The purpose of this study is to evaluate the effect of metallurgical factors on SOHIC of two HIC free linepipe steels in a sour environment. HIC, SSC and SOHIC tests of two steels were performed in reference to NACE TM0284, 0177 and 0103 standard test methods, respectively. Results showed that ferrite-acicular ferrite microstructure is more resistant to SSC and SOHIC than ferrite-pearlite. Inclusions in two steels acted as crack nucleation sites. In the point of view of crack propagation, SOHIC characteristics of two steels were consistent with SSC test results.
Cracking of linepipe steels in the environments containing hydrogen sulfide gas (H2S) is generally categorized into two types; hydrogen induced cracking (HIC) and 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 into the steel. In the steels, hydrogen diffuses to the regions with a high triaxial tensile-stressed condition, or various defects such as inclusions, precipitations or dislocations that act as hydrogen trapping sites and causes embrittlement of steel. [Kimura, M., 1989] 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 linepipe 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 the applied stress.
