The Effects Of Inclusions Of Pipeline Steel On Hydrogen Induced Cracking And Hydrogen Diffusion

In this paper,three experimental pipeline steels were smelt and rolled,controlling the inclusions formation by means of Ti modification,Mg modification and Ti-Mg compound modification.The shape,composition,size and distribution characteristics of inclusions,as well as the HIC susceptibility of three experimental pipeline steels were measured and analyzed.The hydrogen trap density,lattice hydrogen concentration,reversible hydrogen concentration and irreversible hydrogen concentration were calculated according to the hydrogen permeation kinetic parameters by fitting the hydrogen permeation curves.The influence of the shape,composition,size and distribution of inclusions on the HIC susceptibility of three experimental pipeline steels was discussed.The main research progress and results are as follows:(1)The shape and size of inclusions in experimental pipeline steels can be controlled effectively by means of Ti modification,Mg modification and Ti-Mg compound modification.More than 50% inclusions are submicron Mn S.The inclusions in Ti-Mg compound modified experimental steel,which cores are Al-Mg-Ti oxides,hardly gather and grow up.Therefore,the inclusions are fine and distribute dispersedly.(2)Three modification methods can improve the HIC resistance of experimental steels significantly.The HIC resistance of three experimental steels meets the European standard,and the order of HIC resistance is: Ti-Mg compound modification > Ti modification > Mg modification.Due to the fine and dispersive inclusions,Ti-Mg compound modified experimental steel has the best HIC resistance.(3)The direction of banded inclusions influences hydrogen diffusion obviously.Hydrogen atoms diffuse fast when the direction of charging current is parallel to the direction of banded inclusions.Consequently,hydrogen diffusion coefficients increase markedly.Irrespective of the influence of banded inclusions,the HIC susceptibility isproportional to the quantity and area density of inclusions.(4)Irreversible hydrogen is the dominant factor for HIC sensitivity in all forms of hydrogen.The HIC possibility increases as the irreversible hydrogen concentration increases.Ti-Mg compound modification leads to the less large inclusions and more dispersed fine inclusions.The irreversible hydrogen concentration decreases,The HIC resistance,therefore,improves effectively.