API 5L Grade X65 steel pipes, internally clad alloy 625, are commonly utilized in pipelines and risers for subsea oil and gas extraction. Gird welds in such pipes are conventionally made using alloy 625 filler metal. However, alloy 625 weld metal cannot meet the base metal yield strength overmatching requirement for subsea reel lay installation. This study explored materials selection and process development for low-alloy steel girth welds in API 5L Grade X65 steel pipes, internally clad with alloy 625. Welding with a higher melting point filler metal over a lower melting substrate, i.e., low-alloy steel over Ni-based alloy, is impractical due to increased susceptibility to solidification cracking and solidification shrinkage porosity. Pseudo-binary phase diagrams developed for various combinations of low alloy steel filler metals and Ni-based alloy substrates identified good compatibility between ER80S-G filler metal and alloy 686. The solidification temperature range and the tendency for partitioning of alloying elements were significantly lower throughout the entire ER80S-G/alloy 686 dilution range than in the low alloy steel filler metals/alloy 625 combinations. Extensive process optimization effort to reduce the dilution of alloy 686 root pass in the low-alloy steel weld metal and avoid incomplete fusion defects allowed for the production of defect-free girth welds. These welds met the yield strength and ductility requirements for subsea reel lay installation of pipelines. Process optimization for bead tempering significantly narrowed the high hardness region in the ER80S-G/alloy 686 partially mixed zone.
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Characterization of the interpass microstructure in low alloy steel/Alloy 625 HW-GTAW narrow groove welds
A dissimilar weld between a low alloy steel (LAS) butter weld joined to a F65 steel pipe using a narrow groove hot wire gas tungsten arc welding (HW-GTAW) procedure with Alloy 625 filler metal was investigated. The weld interpass microstructure is comprised of large swirls formed by a macrosegregation mechanism involving partial, non-uniform mixing of liquid base metal with the lower melting temperature weld pool, followed by fast solidification. This mechanism produces steep gradients in composition and solidification behavior. The resulting swirls are composed of alternating iron-rich peninsulas and partially mixed zones (PMXZ) that are surrounded by planar and cellular zones exhibiting multiple solidification directions. Large austenitic grains, encompassing planar, cellular, and dendritic morphologies, nucleate off peninsulas in direct contact with the weld pool. The highest hardness was found in nickel and chromium rich PMXZs that exhibited a lath martensite microstructure. In the event of exposure to hydrogen containing environments, the PMXZs could serve as nucleation sites for hydrogen assisted cracking.
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- Award ID(s):
- 2052747
- NSF-PAR ID:
- 10506625
- Publisher / Repository:
- ScienceDirect
- Date Published:
- Journal Name:
- Materials Characterization
- Volume:
- 170
- Issue:
- C
- ISSN:
- 1044-5803
- Page Range / eLocation ID:
- 110638
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.matchar.2020.110638
