THE EFFECT OF METALLURGICAL FACTORS ON SSC OF HIGH STRENGTH PIPELINE STEEL

摘要

Sulfide stress cracking (SSC) is a kind of hydrogen related problem of pipeline steels for transportation of crude oil and natural gas containing hydrogen sulfide. To improve SSC resistance, the various techniques during steel making processes are required such as: addition of proper alloying elements, microstructure control, high cleanness and Ca treatment. However, the effect of metallurgical factors on SSC is not clearly understood in a lot of reports for improvement of SSC resistance. In this study, SSC resistance of three high strength steel plates with different microstructures was evaluated by using NACE TM0177A method. Failure behavior of SSC was investigated by observing fractured surfaces using optical microscopy (OM) and scanning electron microscopy (SEM). SSC property was discussed in terms of metallurgical parameters such as: primary microstructure, second phases and inclusions. Results showed that in the case of steels having the same oxide inclusion level, the hard microstructure is more resistant than the soft microstructure to SSC. SSC fracture mode is the type I SSC. Cracks parallel and perpendicular to the loading axis were primarily related with inclusions including Al, Ca and oxide. In particular, TiNb(C.N) with size over 2 μm as a large precipitate acted as nucleation sites of cracks perpendicular to the loading axis and it increased SSC sensitivity.