Metallurgical Design of Steel Plates With Advanced Cryogenic Properties For Fabrication of Pressurized-LNG Containers

This paper discusses the plate development technology for manufacture of the containment system for a new concept of transporting gas by Pressurized LNG. The plates to produce the cylindrical containment system were required to exhibit a combination of strength and toughness at cryogenic temperatures down to −101˚C (172 K). The plate toughness was achieved over a range of nickel contents from 2% to 5%, by chemistry control, thermo-mechanical processing and controlled cooling to achieve a combination of marten site, fine granular bainite (FGB) and retained austenite. It was observed that the formation of FGB was required to achieve the toughness.



INTRODUCTION

Critical to the development of the Pressurized LNG (PLNG) technology described in this conference (Bowen et al., 2005; Fairchild et al., 2005; US Patents 2000, 2001) were the development of plates and weldments that exhibit high strength and toughness at a service temperature of 172 K to manufacture the containers for transport. Specifically, the steel plates, weld metal and HAZ have to meet the fracture-toughness target of 0.08 MPa-√m at 172 K and the tensile strength target of 1000 MPa. In order to maintain the economic advantage of the PLNG technology, it was also required that the cost of the plates, the major volume component of the containers, be kept low. Because steel plates based on very high nickel contents ( >9%) were considered uneconomical, a maximum level of 5% Ni was identified to preserve the economic incentives. Hence a cost-effective approach was required to impart toughness to steels containing relatively low levels of nickel (i.e. ≤5%). This paper describes a novel approach, consisting of a combination of steel cleanliness and thermo-mechanical processing coupled with phase transformation to achieve the required low-temperature toughness in low nickel steels.