Effect of tempering temperature on hydrogen embrittlement of X60 pipeline steel

In this paper, the effects of microstructure changes on the hydrogen diffusion behavior and hydrogen embrittlement (HE) of X60 pipeline steel at different tempering temperatures were systematically investigated through slow strain rate tensile (SSRT) tests, gas phase hydrogen penetration tests and thermal desorption spectroscopy (TDS) tests, combined with scanning electron microscope (SEM), electron backscattering diffraction (EBSD), x-ray diffraction (XRD) and transmission electron microscope (TEM) characterization methods. The results show that with increasing tempering temperature, the I HEA of X60 pipeline steel in H 2 gradually decreases, while the area of the ductile fracture zone on the fracture surface increases. This indicates that the HE of the steel decreases significantly with increasing tempering temperature. When the tempering temperature reaches 660 °C, the comprehensive mechanical properties and HE resistance of X60 steel reach the optimal value. As the tempering temperature increases, the effective grain size of the test steel does not change significantly, while the dislocation density decreases, and the degenerated pearlite (DP) decomposes. Meanwhile, Nb–V composite microalloying promotes the extensive precipitation of the nanoscale (Nb, V, Ti)C second phase particles during the high-temperature tempering. This results in a significant increase in the number of hydrogen traps (N T ) in the steel and a substantial decrease in the apparent diffusion coefficient (D app ). Essentially, this is attributed to the coupling effect of the reduction in reversible hydrogen traps and the increase in irreversible hydrogen traps. • First study of hydrogen embrittlement (HE) in X60 steel vs. tempering temperature. • Fewer reversible and more irreversible traps enhance HE resistance in tempered X60 steel. • The HE resistance and strength of X60 steel peak at a 660 °C tempering temperature.