Effects of alloying elements on mechanical and fracture properties of base metals and simulated heat-affected zones of S
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I. INTRODUCTION
LOW-ALLOY steels, such as SA 533 and SA 508, are key materials used in nuclear reactor facilities, such as pressure vessels, compressors, and steam generators.[1,2] The most crucial mechanical properties for these steels are sufficient strength to withstand internal pressure and high fracture toughness to assure safety from momentary shock due to unexpected accidents. Particularly, fracture toughness deteriorates because of continuous neutron irradiation during the operation of a nuclear reactor, and embrittlement can occur, resulting in ductile-brittle transition temperature (DBTT) moving toward high temperatures.[3,4] Thus, the use of steels having high fracture toughness is extremely important to guarantee sufficiently the safe operation of the nuclear reactor. It is also critical to use steels whose mechanical properties in the welded region are excellent since large-scale nuclear reactor structures are usually fabricated by welding.[5,6,7] Ways to improve mechanical properties of the low-alloy steels can be largely divided into grain refinement and carbide control. It was reported that the grain refinement could be achieved by grain boundary pinning by aluminum nitrides or by controlling austenitization temperature and quenching rate.[7–11] The carbide control can be achieved by adding alloying elements, such as strong carbide formers. However, studies on the effect of each alloying element on mechanical properties are yet to be made. Furthermore, even when the SANGHO KIM, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. YOUNG-ROC IM, Research Assistant, and HU-CHUL LEE, Professor, are with the School of Materials Science and Engineering, Seoul National University, Seoul, 151-742 Korea. YONG JUN OH, Senior Researcher, and JUN HWA HONG, Director, are with the Nuclear Materials Technology Developments Team, Korea Atomic Energy Research Institute, Dajeon, 305-600 Korea. Manuscript submitted April 14, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
low-alloy steels with excellent mechanical properties are developed, their mechanical properties may deteriorate after welding, which is unavoidable in actual use.[12–15] Therefore, the effect of alloying elements should be interpreted in relation to mechanical properties of heat-affected zones (HAZs). Since sufficient fracture toughness in the HAZs and quantitative evaluation on them are highly critical to the safety of nuclear reactor structures, their mechanical properties should be investigated. The present study is aimed at the development of lowalloy steels with excellent mechanical properties for nuclear reactor pressure vessels. Four kinds of steels having different compositions were fabricated, and their microstructures and mechanical properties were comparatively analyzed to investigate the effect of alloying elements. A steel whose composition is within the composition specification of SA 508 steel (class 3) was first fab
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