| With the continuous development of the construction of oil and gas pipelines,the challenges facing the safe operation of pipelines are also increasing.Hydrogen-induced cracking(HIC),as one of the main failure forms of pipeline steel,has been widely concerned by the petroleum industry and academia.Improving the HIC resistance of pipeline steel has always been the focus and difficulty of pipeline engineering construction.At present,the pipeline steel with ferrite(F)as the main structure is still widely used.Exploring the relationship between its microstructure and HIC behavior can provide theoretical basis and data support for guiding the production process of pipeline steel and further optimizing its HIC resistanceIn this paper,the X65 pipeline steel with F as the main structure was used as the research object.Electrochemical hydrogen charging,hydrogen permeation,dynamic hydrogen charging test,and silver micro-print technology were used to evaluate the hydrogen induced cracking resistance and hydrogen diffusion behavior of pipeline steel.By means of optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron backscatter diffraction(EBSD),the influence law and mechanism of microstructure,grain size,grain orientation and grain boundary structure on HIC behavior of pipeline steel were discussed.The main results can be drawn as followed(1)The hydrogen trapping efficiency at the interface of different structures in pipeline steel is M/A island/F interface>P/F interface>F grain boundary,and the HIC susceptibility is F+M/A structure>F+P structure>F structure.(2)The irreversible hydrogen trap density in pipeline steel changes with the grain size in order of 31.0 μm>5.5 μm>9.5 μm,but the HIC susceptibility decreases with the decrease of grain size.A sufficient number of trigeminal grain boundaries as irreversible hydrogen traps increase the hydrogen trapping efficiency on the one hand,and can significantly disperse the hydrogen pressure on the other hand.(3)Samples with different F grain orientations have different HIC susceptibility:{112}//ND and {001}//ND orientation both increase the HIC susceptibility of pipeline steel,while {110}//ND orientation can reduce hydrogen trapping efficiency and improve the HIC resistance of pipeline steel.(4)High-angle grain boundaries as irreversible hydrogen traps mainly promote the initiation of hydrogen-induced cracks,while low-angle grain boundaries as reversible hydrogen traps mainly promote the propagation of hydrogen-induced cracks When the material is dominated by low-angle grain boundaries or the ratio of high-angle and low-angle grain boundaries is about 1:1,hydrogen trapping efficiency and HIC susceptibility are relatively high. |
