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Fe-15Cr-26Ni-1.25Mo 계 석출 경화형 합금강에서 기계적 성질에 미치는 열처리 효과와 고온 특성 연구
김일봉,강성태,송영석 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.6
1. 서론 2. 실험방법 시편 준비 및 열처리 기계적 특성 및 미세조직 3. 실험결과 및 고찰 미세조직 열처리에 따른 기계적 특성 저주기 피로 시험 크리프 파단 시험 4. 결론 The object of this study is to characterize the high temperature properties and the effect of aging treatment on mechanical properties in modified Fe-15Cr-26Ni-1.25Mo type precipitation hardening alloy containing Ti content of 1.1-1.3 wt%. It has been investigated mainly by tensile test, micro-vickers hardness test, scanning electron microscopy EDAX, charpy n-notch impact, low cycle fatigue, and combination smooth-notch stress-rupture test. The results obtained are summerized as follows; For modified Fe-15Cr-26Ni-1.25Mo type precipitation hardening alloy The optimum heat treatment condition are selected. It is clarified that an optimum Ti content of 1.1-1.3% meets the required mechanical properties of ultra supercritical steam turbine at 649℃. The charpy absorbed energy was almost stable at 13㎏-m∼12㎏-m from R.T to -140℃ The failure cycles(N_f) are 13,022 in Δε_t(%)=0.5 and 1,254 in Δε_t(%)=1 at 650℃. It appers lower notch sensitivity in combination smooth-notch stress-rupture test at 650℃, judging from not fractured at notched part but at smooth part.
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대형 Cr-Mo-V 단강의 기계적 성질에 미치는 Nb 첨가량의 영향
김정태,강성태,송영석 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.10
The effect of niobium(Nb) contents on the mechanical properties of the CrMoV forgings was investigated as a preliminary study to manufacture large forgings having a low fracture appearance transition temperature(FATT) and high tensile strength. The experimental forgings were varied in the Nb contents as 0%(A), 0.03%(B) and 0.05%(C). The tensile strength at room temperature and 600℃ were almost equal regardless of Nb contents, while the FATT lowered in order of alloy C(32℃), B(35℃), and A(62℃). These were resulted from the grain refining. The grain size of alloy A was ASTM No. 5.0 and those of B and C were ASTM No. 6.5 respectively. Based on the preliminary experimental results, large forgings containing 0.03%Nb(D) and 0.05%Nb(E) were manufactured from 30 ton ingot and then those mechanical properties were tested. The higher tensile strength was obtained in both D and E forgings, whereas, the FATT of D and E forgings were 33℃ and 67℃, respectively. The ductile fractured surfaces were developed in the forgings D, while serrated fractographes were alternately occurred with the eutectic niobium carbides and ductile dimples in the forgings E. The eutectic NbC and normal matrix were alternately developed in a severely Nb segregated region of the forgings E. The alternated structure might develop a serrate fracture surfaces, which caused forgings E to increase its FATT. Therefore, in order to manufacture large CrMoV forgings by adding Nb, Nb content to be added should be decided by considering segregation of Nb during solidification. The coarse eutectic niobium carbides were formed in the severely Nb segregated region. The serrated fracture might be caused to increase the FATT of large CrMoV forgings containing Nb.
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