Abstract

Demand for improved pipeline efficiency has directed designs towards larger diameters and higher operating pressures. High strength steel with increased wall thicknesses of the pipe provide for greater available pressure capacity for the pipeline. However, toughness of the line pipe, particularly at low temperature, is challenged when increasing the strength of the steel and with a greater wall thickness of the pipe.

This work discusses the efforts to optimize alloy and thermomechanical controlled processing (TMCP) designs to achieve X80 with good low temperature toughness. Two X80 helical line pipe steel designs are presented; X80-A with low Ni and X80-B with high Ni with gauges of 18.5 mm and 19.1 mm, respectively. This material was cast, rolled, and formed into pipe as part of production trials. Processing data is presented and shows good consistency was achieved in the trials. Results from the tensile and drop weight tear tests (DWTT) are discussed. Both X80-A and X80-B met tensile requirements for an X80 material with an average yield strength of 614 MPa and 586 MPa. In terms of DWTT, X80-A passed −5°C whereas X80-B passed both −20°C and −30°C. In terms of microstructures, nital and LePera etchings along with optical microscopy showed that X80-A and X80-B have fine grain ferrite microstructures with a minimal amount of martensite/retained austenite.

A larger data set consisting of X70 and X80 material from production and trials within the gauge range of 17.8 mm to 20.3 mm are introduced. The data set shows some expected general trends such as decreasing DWTT performance with increases to strength. Various TMCP factors such as roughing last pass temperature and mean flow stress as well as microstructure are also discussed with respect to their impact on DWTT results. Low Ni contents are shown on average to perform better than other levels in the current data set, owing mainly to the optimization efforts of developing a −5°C X80 product.

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