Study on the mechanism of quenching-tempering heat treatment to strengthen the plasticity of X80 long-distance pipeline steel

The present study investigates the microstructural characteristics and mechanical properties of X80 pipeline steel following heat treatment at various temperatures using the quenching-tempering process, while also exploring the interrelationship between these two transformations. The alloy phase transition temperature of X80 pipeline steel was determined using differential scanning calorimetry (DSC), and the heat treatment parameters were subsequently determined. The effect of microstructure on the mechanical properties of heat-treated X80 pipeline steel was investigated using metallographic microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers hardness testing, and slow strain tensile testing. The results demonstrate that an increase in the quenching-tempering temperature leads to a refinement and uniform distribution of grain size within the microstructure of X80 pipeline steel specimens. The grain sizes of the presence of polygonal ferrite, quasi-polygonal ferrite, pearlite, bainitic ferrite, and granular bainitic ferrite in the microstructure of quenched at 950 °C and tempered at 650 °C exhibited reductions of 36.4%, 27.1%, 49.9%, 44.5%, and 50.6% respectively. Additionally, the area fractions of polygonal ferrite, pearlite, bainitic ferrite, and granular bainitic ferrite experienced decreases by 33.9%, 5.7%, 67.2%, and 55.3% correspondingly; whereas the area fraction of quasi-polygonal ferrite demonstrated an increase by 59.5%. Compared to the original X80 pipeline steel, the yield strength and tensile strength of the specimens decreased by 13.2% and 9.1%, respectively, after quenching at 950 °C for 15 min followed by tempering at 650 °C for 15 min. Additionally, the elongation increased by 59.5% while the section shrinkage increased by only 14%. After quenching treatment, X80 pipeline steel can effectively refine its microstructure. Tempering treatment further stabilizes and uniformly distributes the refined grain structure, optimizing the plastic properties of the steel for pipeline applications.