11%CrWMoCo 내열합금의 미세조직과 기계적 성질에 미치는 N첨가 효과
(The) Effect of Nitrogen on the Microstructure and Mechanical properties of 11%CrWMoCo Heat Resistant Steels
내열합금 미세조직 탄화물구조;
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An improvement in thermal efficiency of fossil power plant by increasing the operating temperature and pressure of steam requires the use of structural materials of higher creep rupture strength and improved oxidation and corrosion resistance to aggressive atmofphere. In this investigation, the nitrogen addition effects on the precipitation behavior of 11CrMoWCo (HR1200) were studied. three alloys having nitrogen contents of 83, 439, 682ppm were manufactured by vacuum induction melting in N_(2) gas atmosphere After hot rolling and austenitization, specimens were tempered at 600, 650, and 700℃, respectively, in order to increase the toughness in nitrogen containing ingots. Coarse FeW_(2)B_(2)phase was existed in N80 alloy. However, formation of FeW_(2)B_(2)phase was suppressed with increasing nitrogen contents. It was found that M_(23)C_(6) and MX type of precipitates possessed the major portion in 11CrMoWCo steel, by the XRD and TEM analysis. Aged at 700℃ for 1000 hours, Fe_(2)W (Laves) phase was observed, and some other phases as CrN and Cr_(2)C were also detected. The growth of lath and formation of local subgrains were more pronounced at higher tempering temperature. Tensile strength and hardness of specimens tempered at 700℃ were lower than tempered at 600℃. After creep test at 700℃, specimens containing 439ppm nitrogen showed maximum rupture time. The longer creep life time is due to the fact that the 439ppm nitrogen containing alloy has more MX precipitates and smaller M_(23)C_(6) precipitates, compared to other alloys.