The role of steel chemistry on the microstructure/properties in the heat affected zone for high strength linepipe

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Pipelines to transport energy (natural gas and liquids) are an integral part of the North American energy landscape. Canada alone has more than 75,000 km of natural gas and 25,000 km of liquids (e.g. refined oil and bitumen) pipelines. Further, there are proposals to construct new pipelines such as TransCanada Pipeline's Energy East project to deliver oil products from Alberta to refineries on Canada's east coast (4,500 km 1.1 million barrels of oil per day). The safe and economic construction and operation of this extensive pipeline network is of strategic importance to Canada. There is a trend towards using pipelines of larger diameter and/or increasing the operation pressure of the pipeline to improve the throughput of the system. This leads to using higher strength linepipe steel grades to avoid using thicker wall linepipe steel. The current state of the art uses X70 or X80 steel grades (yield stress of 490 and 560 MPa, respectively). Using higher strength steels translates into less material being required thereby lowering material and transportation cost. One of the engineering challenges for these materials is the development of robust manufacturing and in field joining processes, i.e. girth welds. This project addresses aspects related to the mechanical behavior of the steels used and, in particular, the effects of welding on these properties for a range of different steel chemistries. Currently, the evaluation of steels follows performance based specifications such as the yield stress, i.e. the composition of the steel is not explicitly considered. There are two fundamental challenges that face the industry. For steel producers such as Evraz, the challenge relates to chemistry optimization (in particular, the appropriate alloy additions of Cr, Mo and Nb). On the other hand, pipeline constructors/operators such as TCPL have to understand how the different steels will respond to the heat input from welding during the construction of the pipeline. The results from this project will be i) an increased knowledge base for the microstructure/mechanical properties in the heat affected zone for advanced, ii) transfer of this knowledge to our industry partners and iii) the education of 5 high qualified personal (3 Ph.D., 2 PDFs)