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Monday 23 January 2006
DEVELOPMENT OF Q&T SEAMLESS X100 PIPES
E. Anelli, A. Di Schino, G. Porcu, A. Izquierdo, H. Quintanilla, G. Cumino, M. Tivelli
Referee: P.E. Di Nunzio
Paper presented to Proceedings of the International Symposium on Microalloyed Steels for the Oil & Gas Industry,
Araxà (Brasil), 23-25 January 2006.
The Joint Industrial Program “Seamless 100 ksi weldable” was launched by Tenaris in June 2003 in order to address the complex design issues of high strength seamless Q&T pipes. The JIP was split in two mains phases, the first one devoted to the development and production of seamless pipes intended for deepwater top tension riser (TTR), with yield strength greater than 100 ksi (690 MPa), and the second one to evaluate their field weldability. Phase I was recently completed.
The role of chemical composition and Q&T process conditions on microstructure and precipitation was analyzed, together with relevant effects on strength and toughness, for both laboratory and industrial steels. The main microstructural features which control the strength-toughness combination of these high grade Q&T steels were identified:
• The sub-grain size is the key microstructural parameter in defining the yield strength of the various materials.
• The toughness was related to the inverse square root of the packet size.
• For a given prior austenite grain size, the increase of the martensite volume fraction formed after quenching leads to a finer packet, enhancing toughness.
• Fine packets and sub-grains, suitable to achieve the target strength-toughness combination, i.e YS > 690 MPa (100 ksi) and FATT < – 50 °C (– 45 °F), were obtained when the as-quenched microstructure was mainly constituted of low-C martensite (M > 60%).
These results can be exploited for the set-up of a production route of Q&T seamless pipes for deepwater TTR of 100 ksi grade.
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