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Abstract
Nonmetallic inclusions have a strong influence on the hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) in pipeline steels, which should be well controlled to improve the steel resistance to HIC and SSC. The effects of cooling rate on the formation of nonmetallic inclusions have been studied both experimentally and thermodynamically. It was found that the increasing cooling rate increased the number density and decreased the size of the inclusions, while the inverse results were obtained by decreasing the cooling rate. Furthermore, as the cooling rate decreased from 10 to 0.035 K/s, the inclusions were changed from Al2O3-CaO to Al2O3-CaO-MgO-CaS. At a high cooling rate, the reaction time is short and the inclusions cannot be completely transformed which should be mainly formed at high temperatures. While, at low cooling rate, the inclusions can be gradually transformed and tend to follow the equilibrium compositions.
Highlights
High-grade pipeline steels have been widely used in the construction of long-distance oil and gas transportation systems, which requires good combination of high strength, toughness, corrosion resistance and weldability
Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) have been recognized as the vital threats to the safety of the pipeline operation, and caused significant economic losses throughout the world [1,2,3,4,5,6,7,8]. It has been acknowledged [5,9] that nonmetallic inclusions are one of the dominant factors affecting the HIC and SSC in pipeline steels, especially the large inclusions with string shape
It was reported that the calcium aluminate with low melting temperature can be elongated into string shape after rolling, which is harmful to the performance of pipeline steels [11,12,13]
Summary
High-grade pipeline steels have been widely used in the construction of long-distance oil and gas transportation systems, which requires good combination of high strength, toughness, corrosion resistance and weldability. Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) have been recognized as the vital threats to the safety of the pipeline operation, and caused significant economic losses throughout the world [1,2,3,4,5,6,7,8]. It has been acknowledged [5,9] that nonmetallic inclusions are one of the dominant factors affecting the HIC and SSC in pipeline steels, especially the large inclusions with string shape. Researchers proposed that the inclusions should be controlled to CaO-Al2 O3 -CaS, CaO-CaS type [11] or Al2 O3 -CaS type [14,15] to reduce their detrimental effects
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