• Corrosion Science and Technology
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• 제5권5호
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• pp.181-188
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• 2006

In this study, the characteristics of the experimentally produced high N-high Cr bearing stainless steels are discussed as a part of applications of materials for FGD (Fuel Gas Desulfurization) system of thermal power plants or for power plants using seawater as coolant. Corrosion resistance of developed alloys is especially investigated in detail. Corrosion characteristics of vacuum melted cast are shown to be superior to that of air melted one. From the viewpoint of CPT, It is estimated that the differences of corrosion resistance are $21.8^{\circ}C{\sim}24.6^{\circ}C$ at PRE 40 and $8^{\circ}C{\sim}12.4^{\circ}C$ at PRE 50, and the gaps becomes bigger as the PRE values are lower. In the evaluation of corrosion resistance in alloy A2501, Z3101, and A3301 according to Cr concentration, alloy A3301 shows a deviation from the general tendency in chloride solutions. It has relatively high PRE value as 48.6, but it has relatively poor pitting resistance. It is, however, difficult to observe a specific phase except ferrite in microstructure analysis and neither detects special phase such as sigma phase.

• Corrosion Science and Technology
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• 제19권4호
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• pp.203-210
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• 2020

The corrosion behavior of super austenitic stainless steel was studied by examining the characteristics of the sigma phase formed in the steel. A range of experimental and analytical methods was employed, including potentiodynamic polarization tests, critical pitting temperature tests, transmission electron microscopy, and energy-dispersive spectroscopy. Three steel samples with different sigma phase levels were obtained by intentionally adjusting the manufacturing process. The results showed that the corrosion resistance of the samples was strongly dependent upon the size and distribution of the sigma phase precipitated in the samples. The larger the size of the sigma phase, the higher the Mo content in the sigma phase and the higher the depletion level of Mo at the interface between the matrix/sigma phase, the more samples with a coarse-sized sigma phase were susceptible to localized pitting corrosion at the interface. These results suggest that various manufacturing processes, such as welding and the post-heat treatment of the steel, should be optimized so that both the size and fraction of the sigma phase precipitated in the steel are small to improve the resistance to localized corrosion.

• 대한금속재료학회지
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• 제46권4호
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• pp.209-216
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• 2008

Super austenitic stainless steels shows the high PRE (Pitting Resistance Equivalent) number and the good corrosion resistance. This work controlled the Co contents in Fe-31Cr-1.7Mo-27Ni-0.25N alloys to elucidate the effect of cobalt contents on the biocompatibility and corrosion resistance. Increasing Co contents, the hardness of the annealed alloys tends to be reduced. In aged alloys, cobalt decreased the increments of hardness by aging treatment. Cobalt decreased the critical pitting temperature (CPT) in 6% $FeCl_3$ + 1% HCl solution, but improved the anodic polarization behavior in Hanks' balanced salt solution and artificial saliva solution. Repassivation rate in artificial body solutions was improved by increasing cobalt contents, but didn't show the linear relationship to PRE number of the alloys. The experimental alloys showed the non-cytotoxicity because of its high corrosion resistance.

• 한국표면공학회지
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• 제54권3호
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• pp.102-111
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• 2021

The corrosion behaviors of laser-welded super duplex stainless steel tubes with post-weld heat treatment(PWHT) conditions(950, 1000, 1050, 1100 ℃ for 5 and 30 min) were evaluated by electrochemical potentiodynamic polarization and critical pitting temperature measurements. This study showed that the critical metallurgical factors affecting the degradation of corrosion resistance of a steel tube in as-welded condition were the unbalanced phase fraction(ferrite:austenite = 94:4), Cr₂N precipitation, and phase transformation from the austenite phase to ɛ-martensite(via stress-induced phase transformation). The improvement in the corrosion resistance of the welded specimen depends greatly on the PWHT conditions. The specimens after PWHT conducted below 1000 ℃ showed inferior corrosion resistance, caused by precipitation of the sigma phase enriched with Cr and Mo. At 1100 ℃ for a longer duration in PWHT, the ferrite phase grows, and its fraction increases, leading to an unbalanced phase fraction in the microstructure. As a result, pitting can be initiated primarily at the interface between the ferrite/austenite phase, particularly in base metal.

• 한국주조공학회지
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• 제43권5호
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• pp.230-240
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• 2023

염화물 환경에서 329LD 린 듀플렉스 스테인리스강과 CF3M 오스테나이트 스테인리스강 주물재의 공식 거동을 조사하였다. CF3M 합금에 비해 상대적으로 낮은 Ni 및 낮은 Mo 함량을 갖는 329LD 합금의 공식 전위, 부동태 영역 및 임계 공식 온도는 상용 CF3M 합금보다 높기 때문에 329LD 합금의 공식 저항성은 CF3M 합금보다 우수하였다. CF3M 합금에 비해 높은 Cr과 Mo 와 중간 정도의 N 함량을 갖는329LD 합금의 공식 저항성이 향상된 이유는 크게 두 가지이다. 첫째, 329LD 합금의 공식 저항 당량지수 (PREN δ+γ) 값이 CF3M 합금보다 높다. 둘째, 329LD 합금의 부동태 영역은 CF3M 합금의 부동태 영역보다 크다. 이는 329LD 합금에 높은 Cr과 낮은 Mo 및 중간 정도의 N 함량을 첨가함으로써 세 원소의 시너지 효과가 부동태 피막의 부동태를 향상시켜 내식성을 증가시키기 때문이다. 공식저항 당량지수에서 N factor 16을 이용하여 계산된 오스테나이트 (γ)의 PREN_γ과 페라이트 (δ) 값의 PREN_δ을 계산한 결과 오스테나이트 (γ)의 PREN_γ 값이 페라이트 (δ)보다 작으므로 329LD 합금의 공식은 γ상에서 선택적으로 시작되어 최종적으로는 γ상에서 δ상으로 전파됨을 확인하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.268-273
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• 2001

