• Journal of the Korean Society for Heat Treatment
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• v.22 no.5
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• pp.298-306
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• 2009

Gas nitriding and post oxidation were performed on stainless steels and SACM 645 steel. With increasing gas nitriding time, the increasing rate of nitrided layer was most rapid on SACM 645 steel and the nitriding depth of nitrided layer was most narrow on STS 304 steel among three steels. Corrosion resistance was increased with post oxidation on stainless steels and with increasing time the effect of corrosion resistance was decreased to compare with relatively short gas nitriding time. An improvement effect of corrosion resistance was consisted of predominantly on austenitic stainless steel by post oxidation after gas nitriding among three steels and it was relatively less influenced on martensitic stainless steel.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.3
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• pp.91-96
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• 2018

Microstructural development of ferritic 11Cr-3.45W heat-resistance steel for ultra-supercritical power plant during creep and thermal aging was investigated using electron microscopy. The test samples were isothermally aged at $700^{\circ}C$ for up to 4000 hours and subjected to creep loading at $700^{\circ}C$ for predetermined periods of lifetime to prepare the damaged materials. In this structural material, a various secondary phases are the primary influence on mechanical properties of ferritic heat-resistance steel. The typical precipitates of $M_{23}C_6$, MX and $M_2X$ secondary phases had been analyzed through qualitative and quantitative manner. Coarsening of precipitates and increase of lath width were observed during creep and thermal aging. This phenomenon was remarkable for creep process compared with isothermal aging process.

• Journal of Welding and Joining
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• v.24 no.6
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• pp.39-43
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• 2006

A ferritic stainless steel, STS430, and a galvanized steel for case and frame of electronic goods are welded by resistance spot welding and resistance projection welding methods. In this study, resistance spot welding has been performed to investigate the weldability of dissimilar materials such as a ferritic stainless steel and a galvanized steel. Tensile load of the spot weld without heat treatment was 196 kN and brittle fracture occurred at interface between STS430 and nugget due to the high hardness, 380 Hv, in nugget. It was found that the hardness of as-welded nugget increased with higher cooling rate during resistance spot welding by comparing with GTA weld of same materials. Heat treatment by applying second current made nugget softened. Tensile load of heat treated weld increased and reached to 303 kN and fracture occurred at base metal, a galvanized steel.

• Transactions of Materials Processing
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• v.12 no.4
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• pp.364-369
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• 2003

Non-heat treated steels are attractive in the steel-wire industry since the spheroidization and quenching-tempering treatment are not involved during the processing. However, non-heat treated steels should satisfy high strength and good formability without performing heat treatment. Therefore, it is important to investigate optimum materials showing a good combination of strength and formability after the drawing process. In this study, three different steels such as dual phase steel, low-Si steel, and ultra low carbon bainitic steel were used to study their mechanical properties and the cold formability. The cold formability of three steels was investigated by estimating the deformation resistance and the forming limit. The deformation resistance was estimated by calculating the deformation energy, and the forming limit was evaluated by measuring the critical strain revealing crack initiation at the notch tip of the specimens. The results showed that deformation resistance was the lowest in the low-Si steel, and the forming limit strains of ultra low carbon bainitic steel and low-Si steel were higher than that of commercial SWRCH45F steel.

• Corrosion Science and Technology
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• v.23 no.4
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• pp.283-288
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• 2024

Hot-dip Zn-Mg-Al coatings have a complex microstructure consisting of Zn, Al, and MgZn₂ phases. Its crystal structure depends on alloy content and cooling rates. Microstructure and corrosion resistance of these coatings might be affected by heat treatment. To investigate effect of heat treatment on microstructure and corrosion resistance of Zn-Mg-Al coatings, Zn-1.5%Mg-1.5%Al coated steel was heated up to 550 ℃ at a heating rate of 80 ℃/s and cooled down to room temperature. At above 500 ℃, the ternary phase of Zn-MgZn₂-Al was melted down. Only Zn and MgZn₂ phases remained in the coating. Heat- and non-heat-treated specimens showed similar corrosion resistance in Salt Spray Test (SST). When a Zn-3.0%Mg-2.5%Al coated steel was subjected to heat treatment at 100 ℃ or 300 ℃ for 200 h and compared with GA and GI coated steels, the microstructure of coatings was not significantly changed at 100 ℃. However, at 300 ℃, most Al in the coating reacted with Fe in the substrate, forming a Fe-Al compound layer in the lower part of the coating. MgZn₂ was preferentially formed in the upper part of the coating. As a result of SST, Zn-Mg-Al coated steels showed excellent corrosion resistance, better than GA and GI.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.307-310
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• 2003

