Mechanical property-microstructure relationships of an X70 pipeline steel at elevated temperatures

Abstract Mechanical property-microstructure relationships of an X70 pipeline steel were evaluated at temperatures up to 400 °C using tensile testing, scanning and transmission electron microscopy, and synchrotron wide-angle X-ray scattering characterization techniques. The X70 steel had a bainitic microstructure consisting of quasi-polygonal ferrite, retained austenite, martensite-austenite islands, and carbide/nitride microconstituents. Results are compared to an X52 steel with a ferrite-pearlite microstructure. Both steels exhibited dynamic strain aging (DSA) as evidenced by serrated yielding, reductions in strain rate sensitivity, increased strengths, and reduced dutilities in the approximate temperature ranges of 100–250 °C. For temperatures above the DSA range, the X70 steel exhibited unique properties of an increase in strength simultaneously with an increase in uniform strain, both features interpreted due to microstructural changes during testing due to dynamic tempering, indicated by decomposition of the retained austenite and associated cementite and transition carbide nucleation and coarsening, i.e. microstructural changes normally associated with static tempering at temperatures above 400 °C. However, if the X70 steel was tempered at the test temperature prior to testing, dynamic microstructural changes were absent and the steel exhibited behavior similar to that observed for the X52 steel, i.e. a decrease in strength and increase in ductility at temperatures above the DSA range. New alloying strategies are suggested to improve microstructure stability during isothermal holds at elevated temperature, while maintaining the strength benefits from strain assisted bainitic tempering during plastic deformation.