The role of continuous cooling transformation diagrams in material design for high strength oil and gas transmission pipeline steels

The economical, environmental, and safe movement of gas and oil to the marketplace requires transmission pipelines to be designed to operate at higher pressures and/or with improved toughness over a variety of temperature ranges. To meet the higher strength and toughness specification requirements of these transmission pipelines, appropriate materials and processes must be used in their design and construction. This includes selection of appropriate alloy composition, processing routes, microstructure control, and cost. A continuous cooling transformation (CCT) diagram is a tool that can be used to select alloy composition and processing route in order to obtain a specific, desirable microstructure for transmission linepipe steels in a cost-effective manner. In the past, CCT diagrams were developed experimentally under laboratory conditions, thus requiring extensive time and effort. However, with the vast data available and improved computational tools, reasonably accurate computer generated CCT diagrams can be produced quickly. These computer generated diagrams can give the materials design engineer a reasonable understanding of the effect of subjecting a given alloy to various processing routes and hence the resultant microstructures. Since final microstructure is a key variable in determining the linepipe steel material properties, the chosen alloy/processing route and its effect on the final microstructure needs to be understood. This paper will discuss the role of CCT diagrams in the design of steels (cost, alloy, processing, and microstructure) for oil and gas transmission pipelines. Examples of computer generated CCT diagrams for various API alloy designs are included.