Abstract
In this paper, the electrochemical hydrogen charging for X70 pipeline steel in H 2SO 4 solution was carried out and the influence of non-metallic inclusions and microstructure on the hydrogen-induced cracking (HIC) and healing behavior of X70 pipeline steel was investigated by SEM and TEM technique. Based on the results of hydrogen permeation tests, the diffusivity of hydrogen at room temperature and the density of hydrogen trapping sites in X70 steel were estimated. Heat treatment was used to heal the hydrogen-induced cracks in X70. It was found that non-metallic inclusions, such as nitrides and oxides, played different roles on HIC of X70 steel. Oxides are more sensitive than nitrides to HIC in X70 steel. Cracks initiated and grew preferentially along the grain boundary of polygonal ferrite instead of the acicular ferrite when the sample was charged of enough hydrogen. It also demonstrated that proper heat treatment can heal such cracks. The healing process was divided into two steps: firstly, many cracks dissociated into several detached segments; secondly, theses segments regressed into spherical voids. The mechanism of crack healing was also discussed.
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