Hydrogen sulfide contained in crude oil and natural gas causes hydrogen related problems such as hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) of pipeline steels. In particular, HIC is a more serious problem because it occurs without applied stress. To improve HIC resistance, addition of proper alloying elements, high cleanness and Ca treatment during steel making processes are required. However, the effects of metallurgical factors on HIC are not well understood to improve HIC resistance of pipeline steels.
In this study, HIC resistance of two high strength steel plates was evaluated by using NACE TM0284- 02 methods. After the tests, HIC resistance was measured in terms of crack area ratio (CAR) using ultrasonic detector. Fracture behavior of HIC was investigated by observing fractured surfaces using optical microscopy (OM) and scanning electron microscopy (SEM). HIC property was discussed in terms of metallurgical parameters such as primary microstructure, second phases and inclusions. Results showed that cracks nucleated in the steels are affected not only by the presence of inclusions but also by the microstructure surrounding it. In this study, the effect of various metallurgical factors on nucleation and propagation of HIC was discussed in detail.
Cracking that line pipe steels may encounter in sour environments containing hydrogen sulfide gas (H2S) is generally categorized into two types; HIC and SSC. Particularly, HIC is a more serious problem because it occurs even without applied stress. It has been reported that HIC is caused by hydrogenmetal interaction.1 That is, hydrogen atoms formed by steel corrosion process are adsorbed on the steel surface and diffuse in steels. The diffused hydrogen atoms are accumulated and form molecules at segregated structure or defects in steels such as elongated MnS inclusion. The molecules of hydrogen atoms lower the critical stress for cracking. Consequently, steels are fractured by critical hydrogen pressure. HIC occurs in the form of surface blisters and/or internal cracks in the absence of applied stress. HIC is commonly found in steels with high impurity levels that have a high density of large planar inclusions and/or regions of anomalous microstructure produced by segregation of impurity and alloying elements in the steel. In steels, the development of internal cracks tends to link with other cracks. The link-up of these cracks on different planes in steels is often referred to as stepwise cracking.1, 2
HIC is affected by two factors. One is an environmental factor, such as pH, H2S partial pressure, CO2, Cl- and temperature in aqueous solution. The other is a metallurgical factor including alloying elements, microstructure, inclusion, precipitates, etc.3 To improve HIC resistance of pipeline steels, .the effect of metallurgical factors on HIC should be clearly elucidated. However, the effects of those on HIC are not well understood. The objective of this paper is to clarify the effect of metallurgical factors developing HIC in pipeline steels. In this study, the effect of metallurgical factors such as primary microstructure, second phases and inclusions on nucleation and propagation of HIC was investigated in detail.
