Research On Hydrogen Induccd Cracking Behaviors Of Different Microstructure Pipeline Steels

Pipeline steel in the process of transporting oil and gas, it is easy to occur HydrogenInduce Cracking (HIC) and lead to lose efficacy because of the influence of H2S contained inthe oil, it will cause great potential safety hazard to environment and security of person. So itwill have great practical significance to study the mechanism and influence factors of HIC, itwill help to enhance the HIC resistance and prolong the service life of the pipeline steel.This article chooses three typical microstructure of pipeline steel:1#(ferrite+pearlite),2#(Acicular ferrite),3#(ultra low carbon bainite). The NACE standard TM-0284-2003was usedto compare the HIC sensitivity of the three kinds of pipeline steels. Use optical microscope andscanning electron microscope and EBSD to observe the crack. Combined the theory ofhydrogen pressure to analyze the nucleation mechanism of the HIC, and analyzed themetallographic structure characteristics and the grain boundary structure on the influence ofcrack propagation. The converted Davanathan-Stachursky device was used to measure thehydrogen permeation kinetics parameters of the steels, the relationship between HIC sensitivityand hydrogen trapping efficiency was analyzed. The effect of microstructure of differentthickness on HIC behaviour was studied by observing the crack location. The results showedthat:(1) Under the environment of acid wet H2S, the HIC in the steel usually nucleate on theinclusion and banded structure, and the banded structure’s HIC sensitivity was stronger than theinclusion.(2) The HIC crack in F+P steel mainly propagate along the pearlite banded structure. The crackin AF steel mainly along the boundary of the acicular ferrite and polygon ferrite while it mainlyalong the original austenite grain boundary in ULCB steel.(3) Hydrogen permeation kinetics parameters can be used to well explain the HIC sensitivity ofthe materials. The low angle boundary have resistance to HIC in the pipeline steel, the higherthe proportion of the low angle boundary, and the better the steel to resist HIC.(4) The more the value of hydrogen flux J∞, hydrogen diffusion coefficient Deffand the lowerthe value of solubility of hydrogen in the steel, the less pipelines steel is prone to HIC.(5) The different pipeline steel has the different HIC sensitivity in different thickness. TheULCB steel’s HIC was mainly occur in the1/4thickness, because the microstructure in theseplace has the strong ability to capture hydrogen atom and has the high HIC sensitivity.