• 한국세라믹학회지
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• 제45권1호
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• pp.75-81
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• 2008

In this study, machining characteristics of AIN-hBN composites were evaluated in end-milling process. As a first step, AIN-hBN composite specimens with various hBN contents were prepared using hot press method. Material properties of the composites, such as relative density, Young's modulus and fracture toughness, were measured and compared. Then, a series of end-milling experinients were performed under various cutting conditions by changing cutting speed, depth-of-cut and feed rate. Cutting force variations were measured using a tool dynamometer during the cutting experiments. Machined surfaces of the specimens were observed using SEM and a surface pro filer to investigate the surface integrity changes. The cutting force decreased with an increases of hBN content. The cutting process was almost impossible for monolithic AIN, owing to severe chipping. In contrast, at high content of hBN, surface damage and chipping decreased, and better surface roughness can be obtained.

• 한국분말재료학회지
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• 제12권2호
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• pp.106-111
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• 2005

The effect of post-heat treatment on the coating characteristics and the fatigue strength of the gas flame thermally sprayed Stellite alloy coatings on $0.35\%$ carbon steel were investigated. The fatigue fracture surfaces of the heat treated samples were observed using SEM (Scanning Electron Microscopy). For as-sprayed samples, there was considerable scattering in the fatigue life due to the presence of the pores in the coating. After the post-heat treatment to improve the microstructural characteristics of the coating layer, the fatigue strength of the specimens was greatly improved, increasing with increasing the coating thickness. For the specimens with the 0.3mm and 0.5mm thick coating, the fatigue cracks originated in the substrate region just below the interface. On the contrary, for the specimens with the 1.0mm thick coating, they nucleated at the pore within the coating, and the fatigue strength was 2.6 times higher than that of the substrate due to the high fatigue resistance of the coating.

• 동력기계공학회지
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• 제17권4호
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• pp.94-102
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• 2013

Manufactured $SiC_f/SiC$ composites by NITE method was investigated fracture characteristics according to the size of the surface crack. Coated surface crack with a $SiO_2$ colloid in several ways was evaluating the possibility of healing. The strength of CCS and UCS is 313 and 230MPa, respectively and it is about 1/3 of the SPS. Bending strength of $SiC_f/SiC$ composites has no effect with the pre-crack size to the critical crack size. $SiC_f/SiC$ composites can not generate large amount of $SiO_2$ oxides to the bottom of crack, and is only generated randomly on surfaces, and can not contribute to the recovery of bending strength.

• 대한금속재료학회지
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• 제46권6호
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• pp.363-369
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• 2008

Effects of top coat morphology and thickness on thermal fatigue behavior of thermal barrier coatings (TBC) were investigated in this study. Thermal fatigue tests were conducted on three coating specimens with different top coat morphology and thickness, and then the test data were compared via microstructures, cycles to failure, and fracture surfaces. In the air plasma spray specimens (APS1, APS2), top coat were 200 and $300{\mu}m$ respectively. The thickness of top coat was about $700{\mu}m$ in the perpendicular cracked specimen (PCS). Under thermal fatigue condition at $1,100^{\circ}C$, the cycles to top coat failure of APS1, APS2, and PCS were 350, 560 and 480 cycles, respectively. The cracks were initiated at the interface of top coat and thermally grown oxide (TGO) and propagated into TGO or top coat as the number of thermal fatigue cycles increased. For the PCS specimen, additive cracks were initiated and propagated at the starting points of perpendicular cracks in the top coat. Also, the thickness of TGO and the decrease of aluminium concentration in bond coat do not affect the cycles to failure.

• 한국산업융합학회 논문집
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• 제22권1호
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• pp.11-20
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• 2019

This study dealt with the tensile strength characteristics of stainless steel 304L steel by hydrogen charging. Especially, the effect of hydrogen charging time on the tensile strength and ductility of 304L stainless steels was evaluated, in conjunction with the observation of their fracture surfaces. The tensile properties of hydrogen-charged 304L stainless steels were also investigated with the variation of tensile loading speeds. The hydrogen amount of 304L stainless steels obviously increased with the increase of hydrogen charging time. The tensile properties of 304L stainless steels were clearly affected by the short term charging of hydrogen. In particular, the elongation of 304L stainless steels decreased with increasing hydrogen charging time, due to the hydrogen embrittlement. It was also found that the tensile properties of hydrogen-charged 304L stainless steels were very sensitive to the crosshead speed for tensile loading.

