Development of Q&T weldable seamless pipes of 100 ksi grade

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 into two mains phases, the first one devoted to the development and production of seamless pipes intended for deepwater top tension risers (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 defining the yield strength of the various materials; toughness was related to the inverse square root of the packet size; for a given prior austenite grain size, an increase in the martensite volume fraction formed after quenching leads to a finer packet, thereby 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 to establish a production route for Q&T seamless pipes for deepwater TTR in 100 ksi grade. (AU)