• Journal of the Korean Society for Heat Treatment
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• v.14 no.5
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• pp.286-291
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• 2001

This study was carried out to investigate the influence of ${\sigma}$ phase on the microstructure and mechanical properties in super duplex stainless steel. The precipitation of ${\sigma}$ phase during isothermal heat treatment showed the type S curves with a certain incubation period. The precipitation of ${\sigma}$ phase was precipitated at ferrite phase and interface of ferrite and austenite. Under the state of isothermal transformation, the precipitation of ${\sigma}$ phase was stimulated by applied stress. With increasing of volume fraction of precipitated ${\sigma}$ phase, tensile strength was increased and elongation was decreased with linear relationship, while in case of precipitated ${\sigma}$ phase was 5% over, impact value was rapidly decreased.

• Korean Journal of Materials Research
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• v.26 no.4
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• pp.200-206
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• 2016

This study was carried out to investigate the effect of W substitution on the precipitation behavior of ${\chi}$ and ${\sigma}$ phases in super duplex stainless steel. The ${\chi}$ phase was precipitated at the interface of ferrite / austenite phases and inside the ferrite phase at the initial stage of aging. With an increase in the aging time, the volume fraction of the ${\chi}$ phase increased, and then decreased with the transformation from the ${\chi}$ phase to the ${\sigma}$ phase. The ${\sigma}$ phase was precipitated later than the ${\chi}$ phase, and the volume fraction of x phase increased with the increase in the aging time. The ferrite phase was decomposed into the new austenite (${\gamma}2$) and ${\sigma}$ phases by aging treatment. The decomposition of the ferrite phase into the ${\gamma}2$ and ${\sigma}$ phases was retarded by W substitution for Mo. The volume fraction of the ${\chi}$ phase increased and that of the ${\sigma}$ phase decreased due to W substitution. The ${\chi}$ and ${\sigma}$ phases were intermetallic compounds, which had lower nickel concentration, and higher chromium, molybdenum, and tungsten concentrations. The ${\chi}$ phase has higher molybdenum and tungsten concentrations than those of the ${\sigma}$ phase. The amounts of chromium and nickel in the ${\chi}$ and ${\sigma}$ phases did not change, but these phases have higher concentrations of molybdenum and tungsten due to W substitution for Mo.

• Korean Journal of Materials Research
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• v.11 no.8
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• pp.664-664
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• 2001

The mechanical alloying (MA) effect in $\sigma$-VFe intermetallic compound was studied by neutron and X-ray diffraction. The structure of MA $\sigma$-VFe powders were characterized by the X- ray diffraction with Cu- $K\alpha$ radiation and neutron diffraction with monochromatic neutrons of $1.835\AA$ using a high resolution powder diffractometer (HRPD). Mechanical alloying of $\sigma$-VFe compound gives rise to a dramatic structural change. After 60 hours of MA, the Fe-Fe distribution of the $\sigma$- phase VFe tetragonal structure with 30 atoms in a unit cell is found to change into that of the $\sigma$-(V,Fe) solid solution with bcc structure, which is a stable phase at elevated temperature above $1200^{\circ}C$. A comparison of X-ray diffraction data for the $\alpha$-phase has been also made with the corresponding neutron diffraction data. The (101) and (111) diffraction peaks of the $\sigma$-phase was clearly observed only in neutron diffraction pattern, which is believed to be a characteristic feature due to the chemical atomic ordering of $\sigma$- VFe phase.

• 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.

• Corrosion Science and Technology
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• v.2 no.1
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• pp.18-25
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• 2003

Effects of the precipitation of $\sigma$ phase on the metastable pitting as a precursor of stable pitting corrosion and also on the progress of stale pitting of the 25Cr-7Ni-3Mo-0.25N duplex stainless steel were investigated in chloride solution. Electrochemical potential and current noises of the alloy were measured in 10 % ferric chloride solution ($FeCl_3$) with zero resistance ammeter (ZRA), and then analyzed by power spectral density (PSD) and by corrosion admittance ($A_c$) spectrum. With aging at $850^{\circ}C$, the passive film of the alloy was found to get significantly unstable as represented by power spectral density (PSD) and a transition from metastable pitting state to stable one was observed. In the corrosion admittance spectrum, the number of negative $A_c$ corresponding to the state of localized corrosion increased with aging, suggesting that the precipitation of $\sigma$ phase considerably degraded the passive film by depleting Cr and Mo around it at $\alpha/\sigma$ or $\gamma/\sigma$ phase boundaries, thereby leading to the initiation of the pitting corrosion. However, the Cr and Mo at $\alpha/\sigma$ or $\gamma/\sigma$ phase boundaries which were once depleted due to the precipitation of the $\sigma$ phase were partly replenished by the diffusion of Cr and Mo from the surrounding matrix with aging time longer. The initiation of pitting seems to be associated with the precipitation density of the $\sigma$ phase with an effective size needed to induce the sufficient depletion of Cr and Mo around it.

