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12Cr 강의 화염경화 표면처리 층의 초음속 물방울 충격에 의한 손상 거동
김광호,이민구,김경호,김흥회,김길무 대한금속재료학회 2005 대한금속·재료학회지 Vol.43 No.1
Water drop impact erosion properties of the 12Cr steel, currently used as nuclear steam turbine blade material, surface-hardened by the flame hardening process have been characterized. For this, variations of both the maximum erosion depth de,,,,ax and volume loss V with the cumulative impacts n at the supersonic impact velocity (~350 m/s) have been investigated for the as-received and flame-hardened 12Cr steels with different hardnesses. Typically all the samples showed an erosion-time characteristic involving the incubation period initially followed by the steady state period. It is also found that the d_(e,max) increases stepwise-like with the number of impacts n, which results from a sudden formation of craters by crack propagation. Compared to those for the as-received 12Cr steel, the flame-hardened ones showed an excellent resistance to water drop impact erosion with 2.2~2.8 times higher incubation time n, and 115~1/8 times lower erosion rate a. In the incubation period the as-received 12Cr steel was damaged by ductile depression and ploughing, while the flame-hardened 12Cr steel by fatigue cracks and brittle platelet deformation. Erosion in the steady state period was developed by the cleavage fracture commonly. (Received July 22, 2004)
화염경화 표면처리 공정에 의한 12Cr 강의 잔류응력 거동
이민구(M. K. Lee),김광호(G. H. Kim),김경호(K. H. Kim),김흥회(W. W. Kim) 한국표면공학회 2004 한국표면공학회지 Vol.37 No.4
The residual stresses on the surfaces of low carbon 12Cr steels used as a nuclear steam turbine blade material have been studied by controlling the flame hardening surface treatments. The temperature cycles on the surfaces of 12Cr steel were controlled precisely as a function of both the surface temperature and cooling rate. The final residual stress state generated by flame hardening was dominated by two opposite competitive contributions; one is tensile stress due to phase transformation and the other is compressive stress due to thermal contraction on cooling. The optimum processing temperatures required for the desirable residual stress and hardness were in the range of 850℃ to 960℃ on the basis of the specification of GE power engineering. It was also observed that the high residual tensile stress generated by flame hardening induced the cracks on the surfaces, especially across the prior austenite grain boundaries, and the material failure virtually, which might limit practical use of the surface engineered parts by flame hardening.
김광호(Gwang-Ho Kim),이민구(Min-Ku Lee),김경호(Kyeong-Ho Kim),김흥회(Whung-Whoe Kim),이창규(Chang-Kyu Rhee),김길무(Gil-Mu Kim) 한국표면공학회 2006 한국표면공학회지 Vol.39 No.2
In this study, the movable flame hardening process of 12Cr steel for a uniform hardness and desirable residual stress have been investigated. For this, the temperature cycles have been controlled accurately as a function of the three processing variables, the flame intensity If, the scanning velocity Vs and the initial flame holding time th, where the standard surface temperature Ts, max was maintained at 960°C. The optimized conditions were Vs = 0.68 ㎜/s and th = 67 sec for the C₃H?:O₂ = 5:20 l/min, Vs = 0.80 ㎜/s and th= 56 sec for the C₃H?:O₂ = 6:24 l/min, Vs = 1.01 ㎜/s and th= 48 sec for the C₃H?:O₂= 7:28 l/min, and Vs = 1.15 ㎜/s and th= 39 sec for the C₃H?:O₂= 8:32 l/min. The optimally flame-hardened surface exhibited uniform distributions of the hardness and residual compressive stress over the treated area with moderate levels of 470~490HV0.2 in hardness and ?300~?450 MPa in residual stress, which were acceptable on the basis of the acceptance criteria of Siemens AG-KWU and GE Power Generation Engineering.
증기 터빈 동익 재료인 12Cr 강의 화염표면 처리에 의한 열 이력 제어 및 경화 특성에 관한 연구
이민구,김광호,김경호,김흥회 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.11
Low carbon 12Cr steel used as a nuclear steam turbine blade material have been surface-modified by the flame hardening process and the properties of hardness, hardening depth, and residual stress have been studied. For this, the thermal cycles of 12Cr steel have been controlled precisely as a function of the surface temperature, the exposed height from the water surface, and the cooling rate. The application of flame hardening increases the hardness of 12Cr steel (base value, 250 HV) to 420~550 HV considerably, forming relatively sharp hardening boundaries. Both the hardening depth and the austenitization period are linearly correlated well within the range of processing conditions employed. The rapid cooling rate also increases the hardening depth as well as the surface hardness. The properties of residual stress are discussed in terms of contributions of both the thermal and transformation stress components. The optimum processing conditions for the desirable residual stress state in the material are as follows : less than 1200℃ for the 18 mm-exposed and less than 1150℃ for full-exposed conditions by the criteria of Siemens AG-KWU, and on the basis of the specification of GE Power Generation Engineering, 944~1050℃ for the 10 mm-exposed, 883~1150℃ for the 14 mm-exposed, 833~1134℃ for the 18 mm-exposed, and 785~1097℃ for the full-exposed.
원전 증기 터빈재료의 화염경화공정에 따른 액적충격침식특성
김경호(Kyeong Ho Kim),이민구(Min Ku Lee),이창규(Chang Kyu Rhee),김홍희(Whung Whoe Kim),김광호(Kwang Ho Kim),위명용(Myeong Yong Wey) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.8
To improve liquid impact erosion resistance by strengthening 'the surface of 12Cr steel(l2Cr, 0,3Mn, 01.C, bal. Fe) which is currently used as steam turbine blade material, flame hardening method which is simple process, without having interface, possible to apply to large surface area has been studied to find its optimal condition as a function of flame temperature and cooling conditions. Erosion test is conducted for flame hardened material with maximum water jet speed of about 600m/s by using liquid impact erosion tester. Erosion resistance is analysed by optical and scanning electron microscopes to measure the depth of eroded specimens and its decreased mass. Degree of the surface hardened 12Cr steel was not observed below specimen's surface temperature of 900℃, whereas hardening is observed by martensite transformation above 1,000℃ in austenite region. The surface hardness in hardened layer of 12Cr steel is about two times higher than that of as-received specimen(300KHN). This phenomena would be due to the non-uniformed strain and the increasing of carbon solid solution in the specimen. The result of X-ray diffraction pattern analysis showed that peak-broadening and lattice expansion is occurred with increasing of flame hardening temperature.