• Corrosion Science and Technology
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• 제18권1호
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• pp.33-38
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• 2019

Alloy 600/182 with excellent mechanical/chemical properties have been utilized for nuclear power plants. Although both alloys are known to have superior corrosion resistance, stress corrosion cracking failure has been an issue in primary water environment of nuclear power plants. Therefore, primary water stress corrosion crack (PWSCC) growth rate tests were conducted to investigate crack growth properties of Alloy 600/182. To investigate PWSCC growth rate, test facilities including water chemistry loop, autoclave, and loading system were constructed. In PWSCC crack growth rate tests, half compact-tension specimens were manufactured. These specimens were then placed inside of the autoclave connected to the loop to provide primary water environment. Tested conditions were set at temperature of $360^{\circ}C$ and pressure of 20MPa. Real time crack growth rates of specimens inside the autoclave were measured by Direct Current potential drop (DCPD) method. To confirm inter-granular (IG) crack as a characteristic of PWSCC, fracture surfaces of tested specimens were observed by SEM. Finally, crack growth rate was derived in a specific stress intensity factor (K) range and similarity with overseas database was identified.

• Nuclear Engineering and Technology
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• 제51권7호
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• pp.1816-1821
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• 2019

This research investigates the feasibility of using the thermoelectric power to monitor the thermal embrittlement in 2507 super duplex stainless steel (SDSS) exposed to a temperature between $280^{\circ}C$ and $500^{\circ}C$. It is well known that the precipitation of Cr-rich ${\alpha}^{\prime}$ phase as a result of the spinodal decomposition is the major cause of the embrittlement and the loss of corrosion resistance in this material. The specimens are thermally aged at $475^{\circ}C$ for different holding times. A series of mechanical testing including the tensile test, Vickers microhardness measurement, and Charpy impact test are conducted to determine the property changes with holding time due to the embrittlement. The mechanical strengths and ferrite hardness exhibit very similar trends. Scanning electron microscopy images of impactfractured surfaces reveal a ductile to brittle transition in the fracture mode as direct evidence of the embrittlement. It is shown that the thermoelectric power is highly sensitive to the thermal embrittlement and has an excellent linear correlation with the ferrite hardness. This paper, therefore, demonstrates that the thermoelectric power is an excellent nondestructive evaluation technique for detecting and evaluating the $475^{\circ}C$ embrittlement of field 2507 SDSS structures.

• 한국분말재료학회지
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• 제28권4호
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• pp.301-309
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• 2021

In this study, an AISI 316 L alloy was manufactured using a selective laser melting (SLM) process. The tensile and impact toughness properties of the SLM AISI 316 L alloy were examined. In addition, stress relieving heat treatment (650℃ / 2 h) was performed on the as-built SLM alloy to investigate the effects of heat treatment on the mechanical properties. In the as-built SLM AISI 316 L alloy, cellular dendrite and molten pool structures were observed. Although the molten pool did not disappear following heat treatment, EBSD KAM analytical results confirmed that the fractions of the low- and high-angle boundaries decreased and increased, respectively. As the heat treatment was performed, the yield strength decreased, but the tensile strength and elongation increased only slightly. Impact toughness results revealed that the impact energy increased by 33.5% when heat treatment was applied. The deformation behavior of the SLM AISI 316 L alloy was also examined in relation to the microstructure through analyses of the tensile and impact fracture surfaces.

• 한국재료학회지
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• 제31권5호
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• pp.255-263
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• 2021

P91 steel has been a highly researched material because of its applicability for high-temperature applications. Considerable efforts have been made to produce experimental creep data and develop models for creep life prediction. As creep tests are expensive and difficult to conduct, it is vital to develop authenticated empirical methods from experimental results that can be utilized for better understanding of creep behavior and can be incorporated into computational models for reliable prediction of creep life. In this research, a series of creep rupture tests are performed on the P91 specimens within a stress range of 155 MPa to 200 MPa and temperature range of 640 ℃ (913 K) to 675 ℃ (948 K). The microstructure, hardness, and fracture surfaces of the specimens are investigated. To analyze the results of the creep rupture tests at a macro level, a parameter called creep work density is derived. Then, the relationships between various creep parameters such as strain, strain rate, time to rupture, creep damage tolerance factor, and creep work density are investigated, and various empirical equations are obtained.

• Corrosion Science and Technology
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• 제22권4호
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• pp.273-286
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• 2023

Rebar is embedded in concrete to create reinforced concrete (RC). Rebar carries most of the tensile stress and gives compressively loaded concrete fracture resistance. However, embedded steel corrosion is a significant cause of concern for RC composite structures worldwide. It is one of the biggest threats to concrete structures' longevity. Due to environmental factors, concrete decays and reinforced concrete buildings fail. The type and surface arrangement of the rebar, the cement used in the mortar, the dosing frequency of the concrete, its penetrability, gaps and cracks, humidity, and, most importantly, pollutants and aggressive species all affect rebar corrosion. Either carbonation or chlorides typically cause steel corrosion in concrete. Carbonation occurs when carbon dioxide in the atmosphere combines with calcium within the concrete. This indicates that the pH of the medium is falling, and the steel rebar is corroding. When chlorides pass through concrete to steel, corrosion rates skyrocket. Consideration must be given to concrete moisture. Owing to its excellent resistance, dry concrete has a low steel corrosion rate, whereas extremely wet concrete has a low rate owing to delayed O₂ transfer to steel surfaces. This paper examines rebar corrosion causes and mechanisms and describes corrosion evaluation and mitigation methods.