Alloy designs for high strength oil and gas transmission linepipe steels

The economical movement of gas and oil to the marketplace from remote and rugged locations requires transmission pipelines to be designed to operate at higher pressures with improved toughness over a variety of temperature ranges. This is accomplished by increasing either pipe wall thickness or strength, or a combination of both. Increasing wall thickness adds cost to the installation of the pipeline; therefore, specifying higher strength has been the typical pipeline designer’s standard practice, with API X70, X80 and beyond being routinely specified over the past 10 years, coupled with increased toughness requirements at various design temperatures. Numerous alloy designs have been used for the production of the higher strength and toughness grades, but these have yielded only two basic types of microstructure: ferrite/pearlite and ferrite/acicular ferrite - each of which behaves fundamentally differently through the pipe making process. Rolling mill and pipe-making equipment capabilities, in addition steel cost should be used to determine which of the two microstructures is most suitable for meeting the requirements of a particular pipeline project, and computer models have been developed to assist in this. There are two distinctly different niobium-based alloying approaches to produce the ferrite/acicular ferrite microstructure. One design that has been well documented over the past 15 years uses molybdenum additions. This approach also relies on low temperature controlled rolling which can lead to issues with mill equipment and productivity, depending on the particular plate mill age and design. The other, more recent alloy design utilizes higher niobium (with moderate levels of other strengthening elements). This latter chemistry is unique in that it has the ability to produce the high pipe strength and body impact toughness required for modern transmission pipelines utilizing a higher than normal processing temperature during plate rolling – coined ‘High Temperature Processing’ or HTP. This, in turn, improves productivity and alleviates certain rolling issues associated with the traditional alloy design. This paper discusses the two different microstructure designs and the alloying/rolling approaches used to generate them, with a focus on the HTP concept. Actual pipeline project results, together with rolling/pipe making equipment and testing issues, and the use of modeling for the prediction of plate and pipe strength are also presented. (AU)