Hydrogen-induced cracking (HIC) is a hydrogen embrittlement phenomenon that occurs in a line pipe exposed to the sour environment. Generally, the initiation site of HIC is estimated by observing the fracture surface after the test. It is reported that typical initiation sites of HIC are non-metallic inclusions, such as elongated MnS, in the center segregation region. However, there is no in-situ observation technique that can establish a direct link between the initiation site and inclusion. In this study, an in-situ HIC measurement technique was developed to observe HIC initiation behavior and identify the initiation site of HIC. This technique is based on the combination of an automatic ultrasonic wave inspection (UT) system and energy dispersive X-ray spectroscopy (EDS). By overlaying the C-scan movie obtained using the UT system on the EDS map of the HIC fracture surface, as in projection mapping, the initiation site of HIC was related to the non-metallic inclusion directly. Additionally, the hydrogen concentration at the HIC initiation site was calculated using finite element analysis, whereas the critical hydrogen concentration (Cth) was obtained as the hydrogen concentration at the time of HIC initiation.
In the oil and gas industry, oil country tubular goods or line pipes are exposed to wet H2S environments (sour environments) in some cases. The presence of H2S in the sour environment enhances hydrogen entry into the steel due to the catalytic action of H2S. The absorbed hydrogen enhanced by H2S affects hydrogen embrittlement. Hydrogen-induced cracking (HIC) is a hydrogen embrittlement phenomenon observed in sour conditions.
To evaluate HIC susceptibility, NACE TM0284 was established.1 With the establishment of the test method, it has become clear that HIC susceptibility is affected by inclusions, precipitates, microstructure, and hardness.2-6 Elongated non-metallic inclusions (especially MnS) and insoluble coarse particles (Nb (C, N), TiN) induce the initiation of HIC. As HIC susceptibility increases with the hardness (local maximum hardness) of the steel, a banded microstructure, such as ferrite-pearlite dual microstructure, and center segregated zone also decrease the HIC resistance. Therefore, almost all researchers and manufacturers agree that there are three important factors to achieve resistance to HIC: clean steel, uniform microstructure, and low hardness.
