Crack-initiation toughness and crack-arrest toughness in advanced 9 pct Ni steel welds containing local brittle zones
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I. INTRODUCTION
NATURAL gas is expected to be one of this century’s most important energy sources, because it provides clean energy with a high energy density, and, thus, the global demand for liquefied natural gas (LNG) has been increasing continuously. Because LNG is stored at or below its boiling temperature (111 K), the inner walls of LNG storage tanks must be constructed with a material which possesses high strength and suitable fracture toughness at cryogenic temperatures. The 9 pct Ni steel has been widely used for the construction of the inner walls, because of its excellent fracture toughness at the LNG temperature. Recently, in response to increasing demand for large-scale LNG storage tanks, advanced 9 pct Ni steels exhibiting a higher cryogenic toughness have been developed. One of these newly developed cryogenic steels is a quenched, lamellarized, and tempered (QLT) 9 pct Ni steel now used for LNG storage tanks in Korea.[1,2] The QLT process, originally developed for lowerNi steel such as 5.5 pct Ni steel,[3] enhances cryogenic toughness considerably compared to other conventional processes, such as quenching and tempering or double-normalizing and tempering, due to the increased amount of stable austenite and the refinement of the effective grain size. During the construction of LNG storage tanks, however, the excellent cryogenic fracture performance of QLT-9 pct Ni steel can be upset in the heat-affected zones (HAZs) near JAE-IL JANG, Senior Researcher, Frontics, Inc., Research Institute of Advanced Materials, and BAIK-WOO LEE and JANG-BOG JU, Research Associates, and DONGIL KWON, Professor, School of Materials Science and Engineering, are with Seoul National University, Seoul 151-742, Korea. Contact e-mail: [email protected] WOO-SIK KIM, Principal Researcher, is with the Research and Development Center, Korea Gas Corporation, Ansan 425-150, Korea. Manuscript submitted October 31, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the fusion line (FL) by welding thermal cycles that can produce small areas, called local brittle zones (LBZs), which exhibit abnormally poor fracture resistance. Many studies on the influence of LBZs on the fracture performance of the welds have shown that LBZs cause low toughness values in multipass welded structural steel in various toughness tests, such as the Charpy impact test and the crack-tip opening displacement (CTOD) test, and reduce the resistance to brittle fracture initiation.[4–8] Based upon these studies, some industry standards, such as API RP 2Z,[9] have been established, containing, in some form, a requirement that certain HAZ CTOD specimens must sample at least 15 pct of the coarse-grained HAZ (CGHAZ) microstructure. Meanwhile, steel manufacturers have been performing extensive research on LBZ phenomena in newly developed structural steel. Paradoxically, however, it is very interesting to note that LBZs have not been reported to be a significant cause of actual failure in practical welded structures, although many researchers have pointed out their dele
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