• Proceedings of the Korean Geotechical Society Conference
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• 2010.03a
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• pp.1256-1263
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• 2010

Suction Drain Method is soft ground improvement technique, in which a vacuum pressure can be directly applied to the Vertical Drain Board to promote consolidation and strengthening the soft ground. This method does not require a surcharge load, different to embankment or Preloading Method. In this study, ground improvement efficiency of suction drain method was estimated when duplex loading pressure with vacuum and pressure. During suction drain method process, surface settlement and pore pressure were monitored, and cone resistance test as well as water content were also measured after the completion of Suction Drain Method treatment.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2015.11a
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• pp.291-292
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• 2015

Duplex Stainless Steel의 Plasma Nitriding 처리 시 가스량과 처리온도가 표면 특성에 미치는 영향을 조사하였다. $N_2$함량 및 처리 온도가 각각 10%에서 25%로 $400^{\circ}C$에서 $430^{\circ}C$로 증가함에 따라서 질소가 과고용된 S-phase의 두께 및 표면 경도가 증가하였으나, 내부 식성은 $Cr_2N$ 및 $(Fe,Cr)_4N$이 석출하여 감소하였다. 질소를 10%로 고정하고 $CH_4$함량을 증가시키면 1%일 때 S-phase의 두께가 최대가 되며 그이후로 감소하였다. 처리온도 $400^{\circ}C$일 때 질소함량이 10%, $CH_4$ 함량이 5%일 경우 내식성이 모재보다 증가하였다.

• Korean Journal of Materials Research
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• v.24 no.11
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• pp.610-616
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• 2014

In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging and its effect on the pitting corrosion of 25%Cr-7%Ni-4%Mo super duplex stainless steel. The R-phase in super duplex stainless steel was mainly precipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging, and it was transformed into the ${\sigma}$-phase with an increase in aging time. The ferrite phase was decomposed into a new austenite phase and ${\sigma}$-phase. The R phase was an intermetallic compound, which represented a lower Ni and higher Mo than the matrix, and also had a higher Mo and Cr concentration than the ${\sigma}$ phase. With an increasing aging time, the pitting potential $E_p$ was increased slowly by the precipitation of the R-phase, and it was then steeply decreased by the precipitation of the ${\sigma}$-phase. The R-phase was decreased the pitting potential, but its effect was smaller than effect of ${\sigma}$-phase.

• Journal of Ocean Engineering and Technology
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• v.24 no.5
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• pp.55-59
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• 2010

The effect of aging on the precipitation of the $\sigma$ phase and corrosion resistance in W-substituted, super-duplex stainless steel was investigated. The volume fraction of the $\sigma$ phase and the current density increased as the aging temperature increased up to $750^{\circ}C$, and, then, they decreased. As aging time increased, the volume fraction of the $\sigma$ phase and the current density also increased. The $\sigma$ phase considerably influenced to corrosion resistance. When Mo was substituted for W in super-duplex stainless steel, the volume fraction of the $\sigma$ phase and the current density both decreased markedly.

• Korean Journal of Metals and Materials
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• v.50 no.11
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• pp.793-800
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• 2012

The effect of cold rolling on the microstructural evolution in 22Cr-0.2N micro-duplex stainless steel was investigated. The 22Cr-xNi-yMn-0.2N duplex stainless steel plates with various Ni and Mn contents were fabricated. The steels were vacuum induction melted and hot rolled, followed by annealing treatment at the temperature range of $1000-1100^{\circ}C$, in which both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase depending on the alloy compositions of Ni and Mn increased with an increase in the annealing temperature. Grain growth in the ferrite phase occurred markedly after cold rolling followed by annealing, while fine recrystallised grains were still found in the austenite phase. A large number of martensite laths was found in the microstructure of cold rolled steels, which should be formed by strain-induced martensite from the austenite phase. The intersections of stacking faults were revealed by TEM observation. The volume fraction of the martensite phase increased with an increase of the reduction ratio by cold rolling.

• Journal of the Korean institute of surface engineering
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• v.45 no.6
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• pp.219-225
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• 2012

The aim of the present study was to investigate the corrosion resistance and characteristics of passive layer between naturally passivated and chemically passivated duplex stainless steel, UNS S31803 (EN 1.4462) using CPT, XPS, and EIS. The treatment of $HNO_3$(II) and $HNO_3$(III) in ASTM A 967 was applied. In case of chemically passivated specimen, CPT of $HNO_3$(II) and $HNO_3$(III) were higher than that of naturally passivated specimen. In addition, from XPS results, the protectiveness index (Cr/(Fe+Cr)) of chemically passivated specimens was also higher than that of naturally passivated specimen. The reason for this result is considered due to post-cleaning treatment in chemical passivation process, that is, immersion in $Na_2Cr_3O_7$ solution. The fact that $HNO_3$(II) passivation treatment showed the highest film resistance and 'n', which is exponent related with constant phase element (CPE) of passivation film, was in good agreement with results of CPS and XPS. The chemical passivation treatment was an effective method to improve corrosion resistance of duplex stainless steel.

• Journal of Welding and Joining
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• v.32 no.2
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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 $1050{\sim}1150^{\circ}C$.

• Journal of the Korean institute of surface engineering
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• v.54 no.5
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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.

• Korean Journal of Metals and Materials
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• v.50 no.10
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• pp.735-743
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• 2012

The aim of this study was to analyze the effect of the cooling rate after heat treatment on the microstructure and mechanical properties of 2507 duplex stainless steels. Heat treatment was carried out at $1050^{\circ}C$ for 1 hr, followed by controlled cooling. The cooling rates were $175.6{\times}10^{-3}^{\circ}C/s$, $47.8{\times}10^{-3}^{\circ}C/s$, $33.3{\times}10^{-3}^{\circ}C/s$, $16.7{\times}10^{-3}^{\circ}C/s$, $11.7{\times}10^{-3}^{\circ}C/s$, $5.8{\times}10^{-3}^{\circ}C/s$ and $2.8{\times}10^{-3}^{\circ}C/s$, which resulted in variations of the microstructure, such as the fractional change of the ferrite phase and sigma phase formation. Fatigue, hardness, impact and tensile tests were performed on the specimens with different cooling rates. The precipitation of the ${\sigma}$ phase caused a hardness increase and a sharp decrease of toughness and tensile elongation. The fatigue limit of the sample with a cooling rate of $5.8{\times}10^{-3}^{\circ}C/s$ was 26 MPa higher than that of the sample with a cooling rate of $175.6{\times}10^{-3}^{\circ}C/s$. Our observations of the fracture surface confirmed that the higher fatigue resistance of the specimen with a cooling rate of $5.8{\times}10^{-3}^{\circ}C/s$ was caused by the delay of the fatigue crack growth, in addition to higher yield strength.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.47 no.1
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• pp.1-9
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• 2015

Wood fibers for medium density fiberboard (MDF) was used in the filler layer of the white duplex board for increasing thickness and bulk of the board. The MDF fibers were treated with ozone (3% based on dry weight of the fibers), and mixed together with OCC (old corrugated container) to form paper. Ozone-treated MDF fibers gave high bulk, high tensile strength, high internal bond and fast drainage to the furnish mixed with OCC. It was shown that there were possibilities to reduced basis weight of the filler layer without loss of thickness, stiffness, and tensile strength. Furthermore, it showed the possibility to develop a new kind of board product that has high stiffness as well as high strength properties with light basis weight by application of the ozone-treated MDF fibers.