Effects of Nano-sized Microalloyed Carbonitrides and High-density Pinned Dislocations on Sulfide Stress Cracking Resista
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Ke Yang Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China (Received 24 January 2005; accepted 25 May 2005)
Sulfide stress cracking (SSC) resistance was investigated by comparing acicular ferrite (AF) and ferrite-pearlite (FP) in a microalloyed steel and in a non-microalloyed steel. In microalloyed steel, AF exhibited better SSC resistance than FP, while in non-microalloyed steel, AF presented far worse SSC resistance than FP. In microalloyed steel, nano-sized carbonitrides and high-density pinned dislocations in AF were analyzed to behave as innocuous hydrogen traps, offering numerous sites for hydrogen redistribution and modifying critical cracking conditions. Dislocations in AF of microalloyed steel in the final analysis are attributed to pinning by the nano-sized carbonitrides.
Currently, increasing demand for oil and natural gas promotes active attention to pipeline steels. Because of their high strength and toughness, acicular ferrite (AF) pipeline steels provide a large potential for a variety of applications,1,2 but their sulfide stress cracking (SSC) mechanism is still not fully understood. By far, most work concerning the role of microstructures on the SSC resistance has focused on the microstructures obtained by conventional hot working or heat treatment processes, such as ferrite-pearlite (FP), upper/lower bainite, and quenched-tempered martensite.3,4 The SSC resistance for the microstructures of pipeline steels obtained by thermo-mechanical control process (TMCP) has not been extensively studied, especially for AF. In this work, the SSC susceptibilities of both AF and FP obtained by TMCP were compared, with special attention paid to nano-sized microalloyed carbonitrides and high-density pinned dislocations. Chemical compositions (wt%) of the test steels are identified as steel A (microalloyed steel) with 0.07C, 0.25Si, 0.9Mn, 0.04Nb, 0.04V, 0.015Ti, 0.2Cu, 0.2Ni, 0.023Al, and 0.0165 (S + P + O + N), and steel B (nonmicroalloyed steel) with 0.045C, 0.3Si, 1.94Mn, 0.0058 (S + P + O + N). The samples for microstructural characterization, mechanical property characterization, and SSC testing were cut from as-rolled plates. SSC susceptibilities of the test steels were evaluated at room temperature by NACE TM0177 (96)-method B,5 and the a)
Address all correspondence to this author. e-mail: [email protected] or [email protected] DOI: 10.1557/JMR.2005.0321 2248
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J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 13 Mar 2015
single-edge notched tensile method.6,7 The test solution, consisting of 0.5% glacial acetic acid dissolved in distilled water, was saturated with H2S. For NACE TM0177 (96)-method B,5 bent beam specimens with size of 1.5 × 4.6 × 68 mm were cut from the middle of hot-rolled plates in the transverse direction. Two holes 0.7 mm in diameter were drilled at the mid-length of each test specimen, centered 1.59 mm from each side edge. The tests were performed for the specimens under a certain pseudo-stress for
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