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지르코늄 압력관에서 Hydride Blister에 의한 잔류응력 측정
정용무(Yong-Moo Cheong),공운식(Un-Sik Gong),김상재(Sang-Jai Kim),김영석(Young-Suk Kim) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.3
When the pressure tubes (PT) contact to the calandria tube (CT) in the pressurized heavy water reactor(PHWR), the temperature difference between inner wall and outer wall of PT results in a thermal diffusion of hydrogen (deutrium) and hydride blisters are formed on the outer surface of PT. The volume expansion associated with the formation and growth of zirconium hydride blister creates localized residual stresses in the PT. Residual stresses of localized area near the hydride blister were measured by micro-focused x-ray diffraction(XRD). The specimen were prepared using a cold spot on the outer surface of PT and the diameter of XRD beam was set as 100 ㎛. When we measured residual stress on the outer surface of PT, residual stress state of zirconium matrix just outside the blister were compressive and become tensile as moving to the inside the blister. The cross-sectional residual stress state of blister also revealed that compressive stress state at zirconium matrix just outside the blister and tensile stress state at the inside blister. These results are compared with the finite element model and discussed with the possibility of crack propagation to the zircoium matrix.
김영석(Young Suk Kim),최승준(Seung Jun Choi),정용무(Yong Moo Cheong) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11
A Ti-6Al-4V sheet with the α-Ti and the β phase was subjected to electrolytic charging of 1000 ppm H and to constant load tests at temperatures varying from 10 ℃ to 200 ℃. The delayed hydride cracking velocity (DHCV) was determined using compact tension specimens with the pre-fatigued crack growing along the rolling direction of the sheet. The DHCV of the Ti-6Al-4V alloy decreased with the test temperature increasing from 20 to 100 ℃ and also with the test temperature decreasing below the zero temperature. This fact is quite in contrast with the DHCV of Zr-2.5Nb alloys that increases with the increasing temperature from RT to 300 ℃ and decreases at temperatures above 300 ℃. Based on Kim’s DHC model, we have discussed the different temperature dependency of the DHCV for the Ti-6Al-4V alloys and the Zr-2.5Nb both of which have the microstructure of the α- and β-phases.
Zr-2.5Nb 압력관의 수소지연균열 속도에 미치는 균열선단응력 효과
김영석(Young Suk Kim),정용무(Yong Moo Cheong) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10
Using our new DHC model, we analyze Pan’s in-reactor tests results on a cold-worked Zr-2.5Nb tube showing that DHC velocity has been dictated by the Nb concentration in the β-Zr, not by the tensile stress, which remains as an unresolved issue with the old DHC models. This fact suggests that hydrogen diffusion through the β-Zr is more influential in governing the DHC velocity of the Zr-2.5Nb tube than the crack tip stress, which is consistent with our new DHC model. Normalization of the DHC velocity by hydrogen diffusivity or DH is found to lessen the apparent yield stress effect on the DHC velocity in zirconium alloys. Consequently, it is demonstrated that the crack tip stress directly dependent upon the magnitude of the yield or tensile stresses has little effect on the DHC velocity of zirconium alloys, proving that our new DHC model is valid.
Better Understanding of Delayed Hydride Cracking in Zr-2.5Nb TUbes
김영석(Kim Young Suk),Soon Sam Park,김성수(Sung Soo Kim),정용무(Yang Moo Cheong),안상복,임경수(Kyung Sao Im) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.8
This study focuses on the elucidation of delayed hydride cracking (DHC) of zirconium alloys, which has not been clearly understood up to date. DHC tests were conducted on the compact tension (CT) specimens of a CANDU Zr-2.5Nb pressure tube with hydrogen concentration varying from 12 to 100 ppm. DHC tests were conducted on the compact tension specimens of Zr-2.5Nb with hydrogen concentration varying from 12 to 100 ppm. Hydrogen was charged electrolytically into the CT specimens followed by a homogenization treatment. The DHCV for Zr-2.5Nb pressure tube had a temperature dependence with an activation energy of 49 KJ/mol in a temperature range from 144 to 250℃ and decreased with decreasing hydrogen concentration. The threshold stress intensity factor, KIH in the axial direction of Zr-2.5Nb tube also had a dependence of hydrogen concentration: a drastic decrease of KIH to a constant with increasing hydrogen concentration. Thus, DHCV and KIH were nicely described as a function of the supersaturated hydrogen concentration over TSSD independent of temperatures. Based on these results, we propose that KIH is a critical stress intensity factor to nucleate hydrides at the crack tip region, reducing a supersaturated hydrogen concentration at the crack tip region to the equilibrium terminal solid solubility for dissolution (TSSD) at any temperatures. Therefore, we conclude that the gradient of the equilibrium hydrogen concentration or TSSD at the crack tip region and the supersaturated one at the matrix region is a governing factor to initiate DHC.
