Understanding the Role of Copper Addition in Low-Temperature Toughness of Low-Carbon, High-Strength Steel
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THE strengthening effect of Cu-rich nanoprecipitates in the Fe-based alloys has been known for decades.[1] It has a wide range of applications in offshore engineering, oil and gas pipelines, bridges, infrastructure, etc. Use of Cu-rich precipitates instead of carbides to achieve high strength has been an important strategy to develop precipitation-hardened maraging steels, which is the fundamental basis for HSLA80, HSLA100, and NUCu-X for naval applications.[2,3] Moreover, weldability and corrosion resistance can be simultaneously improved. In the process of tempering for maraging steels, Cu precipitates in the matrix and undergoes changes in the crystal structure in different heat treatment conditions.[4] The researchers agree that Cu precipitates begin first as Cu cluster in the matrix, followed by bcc Cu-rich precipitates in coherence with
XIAOHUI XI, JINLIANG WANG, LIQING CHEN, and ZHAODONG WANG are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 26, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the bcc matrix. At this point, the optimum strengthening effect is achieved.[5] As aging proceeds, the bcc Cu retransform to 9R, 3R, and finally to fcc Cu, while the strengthening effect experiences a complex change along with the aging process.[6] Also, the distribution of Cu precipitates has a strong dependence on heat processing. Therefore, one can regulate the structure and distribution of Cu precipitates by adjusting the heat treatments to achieve the desired strengthening effect. In addition to the effect of Cu precipitates on strength, Cu precipitates exert a crucial impact on toughness. As is widely accepted, the precipitates, including carbide and Cu precipitates, greatly deteriorate the impact toughness.[7,8] On one hand, the stress concentration around the precipitates is induced by deformation, which initiates the crack. On the other hand, the segregation of precipitates at grain boundaries reduces the cohesion between the matrices and, consequently, promotes the propagation of crack, leading to toughness deterioration. Kunishige et al. proposed that the effect of precipitates on toughness is highly dependent on their distribution features[9]; i.e., fine coherent precipitates called ‘‘secondary hardening’’ are accompanied by a huge loss in toughness, while other incoherent precipitates called ‘‘dispersion hardening’’ result in a slight loss in toughness.
Different from carbides, Cu precipitates are softer than the matrix.[10] Also, Cu precipitates undergo complex crystal structural changes based on different heat treatments. Thus, the effect of Cu precipitates on the impact toughness is considerably complicated. Ghosh et al. suggested that the Cu precipitates have a detrimental impact on toughness in terms of the pinning effect of dislocations in the deformed ferrite.[8] However, the presence of an adequate amount of Cu in steel has been found to be useful for retarding c fi a transf
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