발간논문

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This study was carried out to investigate the effect of reverse transformation on the mechanical properties in high manganese austenitic stainless steel. Over 95% of the austenite was transformed to deformation-induced martensite by 70% cold rolling. Reverse transformation became rapid above an annealing temperature of 550℃, but there was no significant transformation above 700℃. In addition, with an increasing annealing time at 700℃, reverse transformation was induced rapidly, but the transformation was almost completed at 10 min. There was a rapid decrese in strength and hardness with annealing at temperature above 550℃, while elongation increased rapidly above 600℃. At 700℃, hardness and strength decreased rapidly, and elongation increased steeply with an increasing reverse treatment time up to 10 min, whereas there were no significant change with a treatment time after 10 min. The reverse-transformed austenite showed an ultra-fine grain size less than 0.2 ㎛, which made it possible to strengthen the high manganese austenitic stainless steel.

keyword : high manganese austenite stainless steel, reverse transformation, reversed austenite, stability of austenite, deformation induced martensite

The purpose of this research was to develop a low Ni austenitic cast iron through replacing Ni by Cu and Mn because they are cheaper than Ni. The effect of the Cu content (6-12 wt%) on the microstructure characteristics was investigated in Fe-3%C-16%(Ni+Cu+Mn) cast iron. Contrary to general effect of the Cu on cast iron, the result of the microstructure analysis indicated that bainite and cementite were formed in high Cu content (>8 wt%Cu). A crystallized Cu-solution (Cu-Mn) phase and MnS in the Cu-solution were formed. The quantity of those phases increased as the Cu content increased. Consequently, the high Cu content in the composition ratio (Ni+Cu+Mn=16%) caused the formation of Cu-Mn/MnS and those phases decreased the effect of Cu and Mn on austenite formation. For this reason, bainite and cementite were formed in high Cu content.

keyword : austenitic cast-iron, casting, microstructure, scanning electron microscopy, electron probe microanalyzer

This paper presents the material properties of carbon steels for ships, and offshore structures (ASTM A131) are tested under a series of arctic and cryogenic temperature conditions. For material tension tests, among the ASTM 131 steels, Grades A and B of mild steel and Grade AH of high tensile steel have been used. The obtained mechanical properties of the materials from the material tension tests were applied in a 13,000TEU class container ship to define the effect of low temperature on the ultimate longitudinal strength of the target structure by using the ALPS/HULL intelligent supersize finite element method. The tensile coupon test results showed increased strength and nonuniform fracture strain behaviors within different grades and temperatures. Increasing the material strength resulted in increasing the ultimate longitudinal strength of the ship.

keyword : Low temperature, arctic, cryogenic condition, ultimate strength, material property

Strength change in ultra low carbon steel carburized at 880℃ and 930℃ for 10, 30, 60 and 120 minutes was investigated. The results were analyzed by a tensile test, chemical composition analysis, optical microscopy and scanning electron microscopy. Stress in the 0.5% strain specimen in the tensile test increased as the time treated at 880℃ and 930℃ increased, because the carbon diffusion layer and the martensite of the specimen increased with increasing treatment time. Martensite was found in the ferrite region in the specimen treated at 880℃, which is attributed to grain boundary diffusion.

keyword : HPF, hot press forming, strength, stress, ultra Low carbon steel, carburizing heat treatment

YAG:Ce phosphor were prepared in a self-propagating high-temperature synthesis (SHS) using a 1.5Y2O3+2.5Al2O3+0.116CeO2+3.0KClO3+kCO(NH2)2+m(C2F4)n precursor mixture. The heat for the combustion propagation was provided by the reaction of a KClO3+CO(NH2)2+(C2F4)n mixture. Pure-phase YAG phosphor was synthesized at the combustion temperature of 1210℃ from k=3.6 mole and m=0.3 mole. The as-prepared YAG:Ce phosphor had a particle size of 2-10 ㎛. The addition of Teflon to the precursor mixture increased the YAG particle size and its luminescent intensity. The emission peak of the YAG phosphor was blue-shifted with an increase of Teflon concentration.

