• 대한금속재료학회지
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• 제48권2호
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• pp.122-132
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• 2010

Higher strength and fracture toughness of reactor pressure vessel steels can be obtained by changing the material specification from that of Mn-Mo-Ni low alloy steel (SA508 Gr.3) to Ni-Mo-Cr low alloy steel (SA508 Gr.4N). However, the operation temperature of the reactor pressure vessel is more than $300^{\circ}C$ and the reactor operates for over 40 years. Therefore, we need to have phase stability in the high temperature range in order to apply the SA508 Gr.4N low alloy steel for a reactor pressure vessel. It is very important to evaluate the temper embrittlement phenomena of SA508 Gr.4N for an RPV application. In this study, we have performed a Charpy impact test and tensile test of SA508 Gr.4N low alloy steel with changing impurity element contents such as Mn and P. And also, the mechanical properties of these low alloy steels after longterm heat treatment ($450^{\circ}C$, 2000hr) are evaluated. Further, evaluation of the temper embrittlement by fracture analysis was carried out. Temper embrittlement occurs in KL4-Ref and KL4-P, which show a decrease of the elongation and a shifting of the transition curve toward high temperature. The reason for the temper embrittlement is the grain boundary segregation of the impurity element P and the alloying element Ni. However, KL4-Ref shows temper embrittlement phenomena despite the same contents of P and Ni compared with SC-KL4. This result may be caused by the Mn contents. In addition, the behavior of embrittlement is not largely affected by the formation of $M_3P$ phosphide or the coarsening of Cr carbides.

• 대한금속재료학회지
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• 제47권9호
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• pp.533-541
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• 2009

Materials used for a reactor pressure vessel(RPV) are required high strength and toughness, which determine the safety margin and life of a reactor. Ni-Mo-Cr low alloy steel shows better mechanical properties than existing RPV steels due to higher Ni and Cr contents compared to the existing RPV steels. The present study focuses on effects of Ni, Cr contents on the cleavage fracture toughness of Ni-Mo-Cr low alloy steels in the transition temperature region. The fracture toughness was characterized by a 3-point bend test of precracked Charpy V-notch(PCVN) specimens based on ASTM E1921-08. The test results indicated that the fracture toughness was considerably improved with an increase of Ni and Cr contents. Especially, control of Cr content was more effective in improving fracture toughness than manipulating Ni content, though Charpy impact toughness was changed more extensively by adjusting Ni content. These differences between changes in the fracture toughness and that in the impact toughness were derived from microstructural features, such as martensite lath size and carbide precipitation behavior.

• 대한금속재료학회지
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• 제48권3호
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• pp.194-202
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• 2010

Application of a stronger and more durable material for reactor pressure vessels (RPVs) might be an effective way to insure the integrity and increase the efficiency of nuclear power plants. A series of research projects to apply the SA508 Gr.4 steel in ASME code to RPVs are in progress because of its excellent strength and durability compared to commercial RPV steel (SA508 Gr.3 steel). In this study, the microstructural characteristics and mechanical properties of SA508 Gr.3 Mn-Mo-Ni low alloy steel and SA508 Gr.4N Ni-Mo-Cr low alloy steel were investigated. The differences in the stable phases between these two low alloy steels were evaluated by means of a thermodynamic calculation using ThermoCalc. They were then compared to microstructural features and correlated with mechanical properties. Mn-Mo-Ni low alloy steel shows the upper bainite structure that has coarse cementite in the lath boundaries. However, Ni-Mo-Cr low alloy steel shows the mixture of lower bainite and tempered martensite structure that homogeneously precipitates the small carbides such as $M_{23}C_6$ and $M_7C_3$ due to an increase of hardenability and Cr addition. In the mechanical properties, Ni-Mo-Cr low alloy steel has higher strength and toughness than Mn-Mo-Ni low alloy steel. Ni and Cr additions increase the strength by solid solution hardening. In addition, microstructural changes from upper bainite to tempered martensite improve the strength of the low alloy steel by grain refining effect, and the changes in the precipitation behavior by Cr addition improve the ductile-brittle transition behavior along with a toughening effect of Ni addition.

