• Corrosion Science and Technology
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• 제16권6호
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• pp.278-284
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• 2017

The effects of TIG welding and post-treatment procedures on the microstructure and the pitting corrosion resistance of welded lean duplex stainless steel S82441 were investigated. Autogenous TIG welding was used with different amounts of heat input and shielding gases such as Ar, and mixtures of $Ar-N_2$ and Ar-He. The addition of 5% to 15% of nitrogen to argon practically did not affect the level of the pitting corrosion resistance. However, the application of gas mixtures (50% Ar + 50% He) resulted in a significant decrease in pitting corrosion resistance. We found that increased current (200 A and 250 A) led to lower values of CPT of welds compared with welds obtained with 50 A, 100 A and 150 A. In addition, the removal of the weld surface layer (0.2 ~ 0.3 mm thickness) in most cases not only resulted in a significant increase in resistance to the pitting corrosion but also post-treatment of weld, implying that corrosion resistance depended on factors such as surface roughness or the presence of undesirable oxides.

• 한국해양공학회지
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• 제23권3호
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• pp.40-45
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• 2009

With the increase in the annealing temperature, the volume fraction of austenite phase increased and the volume fraction of ferrite phase decreased. In compliance with the addition of N, not only the volume fraction of austenite phase was increased but also the austenite structure was made larger. Volume fraction of ${\sigma}$ phase was increased by decreasing of the volume fraction of ferrite phase, with the increase in the aging time and in compliance with the addition of N. As increasing in volume fraction of ${\sigma}$ phase, tensile strength and hardness increased, while elongation and impact value decreased. Elongation slowly decreased and impact value rapidly decreased at the early stage of aging. By the added N, tensile strength, elongation, hardness and impact value was increased.

• 한국재료학회지
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• 제9권12호
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• pp.1252-1262
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• 1999

자동차 휠, 멤버 등의 소재로 사용되는 인장강도 580MPa급 열연강판의 버링성형성을 향상시키기 위하여 강판의 신장 플랜지성(stretch-flangeability)에 대한 미세조직 및 열간압연후 냉각인자의 영향에 대하여 검토하였다. 열간압연후 3단 냉각제어 및 권취온도의 극저온화에 의하여 신장플랜지성이 우수한 페라이트-베이나이트 복합조직강의 제조가 가능하며, 3단 제어냉각에서 강판의 온도를 $Ar_3$ 직하의 페라이트변태역에서 일정 시간 유지하면 페라이트 변태 및 NbC의 석출이 조장됨을 확인하였다. 페라이트-베이나이트 복합조직 열연강판의 우수한 신장플랜지성은 3단 냉각 및 극저온 권취에 의한 등축 페라이트 분율의 증가, 입계 세멘타이트의 미세화 및 구성 상간의 경도차 저하에 의하여 타발공정에서의 미소균열 생성 및 전파가 억제되기 때문인 것으로 판단되었다. 아울러 0.08wt%C-1.5wt%Mn-0.04wt%Nb 성분계를 이용하여 제조된 인장강도 580MPa급 페라이트-베이나이트 복합조직 열연강판은 연신율 22% 이상, 구멍확장율 (신장플랜지성) 90% 이상의 재질특성을 가지며, 버링비 60% 이상의 자동차 휠 디스크에 적용 가능한 것으로 판명되었다.

• 한국해양공학회지
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• 제28권4호
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• pp.306-313
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• 2014

Super duplex stainless steels (sDSS) are excellent for use under severely corrosive conditions such as offshore and marine applications like pipelines and flanges. sDSS has better mechanical properties and corrosion resistance than the standard duplex stainless steel (DSS) but it is easier for a sigma phase to appear, which depresses the mechanical property and corrosion resistance, compared to DSS, because sDSS has a higher alloy element than DSS. In addition, sDSS has a feeble ductile-brittle transition temperature (DBTT) because it has a 50% ferrite microstructure. In the actual operating environment, sDSS would be thermally affected by welding and a sub-zero temperature environment. This study analyzed how precipitated sDSS behaves at a sub-zero temperature through annealing heat treatment and a sub-zero tensile test. Six types of specimens with annealing times of up to 60 min were tested in a sub-zero chamber. According to the experimental results, an increase in the annealing time reduced the elongation of sDSS, and a decrease in the tensile test temperature raises the flow stress and tensile stress. In particular, the elongation of specimens annealed for 15 min and 30 min was clearly lowered with a decrease in the tensile test temperature because of the increasing sigma phase fraction ratio.

• 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.

