ICF13A

13th International Conference on Fracture June 16–21, 2013, Beijing, China Crystallographic texture helps reduce HIC cracking in pipeline steels José M. Hallen, Victoria Venegas, Francisco Caleyo* DIM, IPN-ESIQIE, UPALM, Edif. 7, Zacatenco, México D.F., 07738, México *Corresponding author: fcaleyo@gmail.com Abstract The resistance of sour-service steels to hydrogen-induced cracking (HIC) has been traditionally improved through reduction in sulphur content, control of inclusion morphology, and use of low segregated, uniform microstructures. Other approaches are currently investigated; among them, the control of crystallographic texture as a mean to further reduce the susceptibility of pipeline steels to HIC. In this study, low-carbon steel samples, all within API specifications, were produce using different rolling/recrystallization schemes. These samples showed similar microstructure, but differed in their crystallographic textures. After cathodic hydrogen charging, HIC was detected in the cold- and hot-rolled/recrystallized steels, whereas the warm-rolled/recrystallized steels proved resistant to this damage. These results can be related to the differences in texture and grain boundary distribution observed in these groups of samples. It is concluded that it is feasible to improve the HIC resistance of pipeline steels through crystallography texture control and grain boundary engineering. The use of warm rolling schedules has been proven an effective solution in achieving this goal as they lead to crystallographic texture dominated by the {111}ND-fiber texture, and to a high proportion of low-angle grain boundaries. These two characteristics are necessary to further reduce, beyond traditional methods, the susceptibility of pipeline steels to HIC. Keywords HIC, pipeline steel, crystallographic texture, warm rolling 1. Introduction The origin, mechanisms, and consequences of Hydrogen-induced cracking (HIC) have been documented over the last several decades [1]. The strategies to reduce the incidence of this damage have not proven to be totally effective for severe operating conditions [1,2], so that the control of crystallographic texture and grain-boundary distribution of sour-service steels has recently been proposed as a means to reduce their susceptibility to HIC [3]. In recent works the authors have shown that, at a grain scale, crystallographic texture and grain-boundary character can play a significant role in HIC propagation in pipeline steels [4–6]. From the results of these studies it was postulated that the resistance of low-carbon steels to HIC could be improved by controlling their local texture and grain-boundary distribution [7]. Controlled warm-rolling, in the 600–800 °C range, was proposed as a mean to produce crystallographic textures suitable for reducing HIC because of (i) a reduction in the number of transgranular and intergranular cleavage paths along {001}ND-oriented1 grains, (ii) a reduction in crack coalescence and stepwise HIC propagation, and (iii) an increase in the number of high-resistance, intergranular crack paths along coincidence site lattice and low-angle grain boundaries between {111}ND-oriented grains. New experimental evidence is presented in this work, which supports the hypothesis that the resistance of pipeline steels to HIC can be improved at a macroscopic scale through controlled warm rolling. Several samples of pipeline steel, all within API 5L specifications [7], were obtained through different thermo-mechanical processes. These samples, all with similar microstructures, developed different crystallographic textures and grain-boundary statistics. Cathodic hydrogen charging was used to investigate their HIC behavior. The results of this study show that a strong {111}ND fiber texture, which results from warm rolling, reduces the HIC damage in sour-service pipeline steels. 1 ND, RD, and TD refer to the pipe’s Normal (radial), Rolling (axial), and Transverse (hoop) directions, respectively. {hkl}ND refers to a grain orientation for which the {hkl} plane lies parallel to the ND-TD plane. -1-

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