• International Journal of Ocean System Engineering
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• v.1 no.1
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• pp.46-51
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• 2011

In a line heating process for hull forming, the phase of the steel transforms from austenite to martensite, bainite, ferrite, or pearlite depending on the actual speed of cooling following line heating. In order to simulate the water cooling process widely used in shipyards, a heat transfer analysis on the effects of impinging water jet, film boiling, and radiation was performed. From the above simulation it was possible to obtain the actual speed of cooling and volume percentage of each phase in the inherent strain region of a line heated steel plate. Based on the material properties calculated from the volume percentage of each phase, it should be possible to predict the plate deformations due to line heating with better precision. Compared to the line heating experimental results, the simulated water cooling process method was verified to improve the predictability of the plate deformation due to line heating.

• Journal of Korea Foundry Society
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• v.9 no.6
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• pp.463-473
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• 1989

This study is aimed to investigate the effect of cyclic heat treatment which is special heat treatment on the strength and toughness in Multi-phase(Ferrite-Bainite-Martensite) Ductile cast Irons. Spceimens were austenitized at eutectoid transformation temperature range(${\alpha}+{\gamma}$) for 30min and austempered at $300^{\circ}C$ and $400^{\circ}C$ for different holding times, and then quenched in ice water to obtain the multi-phase(Ferrite-Bainite-Martensite) structure from various prior structures, which was obtained by various cyclic heat treatments. As the number of cycle in cyclic heat treatment increased, volume fraction of pearlite increased and the its morphology was refined. As the number of cycle in cyclic heat treatment increased, the multi-phase(Ferrite-Bainite-Martensite) was dispersed in whole matrix as refined island phase. Particularly, martensite among the multi-phase gradually became a spherical shape. Good combination in impact energy and tensile strength was detained in $840^{\circ}C-300^{\circ}C-15min$ condition after 10 cycles in cyclic heat treatment, and its multi-phase volume fraction is Ferrite(50%)-Martensite(l3%)-Bainite(37%).

• Journal of Korea Foundry Society
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• v.28 no.3
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• pp.119-123
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• 2008

Wear test on two die steels for aluminum die casting was carried out by dipping and rotating the specimens into the molten aluminum maintained $680^{\circ}C{\sim}780^{\circ}C$. The rotating speed of the specimen was $4.5rpm{\sim}20.0rpm$. Diffusion layer was formed between the die steel and molten aluminum, and became thicker with dipping time. Wear rate was not proportional with the thickness of the diffusion layer, but was closely related to the density of the diffusion layer. Wear rate was little affected by the kind of die steel and by the microstructure such as martensite, tempered martensite, and pearlite. Specimen with nitrided surface showed good wear resistance, and its wear rate was decreased with increase in the thickness of nitrided layer. While whole surface was worn in heat treated specimens, wear of nitrided specimens was proceeded by pitting partially.

• Journal of Welding and Joining
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• v.31 no.6
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• pp.44-49
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• 2013

The influence of thermo-mechanical cycling on the microstructure and strength in the weld coarse-grained heat affected zone (CGHAZ) of Ti-Nb added low carbon HSLA steel was explored through Vickers hardness tests, nanoindentation experiments, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Undeformed and deformed CGHAZs were simulated using Gleeble simulator with different heat inputs of 30kJ/cm and 300kJ/cm. At high heat input of 300kJ/cm, the CGHAZ consisted of ferrite and pearlite and then their grain sizes were not affected by deformation. At low heat input of 30kJ/cm, the CGHAZ consisted of lath martensite and then the sizes of prior austenite grain, packet and lath width decreased with deformation. In addition, the fraction of particle increased with deformation and this is because the precipitation kinetics was accelerated by deformation. Meanwhile, the Vickers and nanoindentation hardness of deformed CGHAZ with 30kJ/cm heat input were higher than those of undeformed CGHAZ, which are due to the effect of grain refinement and precipitation strengthening.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.3
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• pp.144-151
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• 1993

For the purpose of studying the influence of stress relieving on microstructures and mechanical properties of weld metal, manual arc welding, onepole and twopole submerged arc welding were accomplished on $60kg/mm^2$ quenched and tempered high strength steel. After stress relieving, a lot of carbides were precipitated, developed and subsequently coarsened at the grain boundaries and within matrix due tn multiple tempering effect in manual arc welding, resulting in deterioated toughness. Meanwhile pearlite and cementite films were spheroidized and shortened in submerged arc welding, resulting in improved considerable toughness. It was observed that main effect of stress relieving was to reduce solut supersaturation by nucleation and growth of carbide precipitates, and stress relieving led to some reduction in the yield and tensile strenath but did not significantly affect elongation.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.1
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• pp.47-54
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• 1997