Fossil power plants operated in high temperature condition are composed of components such as turbine, boiler, and piping system. Among these components, turbine blades made with 12%Cr steel operate at a temperature above $500^{\circ}C$. Due to the long term service, turbine blades experience material degradation manifested by change in mechanical and microstructural properties. The need to make life assessment and to evaluate material degradation of turbine blade is strongly required but in reality, there is a lack of knowledge in defining failure mechanism and fundamental data for this component. Therefore, in making life assessment of turbine blade, evaluation of material degradation must be a priority. For this purpose, evaluation of toughness degradation is very important. The major cause of toughness degradation in 12Cr turbine blade is reported to be critical corrosion pitting induced by segregation of impurity elements(P etc.), coarsening of carbide, and corrosion, but the of materials for in-service application. In this study, the purpose of research is focused on evaluating toughness degradation with respect to operation time for 12%Cr steel turbine blade under high temperature steam environment and quantitatively detecting the degradation properties which is the cause of toughness degradation by means of non-destructive method, electrochemical polarization.

• Corrosion Science and Technology
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• 제3권5호
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• pp.179-186
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• 2004

Recently the alloy development of duplex stainless steel has been done. On this study we studied the effect of the corrosion resistance according to the composition variety of C, Cr, N in the alloy elements of duplex stainless steel. materials which have below 0.1[mm/year] corrosion rate enable to use for corrosion-resisting materials, generally. On this experiment we inspected the effect of the composition variety of C, Cr, N in duplex stainless steel and the heat treatment, which the condition was the water quenching after the heat treatment for 1hr. The experiment was done on the basis of the ASTM G48A test, Critical pitting temperature(CPT), and ASTM G-61(Electrochemical tests for cyclic polarization).

• Corrosion Science and Technology
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• 제23권2호
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• pp.166-178
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• 2024

The localized corrosion resistance of UNS N07718 alloy was investigated after solution heat treatment. When the alloy was heat-treated at 1050 ℃ for 2.5 hours, it experienced an increase in average grain diameter, a reduction in grain boundary area, and the dissolution of delta phases along grain boundaries. Additionally, primary metallic nitrides (MN) and metallic carbides (MC), enriched with either Ti or Nb, were identified and exhibited a random distribution within the microstructures. Despite the solution heat treatment, the composition, diameter, and abundance of MNs and MCs remained relatively consistent. The critical pitting temperature (CPT), as determined by the ASTM G48-C immersion test, revealed similar values of 45 ℃ for both treated and untreated alloys. However, a decrease in maximum pit depth and corrosion rate was observed after the solution heat treatment. The microstructural changes that occurred during the heat treatment and their potential implications were discussed to understand the influence of the solution heat treatment.

• Nuclear Engineering and Technology
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• 제53권9호
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• pp.2982-2989
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• 2021

One of the consequences of the 475 ℃ embrittlement of duplex stainless steels is the reduction of the resistance to localized corrosion. Therefore, the detection of this type of embrittlement before the material exhibits significant loss in toughness, and corrosion resistance is important to ensure the structural integrity of critical components under corrosion threats. In this research, conductivity measurements are performed using the alternating current potential drop (ACPD) technique with using a portable four-point probe as a nondestructive evaluation (NDE) method for detecting the embrittlement in a 2507 (UNS S32750) super duplex stainless steel (SDSS) aged at 475 ℃ from as-received condition to 300 h. The electric conductivity results were compared against two electrochemical tests namely double loop electrochemical potentiokinetic reactivation (DL-EPR) and critical pitting temperature (CPT). Mechanical tests and the microstructure characterized using scanning electron microscopy (SEM) imaging are conducted to track the progress of embrittlement. It is shown that the electric conductivity correlates with the changes in impact energy, microhardness, and CPT corrosion tests result demonstrating the feasibility of the four-point probe as a possible field-deployable method for evaluating the 475 ℃ embrittlement of 2507 SDSS.

• 한국표면공학회지
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• 제54권5호
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• pp.248-259
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• 2021

This study examined the influence of post weld heat treatment (PWHT) conditions on corrosion behaviors of laser-welded super duplex stainless steel tube. Due to the high cooling rate of laser welding, the phase fraction of ferrite and austenite in the weld metal became unbalanced significantly. In addition, the Cr₂N particles were precipitated adjacent to the fusion line, which can be susceptible to the localized corrosion. On the other hand, the phase fraction in the weld metal was restored at a ratio of 5:5 when exposed to temperatures above 1060 ℃ during the post weld heat treatment. Nevertheless, the high beltline speed during the PWHT, leading to the insufficient cooling rate, caused a precipitation of σ phase at the interface between ferrite/austenite in both weld metal and base metal. This resulted in the severe corrosion damages and significant decrease in critical pitting temperature (CPT), which was even lower than that measured in as-welded condition. Moreover, the fraction of σ phase in the center region of post weld heat treated steel tube was obtained to be higher than in the surface region. These results suggest that the PWHT conditions for the steel tube should be optimized to ensure the high corrosion resistance by excluding the precipitation of σ phase even in center region.