• Korean Journal of Materials Research
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• v.29 no.1
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• pp.52-58
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• 2019

The microstructure, hardness, and wear behaviors of a High Velocity Oxygen Fuel(HVOF) sprayed WC-CoFe coating are comparatively investigated before and after laser heat treatments of the coating surface. During the spraying, the binder metal is melted and a small portion of WC is decomposed to $W_2C$. A porous coating is formed by evolution of carbon oxide gases formed by the reaction of the free carbon and the sprayed oxygen gas. The laser heat treatment eliminates the porosity and provides a more densified microstructure. After laser heat treatment, the porosity in the coating layer decreases from 1.7 % to 1.2 and the coating thickness decreases from $150{\mu}m$ to $100{\mu}m$. The surface hardness increases from 1440 Hv to 1117 Hv. In the wear test, the friction coefficient of coating decreases from 0.45 to 0.32 and the wear resistance is improved by the laser heat treatment. The improvement is likely due to the formation of oxide tribofilms.

• Nuclear Engineering and Technology
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• v.51 no.1
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• pp.249-256
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• 2019

The 9Cr-1Mo ferritic-martensitic ODS steel is a promising structural material for the next generation nuclear power plants including fast reactors for application in reactor vessels and nuclear fuel. The ODS steel was cooled down by furnace cooling, air cooling, oil quenching and water quenching, respectively, after normalizing it at $1150^{\circ}C$ for 1 h and then tempering at $780^{\circ}C$ for 1 h. It is found that grain size, a relative portion of ferrite and martensite, martensitic lath configuration, behaviors of carbide precipitates, and hardness of the ODS steel are strongly dependent on a cooling rate. The grain size and martensitic lath width become smaller with the increase in a cooling rate. The carbides were precipitated at the grain boundaries formed between the ferrite and martensite phases and at the martensitic lath interfaces. In addition, the carbide precipitates become smaller and more widely dispersed with the increase in a cooling rate, resulting in that the faster cooling rate generated the higher hardness of the ODS steel.

• Korean Journal of Metals and Materials
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• v.56 no.12
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• pp.854-860
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• 2018

In spite of the many attractive properties of (W,Ti)C, its low fracture toughness limits its wide application. To improve the fracture toughness generally a second phase is added to fabricate a nanostructured composite. In this regard, graphene was considered as the reinforcing agent of (W,Ti)C. (W,Ti)C-graphene composites that were sintered within 2 min using pulsed current activated heating under a pressure of 80 MPa. The rapid consolidation method allowed retention of the nano-scale microstructure by blocking the grain growth. The effect of graphene on the hardness and microstructure of the (W,Ti)C-graphene composite was studied using a Vickers hardness tester and FE-SEM. The grain size of (W,Ti)C was reduced remarkably by the addition of graphene. Furthermore, the hardness decreased and the fracture toughness improved with the addition of graphene.

• Korean Journal of Metals and Materials
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• v.49 no.5
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• pp.380-387
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• 2011

Wire brushing, which is a typical surface preparation method for roll bonding, has recently been highlighted as a potentially effective method for surface nanocrystallization. In the present study, the microstructure evolution and hardness of the wire-brushed surface and roll-bonded interface of a 1050 aluminum sheet were investigated. Wire brushing formed protruded layers with a nanocrystalline structure and extremely high surface hardness. After roll bonding, the protruded layers remained as hard layers at the interface. Due to their hardness and brittleness the interface hard layers, can affect the interface bonding properties and also play an important role determining the mechanical properties of multi-layered clad sheets.

• Journal of the Korean institute of surface engineering
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• v.34 no.2
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• pp.97-104
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• 2001

($Ti_{ 1-x}$$Al_{ x}$)N has been deposited on high speed steel (HSS) substrate using PECVD from the gas mixture of $TiC1_4$, $AlC1_4$, $NH_3$, $H_2$, and Ar. The correlation between the microstructure and the mechanical properties was investigated. ($Ti_{1-x}$$Al_{ x}$)N showed single phase NaCl-structure up to X=0.87, while a mixed phase of NaCl Type (Ti, Al) N and wurtzite structure AlN was observed for 0.87$Ti_{1-x}$ $Al_{x}$ )N became by degrees as increasing X, which made the hardness of the coating higher by Al addition. When the coating was composed of a mixed phase, however, the hardness decreased abruptly due to the effect of soft AlN phase. The wear volume of the coatings could be obtained as the concentration of the coating was varied, and the relation between the wear volume and hardness or the adhesion strength was discussed.

• Korean Journal of Materials Research
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• v.30 no.1
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• pp.8-13
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• 2020

Effect of heat treatment on microstructure and mechanical properties of an Fe-6.5Mn-0.08C medium-manganese steel is investigated in this study. Three kinds of medium-manganese steel specimens are fabricated by varying heat treatments of intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA). Hardness and tensile tests are performed to examine the correlation of microstructure and mechanical properties for the Fe-6.5Mn-0.08C medium-manganese steel specimens. The IQ and SQ specimens have microstructures of martensite matrix with ferrite, whereas IA specimen exhibits microstructure of acicular ferrite matrix with martensite. The tensile test results show that the SQ specimen with martensite matrix has the highest yield strength and the lowest elongation. On the other hand, the SQ specimen has the highest hardness due to the relatively lower reduction of carbon content in martensite during intercritical annealing. According to the fractography of tensile tested specimens, the SQ specimen exhibits a dimple and quasi-cleavage fracture appearance while the IQ and IA specimens have fully ductile fracture appearance with fine-sized dimples caused by microvoid coalescence at ferrite and martensite interface.

• Korean Journal of Materials Research
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• v.17 no.10
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• pp.538-543
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• 2007

We fabricated the Ni plate by electroforming process and evaluated the microstructure, mechanical properties and wear behavior of the Ni plate. Specifically, the effects of addition of wetting agents, SF 1 and SF 2 solutions, on the microstructure and properties were investigated. The microstructure and surface morphology were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM), respectively, and friction coefficient was measured by the ball-on-disk method. We found that the microstructure and mechanical properties of Ni plate were changed with kind and amount of wetting agents used. The hardness and tensile strength of Ni plate formed without wetting agents was 228 Hv and 660.7 MPa, respectively, whiled when wetting agent was added, those were improved to be 739 Hv and 1286.3 MPa. These improvements were probably due to the finer grain size and less crystallization of Ni. In addition, when both wetting agents were added, the friction coefficient was reduced from 0.73 to 0.67 which is partially caused by the improved hardness and smooth surface.

• Proceedings of the KWS Conference
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• 2000.10a
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• pp.290-293
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• 2000

• Proceedings of the Korean Nuclear Society Conference
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• 1999.05a
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• pp.294-294
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• 1999

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2003.05a
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• pp.50-51
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• 2003