• 동력기계공학회지
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• 제16권5호
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• pp.76-82
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• 2012

Duplex stainless steels have the dual microstructure of austenite and ferrite phases. This steel exhibits generally a high corrosion resistance and higher mechanical strength compared with austenitic stainless steels. The steels used in the investigation have the chemical composition of Fe-22Cr-xNi-yMn-0.2N in which the contents of Ni and Mn were varied with maintaining the equal [Ni/Cr] equivalent. The fraction of ferrite phase was increased with the increase of annealing temperature. The impact factor of Mn element on the [Ni] equivalent was amended on the basis of the results of the investigation. 4Mn-2Ni alloy showed the highest pitting corrosion resistance. The fraction ratio, grain size and misorientation angle between grains were measured, and the correlation with pitting potential was investigated.

• Journal of Advanced Marine Engineering and Technology
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• 제36권8호
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• pp.1069-1075
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• 2012

레이저는 고밀도 열원으로써, 입열을 최소화하면서 고속용접이 가능한 장비이다. 특히 최근 상업화된 고출력 파이버 레이저는 특유의 발진원리 때문에 높은 빔품질과 소형화된 시스템이 가능하다. 이러한 장점들로 인해 파이버 레이저는 LNG 카고탱크에 대해 선박 내부에서 용접이 가능한 가장 적합한 열원이라고 판단된다. 따라서 본 연구에서는 선박산업에 레이저 용접기술을 적용하기 위해 파이버 레이저를 이용하여 LNG 선용 스테인리스강의 용접을 진행하였다. 연구에서 사용한 재료는 카고탱크에 사용되고 있는 오스테나이트계 스테인리스강인 두께 1.2mm의 STS304L였다. 레이저 출력 및 용접속도를 변화시켜가면서 겹치기 및 맞대기 용접을 실시한 후 용입특성을 분석하여 최적의 접합조건을 도출하였다. 그 결과 용접부에서 모재보다 우수한 기계적 성질을 얻을 수 있었다.

• Journal of Welding and Joining
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• 제32권2호
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• pp.48-53
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• 2014

This study was carried out to investigate the effect of postweld heat treatment(PWHT, 930, 1080, $1230^{\circ}C$) on the microstructure, phase formation, pitting corrosion and mechanical properties such as hardness, tensile strength and impact values of super duplex stainless steel(UNS S32750) multipass welds. Based on the microstructural examination and X-ray diffraction analysis, it was found that the ${\sigma}$ phase was formed in the welds heat treated at $930^{\circ}C$ in which the ferrite content greatly decreased into 5~10% in the welds. The secondary austenite was formed in the reheated zone of welds and redissolved into ferrite with increasing heat treatment temperatures. The tensile strength and impact values of welds heat treated at $930^{\circ}C$ were the lowest and revealed the brittle fracture surface. The weight loss by pitting corrosion increased with test temperatures. It was confirmed that pitting corrosion occurred mainly in secondary austenite of reheated zones. The postweld heat treatment temperature is recommended to be in the range of $1050{\sim}1150^{\circ}C$.

• 한국재료학회지
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• 제31권1호
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• pp.29-37
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• 2021

Martensitic stainless steel is commonly used in the medical implant instrument. The alloy has drawbacks in terms of strength and wear properties when applied to instruments with sharp parts. 440C STS alloy, with improved durability, is an alternative to replace 420 J2 STS. In the present study, the carbide precipitation, and mechanical and corrosion properties of STS 440C alloy are studied as a function of different heat treatments. The STS 440C alloy is first austenitized at different temperatures; this is immediately followed by oil quenching and sub-zero treatment. After sub-zero treatment, the alloy is tempered at low temperatures. The microstructures of the heat treated STS 440C alloy consist of martensite and retained austenite and carbides. Using EDX and SADP with a TEM, the precipitated carbides are identified as a Cr23C6 carbide with a size of 1 to 2 ㎛. The hardness of STS 440C alloy is improved by austenitization at 1,100 ℃ with sub-zero treatment and tempering at 200 ℃. The values of Ecorr and Icorr for STS 440C increase with austenitization temperature. Results can be explained by the dissolution of Cr-carbide and the increase in the retained austenite. Sub-zero treatment followed by tempering shows a little difference in the properties of potentiodynamic polarizations.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2007년도 추계학술대회 논문집
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• pp.139-141
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• 2007

Compression tests were carried out to investigate morphologies of compressed specimen, deformation microstructure and stress-strain relation in high-nitrogen austenite stainless steel. Tests were performed under a wide range of temperature and, with true strain rates up to $\dot{\varepsilon}$ =0.05, 0.1, 0.5 and $1.0s^{-1}$. The activation energy of loading force was equal to plastic deformation energy within the temperature range of $900^{\circ}C$ to $1250^{\circ}C$. Dynamically recrystallized grain size decreased with an increasing strain rate and temperature. Flow stresses and deformation microstructures, were used to quantify the critical strain rate and recrystallized grain size. The grain size versus strain rate-temperature map obtained in the study was in good agreement with the deformation microstructures of compressed specimens.

