• 한국재료학회:학술대회논문집
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• 한국재료학회 2011년도 춘계학술발표대회
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• pp.57.2-57.2
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• 2011

MX 석출물 형성 원소인 V과 Nb을 첨가한 11Cr-1Mo-VNb 페라이트계 내열강의 각각의 시험 온도에서 파단 시간에 따른 미세조직의 변화를 연구하였다. 초기 조직은 템퍼드 마르텐사이트 조직을 갖고 있었으며, 고온 강도를 저하시키는 ${\delta}$-페라이트는 존재하지 않았다. 주 석출상은 $M_{23}C_6$와 MX로 확인되었다. 또한, 모든 크리프 파단재에서 응력을 받은 게이지 부분이 응력을 받지 않은 그립 부분보다 석출물 및 마르텐사이트 래스 폭의 성장이 가속되는 결과를 나타내었다. 이는 크리프 변형 중 응력 집중에 의해 래스 경계를 따라 전위가 집적되고, 이에 따른 용질원자의 확산 속도가 증가하여 석출물의 성장에 따른 래스 경계의 이동이 일어나 게이지 부분이 그립 부분보다 마르텐사이트 회복이 가속된 것으로 판단된다.

• 한국재료학회지
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• 제8권1호
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• pp.64-70
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• 1998

자동차의 엔진밸브 스프링으로 사용되는 Si-Cr 스프링강의 영구 변형 저항성과 내피로성이 우수한 고강도강을 개발하기 위하여 탄소함량을 증대시키고 Mo, V, W와 같은 합금원소를 기존의 SAE 9254 스프링강에 첨가하여 개발강을 제조했다. SEM및 EDX가 부착된 TEM을 이용하여 미세조직을 관찰했고, 크립시험 및 피로시험기를 이용하여 스프림의 영구 변형 저항성 및 스프링의 내피로성을 조사했다. 실험결과, 개발강은 피로 특성치는 기존강과 동등 수준이면서 인장강도가 기존강의 것보다 10%가 더높은 2100MPa 급의 고강도를 나타내었으며 또한 영구 변형 저항성도 현저하게 개선되었는데 이는 W, Mo의 첨가로 인해 템퍼링시에 세멘타이트의 성장이 억제되어서 세멘타이트의 석출물이 미세하게 되었기 때문이다.

• Journal of Mechanical Science and Technology
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• 제16권8호
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• pp.1109-1116
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• 2002

To investigate the effect of nitriding layer on both fatigue crack initiation and fatigue life, turbine rotor steel ( IC.- 1Mo-0.25V steel) specimens were nitrided by the nitemper method and then put to a rotary bending fatigue test at room and elevated temperatures. In nitriding, temperature and time were controlled to obtain a different nitrided thickness. Microstructure analysis, micro-Victors hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at inclusion near the interface between nitrided layer and substrate, which showed fish-eye type appearance in fractograph. The fatigue life of nitrided specimens at every temperature was prolonged compared to that of the non-nitrided. However, there was not observable improvement in fatigue characteristics with the increase of a nitrided thickness.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2005년도 춘계학술발표대회 개요집
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• pp.250-252
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• 2005

T/P92 steels are created for using USC boiler tube and header in next generation power plant. SP-Creep test and tensile creep test are performed to characterize creep for local structures of T/P92 steel weldment. The results are shown that P92 steel weldment is clearly superior than that of X20CrMoV121 steel weldment, which is widely used in supercritical power Plant. while fine gram HAZ is most weakest in X20 steel weldment, coarse grain HAZ is most weakest in P92.

• 대한기계학회논문집A
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• 제26권7호
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• pp.1241-1249
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• 2002

The extent of materials deterioration can be evaluated accurately by mechanical test such as impact test or creep test. But it is almost impossible to extract a large test specimen from in-service components. Thus material degradation evaluation by non-destructive method is earnestly required. In this paper, the material degradation for virgin and several aged materials of a Cr-Mo-V steel, which is an candidated as structural material of the turbine casing components for electric power plant, is nondestructively evaluated by reactivation polarization testing method. And, the results obtained from the test are compared with those in small punch(SP) tests recommended as a semi-nondestructive testing method using miniaturized specimen. In contrast to the aged materials up to 1,000hrs which exhibit the degradation behaviors with increased ${\Delta}[DBTT]_{SP}$, the improvement of mechanical property can be observed on the 2,000hrs and 3,000hrs aged materials. This is because of the softening of material due to the carbide precipitation, the increase of ferritic structures and the recovery of dislocation microstructure by long-time heat treatment. The reactivation rates($I_R/I_{Crit},\;Q_R/Q_{Crit}$) calculated by reactivation current densityt ($I_R$) and charge($Q_R$) in the polarization curves exhibit a good correlation with ${\Delta}[DBTT]_{SP}$ behaviors.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집A
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• pp.105-110
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• 2000

The Micromechanics test is new test method which uses comparatively smaller specimen than that required in conventional material tests. There are several methods, such as small-specimen creep test, the continuous indentation test, and small punch(SP) test. Among them, the small punch(SP) test method has been applied to many evaluation fields, such as a ductile-brittle transition temperature, stress corrosion cracking, hydrogen embrittlement, and fracture properties of advanced materials like FGM or MMC. In this study, the small punch(SP) test is performed to evaluate the mechanical properties at high/low temperature from $-196^{\circ}C$ to $650^{\circ}C$ and the material degradation for virgin and aged materials of 9Cr1MoVNb steel which has been recently developed. The ${\Delta}P/{\Delta}{\delta}$ parameter defined a slope in plastic membrane stretching region of SP load-displacement curve decreases according to the increase of specimen temperature, and that of aged materials is higher than the virgin material in all test temperatures. And the material degradation degrees of aged materials with $630^{\circ}C$ -500hrs and $630^{\circ}C$ -1000hrs are $36^{\circ}C$ and $38^{\circ}C$ respectively. These behaviors are good consistent with the results of hardness($H_v$) and maximum displacement(${\delta}_{max}$).