• 대한금속재료학회지
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• 제47권3호
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• pp.139-146
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• 2009

The heat-affected zone (HAZ) of SA508 Gr.4N Ni-Mo-Cr low alloy steel, which has higher Ni and Cr contents than SA508 Gr.3 Mn-Mo-Ni low alloy steel, was investigated on the microstructure and mechanical properties. The HAZ was categorized into seven characteristic zones (CGCG, FGCG, ICCG, SCCG, FGFG, ICIC and SCSC-HAZ) according to the peak temperature from the thermal cycle experienced during multi-pass welding. Post Weld Heat Treatment (PWHT) was conducted in the temperature range of $550{\sim}610^{\circ}C$ for 30 hours to evaluate the effect of PWHT conditions on the microstructure and mechanical properties. Before PWHT, CGHAZ and FGFGHAZ showed high yield strength (YS) ranging from 1000 to 1250 MPa, while YS of SCSCHAZ decreased from 607 MPa (observed for base metal) to 501 MPa. The Charpy impact energies of sub-HAZs fell below 100J at $-29^{\circ}C$, except in the SCSCHAZ. By applying PWHT to sub-HAZ specimens, YS decreased as the PWHT temperature increased. In the case of CGHAZs and FGFGHAZ heat-treated at $610^{\circ}C$, YS dropped drastically to the range of 654~686 MPa. From the Charpy impact test, the upper-shelf energy (USE) increased to approximately 250J and Index temperature ($T_{68J}$) decreased below $-50^{\circ}C$. Specifically, in FGFG, ICIC and SCSC-HAZ, $T_{68J}$ was below -110, which was lower than the case of base metal.

• 대한금속재료학회지
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• 제46권12호
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• pp.771-779
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• 2008

An effective way of increasing the strength and fracture toughness of reactor pressure vessel steels is to change the material specification from that of Mn-Mo-Ni low alloy steel(SA508 Gr.3) to Ni-Mo-Cr low alloy steel(SA508 Gr.4N). In this study, we evaluate the effects of alloying elements on the microstructural characteristics of Ni-Mo-Cr low alloy steel. The changes in the stable phase of the SA508 Gr.4N low alloy steel with alloying elements were evaluated by means of a thermodynamic calculation conducted with the software ThermoCalc. The changes were then compared with the observed microstructural results. The calculation of Ni-Mo-Cr low alloy steels confirms that the ferrite formation temperature decreases as the Ni content increases because of the austenite stabilization effect. Consequently, in the microscopic observation, the lath martensitic structure becomes finer as the Ni content increases. However, Ni does not affect the carbide phases such as $M_{23}C_6 $ and $M_7C_3$. When the Cr content decreases, the carbide phases become unstable and carbide coarsening can be observed. With an increase in the Mo content, the $M_2C$ phase becomes stable instead of the $M_7C_3$ phase. This behavior is also observed in TEM. From the calculation results and the observation results of the microstructure, the thermodynamic calculation can be used to predict the precipitation behavior.

• 한국표면공학회지
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• 제56권1호
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• pp.94-103
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• 2023

The effects of carburizing pressure and gas ratio on vacuum carburizing properties (uniformity and surface characteristics) have been studied through the analyses of carbon concentration, hardness, surface color, surface roughness and type of carbon bonding. AISI 4115 steel specimens were carburized with various pressures (1, 5, and 10 Torr) at different locations (P₁, P₂, P₃, P₄, P₅, and P₆) inside a furnace held at 950 ℃. Since the carburizing pressure represents the density of the carburizing gas, it plays an important role in improving the carburizing uniformity according to locations in the furnace. As the carburizing pressure increased, the carburizing uniformity according to the sample location was improved, but the surface of the carburized specimen was discolored due to the residual acetylene gas, which does not contribute to the carburizing reaction. Therefore, the carburizing uniformity and surface discoloration have been improved by injecting acetylene gas (carburizing gas) and nitrogen gas (non-reactive gas) in a specific ratio.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2009년 추계학술발표대회
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• pp.12-12
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• 2009