• Nuclear Engineering and Technology
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• 제51권1호
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• pp.207-213
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• 2019

Corrosion and mechanical behavior of the hot-rolled 0.5%Gd-0.8%B-stainless steel to develop a spent nuclear fuel storage material was studied in a simulated nuclear waste treatment condition with rolling condition. The austenite and ferrite phases of the 0.5%Gd-0.8%B-stainless steels are about 88:12. The average austenite and ferrite grain size of the plane normal to rolling, transverse and normal directions of the hot rolled specimens are about 5.08, 8.94, 19.35, 23.29, 26.00 and 18.11 [${\mu}m$], respectively. The average micro-hardness of the as-cast specimen is 200.4 Hv, whereas, that of the hot-rolled specimen are 220.1, 204.7 and 203.5 [$H_v$] for the plane normal to RD, TD and ND, respectively. The UTS, YS and elongation of the as-cast and the hot-rolled specimen are 699, 484 [MPa], 34.0%, and 654, 432 [MPa] and 33.3%, respectively. The passivity was observed both for the as-cast and the hot rolled specimens in a simulated nuclear waste solution. The corrosion potential and corrosion rate of the as-casted specimens are $-343[mV_{SHE}]$ and $3.26{\times}10^{-7}[A/cm^2]$, whereas, those of the hot rolled specimens with normal to ND, RD and TD are -630, -512 and -620 [$mV_{SHE}$] and $6.12{\times}10^{-7}$, $1.04{\times}10^{-6}$ and $6.92{\times}10^{-7}[A/cm^2]$, respectively. Corrosion tends to occur preferentially Cr and B rich area.

• 한국압력기기공학회 논문집
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• 제20권1호
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• pp.56-65
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• 2024

Cast austenitic stainless steels (CASS) and austenitic stainless steel weldments with a ferrite-austenite duplex structure are widely used in nuclear power plants, incorporating ferrite phase to enhance strength, stress relief, and corrosion resistance. Thermal aging at 290-325℃ can induce embrittlement, primarily due to spinodal decomposition and G-phase precipitation in the ferrite phase. This study evaluates the effects of thermal aging by collecting and analyzing various mechanical properties, such as Charpy impact energy, ferrite microhardness, and tensile strength, from various literature sources. Different model expressions, including hyperbolic tangent and phase transformation equations, are applied to calculate activation energy (Q) of room-temperature impact energies, and the results are compared. Additionally, predictive models for Q based on material composition are evaluated, and the potential of machine learning techniques for improving prediction accuracy is explored. The study also examines the use of ferrite microhardness and tensile strength in calculating Q and assessing thermal embrittlement. The findings provide insights for developing advanced prediction models for the thermal embrittlement behavior of CASS and the weldments of austenitic steels, contributing to the safety and reliability of nuclear power plant components.

• 한국재료학회지
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• 제25권6호
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• pp.293-299
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• 2015

In this study, nitrogen ions were implanted into STS 316L austenitic stainless steel by plasma immersion ion implantation (PIII) to improve the corrosion resistance. The implantation of nitrogen ions was performed with bias voltages of -5, -10, -15, and -20 kV. The implantation time was 240 min and the implantation temperature was kept at room temperature. With nitrogen implantation, the corrosion resistance of 316 L improved in comparison with that of the bare steel. The effects of nitrogen ion implantation on the electrochemical corrosion behavior of the specimen were investigated by the potentiodynamic polarization test, which was conducted in a 0.5 M $H_2SO_4$ solution at $70^{\circ}C$. The phase evolution and texture caused by the nitrogen ion implantation were analyzed by an X-ray diffractometer. It was demonstrated that the samples implanted at lower bias voltages, i.e., 5 kV and 10 kV, showed an expanded austenite phase, ${\gamma}_N$, and strong (111) texture morphology. Those samples exhibited a better corrosion resistance.

• Journal of Welding and Joining
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• 제33권3호
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• pp.68-74
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• 2015

Superduplex stainless steels are important materials to the oil and gas industry, especially for off-shore production. TIG welding of super duplex stainless steels to obtain the optimal phase balance between austenite and ferrite is mainly achieved by controlling the cooling rate and the weld chemistry. The latter depends on the filler wire chosen and the shielding gas used. If TIG welding of superduplex stainless steels is performed with argon shielding gas only, then nitrogen gets lost from the weld pool, which can result in a ferrite-rich weld metal, with an inferior corrosion resistance than parent metal. In the present study, nitrogen permeation model from the shield gas which gets into the weld metal in DCEN-TIG welding has suggested. This plasma stream model shows characteristics of permeation of nitrogen ions into the molten metal due to the strong physical effect of plasma stream which formed by the arc pressure rather than the permeation of nitrogen ions caused by electric effect.

• 열처리공학회지
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• 제21권5호
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• pp.244-250
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• 2008

This study has investigated the surface phase change, hardness variation, surface precipitates, nitrogen content and corrosion resistance in STS 431 (17Cr-2Ni-0.2C-0.01Nb) martensitic stainless steel after high temperature gas nitriding (HTGN) treatment at the temperature range between $1050^{\circ}C$ and $1150^{\circ}C$. The HTGN-treated surface layer appeared $Cr_2N$ of rod type, carbo-nitride of round type and fine precipitates in the austenite matrix. On the other hand the interior region where the nitrogen was not permeated, exhibited martensite phase. The surface hardness showed 250~590 HV, depending on the HTGN treatment conditions, while the interior martensitic phase represented 520 HV. The permeation depth of nitrogen increased with increasing the HTGN-treated temperature. The nitrogen concentration of the surface layer appeared approximately ~0.17% at $1100^{\circ}C$. On comparing the corrosion resistance between solution-annealed and HTGN-treated steels, the corrosion resistance of HTGN-treated steel was superior to that of solution-annealed specimens.