| In the thesis,X52,20,X65,X70and high strength pipeline steel(number:36#,38#,42#) were used as experimental materials. Hydrogen was charged into X52,20and X70pipeline steel in different time.The inclusions were characterized by optical microscopey(OM) scanning electron microscopey(SEM) and energy-dispersive X-ray analyzer(EDXA),the correlation between inclusions and Hydrogen-induced cracking (HIC) was studied. HIC test was measured accroding to the NACE standard TM0284-2003《Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-induced cracking》"A" Solution, used in the evaluation of HIC susceptibility of X65and three kinds of high strength pipeline steels.The micro structures and hardness distribution of X65sour pipeline steel were characterized by using OM,SEM,EDXA,transmission electron microscopy(TEM) and FM-300microhardness tester, their correlations with the HIC susceptibility were studied.another work first use of LIBS(Laser Induced Breakdown Spectroscopy)-original position statistic(OPA)-100to HIC area in C and other alloying elements to segregation detection, studied the correlation between segregations and HIC; The influence rule of M/A on the hydrogen sulfide corrosion resistance were studied by OM,SEM,EDXA,TEM and LIBS-OPA-100.The Electron Back Scattering Diffraction (EBSD) technique was used to analyze the microstructure misorientation of high strength pipeline steel,studying the correlation between misorientation and HIC.The electrochemical parameters of high strength pipeline steels were measured by Princeton Applied Research Model273A,comparative analysising before corrosion and after corrosion.The main conclusion were as followed.(1) The HIC sensitivity in the same time was in order of20>X70>X52. The HIC of tested steels(X52,20,X70) is nucleated and extended at the inclusions(mainly Al-O).The HIC of X52is initiated in the Al-O and spherical oxide inclusion. Microscopic observation showed that there is no dependent relation between cracks induced by hydrogen and the Mns inclusions of20steel. Otherwise, the Al-O, composite oxides/Fe-C and Si-C are more harmful as the sources of hydrogen induced cracks; The HIC of X70are primarily associated with the Al-0and Fe-C inclusions, large cracks formed due to the growth and merging of the crack source at adjacent inclusions.Ti-C inclusion is not easy to became source of HIC except for Al2O3-Ti inclusion.(2) There are higher hardness value and some points above the critical value of HRC22(HV248) in center banded structure area. The center banded ferrite/MA structure have higher sensitive to HIC than ferrite. The result shows that both banded structure formed by C, Mn, S, Si elements in the center and higher hardness lead to hydrogen-induced cracking.(3) M/A microstructure of high strength pipeline steel is main source of HIC and immediate cause for unqualified HIC, and Al-0inclusion also decreased the resistance to HIC. Mo promotes acicular ferrite and restrains banded structure to the benefit of HIC resistance, on the other hand, Mo also accelerates M/A to the disadvantage of HIC, which should be controlled in appropriate scope. Ti promotes acicular ferrite to the benefit of HIC resistance.(4) High-angle easily become the channel for cracking propagation, there is a crack arrest effect of the small angle grain boundary.No Mo high titanium(42#) steel corrosion resistance is better,the current density of value is in an order of magnitude. |
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