Non-heat treated steels are attractive in the steel-wire industry since the spheroidization and quenching-tempering treatment are not involved during the processing. In this study, three different steels such as dual phase steel, low-Si steel, and ultra low carbon bainitic steel were used to investigate their deformation resistance and forming limit. Deformation resistance was estimated by calculating the deformation energy and the forming limit was evaluated by measuring the critical strain revealing crack initiation at the notch tip of the specimens. The results showed that deformation resistance was the lowest in the low-Si steel, and the forming limit strain was the highest in the ultra low carbon bainitic steel.

• Corrosion Science and Technology
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• v.19 no.3
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• pp.146-155
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• 2020

To elucidate the effect of heat inputs on phase transformation and resistance to intergranular corrosion of F316 austenitic stainless steel (ASS), thermodynamic calculations of each phase and time-temperature-transformation diagram were conducted using JMaPro simulation software, oxalic acid etch test, double-loop electrochemical potentiokinetic reactivation test (DL-EPR), field emission scanning electron microscopy with energy dispersive spectroscopy, and transmission electron microscopy analyses of Cr carbide (Cr₂₃C₆), austenite phase and ferrite phase. F316 ASS containing a relatively low C content of 0.043 wt% showed a slightly sensitized microstructure (acceptably dual structure) due to a small amount of Cr carbide precipitated at heat affected zone irrespective of heat inputs. Based on results of DL-EPR test, although heat input was increased, the ratio of I_r to I_a was only increased very slightly due to a slight sensitization. Therefore, heat inputs have little influences on resistance to intergranular corrosion of F316 austenitic stainless steel containing 0.043 wt% C.

• Journal of Ocean Engineering and Technology
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• v.23 no.5
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• pp.85-91
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• 2009

A stainless steel that contains aggressive negative ion was known to decrease the corrosion resistance. Stainless steel with super corrosion resistance was developed for improvement of corrosion resistance. Super duplex stainless steel is widely used under sever environment because of good mechanical properties and corrosion resistance. Also, Super duplex stainless steel has long life in severe environments by showing the enough strength and corrosion resistance. But duplex stainless steel is not stabilized compared to austenite stainless steel in corrosion resistance. In this study, corrosion characteristic were investigated to super duplex stainless steel with additive 0.2% nitrogen with $SiO_2$ thin films coated or no coated by sol-gel method in 3.5% NaCl. From test results, corrosion current density in the heat-treated specimen for ${\sigma}$ phase precipitation was higher than that of different heat-treated specimen. Also, $SiO_2$ colloidal-coated specimen had not occurred almost corrosion.

• Journal of Energy Engineering
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• v.24 no.4
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• pp.71-80
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• 2015

Steel stiffener is required for reinforcing the structure of the window frame made of versatile but weak PVC material. Steel stiffener however becomes a source of greater heat loss and frequently plays a role of thermal bridge due to its high thermal conductivity. To maintain thermal resistance similar to PVC frame, steel stiffener is perforated to reduce the effective heat transfer area. To compensate the structural strength of the steel stiffener which is weakened by the perforation, the thickness is increased. Increase in thickness will also increase the thermal heat resistance. Five samples which are PVC frame, PVC frame + original steel stiffener, PVC frame + 30% perforated steel stiffener, PVC frame + 50% perforated steel stiffener, PVC frame + 65% perforated steel stiffener are modeled and simulated for 2nd moment of area and thermal resistance. Therm/window version 6.3 is used for thermal analysis. The results show that among the five samples analyzed, PVC frame + 65% perforated steel stiffener best satisfies both structural strength and thermal resistance.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.1
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• pp.17-20
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• 2017

Mechanical properties of STD61 steel are compared with those of carburized STD61 steel when both are quenched and tempered in vacuum heat treatment. Mechanical properties of carburized STD61 steel are improved better than STD61 steel in hardness, tensile strength, impact energy and wear resistance.