• 비파괴검사학회지
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• 제22권3호
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• pp.267-274
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• 2002

본 연구는 Cr-Mo-V강의 다양한 분극특성들을 이용하여 비파괴적으로 재질열화를 평가하는 전기화학적 기법에 대해서 서술한 것이다. 사용된 전기화학적 기법은 스테인레스강의 열 이력에 의해 주로 야기되는 강화원소 결핍 영역에서의 예민화 그리고 부식속도를 평가하기 위해 널리 사용되는 양극분극 시험법이다. 재질열화 평가는 $10{\times}10{\times}0.5mm$ 크기의 시험편을 사용하는 미소역학 시험법으로 잘 알려진 SP시험에 의해 수행되었다. $630^{\circ}C$에서 1,000hrs 시효된 재료가 가장 높은 재질 열화도 ${\Delta}[DBTT]_{SP}$을 보였으나, 2,000hrs 그리고, 3,000hrs 시효된 재료는 시효 시간이 증가함에 따라 ${\Delta}[DBTT]_{SP}$의 감소를 보였다. 전류밀도의 차 $({\Delta}I_{FP},\;{\Delta}I_{SP})$가 1,000hrs 시효시간까지는 증가하였고, 그 이후의 시효시간에서는 시효시간의 증가와 더불어 감소됨이 관찰되었다. 이 같은 결과는 시효재료에 대한 경도의 거동과 일치하였다. 덧붙여, ${\Delta}[DBTT]_{sp}$는 ${\Delta}I_{FP}$ 및 ${\Delta}I_{SP}$와 같은 전기화학적 분극특성들과 좋은 상관관계를 가지고 있음을 알 수 있었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.308-313
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• 2004

SP test has been confirmed the availability, however the application of SP test is hampered because the relation of stress-strain and load-displacement is not determined definitely. This study suggested an evaluation technique of plastic flow characteristic for X20CrMoV121 steel weldment through inverse analysis using SP test and finite element analysis(FEA). From the result, good agreement was found in load-displacement curves obtained from SP test and FEA. Also, The behavior of load-displacement curve from FEA show a rule that load is increase with increasing K(strength coefficient) and displacement is increase with increasing n(work hardening index). From the inverse analysis, true stress-strain curve could be obtained for each local structure of weldment. And the CGHAZ and WM, which showed lower load- displacement behavior, have smaller work hardening index, while FGHAZ have the largest index.

• Journal of Welding and Joining
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• 제34권6호
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• pp.55-61
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• 2016

Recently developed ferritic heat resistance steel, T24 was used to evaluate microstructure characteristics of simulated heat affected zone. Also, correlation between the prior austenite grain size and amount of $M_{23}C_6$ carbide dissolution was discussed. With the increasing of peak temperature, Grain size steadily increased up to $1050^{\circ}C$ and then rapidly increased at $1150^{\circ}C$. Of the peak temperature $950{\sim}1050^{\circ}C$, amounts of $M_{23}C_6$carbide dissolution are low. But Most of $M_{23}C_6$ carbide that is inhibited grain growth were dissolved above $1050^{\circ}C$ and decreased volume fraction of carbide. This indicates that grain growth may be achieved through dissolution of carbide in the base material. As of welding, due to very rapid heating rate, $M_{23}C_6$ carbide exists above equilibrium solution temperature that is $800^{\circ}C$, even at $1050^{\circ}C$. So, It was confirmed that close correlation between carbide dissolution in the base material and grain growth. Calculated grain size has a linear relationship with peak temperature, on the other hand, measured grain size discontinuously increased between $950{\sim}1050^{\circ}C$ and above $1050^{\circ}C$. Grain size of heat affected zone at $1350^{\circ}C$ peak temperature showed maximum 67um and minimum 4um. Also, The number of side showed 3 to 10.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 춘계학술대회 논문집
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• pp.164-169
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• 2000

The remote measurement system(RMS) as a new experimental method is limited in its application to crack measurements at elevated temperatures because of the oxide layer on the specimen surface. Since TiAlN and Cr coating layers have a high resistance to oxidation and wear, this paper proposed a TiAlN and Cr coating technique for specimens to facilitate the measurement of crack growth behavior using RMS. To investigate the effects of the coating layer, tension and fatigue tests were carried out at room temperature and at 538$^{\circ}C$, using specimens of 1Cr-1Mo-0.25V steel. From the experimental results, it was found that the mechanical properties of the TiAlN and Cr coated specimens were similar to those of the substrate. Accordingly, the TiAlN and Cr coated layer had hardly any influence on the fatigue crack propagation.