Abstract
In our previous study, intercritical quenching from the dual-phase region of ferrite and austenite regions, which is called lamellarizing (L) treatment, brought a clear improvement of balance between the strength and toughness of Cu-containing low alloy steel based on American Society for Testing and Materials (ASTM) A707 5L grade. Moreover, the results imply that the reverse transformation behavior during L treatment is very important in order to optimize the L treatment temperature. Hence, the purpose of this paper is to clarify the mechanism by which L treatment improves the mechanical properties in terms of reverse transformation behavior. Additionally, offshore structures require good weldability, because the structures generally have a lot of weld joints. Therefore, weldability was also investigated in this study. The investigation revealed that coarse Cu precipitates are observed in the not-transformed α phase, so the strength tendency in relation to the L treatment temperature is relevant to the area ratio of the not-transformed α phase and the transformed γ phase during L treatment. From the in situ electron back scatter diffraction (EBSD) results, it is believed to be possible to enhance the mechanical properties of Cu-containing low alloy steel by controlling the area ratio of the reverse-transformed gamma phase and selecting the appropriate L treatment temperature. Furthermore, the long part forging of Cu-containing low alloy steel has a good weldability, since the maximum hardness of the heat-affected zone (HAZ) is less than 300 HV, and the HAZ of steel has a good crack tip opening displacement (CTOD) property with less than 2.3 kJ/mm of heat input of GTAW.
Highlights
Oil and natural gas are major energy sources, accounting for about 60% of the primary energy in the world
Copper-containing low alloy steel based on the American Society for Testing and Materials (ASTM) A707 5L grade is widely used for structural parts of offshore wells because of its low carbon equivalent (Ceq ) and weld crack sensitivity composition (Pcm ) [2,3,4]
The specimens used in this study were cut from a 50-kg steel ingot made by vacuum induction melting and a full-scale large forging that weighed approximately 10 metric tonnes
Summary
Oil and natural gas are major energy sources, accounting for about 60% of the primary energy in the world. Ultra-deepwater development is expected to increase in the future, as demands for oil and natural gas increase [1]. Copper-containing low alloy steel based on the American Society for Testing and Materials (ASTM) A707 5L grade is widely used for structural parts of offshore wells because of its low carbon equivalent (Ceq ) and weld crack sensitivity composition (Pcm ) [2,3,4]. From the viewpoint of preventing failure and accidents, steel with high strength and excellent low-temperature toughness is required. The forging steel is subjected to quenching (Q) and tempering (T). The steel is reverse-transformed from α to γ by holding the quenching temperature; after that, γ is transformed to bainite or martensite by water cooling.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
