprobe x-ray maps

21. X-ray line scans across the crack Distance in m Distance in m Distance in m

22. 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.

23. Development of Hollow Bead Defect Transverse section of the pore Longitudinal section of the pore

24. Red arrows indicate cellular dendrite growth directions Dark lines are intercellular dendrite regions of segregation Hollow Bead Pore Transverse Section

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

26. Growth direction of pore Hollow Bead Pore Longitudinal Section Red arrow indicates cellular growth direction

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

28. weld centre-line top parent metal parent metal bottom Hollow Bead Defect Schematic diagram of transverse section through the Hollow Bead Defect

29. 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

30. 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

31. 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

32. 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

33. 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.