The Development of Large-Diameter HighStrength Line Pipe for Low-Temperature Service

ABSTRACT The evolution of large-diameter U-O-E submerged-are-welded line pipe is discussed in relation to the changes in both steel composition and processing that have been made over the years to meet' the ever increasing demands for higher strength, better notch toughness, and improved weldability. Continued research in steel development and processing has resulted in a new class of high strength low-alloy steels that are capable of being used for transmission pipe for low-temperature environments. The development of one of the steels for this new class of line pipe, a low-carbon, low-sulfur molybdenumcolumbium steel, is described. The improved strength-toughness relationship in this product is the result of the combination and interdependence of the steel composition and controlled rolling practice. The improved weldability is demonstrated by the results of underbead cracking tests on a number of line-pipe-type steels, and the data are discussed in relation to various carbon-equivalent formulas. Studies of splitting, a phenomenon that has appeared in these newer control rolled steels for line pipe, are also discussed. In addition; some recent work to determine the effects on the strength and toughness of line pipe of the thermal cycles used for pipe coating and the plastic straining to which the pipe is subjected in field bending is described. DISCUSSION The production of large-diameter line pipe made by the U&O process began around 1950. Initially the pipe made by this process was fairly low strength, up to about API Grade X-46, using semi killed carbon-manganese steel plate a typical composition would be 0.25 percent carbon and 1.00 percent manganese. During the 1950's, because of the desire to increase the operating pressure of additional pipelines being planned, pipe having 52 and 56 ksi minimum yield strength were developed. This was followed by the development of pipe having minimum yield strengths of 60 and 65 ksi in the 1960's. Because the trouble free operation of a pipeline is greatly dependent on the integrity of both the seam welds and girth welds, the steel must have good weldability. This fact prevented achieving the desired increases in strength simply by increasing the carbon and manganese contents of the steel. The approach that was adopted was to develop steels that achieved increased strength from small alloying additions of vanadium (up to about 0.08%) and/or columbium (up to about 0.04%). As the number of pipelines constructed from these larger-diameter higher-strength pipe increased, a new problem became evident the consequences of a failure in a pipeline were potentially much more disastrous. Full-scale fracture experiments conducted for the AGA on test pipelines (primarily of Grade X-52 pipe) pressurized with natural gas showed that the distance a fracture would propagate was a function of temperature that is, if the temperature of the pipe in the line was below.