The effect of inclusion type and morphology, sulfur content and Ca/S ratio on Hydrogen Induced Cracking Resistance of seamless line pipes Grades X60 and X65 was studied. The sulfur content ranges between 0.0010% and 0.0040% and the Ca/S ratio varies from 0.6 to 2.7.
HIC resistance was evaluated following NACE TM0264-96, using the solution with pH 2,7. Test duration was 192 hours. Fourteen of the steels tested presented slight cracking after the test. The obtained values of CLR, CTR and CSR were notably lower than the usual acceptance limits for these parameters.
The maximum values of the crack ratios were obtained in materials with Ca/S ratio lower than 1 and S content higher than 0.003 %. In the range of sulfur tested, it was found that, for seamless line pipes, a Ca/S ratio between 0.6 and 1 is adequate to guarantee HIC resistance.
Aggressive environments such as hydrogen sulfide, carbon dioxide, chloride and sulfur compounds are encountered often in crude oil survey and oil and natural gas transportation. In these cases, hydrogen induced cracking (HIC) occurs easily in pipeline and pressure vessel steels.
HIC manifests itself in the form of surface blisters and/or internal cracks in the absence of an applied stress, Hydrogen absorbed into the steel diffuses and can be trapped in defects. At such discontinuities, the hydrogen atoms combine to form molecular hydrogen gas. High pressures can be developed and cracks can initiate and propagate. Numerous research programs have been performed for many years to enhance the hydrogen induced cracking resistance of line pipe steels. However, most of them have been conducted on large size welded pipelines derived from heavy plates.
It has long been recognized that non-metallic inclusions are the main crack initiation sites lA2. Large, elongated and aligned inclusions are more dangerous than small, globular and randomly dispersed inclusions. As the sulfur content decreases, the sulfides become more globular. To levels and inclusion modification by Ca treatment are recommended. To avoid elongated MnS inclusions, low S Too low Ca/S ratio results in the formation of Manganese Sulfide elongated inclusions, whereas too high ratio results in the formation of CaS/CaO clusters. Both are detrimental to HIC resistance 2, because they are preferential sites for crack nucleation. However, such a direct influence of calcium treatment on HIC results has been questioned by other authors In plates, it has been reported that the Ca/S ratio must range between 1 and 4 to obtain adequate inclusion modification and good HIC resistance.
HIC behavior does no depend only on the cleanliness of the steel. The extent of crack propagation depends on the microstructure In plates, the microstructure is not perfectly homogeneous. Segregated areas enriched in alloying elements can be found at mid-thickness of rolled products. Due to the higher hardenability of segregated areas, hard untempered bainitic or martensitic microstructure can be found in these areas This segregation pattern is not present in seamless tine pipes.
Galis and Guntz I3 studied the effect of Ca/S ratio and S on HIC resistance in seamless line pipes. The optimum ratio was between 0.5 and 1 .I with respect to the maximum allowable crack length. In this paper, the effect of some material characteristics (microstructure, inclusion content, types and morphology, sulfer content ans CalS ratio) on HIC of seamless line pipes was studied.
EXPERIMENTAL PROCEDURE
Materials
Nineteen line pipes microalloyed steels of API grades X60 and X65 (Table 1) were used in this study. Steels were aluminum killed and calcium treated. All t
