Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated hea
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5/24/04
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Effects of Alloying Elements on Fracture Toughness in the Transition Temperature Region of Base Metals and Simulated Heat-Affected Zones of Mn-Mo-Ni Low-Alloy Steels SANGHO KIM, SUNGHAK LEE, YOUNG-ROC IM, HU-CHUL LEE, SUNG-JOON KIM, and JUN HWA HONG This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M2C carbides was greatly increased and the number of coarse M3C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M2C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.
I. INTRODUCTION
THE Mn-Mo-Ni low-alloy steels such as SA 533 and SA 508 steels are key materials used in nuclear reactor facilities such as pressure vessels, compressors, and steam generators.[1,2,3] The most crucial mechanical properties for these steels are sufficient strength, to withstand the internal pressure, and high fracture toughness, to assure safety against unexpected accidents. Because of the continuous neutron irradiation that occurs during the operation of a nuclear reactor, fracture toughness deteriorates and the ductile-brittle transition temperature (DBTT) moves toward high temperatures.[4,5] Thus, in order to guarantee the safe operation of a nuclear reactor, steels that have a high fracture toughness should be used. Since large-scale nuclear reactor structures are usually fabricated by welding, it is also important to use steels with excellent welding properties.[6,7,8] The Mn-Mo-Ni low-alloy steels used for nuclear reactor pressure vessels have been continuously developed to establish SANGHO KIM, Researcher, Plate Research Group, and YOUNG-ROC IM, Researcher, Sheet Product & Process Research Group, are with the Technical Research Laboratories, POSCO, Pohang 790-785, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea, is jointly appointed with Materials Science and Engineeri
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