Materials Science and Engineering A 478 (2008) 43–47 Effect of applied stress and microstructure on sulfide stress cracking resistance of pipeline steels subject to hydrogen sulfide Ming-Chun Zhao a,b,∗ , Ming Liu b , Andrej Atrens b , Yi-Yin Shan c , Ke Yang c a School of Material Science and Engineering, Central South University, Changsha, Hunan 410083, PR China b Division of Materials, The University of Queensland, Brisbane, Qld 4072, Australia c Institute of Metal Research, Chinese Academy of Sciences, Shenyang, PR China Received 7 February 2007; received in revised form 16 May 2007; accepted 17 May 2007 Abstract Effects of applied stress and microstructure on sulfide stress cracking resistance of pipeline steels subject to hydrogen sulfide were investigated by the single-edge notched tensile method using a microalloyed steel and a non-microalloyed steel. The failure time increased with the decreasing applied stress, and finally the threshold stress intensity factor was calculated for acicular ferrite (AF) and ferrite-pearlite (FP) in these two steels. The strength was not the dominant factor for the SSC, and aged microalloyed AF had the best SSC resistance in coincidence with the highest strength. The SSC resistance in sort ascending was non-microalloyed AF, non-microalloyed FP, microalloyed FP, microalloyed AF and aged microalloyed AF. The SSC was explained from hydrogen penetration and microstructural characteristic. The localized hydrogen concentration was enhanced by applied stress. The higher the applied stress, the more easily the SSC occurred. Carbonitrides and pinned dislocations contributed in better SSC resistance. © 2007 Elsevier B.V. All rights reserved. Keywords: Pipeline steel; Microstructure; Sulfide stress cracking; Hydrogen trap 1. Introduction Pipeline steels subject to sour oil and gas environments con- taining hydrogen sulfide (H 2 S) are up against a severe sulfide stress cracking (SSC) failure issue under the combined action of externally applied tensile stress and corrosion caused by the aqueous sour media. For conventional industrial applications, pipeline steels with the hardness of higher than HRC 22 (equiv- alent to 550 MPa yield strength) are thought as the high SSC susceptibility [1], which proceeds perpendicular to the load- ing axis in the form of inter-granular to trans-granular cracking under external loads [2]. In recent years, high strength pipeline steels, which are developed due to the proper design of chemi- cal composition and/or the improved thermo-mechanical control processing, have been widely applied in the oil and gas industry driven by cost considerations [3,4]. Their yield strength values may even be higher than the critical SSC susceptibility cri- terion of 550 MPa. In particular, high strength pipeline steels ∗ Corresponding author. Fax: +86 731 8876692. E-mail addresses: mczhao@imr.ac.cn, zmczhao@yahoo.com.cn (M.-C. Zhao). with an acicular ferrite dominated microstructure have lately received considerable attention because of their high strength and toughness behaviors [3–7]. However, their sulfide stress cracking (SSC) resistant mechanism is still not fully under- stood. Wide industrial productions and engineering applications of high strength pipeline steels require an understanding of their SSC resistant behaviors in sour oil and gas environments con- taining H 2 S and how these behaviors are affected by the changes in the other conditions such as applied stress and microstruc- ture. This should also in turn help to better design chemical composition and thermo-mechanical control processing of SSC resistant pipeline steels with high strength. In the present work, the SSC resistance of acicular ferrite (AF) and ferrite-pearlite (FP) that are two typical microstructures applied in high strength pipeline steels is investigated in a contrast by using a microal- loyed steel and a non-microalloyed steel, and the potential role of both applied stress and microstructure on SSC resistance is analyzed. 2. Experimental procedure Chemical compositions of microalloyed steel (steel A) and non-microalloyed steel (steel B) are given in Table 1. Three 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.067