• Corrosion Science and Technology
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• v.19 no.4
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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.

• Journal of Microbiology
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• v.42 no.2
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• pp.103-110
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• 2004

In order to understand the molecular basis of selective expression of stationary-phase genes by RNA polymerase containing$\sigma$$\^$38/ (E$\sigma$$\^$38/) in Escherichia coli, we examined transcription from the stationary-phase promoters, katEP, bo1AP, hdeABP, csgBAP, and mcbP, in vivo and in vitro. Although these pro-moters are preferentially recognized in vivo by E$\sigma$$\^$38/, they are transcribed in vitro by both E$\sigma$$\^$38/ and E$\sigma$$\^$70/ containing the major exponential $\sigma$, $\sigma$$\^$70/. In the presence of high concentrations of glutamate salts, how-ever, oldy E$\sigma$$\^$38/ was able to efficiently transcribe from these promoters, which supports the concept that the promoter selectivity of $\sigma$$\^$38/-containing RNA polymerase is observed only under specific reaction con-ditions. The examination of 6S RNA, which is encoded by the ssr1 gene in vivo, showed that it reduced E$\sigma$$\^$70/ activity during the stationary phase, but this reduction of activity did not result in the elevation of E$\sigma$$\^$38/ activity. Thus, the preferential expression of stationary-phase genes by E$\sigma$$\^$38/ is unlikely the con-sequence of selective inhibition of E$\sigma$$\^$70/ by 6S RNA.

• Journal of Ocean Engineering and Technology
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• v.23 no.6
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• pp.87-92
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• 2009

The effect of aging on the precipitation of the $\sigma$ phase and the corrosion resistance in tungsten substituted super duplex stainless steels was investigated. The volume fraction of the $\sigma$ phase and the current density increased with aging at temperatures up to $750^{\circ}C$ and then decreased. With an increase in aging time, the volume fraction of the $\sigma$ phase and the current density also increased. The $\sigma$ phase hardly influenced the corrosion resistance. With the substitution of tungsten for molybdenum in super duplex stainless steel, the volume fraction of the $\sigma$ phase and the current density decreased remarkably.

• Journal of Power System Engineering
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• v.19 no.6
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• pp.75-81
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• 2015

A deposited metal specimen of ER2594 which is a super duplex steel welding wire used to investigate the effect of sigma(${\sigma}$) phase on electrochemical corrosion characteristics was prepared by gas tungsten arc welding. Aging treatment was conducted for the specimen at the temperature range of $700^{\circ}C$ to $900^{\circ}C$ for 5 to 300 minutes after annealing at $1050^{\circ}C$. Corrosion current density has decreased a little with an increase of aging time over 60 minutes at $700^{\circ}C$ to $900^{\circ}C$ and the uniform corrosion of deposited metal had more influence on the precipitation of ferrite than the precipitation of sigma phase. Therefore, the precipitation of sigma phase did not have much effect on the uniform corrosion. Pitting potential representing pitting corrosion has shown decreasing tendency as the precipitation of sigma phase increased. The degree of sensitization representing intergranular corrosion has shown increasing tendency as the precipitation of sigma phase increased at $700^{\circ}C$ to $800^{\circ}C$, while it has decreased at $900^{\circ}C$ for 60 to 300 minutes.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.1
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• pp.14-20
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• 2013

Duplex stainless steel, which is a kind of stainless steel with a mixed microstructure of about equal proportions of austenite and ferrite, is generally known as a unique material with excellent corrosion resistance and high strength. However, toughness, strength, and corrosion resistance of the steel could be reduced due to precipitation of topologically closed packed phases such as sigma phase during cooling. In case of large forged products, they have strong possibility that ${\sigma}$-phase precipitates due to difference of cooling rate between surface and inner of the products. Investigation on sigma phase precipitation behavior of duplex stainless steel with change of cooling rate was carried out in this study. Forged SAF 2205 duplex stainless steel was used as specimens to examine the cooling rate effect. Dissolution behavior of sigma phase was also discussed through resolution test of duplex stainless steel containing lots of sigma phase. Experimental results revealed that impact energy was very sensitive to precipitation of small amount sigma phase. However, sigma phase could be removed by short term resolution treatment and impact resistance of the duplex stainless steel was restored.