Flake graphite cast irons with the high damping capacity have been used for the control of vibration and noise occuring in the members of various mechanical structures under vibrating conditions. However, the damping capacity which is morphological characteristics of graphite is one of the important factors in reducing the vibration and noise, but hardly any work has deal with this problem. Therefore, the authors have examined the damping capacity of various cast irons with alloying elements and studied the influences of the matrix, mechanical properties and morphological characteristics of graphite. The main results obtained are as follows: Effects of Ni on the damping capacities and mechanical properties are investigated in 3.9%C-0.3% Cu gray cast iron. At 0.2% Ni content, specific damping capacity showed the maximum value, and decreased with further increase in Ni content, Graphite continuity also showed same behavior. This indicates that the specific damping capacity has a close relation with graphite continuity. On the other hand, the damping capacity in pearlite matrix showed superior to that in ferrite. In contrast, with increasing the Ni content, both tensile strength and hardness increased due to the decrease of graphite length and eutectic cell size.

• Journal of the Korean Society for Heat Treatment
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• v.33 no.6
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• pp.271-276
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• 2020

The relationship between the hardness and interlamellar spacing of discontinuous precipitates (DPs) formed by continuous cooling was studied for Mg-9%Al-1%Zn alloy. After solution treatment at 683 K for 24 h, the specimens were cooled to room temperature with different cooling rates ranging from 0.2 to 2 K·min^-1, in order to obtain DPs with various interlamellar spacings. It was found that cooling rate of 2 K·min^-1 yielded only small amount of nodular DPs at the grain boundaries, while cooling rates below 2 K·min^-1 yielded both DPs and continuous precipitates (CPs). The volume fraction of DPs increased with increasing cooling rate up to 0.5 K·min^-1, over which it abruptly decreased. The hardness of DPs was increased with an increase in the cooling rate, whereas the interlamellar spacing of the DPs was decreased with respect to cooling rate. The hardness of the DPs formed by continuous cooling was correlated with the interlamellar spacing and can follow a Hall-Petch type relation as in the case of pearlite with lamellar morphology.

• Korean Journal of Metals and Materials
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• v.50 no.3
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• pp.212-217
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• 2012

This study was carried out to investigate the effect of the hot forging ratio on the microstructure and mechanical properties of ultra high carbon steel. The microstructure of ultra high carbon steel with 1.5%wt.C consisted of a proeutectoid cementite network and acicular microstructure in pearlite matrix. With increasing hot forging ratio, the volume and thickness of the network and acicular proeutectoid cementite decreased. Lamella spacing and the thickness of eutectoid cementite decreased with increasing hot forging raito, and were broken up into particle shapes, which then became spheroidized. When the forging ratio was over 65%, the network and acicula shape of the as-cast state disappeared. With increasing hot forging ratio, hardness, tensile strength, elongation and impact value were not changed up to 50%, and then rapidly increased with the increase of the forging ratio.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.5 s.143
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• pp.472-478
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• 2005

From a rapid cooling to a slow cooling in the actual cooling process in shipyards, the phase of steel becomes martensite, bainite, ferrite, and pearlite. In order to simulate the cooling process, heat transfer analysis was performed considering the effects of impinging water jet, film boiling, and radiation. From above simulation it is possible to find the cooling speed at the inherent strain region and volume percentage of all phases in that region. By the suggested method based on the precise material properties calculated from volume percentage of all phases, it will be possible to predict the plate deformations by line heating more precisely. It is verified by comparing with some experimental results that the present method is very effective and efficient.

• Journal of the Korean Society of Industry Convergence
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• v.24 no.5
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• pp.525-530
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• 2021

The purpose of this study was to investigate the effect of spherodizing heat treatment holding time on the microstructure and mechanical properties of the accelerated cooling-treated API X70 steel, which is mainly used as a structural material for line pipe steel for natural gas pipes. The accelerated cooling-treated API X70 steel was spherodizing treated at 700℃ for 12~48 h. The microstructure was observed using an OM and a FEG-SEM, and mechanical properties were obtained by tensile test. The microstructure of the API X70 steel was banded in the hot rolling direction, and the polygonal ferrite(PF) adjacent to pearlite(P) has mainly a fine size, and coarse PF and fine acicular ferrite were formed in the middle of P and P. As the spherodizing treatment time increased, the number of carbide particles decreased and its distribution interval increased, and the ferrite grain size was coarsened. The tensile strength decreased and the ductility increased with spherodizing treatment time, and the yield point elongation was disappeared in a stress-strain curve after the spherodizing treatment.