• Corrosion Science and Technology
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• v.13 no.3
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• pp.102-106
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• 2014

Nickel electrodeposition from sulfamate bath has several benefits such as low internal stress, high current density and good ductility. In nickel deposited layers, sulfur induces high temperature embrittlement, as Ni-S compound has a low melting temperature. To overcome high temperature embrittlement problem, adding manganese is one of the good methods. Manganese makes Mn-S compound having a high melting temperature above $1500^{\circ}C$. In this work, the mechanical properties of Ni-Mn deposited layers were investigated by using various process variables such as concentration of Mn$(NH_2SO_3)_2$, current density, and bath temperature. As the Mn content of electrodeposited layers was increased, internal stress and hardness were increased. By increasing current density, internal stress increased, but hardness decreased. With increasing the bath temperature from 55 to $70^{\circ}C$, internal stress of Ni deposit layers decreased, but hardness didn't change by bath temperature. It was likely that eutectoid manganese led to lattice deformation, and the lattice deformation increased hardness and internal stress in Ni-Mn layers. Increasing current density and decreasing bath temperature would increase a mount of $H_2$ absorption, which was a cause for the rise of internal stress.

• Corrosion Science and Technology
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• v.7 no.3
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• pp.145-151
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• 2008

Material degradation such as high temperature oxidation of metallic material is a severe problem in energy generation systems or manufacturing industries. The metallic materials are oxidized to form oxide films in high temperature environments. The oxide films act as diffusion barriers of oxygen and metal ions and thereafter decrease oxidation rates of metals. The metal oxidation is, however, accelerated by mechanical fracture and spalling of the oxide films caused by thermal stresses by repetition of temperature change, vibration and by the impact of solid particles. It is therefore very important to investigate mechanical properties and adhesion of oxide films in high temperature environments, as well as the properties in a room temperature environment. The oxidation tests were conducted for Ni and Ni-Co alloy under high temperature corrosive environments. The hardness distributions against the indentation depth from the top surface were examined at room temperature. Dynamic indentation tests were performed on Ni oxide films formed on Ni surfaces at room and high temperature to observe fractures or cracks generated around impact craters. As a result, it was found that the mechanical property as hardness of the oxide films were different between Ni and Ni-Co alloy, and between room and high temperatures, and that the adhesion of Ni oxide films was relatively stronger than that of Co oxide films.

• Journal of the Korean Ceramic Society
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• v.50 no.3
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• pp.226-230
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• 2013

We performed high pressure high temperature (HPHT) sintering for the 20 nm MgO powders at the temperatures from $600^{\circ}C$ to $1200^{\circ}C$ for only 5 min under 7 GPa pressure condition. To investigate the microstructure evolution and physical property change of the HPHT sintered MgO samples, we employed a scanning electron microscopy (SEM), density and Vickers hardness measurements. The SEM results showed that the grain size of the sintered MgO increased from 200 nm to $1.9{\mu}m$ as the sintering temperature increased. The density results showed that the sintered MgO achieved a more than 95% of the theoretical density in overall sintering temperature range. Based on Vickers hardness test, we confirmed that hardness increased as temperature increased. Our results implied that we might obtain the dense sintered MgO samples with an extremely short time and low temperature HPHT process compared to conventional electrical furnace sintering process.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.2
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• pp.67-73
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• 2017

Transformation behavior and hardness change were studied in five kinds of self-control steels; standard, high V, modified Ni, W, and high C-Ni steels. In the cooling rates of $10-100^{\circ}C/min$, the primary ferrite and bainite were formed, and the amount of the former increased with decreasing cooling rate. The bainite transformation temperature, Bs, was measured as 570, 560, 590, 575, and $565^{\circ}C$ in experimental steels, respectively, which was similar to the calculated temperature. The self-control, that is, the consistency in hardness, was observed, in which the hardness increased with the decrease in Bs. In the case of hot compression testing, the lower temperature deformation led to the enhancement in hardness.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.10a
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• pp.792-795
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• 2002

In previous researches, it is reported that Ti-10Ta-10Nb is robuster than Ti-6A1-4V which is used as a biomaterial in a experiment of cytotoxicity. Ti-10Ta-10Nb has enough hardness to be required as a biomaterial because the change of its hardness can be controlled more than 100% according to heat treatment condition and manufacturing condition. There are many hardness changing condition including Cast Homogenization, Solution treatment. Forging, Rolling in this research. The changing form and amount of new Ti-10Ta-10Nb to be developed in this researches, are measured as quantitative. Specially, the changing hardness amount of the specimen that is manufactured in single phase temperature, i.e. 80$0^{\circ}C$, are measured in case of high temperature rolling and high temperature cast condition.

