• 한국재료학회지
• /
• 제16권5호
• /
• pp.318-322
• /
• 2006

Short-term high temperature degradation test was conducted on Hastelloy X, a candidate tube material for high temperature gas-cooled reactors (HTGR), to evaluate the variation of microstructure and mechanical property in air at $1050^{\circ}C$ during 2000 h. The dominant oxide layer was Cr-oxide and a very shallow Cr-depleted region was observed below the oxide layer. At the beginning of degradation, the island shape $M_6C$ precipitate (M=Mo-rich, Fe, Ni, Cr) was observed in matrix region. After 2000 h degradation, precipitate shape was changed to the chain shape and increased amount of precipitate. These results influenced mechanical property of the specimen which exposed in high temperature. Yield strength was decreased from 115MPa to 89 MPa after 24 h and 2000 h exposure, respectively.

• 대한기계학회논문집A
• /
• 제21권10호
• /
• pp.1674-1682
• /
• 1997

Fatigue tests were performed to investigate the effect of ceramic coatings as in TiN and TiCN on fatigue crack growth properties of ceramic coated 1Cr-1Mo-0.25V steel with different coating thickness in laboratory air conditions. The experimental results are described with respect to a Paris equation, da/dN=C(.DELTA.K)$^{m}$ , where the crack growth rate of coated specimens provided as similar growth rate as that of the uncoated specimen regardless of coating thickness. Furthermore, it was observed that the type of coating layer had virtually no effect on crack growth rate in the full region of stress intensity factor range. And it was also appeared that the final crack length of TiCN coated specimens was short compared to that of TiN coated, and the substrate specimens, in which it was inferred due to lowering the toughness of coated material from high hardness of TiCN coating layer itself.

• Applied Microscopy
• /
• 제45권3호
• /
• pp.177-182
• /
• 2015

The boiler tubes of X20CrMoV12.1 used in fossil-fired power plants were obtained and analyzed for the effect of water treatment on the steam corrosion-induced oxide scale in an effort to better understand the oxide formation mechanism, as well as pertinent method of maintenance and lifetime extension. The specimens were analyzed using various microscopy and microanalysis techniques, with focuses on the effect of water treatment on the characters of scale. X-ray diffraction analysis showed that the scales of specimens were composed of hematite ($Fe_2O_3$), magnetite ($Fe_3O_4$), and chromite ($FeCr_2O_4$). Electron backscatter diffraction analysis showed that the oxides were present in the following order on the matrix: outer $Fe_2O_3$, intermediate $Fe_3O_4$, and inner $FeCr_2O_4$. After all volatile treatment or oxygenated treatment, a dense protective $Fe_2O_3$ layer was formed on the $Fe_3O_4$ layer of the specimen, retarding further progression of corrosion.

• 한국주조공학회지
• /
• 제43권5호
• /
• pp.223-229
• /
• 2023

In this study, STS316L produced by a double-melting process involving vacuum induction melting (VIM) and vacuum arc remelting (VAR) was subjected to extrusion and drawing to form a tube and was subsequently electrolytic polished (EP). The grain size of the obtained STS316L without EP was approximately 55 ㎛, with no difference found after EP. The thickness of the EP layer was measured by AES and TEM, showing values of approximately 10 nm and 15 nm, respectively. After EP, the Cr/Fe and CrO/FeO ratios of the passive layer increased from 1.48 to 1.62 and from 2.15 to 2.26, respectively, while the surface roughness decreased significantly from 0.255 to 0.024 ㎛. Consequently, the corrosion rate decreased in both NaCl and HCl solutions after the EP process. Additionally, the amounts of eluted Cr and Fe ions were reduced from 1.2 to 0.8 ppb and 10.3 to 0.8 ppb, respectively. Furthermore, polarization tests revealed that STS316L treated with EP required a lower current density to reach a passive state, indicating that corrosion behavior was retarded.

• Journal of Welding and Joining
• /
• 제11권4호
• /
• pp.91-102
• /
• 1993

The plasma sprayed $Cr_3C_2$-NiCr coatings are widely used as wear-resistant and corrosion-resistant materials. The mechanical and wear properties of the plasma sprayed $Cr_3C_2$-NiCr coating on steel plate were examined in this study. The pore in the coatings could be classified into two types, the one is the intrinsic pore originated from the spraying powder, the other is the extrinsic pore formed during spraying. During the tensile adhesion test, the fracture occured at the interface of top coating and bond coating. It is though that the compressive residual stress increases with the increase of the top coating thickness. From the wear test, it was found that the wear rate increased with the increase of the sliding velocity regardless of the temperature. It is thought that the fracture toughness reduces with the increase of the sliding velocity at $30^{\circ}C$ and that the adhesion amount increases with the increase of the sliding velocity at $400^{\circ}C$ It is concluded that the wear mechanism at $30^{\circ}C$ is the fracture and pull-out of the carbide particles due to the fatigue on sliding surface, while the wear mechanism at $400^{\circ}C$ is the adhesion of the smeared layer formed during wear process.

