• Journal of the Korean Society for Heat Treatment
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• v.10 no.1
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• pp.30-39
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• 1997

After changing the heat treating atmosphere of nitrogen gas, argon gas and vacuum, the nitrogen contents, microstructural changes, hardness and corrosion resistance of 0.25wt.%N alloyed super duplex stainless steel have been investigated in the temperature range from $1050^{\circ}C$ to $1350^{\circ}C$. The nitrogen content showed to be increased up to 0.36wt.% after heat treating the specimen in nitrogen gas at $1200^{\circ}C$, while the decrement of nitrogen content in vacuum atmosphere was shown down to 0.03wt.% at $1350^{\circ}C$. After heat treating in the mixed gas atmosphere of argon and nitrogen at $1250^{\circ}C$, the surface ${\gamma}$ phase existed as ${\alpha}+{\gamma}$ phase increased with increasing nitrogen gas content. The ${\gamma}$ single phase appeared at the surface above $80%N_2$ gas, while the surface ${\alpha}$ single phase was shown below $20%N_2$ gas. When heat treating the specimen in nitrogen gas at $1050^{\circ}C$, the hardness of austenite phases increased above Hv 40 at the surface layer compared to the hardness of the core parts, while decrement of denitriding effect caused to the hardness nearly unchanged between surface and the core parts after heat treating in vacuum atmosphere. The surface ${\gamma}$ single phase specimen showed superior corrosion resistance than the surface ${\alpha}$ single phase specimen. The surface ${\alpha}$ phase existed in the ${\alpha}+{\gamma}$ microstructure showed higher corrosion resistance after heat treating in the nitrogen gas atmosphere than the ${\alpha}$ phase heat treated in the argon gas and vacuum atmosphere.

• Journal of Welding and Joining
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• v.31 no.5
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• pp.20-25
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• 2013

This study was carried out to investigate the influence of a sigma phase precipitation on the pitting corrosion resistance and microstructural change of super duplex stainless steels(UNS S32750). The welds made by FCAW (Flux Cored Arc Welding) were heat treated ($930^{\circ}C$, $1080^{\circ}C$, $1230^{\circ}C$) and quenched. Based on the microstructural examination, it was found that the ${\sigma}$ phase was formed in base metals and welds heat treated at $930^{\circ}C$ while there were little ${\sigma}$phases formed in base metals and weld metal experienced the relatively fast cooling from $1080^{\circ}C$ and $1230^{\circ}C$. On the other hand, the most weight loss due to pitting corrosion occurred in base and weld metals heat treated at $930^{\circ}C$. It was confirmed that the pitting corrosion occurred in the phase boundaries of ferrite/sigma and austenite/sigma. The pitting corrosion resistance decreased owing to an increase in Cr, Mo depleted areas adjacent to the intermetallic phases such as ${\sigma}$phases. The hardness was greatly increased due to the precipitation ${\sigma}$phases.

• Transactions of Materials Processing
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• v.12 no.4
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• pp.308-313
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• 2003

High temperature deformation and softening behavior of SAF 2507 super duplex stainless steel (SDSS) has been investigated in connection with an FEM analysis of hot forging process. Flow curves at various strain rates and temperatures were determined first from compression tests, and the kinetics of dynamic recrystallization were also formulated through the analysis of load relaxation test results. Using the dynamic materials theory proposed by Prasad, the deformation behavior was effectively determined for various conditions. Constitutive relations and recrystallization kinetics formulated from the test results were then implemented in a commercial FEM code. The forming load as well as the distribution of recrystallized volume fraction after forging was successfully predicted by means of the flow stress compensation formulated upon the volume fraction of recrystallization and adiabatic heating.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.233-235
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• 2001

Residual stress in the forged parts affects the resistance to mechanical failure, dimensional uniformity, and the service life of the parts. In order to elucidate the development of residual stress in open-die forging process, elasto-plastic finite element analysis was implemented to radial forging process. Super duplex stainless steel SAF 2507 was selected as workpiece material and a series of mechanical tests followed by numerical compensation to deformation heating was conducted to obtain necessary flow data. The residual stress distributions were calculated using commercial 3-D FEM code and the effects of process design were evaluated from selected results.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.195-198
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• 2003

High temperature deformation and softening behavior of SAF 2507 super duplex stainless steel (SDSS) has been investigated in connection with an FEM analysis of hot forging process. Flow curves at various strain rates and temperatures were determined first from compression tests, and the kinetics of dynamic recrystallization were also formulated through the analysis of load relaxation test results. Applying the dynamic materials and proposed by Prasad et al., it was possible to determine the characteristics of deformation behavior effectively at a given condition of deformation. Constitutive relations and recrystallization kinetics formulated from the test results were then implemented in a commercial FEM code. Flow stress compensation formulated upon the volume fraction of recrystallization and adiabatic heating was found to improve significantly the FEA solutions in predicting the forming load and the distribution of recrystallized volume fraction after forging.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.04a
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• pp.119-122
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• 2000

Dynamic deformation of metallic materials mostly accompanies substantial amounts of deformation heat. Since the flow stress of deformation is sensitive to temperature implication of heat due to plastic work is essential to the evaluation of constitutive relations. In this study a series of compression tests were conducted for SAF 2507 super duplex stainless steel and the accumulation of deformation heat was calculated through numerical integration method. Isothermal flow surfaces were deduced from subsequent logarithmic interpolation. Simple closed die forging process was analyzed and optimized with commercial FEM code applying both raw and calibrated material database.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.4
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• pp.425-431
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• 2015

LNG (liquefied natural gas) is considered the best alternative eco-fuel, and many studies on the LNG fuel system have been performed to use LNG as the fuel for ships. For the LNG fuel supply system, natural gas transfers from the vaporizer to the engine in the gaseous state with a temperature of $50^{\circ}C$ and a pressure of 35MPa. Therefore, a structural safety evaluation of the double-walled pipelines considering thermal load is essential. In this article, an uniaxial tensile test for super duplex stainless steel, material for double-walled pipe, according to the annealing time was carried out to analyze the thermal effect. In addition, thermo-structural analysis of the high temperature-high pressure double-walled pipe with fixed supports that are now used widely was carried out to evaluate the structural safety. To minimize stress concentration of the connection point between the support and inner pipe, the shapes of the new type support that can slip through inner pipe were proposed, and the supports which has best structural performance was selected using the results from the thermo-structural analyses of new supports and an analysis of the whole double-walled pipeline was performed to ensure structural safety. These results can be used as a database for the design of double-walled pipelines and sliding support.