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Ni-Cr-Mo-V계 저합금강의 미세조직과 기계적 특성에 관한 연구
조무근,류성현,김경일,김동응,김정인,김경택,김상섭,조규섭 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.4
Ni-Cr-Mo-V steel alloys for high-speed railway brake discs were prepared to investigate the effect of alloying element contents on the microstructure and mechanical properties. The Cr, Mo, Mn alloying elements were incorporated into the steel alloys, which contained low carbon content in the range 0.16 wt.% ~ 0.21 wt.% to provide sufficient hardenability. The steel alloys were austenitized at 940o C for 1 hour and quenched, and tempered at 610o C. Microstructural study showed a tempered martensitic microstructure with different sized austenite grains and packets. C-Mo alloy with high Mo content and the smallest prior austenite grain size showed the highest hardness and tensile strength. But, the alloy exhibited lower impact toughness than low Mo content alloys. The lowest tensile strength of the low Mo content Mn-Cr alloy, at room temperature and elevated temperature of 600o C, was 1053.4 MPa and 667.2 MPa, respectively. The grain refinement in the C-Mo alloy was considered to be due to the solute drag effect of the Mo element. The absorbed impact energy increased with tempering temperatures, but the impact energy of the three alloys had lower values than the generally guaranteed impact energy of the currently used disk. The low impact toughness of the Mo containing alloys was attributed to the higher Si content and higher tempered hardness of the alloys. A higher thermal conductivity and lower thermal expansion coefficient were obtained in the high Mo content C-Mo alloy, which had a higher Ac3 transformation temperature.
소결 방법에 따른 WC-20wt%Co 초경합금의 인장 특성 및 피로 수명
류성현,조무근,김경일,노우람,홍익제,이광희,김상섭,조규섭 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.3
WC-Co cemented carbide has excellent mechanical properties and is widely used in many industrial applications including cold forging die and cutting tools. WC-Co cemented carbide is manufactured by liquid phase sintering (LPS), -and a new sintering process for densification has been developed. In this study, cylindrical rod shaped tensile and fatigue specimens of WC-20wt%Co cemented carbide were fabricated using commercial sintering methods such as vacuum sintering (VS), low pressure sintering (LP), sinter HIP (S-HIP), and HIP after vacuum sintering(VS+HIP). The effect of the sintering methods on microstructure, tensile properties and fatigue life was investigated. The relative density and average size of the WC particles increased after additional HIP treatment. Also large abnormal WC particles were observed. In the tensile test, the deviation of tensile properties (maximum tensile strength, elongation) was minimized in the VS+HIP specimen due to densification. Based on the uniaxial tensile-compression fatigue test, the SN curve of WC- 20wt%Co cemented carbide was concluded to be logσa = -0.11244logNf +3.66817. Furthermore, the increase in WC particle size in the HIP process caused the VS+HIP specimen to have a lower fatigue life than the VS specimen. However, when high pressure was applied to the WC-Co cemented carbide during the sintering process, internal defects and the standard deviation of fatigue life were reduced.