• Corrosion Science and Technology
• /
• 제2권1호
• /
• pp.7-11
• /
• 2003

Photoelectrochemical response and electrochemical impedance behavior was investigated for passive film formed on sputter-deposited Cr alloy in $0.1kmol{\cdot}m^{-3}$. Photoelectrochemical action spectrum could be separated into two components, which were considered to be derived from $Cr_2O_3$ ($E_g\sim3.6eV$) and $ Cr(OH)_3 $ ($E_g\sim2.5eV$). The band gap energy, $E_g$, of each component was almost constant for various applied potentials. polarization periods and alloying additives. The photoelectrochemical response showed negative photo current for most potentials in the passive region. Therefore, the photo current apparently exhibited p-typesemiconductor behavior. On the other hand, Mort-Schottky plot of the capacitance showed positive slope, which means that passive film formed on Cr alloy has n-type semiconductor property. These apparently conflicting results are rationally explained assuming that the passive film on Cr alloy formed in the acid solution has n-type semiconductor property with a fairly deep donor level in the band gap and forms an accumulation layer in the most of potential region in the passive state.

• 대한치과기공학회지
• /
• 제28권1호
• /
• pp.19-26
• /
• 2006

The effect of Nb on interfacial bonding characteristics of Ni-Cr alloy for porcelain fused to metal crown (PFM) has been studied in order to investigate oxide layer. A specimens of Ni-Cr alloy, which is 0.8mm in thickness, within the porcelain furnace of 1,000$^{\circ}C$ with four tests such as air, vacuum, air for 5 minutes and vacuum for 5 minutes in order to examine an oxide behavior of alloy surface generated by the adding of Nb to be controlled at a rate of 0, 1, 3 and 5. Oxide film was observed form of the fired specimens with scanning electron microscope (SEM), and at the same time it measured Electron Probe Micro Analyzer (EPMA). The result of this study were as follows: 1. Cr oxide film and Nb oxide film were observed from the surface of specimen to be controlled at a rate of Nb 1%. 2. Nb oxide film was observed from the interface of specimens to be controlled at a rate of Nb 1% and 3%. 3. The stability of oxide films that treated in air were more stable than treated under vacuum.

• 대한금속재료학회지
• /
• 제50권8호
• /
• pp.549-556
• /
• 2012

The tribological behavior of CrMoN films with respect to surface chemistry was investigated by using X-ray photoelectron spectroscopy (XPS). All of the films were prepared from a hybrid PVD system consisting of DC unbalanced magnetron (UBM) sputtering and arc ion plating (AIP) sources. The tribological property of the films was evaluated by a friction coefficient using a Ball-on-disk type tribometer. The chemistry of wear track was analyzed by energy dispersive spectroscopy (EDS) and XPS. The friction coefficient was measured to be 0.4 for the CrMoN film, which is lower than that of a monolithic CrN film. EDS and XPS results imply the formation of an oxide layer on the coating surface, which was identified as molybdenum oxide phases, known to be a solid lubricant during the wear test.

• 한국표면공학회지
• /
• 제34권5호
• /
• pp.421-428
• /
• 2001

New WC-CrN superlattice film was deposited on Si substrate (500$\mu\textrm{m}$) using cathodic arc ion plating system. The microstructure and mechanical properties of the film depend on the superlattice period (λ). In the X-ray diffraction analysis (XRD), preferred orientation of microstructure was changed according to various superlattice periods(λ). During the Transmission Electron Microscope analysis (TEM), microstructure and superlattice period (λ) of the WC - CrN superlattice film was confirmed. Hardness and adhesion of the deposited film was evaluated by nanoindentation test and scratch test, respectively. As a result of nanoindentation test, the hardness of WC - CrN superlattice film was gained about 40GPa at superlattice period (λ) with 7nm. Also residual stress with various superlattice period (λ) was measured on Si wafer (100$\mu\textrm{m}$) by conventional beam-bending technique. The residual stress of the film was reduced to a value of 0.2 GPa by introducing Ti - WC buffer layers periodically with a thickness ratio ($t_{buffer}$/$t_{buffer+superlattice}$ ). To the end, for the evaluation of oxidation resistance at the elevated temperature, CrN single layer and WC - CrN superlattice films with various superlattice periods on SKD61 substrate was measured and compared with the oxidation resistance.

• 한국재료학회지
• /
• 제3권6호
• /
• pp.668-677
• /
• 1993

Cu-P계, 4종의 Cu-P-Pn계 및 3종의 Cu-P-Sn-Ag계 용가재를 사용해 Ar분위기 하에서 1003 및 1033K로 1.2Ks동안 노브레이징한 ST304, STS430 및 저탄소강과 동 접합체들을 전단시험 및 조직시험하였다. 계면에서의 미세조직은 제 종류 즉 첫째,균열을 포함하는 반응층 둘째, 분산층 세째, 균열을 포함하는 반응층과 분산층으로 분류된다. 분산층만이 존재할때 40-60MPa 이상의 상대적으로 높은 전단강도가 얻어지며, 동모재파단을 일으킨다. 이 반응층이 형성되었을때는 반드시 균열이 형성되며, 낮은 전단강도를 나타내고 접합부파단을일으킨다. 이 반응층은 Fe-P계의 화합물이다. 이러한 미소조직 및 강도 경향은 용가재내 Sn의 존재 및 모재내 Ni(또한 Cr)의 존재 유무에 따라 변화한다.