터빈에서 핵심부품인 로터는 블레이드를 원심 운동시키는 대형 단조강이며, 고압의 증기 조건에서 고속회전하며 고온에서 운전과 저온에서 과속시험 동안 높은 원심력을 받는다. 또한 기동/정지 천이 동안 열응력을 받기 때문에, 이러한 운전조건에 부합되는 소재로서는 높은 Creep 강도 및 피로강도를 가지는 CrMoV type의 강종이 사용되어져 왔다. 발전소의 대용량화 및 고온화에 따라 종래의 증기조건에서 사용되어져 왔던 1%CrMoV강은 내산화성 및 내부식성이 문제가 되어 더 이상 사용이 불가하며, 고온/고압하에서도 우수한 소재 특성을 가지는 12%Cr강의 사용이 필수적이다. 그러나 12%Cr강으로 제작되는 로타는 Cr 양이 높기 때문에 저널부에 Galling 또는 Scuffing 이라 불리는 부적절한 마모현상과 사용 중 소착이 발생하기 쉬운 단점이 있기 때문에, 저널부에 Cr 함유량 2~3% 이하의 저합금강을 오버레이 용접하여 육성하는 일체형 가공구조의 로타 저널부가 주목되어 왔다. 따라서 본 연구에서는 Large scale 로타가 용접 도중 급열 및 급냉이 되지 않으면서 균일한 온도로 일정 시간 유지할 수 있는 열관리 장치 개발, 최적 오버레이 용접조건 선정 및 용접부 건전성 시험 평가를 통하여 12%Cr 로타 저널부의 최적 오버레이 용접공정을 확립하고자 하였다. 용접 열관리 장치는 전기저항 가열방식을 적용하고 있으며 용접이 최종 완료되기 전까지 로타 제품 전체는 $93^{\circ}C$이상의 온도로 유지 되어져야 하며, 규정 용접후열처리 온도는 $650^{\circ}C{\pm}14^{\circ}C$ 이다. 또한 로타 오버레이 용접은 모재 Set up $\Rightarrow$ 용접예열 $\Rightarrow$ GTA용접 $\Rightarrow$ SA용접 $\Rightarrow$ 용접후열(Post heating) $\Rightarrow$ 용접후열처리(PWHT) $\Rightarrow$ 정삭가공 $\Rightarrow$ NDE(UT) 순으로 수행 되어진다 실제 로타의 1/3 Scale로 시험편을 제작하여, 오버레이 mockup 시험을 수행한 후 화학성분, 경도 분포, 인장강도, 충격인성 및 굽힘시험을 수행한 결과, 오버레이 용접에서 요구되어지는 용접 물성값을 만족하는 것으로 확인되었다. 또한 균열 등의 선형 결함이나 기공, 슬라그 혼입과 같은 결함은 관찰되지 않았으며, 용접 시 아크의 안정성과 슬라그의 박리성은 양호하였으며 비드의 외관도 미려하여 용접 작업성도 양호하였다.

• 한국재료학회지
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• 제9권3호
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• pp.327-332
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• 1999

중성자에 조사 $(fluence: 2.3\times10^{19}ncm^{-2}, 553 K, E\geq1.0 MeV)$된 Mn-Mo-Ni 저 합금강 모재의 열처리 회복 거동을 조사하기 위하여 등시소둔과 등온소둔을 수행하여 회복 활성화에너지, 회복 반응차수 그리고 회복 반응률상수를 결정하였다. 열처리 후 회복은 비커스 미세 고온경도기로 측정하였고 실험결과를 이용, 열처리 회복단계, 회복결함들의 거동 및 회복 kinetics을 분석하였다. 실험결과 2단계의 회복구간(stage I : 703-753K, stage II : 813K-873K)이 나타났으며 각 단계의 회복활성화 에너지는 2.50 eV(1단계) 및 2.93 eV(2단계)이었다. 조사재와 비조사재의 등시소둔 곡선의 비교를 통하여 813K에서 RAH(radiation anneal hardening) 피크를 확인할 수 있었다. 743K 및 833K에서 수행한 등온소둔 결과, 회복의 60%가 모두 120분 이내에 일어나는 것으로 관찰되었다. 회복 반응차수는 두 회복구간에서 모두 2로 나타났으며 회복 반응율상수는 $3.4\times10^{-4}min^{-1}$(1단계)과 $7.1\times10^{-4}min^{-1}$(2단계) 이었다. 이상의 결과와 기 발표된 자료들을 함께 분석한 결과, 본 재료의 회복은 오랜 중성자조사로 형성된 점결함 집합체들이 열처리에 의한 분해와 Fe 기지에 격자간 원자로 존재하던 self-interstitial들과 vacancy들의 재결합에 의해 일어나는 것으로 해석된다.

