IN625

Steel

PWHT F22 BS1

7

Objectives

• Verify the success of the post weld heat treatment (PWHT) at lowering the hardness of the heat affected zone (HAZ) to meet NACE requirements (< 250 VHN).

• Explore the effect of different substrates and weld bead sequences.

Results: Micro-Hardness Maps

Methods

• Eight hot-wire gas tungsten arc welded (GTAW) overlays were prepared as they would be for subsea service.

• Samples were cut from the DMW interface (red boxes in 2 & 3), mounted and prepared for optical microscopy.

• Chromic acid was used as the etchant for the 625 Nickel based alloy, and 10% nital was used to etch the base steel forging.

• After pictures were taken and analyzed, the etchant was polished off and Micro hardness maps were done over the interface.

• From the hardness data, hardness distribution by value graphs were made to display the hardness trends across the DMW interface.

• PWHT• Solved the HAZ hardness issue (from 370 VHN to 250 VHN)• Caused carbon migration to the fusion boundary raising the hardness

(from 300 VHN to 450 VHN)• The base metal hardness may have been lowered below an

expectable amount (from 270 VHN to 170 VHN)• Substrate

• 8630 HAZ had higher hardness than F22 in all combinations (~25VHN higher)

• BS (Bead sequence)• BS1 has higher hardness values than BS3 in all material combinations• BS1 was more susceptible to LOF defects

There have been several failures of Dissimilar metal welded(DMW) pipes in subsea service. These DMW pipe joints are typicallydone in three steps. First a multi-pass butter layer (of Inconel 625) isadded to both of the joint faces in one of two ways; in bead sequenceone (BS1) welds are layered normal to the fusion boundary, in beadsequence three (BS3) welds are layered parallel to the fusion boundary.The base forgings (either F22 or 8630 steel) are then post-weld heattreated (PWHT) to temper the heat affected zone (HAZ). Finally a closureweld is completed in the field using the same welding consumable as inthe buttering layer.

The butter layer applied is thick enough such that the HAZ of theclosure weld does not reach the base metal, and thus not criticallyhardening the base forging.

Subsea pipes are cathodically protected to prevent corrosion. Whilethis keeps the pipes from rusting it also evolves atomic hydrogen at thesurface of the pipes and leads to hydrogen ingress into the pipes. Thefailures mentioned earlier have been attributed to high hardness and thisbuildup of hydrogen in the DMW fusion zone.

Acknowledgements

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8630 BS1

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F22 BS1

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PWHT 8630 BS1

PWHT 8630 BS3

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PWHT F22 BS1

PWHT F22 BS3

IN625

SteelF22 BS3

PWHT 8630 BS1 PWHT F22 BS1

Post weld heat treated (PWHT)reduced the hardness in the heataffected zone (HAZ) (4 & 5).PWHT caused carbon to diffuseto the fusion boundary whichincreased the hardness there (6).

PWHT lowered the hardness of the base metal to below 250 VHN (4 & 6). 8630 had higher hardness than F22 in all combinations (4 & 6).

Schematic of a DMW joint

625

BS3

625

BS1

625

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Step One

Final StepIN625

SteelPWHT 8630 BS1

Results: Hardness Distribution by value Graphs

Bead sequence one (BS1) was harder and hence more brittle in allmaterial combinations than bead sequence three (BS3).

Results: Visual inspection

Bead sequence one (BS1)produces lack of fusion (LOF)defects at the dissimilar metalweld (DMW) interface in allmaterial combinations.

Conclusion

Introduction