김영석(Kim Young Suk),서용찬(Suh Yong Chan),안상복(Sang Bok Ahn),정용무(Cheong Yong Moo),임경수(Im Kyung Soo) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.3
With a view to assessing a change in the creep rate of the Zr-2.5Nb pressure tubes operating in the reactor with the operation time, Zr-2.5Nb sheets were made with four different kinds of manufacturing process to simulate a microstructural change of the pressure tubes with time. The creep tests were conducted on the Zr-2.5Nb sheets at temperatures ranging from 623 to 673 K and constant stress of 120 ㎫. As an indirect evaluation of the Nb content dissolved in the α-Zr grains, the Nb content in the β-Zr phase of Zr-2.5Nb sheets made with 4 different processes was measured using a replica method. Zr-2.5Nb made with process P1 had the lowest creep rate while that made with process P3 or P4 had the highest creep rate. By correlating the Nb content in the α-Zr grains with the creep rate and strength of the Zr-2.5Nb sheets, we conclude that the Nb contents dissolved in the the c-Zr grains governs the creep rate and strength of Zr-2.5Nb alloy. A change in the creep rate of the Zr-2.5Nb pressure tubes was discussed on the basis of a fact that α-Zr grains in the Zr-2.5Nb pressure tubes have the Nb content decreasing with the operation time increasing.
수소량에 따른 Zr-2.5Nb 압력관의 DHC 속도 및 K<SUB>IH</SUB>
김영석(Kim Young Suk),박순삼(Soon Sam Park),김성수(Sung Soo Kim),정용무(Yong Moo Cheong),임경수(Kyung Soo Im) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.3
To obtain a better understanding of DHC phenomenon, this study focused on the effect of DHC velocity and threshold stress intensity factor, K1H, of Zr-2.5Nb tubes as a function of hydrogen concentrations. DHC tests were conducted on the compact tension specimens of Zr-2.5Nb with hydrogen concentration varying from 12 to 100 ppm. Hydrogen was charged electrolytically into the CT specimens followed by a homogenization treatment. The DHCV for Zr-2.5Nb pressure tube had an activation energy of 49 KJ/㏖ in a temperature range of 144 and 250℃ and decreased with decreasing hydrogen concentration. The threshold stress intensity factor, K1H in the axial direction of Zr-2.5Nb tube also had a dependence of hydrogen concentration: a exponential decrease of K1H to a constant with increasing hydrogen concentration. Thus, DHCV and K1H were nicely described as a function of the supersaturated hydrogen concentration over TSSD independent of temperatures. Based on these results, we propose that K1H is a critical stress intensity factor to drop a supersaturated hydrogen concentration at the crack tip region to the equilibrium solubility for dissolution at any temperatures. Hence, the lower the initial hydrogen concentration at the matrix region, the smaller a concentration gradient between the crack tip and the matrix region, lowering DHCV.
항복강도에 따른 Zr-2.5Nb 압력관의 Delayed Hydride Cracking Velocity
김영석(Kim Young Suk),정용무(Cheong Yong Moo),임경수(Im Kyung Soo),김인섭(Kim In Sup),마르켈로프(V. Markelov) 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.8
The objective of this study is to investigate the delayed hydride cracking velocity (DHCV) of Zr-2.5Nb pressure tubes with yield strength. DHC tests were conducted at an initial K₁ of 19 ㎫√m and 250℃ on the compact tension specimens containing 100 ppm hydrogen that were taken from the Zr-2.5Nb tubes with different yield strengths. The crack growth was determined with the direct current potential drop method. A nice correlation of the DHC velocity and the yield strength was obtained at all temperatures by normalizing the DHCV with the terminal solid solubility and diffusion of hydrogen: the DHCV increased exponentially with increased yield strength of the Zr-2.5Nb tubes. In contrast, the texture and the distribution of the β-Zr phase were found to have a comparatively minor effect on the DHCV of the Zr-2.5Nb tubes. Yield strength of the Zr-2.5Nb tubes determines a plastic zone size with a steep gradient of hydrogen concentration ahead of the crack tip where hydrides can be precipitated. The yield strength effect is discussed by correlating the plastic and the striation spacing observed on the fractured surfaces as a function of yield strengths.
실험환경과 수소량에 따른 CANDU Zr-2.5Nb압력관의 DHCV 특성
조선영(Sun Young Cho),정용무(Yong Moo Cheong),김성수(Sung Soo Kim),임경수(Kyung Soo Im),김영석(Young Suk Kim) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
Even though CANDU Zr-2.5Nb Pressure tube has been used in the water under the high temperature and pressure, the evaluation of DHCV for the pressure tube has been limited in the air. Therefore, it is necessary for DHCV of real condition such as in the water under the high temperature and pressure to be evaluated. The test was carried out under the simulated condition using the water of 182℃ and 250℃ for the pressure tubes which were saturated at 34ppm and 60ppm with the hydrogen. At each temperature, DHCV in the air was faster than that in the water. In addition, the difference of DHCV between in the air and in the water was increased as the temperature grew up. In terms of safety, DHCV in the water under the high temperature and pressure was conservative as compared with that in the air. The reason why DHCV in the air was different from that in the water under the high temperature and press따e seemed to be due to the corrosion resistance of pressure tube, the formation of the corrosive film at the crack tip and the influence of compression under the high temperature and pressure water.