• 대한금속재료학회지
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• 제48권6호
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• pp.523-532
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• 2010

Co-application of organic coating and cathodic protection has not provided enough durability to low-alloyed steels inseawater ballast tank (SBT) environments. An attempt has made to study the effect of alloy elements (Al, Cr, Cu, Mo, Ni, Si, W) on general and localized corrosion resistance of steels as basic research to develop new low-allowed steels resistive to corrosion in SBT environments. For this study, we measured the corrosion rate by the weigh loss method after periodic immersion in synthetic seawater at $60^{\circ}C$, evaluated the localized corrosion resistance by an immersion test in concentrated chloride solution with the critical pH depending on the alloy element (Fe, Cr, Al, Ni), determined the permeability of chloride ion across the rust layer by measuring the membrane potential, and finally, we analyzed the rust layer by EPMA mapping and compared the result with the E-pH diagram calculated in the study. The immersion test of up to 55 days in the synthetic seawater showed that chromium, aluminium, and nickel are beneficial but the other elements are detrimental to corrosion resistance. Among the beneficial elements, chromium and aluminium effectively decreased the corrosion rate of the steels during the initial immersion, while nickel effectively decreased the corrosion rate in a longer than 30-day immersion. The low corrosion rate of Cr- or Al-alloyed steel in the initial period was due to the formation of $Cr_2FeO_4$ or $Al_2FeO_4$, respectively -the predicted oxide in the E-pH diagram- which is known as a more protective oxide than $Fe_3O_4$. The increased corrosion rate of Cr-alloyed steels with alonger than 30-day exposure was due to low localized corrosion resistance, which is explained bythe effect of the alloying element on a critical pH. In the meantime, the low corrosion rate of Ni-alloyed steel with a longer than 30-day exposure wasdue to an Ni enriched layer containing $Fe_2NiO_4$, the predicted oxide in the E-pH diagram. Finally, the measurement of the membrane potential depending on the alloying element showed that a lower permeability of chloride ion does not always result in higher corrosion resistance in seawater.

• 대한금속재료학회지
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• 제49권8호
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• pp.628-634
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• 2011

SA508 Gr.4N Ni-Cr-Mo low alloy steel, in which Ni and Cr contents are higher than in commercial SA508 Gr.3 Mn-Mo-Ni low alloy steels, may be a candidate reactor pressure vessel (RPV) material with higher strength and toughness from its tempered martensitic microstructure. The inner surface of the RPV is weld-cladded with stainless steels to prevent corrosion. The goal of this study is to evaluate the microstructural properties of the clad interface between Ni-Cr-Mo low alloy steel and stainless weldment, and the effects of post weld heat treatment (PWHT) on the properties. The properties of the clad interface were compared with those of commercial Mn-Mo-Ni low alloy steel. Multi-layer welding of model alloys with ER308L and ER309L stainless steel by the SAW method was performed, and then PWHT was conducted at $610^{\circ}C$ for 30 h. The microstructural changes of the clad interface were analyzed using OM, SEM and TEM, and micro-Vickers hardness tests were performed. Before PWHT, the heat affected zone (HAZ) showed higher hardness than base and weld metals due to formation of martensite after welding in both steels. In addition, the hardness of the HAZ in Ni-Cr-Mo low alloy steel was higher than that in Mn-Mo-Ni low alloy steel due to a comparatively high martensite fraction. The hardness of the HAZ decreased after PWHT in both steels, but the dark region was formed near the fusion line in which the hardness was locally high. In the case of Mn-Mo-Ni low alloy steel, formation of fine Cr-carbides in the weld region near the fusion line by diffusion of C from the base metal resulted in locally high hardness in the dark region. However, the precipitates of the region in the Ni-Cr-Mo low alloy steel were similar to that in the base metal, and the hardness in the region was not greatly different from that in the base metal.