• Journal of the Korean institute of surface engineering
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• v.50 no.4
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• pp.272-276
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• 2017

The response of AISI 310 type austenitic stainless steel to the novel low temperature plasma carburizing process has been investigated in this work. This grade of stainless steel shows better corrosion resistance and high temperature oxidation resistance due to its high chromium and nickel content. In this experiment, plasma carburizing was performed on AISI 310 stainless steel in a D.C. pulsed plasma ion nitriding system at different temperatures in $H_2-Ar-CH_4$ gas mixtures. The working pressure was 4 Torr (533Pa approx.) and the applied voltage was 600 V during the plasma carburizing treatment. The hardness of the samples was measured by using a Vickers micro hardness tester with the load of 100 g. The phase of carburized layer formed on the surface was confirmed by X-ray diffraction. The resultant carburized layer was found to be precipitation free and resulted in significantly improved hardness and corrosion resistance.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2012.11a
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• pp.175-177
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• 2012

A corrosion resistance and hard nitrocarburized layer was distinctly formed on 310 austenitic stainless steel substrate by DC plasma nitrocarburizing. Basically, 310L austenitic stainless steel has high chromium and nickel content which is applicable for high temperature applications. In this experiment, plasma nitrocarburizing was performed in a D.C. pulsed plasma ion nitriding system at different temperatures in $H_2-N_2-CH_4$ gas mixtures. After the experiment structural phases, micro-hardness and corrosion resistance were investigated by the optical microscopy, X-ray diffraction, scanning electron microscopy, micro-hardness testing and Potentiodynamic polarization tests. The hardness of the samples was measured by using a Vickers micro hardness tester with the load of 100 g. XRD indicated a single expanded austenite phase was formed at all treatment temperatures. Such a nitrogen and carbon supersaturated layer is precipitation free and possesses a high hardness and good corrosion resistance.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.1
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• pp.31-37
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• 2019

This study investigates the effects of long-term artificial-aging on hardness variation in the dissimilar metal weldments for nuclear power plant facilities. These dissimilar welds are inevitably required to join the components in nozzle parts of pressurized vessels, such as austenitic stainless steels and ferritic steels. A artificial thermal aging was conducted in an electrical furnace to simulate material degradation at high temperatures. The test materials were held at the temperature of $600^{\circ}C$ for 10000 hours and interrupted at various levels of degraded specimens. The degradation of hardness is a well-known phenomenon resulting from long-term aging or high-temperature degradation of structural materials. In this study, the variation of hardness at each position was different, and complicated in relation to microstructures such as twins, grains, precipitates, phase transformations, and residual stresses in dissimilar weldments. We discussed the variation of hardness in terms of microstructural changes during long-term aging.

• Korean Journal of Materials Research
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• v.28 no.8
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• pp.459-465
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• 2018

This paper presents a study of the microstructure and mechanical properties of commercial high-hardness armor (HHA) steels tempered at different temperatures. Although the as-received specimens of all the steels exhibit a tempered martensite structure with lath type morphology, the A steel, which has the smallest carbon content, had the lowest hardness due to reduced solid solution hardening and larger lath thickness, irrespective of tempering conditions. As the tempering temperature increases, the hardness of the steels steadily decreases because dislocation density decreases and the lath thickness of martensite increases due to recovery and over-aging effects. When the variations in hardness plotted as a function of tempering temperature are compared with the hardness of the as-received specimens, it seems that the B steel, which has the highest yield and tensile strengths, is fabricated by quenching, while the other steels are fabricated by quenching and tempering. On the other hand, the impact properties of the steels are affected by specimen orientation and test temperature as well as microstructure. Based on these results, the effect of tempering on the microstructure and mechanical properties of commercial high-hardness armor steels is discussed.

• Journal of the korean Society of Automotive Engineers
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• v.10 no.2
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• pp.46-53
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• 1988

Effect of Notch Shape and Hardness Ratio on Characteristics of Impact Fracture in Dual Phase Steels. In this study, it is investigated the effect of notch shape and hardness ratio on the characteristics of impact fracture in dual phase steels. The impact test was carried out at the temperature range from -40.deg. C to room temperature with Instrumented Charpy Impact Tester. The main results obtained are as follows; 1, The maximum impact bending strength (.sigma.$_{max}$) increases with the tensile strength. Also, the impact energy depends on .sigma.$_{max}$. 2, In room temperature, the impact energy depends on crack-initiation energy (E$_{i}$) in case of the high hardness ratio (R=3.4), whereas depends on crack-propagation energy (E$_{p}$) in case of the low hardness ratio (R=1.8) and the dependence of crack-initiation energy of the impact characteristics decreases with increasing test temperature. These phenomena are result from the difficulty of cleavage facet formation.ion.ion.