• Journal of Welding and Joining
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• 제11권4호
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• pp.57-69
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• 1993

In the present study, the microstructure and Charpy V notch toughness of multipass $CO_2$ FCA weldment in three different heat inputs(1-3KJ/mm)were investigated. The weldments using two different domestic FCAW wires(AWS E71T-1 and E71T-5 equivalent) in C-Mn steel were chemically analysed. The following conclusions can be inferred. 1. T-1 wire Showed a stable arc transfer, less spatter and harsh, a better bead spreading and easy slag removal, whereas T-5 wire suffered from the arc stability, which tended to increase spatter and produce a more convex bead. 2.The microsturctures of the top beads of the weldments in three different heat inputs consisted of coarse-grained boundary ferrite and Widmanstatten ferrite side plate with increasing heat inputs. The modest fraction of acicular ferrite in the two wire weldments was observed in the 2KJ/mm heat input. 3.The fine-grained reheated zones of both welds consisted of a duplex microstructure of polygonal ferrite and second phases. 4. The basic flux weldment of T-5wires showed a higher Charpy impact property than that of T-1 wires because of a higher fraction of acicular ferrite in the weld microstructure.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 제4회 압출 및 인발가공 심포지엄
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• pp.55-62
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• 2001

An electrolytic copper rod was drawn up to $90\%$ in area reduction and annealed under various conditions. The EBSD measurement of the drawn wire showed that in the center region the <111> + <100> fiber duplex texture was dominant, while in the middle and surface regions relatively defused textures developed with a little higher density in <11w>//wire axis. The inhomogeneous texture in the deformed wire gave rise to the inhomogeneous microstructure and texture after annealing. The annealing texture could be classified into the recrystallization texture developed during low temperatures and at the early stage at a high temperature and the growth texture developed after a prolonged annealing at the high temperature. The recrystallization temperature could be explained by the strain energy release maximization model and the growth texture was discussed based on the grain boundary mobility anisotropy.

• 한국표면공학회지
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• 제21권4호
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• pp.176-182
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• 1988

Effects of Cr, Ni and W on the anodic polarization behavior were investigated for Fe-Cr-Ni-W alloys in deaerated 1N HCI solution. Surface films formed on the polarization were analysed using AES, SEM and EDAX. A higerconcentration of tungten was found in the surface oxide film compared to the matrix. It played an importanet role on incresing the stability of the passive film. The presence of an adequate amount of Cr was essential to increase the pitting resistance of the alloys in acid chloride media. Under 12 wt%cr,alloys containing 6wt%W did not exhidit any passivity at all. The main role of Ni was to control the microstructure rather than to modify the corrosion resistance. In 23 cr-14Ni-^W alloy, the duplex microstructure of ferrite($\delta$-phase) in an austenic matrix was developed. The reson why proferred pitting appeared in austenite and ferrite/austenite interface was that ferrite had more amount of Cr and W than austenite.

• 열처리공학회지
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• 제23권2호
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• pp.96-103
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• 2010

$Ni_3Al$ is known as a good high temperature structural material because of high yield strength at ambient temperature. However, it is too brittle to use as a structural material because of their weak grain boundary. In this work, orientation measurement and related mechanical properties of directionally solidified NiAl/$Ni_3Al$ two-phase alloys with various compositions (Ni-23~27 at.%Al) were investigated for developing multi-phase DS-processed alloys with the growth rates of 10, 50 and 100 ${\mu}m/s$ in a modified Bridgeman type furnace. It was found that the multi-phase microstructures such as the $\gamma$ dendrite +${\gamma}'$ matrix duplex microstructure was formed in the hypoeutectic composition of 23 at.%Al, $\beta$ dendrite +${\gamma}'$ matrix duplex microstructure in the hypereutectic composition of 26 and 27 at.%Al. And ${\gamma}'$ single phase was formed in the composition of 24.5 and 25 at.%Al. The hypoeutectic alloy including $\gamma$ dendrites with ${\gamma}'$ matrix showed a large elongation of over 70% at room temperature. However, the room-temperature tensile elongation decreased with increasing Al contents because the volume fraction of brittle $\beta$ dendrites in the ductile ${\gamma}'$ matrix increased.

• 열처리공학회지
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• 제10권3호
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• pp.181-187
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• 1997

TiAl intermetallic compound was candidated for the application to the high temperature materials such as a gas turbine exhaust valve in the automobile. However, this material dose not have ductility allowing to machinability to product. To improve the ductility, many researches conduct alloy design and heat treatment methods. We observed that the microstructure of TiAl varied with Ni, Mn elements as well as a heat treatment condition. In the case of Ni element addition, the TiAlNi intermetallic compound was precipitated at the grain boundary. When the heat treatment temperature increased from $1000^{\circ}C$ to $1300^{\circ}C$, the TiAlNi intermetallic compound was uniformly dispersed on the matrix. In the case of Mn element addition, the mixed duplex structure of ${\gamma}$-TiAl and lamellar(TiAl/$Ti_3Al$) was obtained with $1250^{\circ}C$ and $1300^{\circ}C$ heat treatment for 1 hour. When the heat treatment temperature increased from $1250^{\circ}C$ to $1300^{\circ}C$, the lamellar domain of the duplex structure was transformed near-lamellar structure.