The Short and Long Term Effects of Elevated Temperature on the Mechanical Properties of Line Pipe Steels

The mechanical properties of six industrially produced pipeline steels equivalent to API X52, X60, and X70 specifications were evaluated in the temperature range of 200–350 °C. The steels were tested in uniaxial tension at strain rates of 10−4 and 8 × 10−4 s−1 in the as-received condition and after a low temperature 100 h aging process under a 419 MPa tensile stress. Dynamic strain aging was identified in the tensile data with the observation of serrated yielding, minima in ductility and maxima in ultimate tensile strength with respect to temperature. In addition to minima in ductility, higher strength grade steels exhibited maxima in ductility at high temperatures and greater amounts of strengthening compared to the lower strength grade, both which could be attributed to the precipitation of carbides or nitrides during tensile deformation. The low temperature aging process resulted in increased yield strength due to static strain aging, slight changes to ultimate tensile stress and, no observable change in ductility. Thus, based on the results discussed it is suggested that pipeline steels can be designed based on room temperature tensile properties, using established corrections for such properties at elevated temperatures.