keyword : teflon, YAG phosphor, SHS, self-propagating high-temperature synthesis

We studied the effect of temperature and reaction time by investigating the various temperatures and reaction times in the reduction of molybdenum oxide (MoO3) to molybdenum (Mo) powder in hydrogen gas. We also studied the effect of the reaction of reduction according to the various hydrogen gas flow rates. We surveyed the reduction from molybdenum oxide to molybdenum powder in hydrogen gas and checked two temperature ranges, one from 400℃ to 600℃ and the other from 700℃ to 900℃. We found that the reaction ratio of molybdenum oxide increased with an increasing temperature and also increased with an increasing reaction time, but hydrogen gas did not influence the reduction ratio of molybdenum oxide. We examined molybdenum powders fabricated by ball milling for two hours, using with X-ray diffraction (XRD) and a scanning electron microscopy (SEM).

keyword : metals, powder processing, microstructure, scanning electron microscopy, Mo powder

Nanopowder of FeAl and Al2O3 was synthesized from FeO and Al powders by high energy ball milling. Using the pulsed current activated sintering method, the nanocystalline Al2O3 reinforced FeAl composite was consolidated within two minutes from mechanically synthesized powders. The advantage of this process is that it allows very quick densification to near theoretical density and prohibits grain growth in nanostuctured materials. The grain size, sintering behavior and hardness of sintered FeAl-Al2O3 composite were investigated.

keyword : composite, nanomaterials, sintering, mechanical properties

Al-Si coated boron steel and Zn coated DP steel plates were laser-welded to manufacture a Tailor Welded Blank (TWB) for a car body frame. Hot-stamping heat treatment (900℃, 5 min) was applied to the TWB weld, and the microstructural change and transformation mechanism were investigated in the Al-rich area near the bond line of the Al-Si coated steel side. There was Al-rich area with a single phase, Fe3(Al,Si), which was transformed to α-Fe (Ferrite) after the heat treatment. It could be explained that the Fe3(Al,Si) phase was transformed to α-Fe during heat treatment at 900℃ for 5 min and the resultant α-Fe phase was not transformed by rapid cooling. Before the heat treatment, the microstructures around the Fe3(Al,Si) phase consisted of martensite, bainite and α-Fe while they were transformed to martensite and δ-Fe after the heat treatment. Due to the heat treatment, Al was diffused to the Fe3(Al,Si) and this resulted in an increase of Al content to 0.7 wt% around the Al-rich area. If the weld was held at 900℃ for 5 min it was transformed to a mixture of austenite (γ) and δ-Fe, and only γ was transformed to the martensite by water cooling while the δ-Fe was remained unchanged.

keyword : metals, joining, phase transformation, transmission electron microscopy, Al-segregated zone

A 71.5 wt%Mg-23.5 wt%Ni-5 wt%Fe2O3 (Mg-23.5Ni-5Fe2O3) sample was prepared by a quite simple process, reactive mechanical grinding, and its hydriding and dehydriding properties were then investigated. The reactive mechanical grinding of Mg with Ni and Fe2O3 is considered to facilitate nucleation and shorten the diffusion distances of the hydrogen atoms. After the hydriding-dehydriding cycling, the Mg- 23.5Ni-5Fe2O3 sample contained Mg2Ni phase. Expansion and contraction of the hydride-forming materials (Mg and Mg2Ni) with the hydriding and dehydriding reactions are also considered to increase the hydriding and dehydriding rates of the mixture by forming defects and cracks leading to the fragmentation of the particles. The temperature dependence of the hydriding rate of the sample is discussed.

keyword : hydrogen absorbing materials, mechanical alloying, milling, microstructure, X-ray diffraction, Ni and Fe2O3 addition

A high-energy mechanical milling (HEMM) process was introduced to improve sinter-ability, and rapid sintering of spark plasma sintering (SPS) under pressure was used to make ultra fine grain (UFG) of Ti-Nb-Mo-CPP composites, which have bio-attractive elements, for increasing mechanical properties. Ti-Nb- Mo-CPP composites were successfully fabricated by SPS at 1000℃ within 5 minutes under 70 MPa using HEMMed powders. The Vickers hardness of the composites increased with increased milling time and addition of CPP contents. Biocompatibility and corrosion resistance of the Ti-Nb-Mo alloys were improved by addition of CPP, and the Ti-35%Nb-10%Mo-10%CPP alloy had better biocompatibility and corrosion resistance than the Ti-6Al-4V

keyword : biomaterials, sintering, microstructure, SEM, spark plasma sintering