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.
Cracking of pipeline steels in environments containing hydrogen sulfide gas (H2S) is generally categorized into two types; hydrogen induced cracking (HIC), sulfide stress cracking (SSC). Both HIC and SSC belong to hydrogen embrittlement phenomena. Hydrogen atoms generated by a sulfide corrosion process are adsorbed on the steel surface and diffuse to the steel. In the steels, hydrogen diffuses to the regions that have high triaxial tensile-stressed condition or various defects such as: inclusions, precipitations, or dislocations that work as hydrogen trapping sites and cause embrittlement of steel. 1 Unlike HIC which develops at conditions without applied stress, SSC occurs under externally or internally stressed or strained conditions and propagates perpendicularly to the tensile stress direction. SSC of pipeline steels exposed to sour environment under external stress is classified into type I and type II. Type I SSC can be understood in two stages. The first stage is the formation of hydrogen induced internal blister cracks parallel to applied stress. In the second stage, the blister cracks link together perpendicularly to applied stress. Generally, type I SSC is referred to as stress-oriented hydrogen induced cracking (SOHIC) because of formation of the blister cracks parallel to applied stress. On the other hand, type II SSC is recognized to be the cracking which results from the typical hydrogen embrittlement. The final failure occurs in the direction perpendicular to applied stress in the manner of quasi-cleavage. Specifying the maximum hardness of 248 in Vickers hardness has been required to prevent type II SSC. 2, 3, 4 This study focuses on type I . Type II SSC was excluded from the present study because hardness of the tested steels is less than 248 in Vickers hardness.
