Hydrogen Embrittlement of a 1500-MPa Tensile Strength Level Steel with an Ultrafine Elongated Grain Structure
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INTRODUCTION
DURING the last 20 years, demand has increased to strengthen steel with the target of producing nextgeneration steel structures and transportation vehicles for a more efficient use of resources and energy, as well as a reduced amount of CO2 emissions. However, the susceptibility to hydrogen embrittlement (HE) increases with increasing strength in steels and is a drawback for the practical use of high-strength, low-alloy steels with tensile strength (TS) exceeding 1200 MPa. The HE of high-strength, low-alloy steels has been investigated extensively.[1–3] The following aspects are effective in improving the resistance to HE: (1) increase in intrinsic fracture resistance, (2) suppression of hydrogen accumulation at crack initiation sites and crack tips by using effective hydrogen trapping sites such as TiC[4] and VC,[5] and (3) decrease in hydrogen intrusion from the environment. Because HE has often been observed to initiate from the boundaries of prior-austenite grains in tempered martensite steels with TS exceeding 1200 MPa, many attempts have been made to raise intrinsic fracture YIHONG NIE, Senior Researcher, and FUXING YIN, Managing Director, formerly with the National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan, are now with China First Heavy Industries, Tianjin 300457, P.R. China. YUUJI KIMURA, TADANOBU INOUE, and EIJI AKIYAMA, Senior Researchers, and KANEAKI TSUZAKI, Managing Director, are with the National Institute for Materials Science. Contact e-mail: [email protected] Manuscript submitted November 29, 2010. Article published online November 15, 2011 1670—VOLUME 43A, MAY 2012
resistance through suppressing the intergranular fracture by controlling the grain size,[6–11] grain shape,[12] and grain boundary characteristics[13] in the prioraustenite grain structure. Grain refinement has two advantages in suppressing intergranular fracture; the first is to reduce the stress concentration at grain boundaries, and the second is to decrease impurity segregation at grain boundaries by increasing the grain boundary area. Several researchers[6–11] have investigated the influence of prior-austenite grain size on HE. However, conflicting results were found in the reports with respect to the role of prior-austenite grain size. Differences may be caused by the different microstructures (undissolved carbide, retained austenite, grain boundary structure, etc.) that are obtained in the effort to vary grain size. Besides, only a few studies have been carried out for fine-grained steels with a prior-austenite grain size of a few micrometers or less, in which ‘‘the grain refinement effects’’ are expected to become significantly larger. Recently, Hui et al.[11] reported that HE resistance was enhanced in a 0.4 pct C-1 pct Cr-0.5 pct Mo-0.3 pct V-0.04 pct Nb steel with TS from 1300 MPa to 1600 MPa by the prior-austenite grain refinement from 20 to 4 lm through a thermal cycling of rapid heating and quenching. (Percent in this article means mass percent.) However, they pointed out that the prior-austen
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