발간논문

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Effect of Co addition on the carbide microstructure of high speed steels was investigated by calculating the phase equilibria via Thermo-Calc and by analyzing their carbide microstructure via transmission electron microscopy in particular. The calculation showed that the nucleation driving force for MC carbide is much larger than that for M_6C so that MC will be precipitated as secondary carbides during tempering treatment. The result further showed that, with the addition of Co, the stable region of MC carbide in the pseudo-binary phase diagram is enlarged and the volume fraction of fine secondary MC carbides increases. These results were in good agreement with those of the microstructural analysis using particularly TEM showing that the fine secondary carbides are mostly the MC carbides and that their volume fraction increases with the Co addition. Tempering treatment results in a large increase of the hardness and bend strength of the steels. This is primarily because of the precipitation of fine secondary MC carbides during tempering treatment. This tempering effect was more pronounced with the Co addition because of the large amount of precipitation of fine secondary MC carbides.

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As the service time of power plant approaches its design life, the crack initiation and growth in boiler components becomes major concerns. Though there are many research effort for finding out the properties of used materials with the materials artificially aged by heat treatment, the obtained results are nothing but the estimated values to be verified. In this study, characteristics of the crack growth behavior for 1Cr-0.5Mo steel which was served for a boiler header for about 30 years were studied by performing fatigue crack growth tests at 538℃ and room temperature under triangular and trapezoidal wave-shapes. In order to evaluate hold time effect at elevated temperatures, the fatigue test with 10 second hold time (0.083Hz) was performed. From the experimental results, it was found that the increase of the fatigue test temperature had a significant effect on the increase of the rate of fatigue crack propagation. In the case of hold time testing, the oxidation layer formed at the crack wake was thicker than in the case without hold time.

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The mechanism of catastrophic failures in CVD-processed diamond thick films during cooling after fabrication is investigated both theoretically and experimentally in order to develop systematic methodologies for crack prevention. Finite element calculations reveal that high thermal stresses concentrate at the round edge rim of thick film and tensile thermal stress develops in the edge region. They are thought to be the cause of crack initiation and propagation, respectively. However, the cracking as a stress relaxation mechanism is competing interface decohesion between diamond film and substrate. Hence, it is suggested that the cracking occurs conditionally although thermal stress exceeding the fracture strength of film is generated in the thick film. Stress distribution should be so configured that σ_(θθ)/ σ_(zz,i) exceeds the ratio of fracture strength of film to interface adhesion strength. On the basis of the cracking condition, three methodologies were suggested to prevent the cracking; minimization of thermal hoop stress and interface adhesion strength, and the increase of film thickness. This analysis on the cracking mechanism was verified by experimental results of diamond thick film deposited on Si, Mo and W substrates. In particular, even in high thermal stress, it is ascertained experimentally that low adhesion can prevent the cracking by introducing low adhesive interlayer.

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Recently, the ODS ferritic steels are very much noticed as nuclear fusion structural materials and long life fuel claddings of fast breeder reactors. It was investigated on the ion irradiation damage of ferritic 12%Cr ODS steels with the addition of Ti and W in comparison with 17%Cr ODS and 12%Cr-3W ODS steels that cavitied were formed during simultaneous irradiation of 3.2MeV Fe^+ ion and 330KeV He^+ ion at 973K. It was found in the 12%Cr ODS steels with addition of Ti and W that cavity formation was restrained by ultrafine oxide and increase in dislocation density. But it was also found in the 17%Cr ODS and 12%Cr-3W ODS steels that large cavity was formed due to coarse oxide and decreased density of dislocations.

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We developed a new dynamic measurement system therein was especially designed by assembling a scanning laser extensometer, a frequency response analyzer for detecting the amplitude and phase lag of strain in response to a sinusoidal time-varying stress and a standard servo-hydraulic mechanical testing machine equipped with an infrared lamp heater. We developed a $quot;non-contacting$quot; extensometric technique based on a potable scanning laser caliper. This opto-electronic instrument, as such, did not suffice for our needs because a displacement signal corresponding to 1㎛ was comparable to a scatter band of noise. To improve the signal-to-noise ratio to a desired level, a dual-channel programmable active filter, together with a set of optical filters, was employed. As a result of this and some other technical improvements requisite for the intended measuring a low frequency cyclic micro deformation response of, say, 3㎛ in displacement amplitude within an accuracy of ±0.1㎛ was established. Anelastic behavior of a polycrystalline aluminum has been examined through the measurements of the dynamic Young`s modulus and internal friction over a temperature range from 25 to 400℃ at frequencies of 0.05 and 0.1㎐. One broad peak of internal friction is observed over the ranges 240 to 260℃, together with marked decreases in the experimental data. It is concluded that the peak phenomenon is due to grain-boundary relaxation.

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The objective of this study is to investigate the correlation of microstructure, wear resistance, and surface roughness in ferrous coated layers applicable to cylinder bores. Four kinds of ferrous spray powders, two of which were STS 316 steel powders and the others were blend powders of ferrous powders mixed with 20wt.% Al₂O₃-ZrO₂powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coated layers showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coated layers, while Al₂O₃-ZrO₂oxides were mainly formed in the martensitic matrix for the blend coated layers. The wear test results revealed that the blend coated layers showed the better wear resistance than the STS 316 coated layers because they contained a number of hard Al₂O₃-ZrO₂oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during the wear process. The STS 316 coated layers showed the slightly worse wear resistance than the blend coated layers, but they showed the excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material and the surface roughness, the hardness difference between the oxides and matrix should be minimized, maintaining the hardness of the coated layer at a certain level by forming an appropriate amount of oxides.

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High chromium white cast irons are commonly used where abrasion resistance is needed. Several investigators have reported the low stress abrasion resistance of white cast irons containing a various amount of carbide using rubber wheel abrasion tester(RWAT) with SiO₂and Al₂O₃ abrasives. In this study the role of MC type carbides with high hardness as well as M_7C₃type carbides in improving abrasive wear resistance was investigated. The weight percent of V was fixed as 5wt.%. However, the volume fraction of MC type carbide was decreased with increasing the volume fraction of primary (Cr, Fe, V)_7C₃type carbides due to the increase of higher Cr and C alloying content in hyper high chromium white cast irons. As a result, the specimen with many primary M_7C₃ type carbides and the lowest volume fraction of MC type carbide had the best abrasion wear resistance against SiO₂abrasive, but it was not effective for abrasive wear resistance against Al₂O₃ abrasives.

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Carbon nitride thin films were deposited on the substrates of 0.55wt%C-1.68wt%Mn induction-hardened steels by ion beam assisted deposition. Rolling contact fatigue tests were conducted using Polymet RCF-1 machine under elastohydrodynamic lubricating condition at room temperature. Failure modes during rolling contact were investigated on the basis of surface and subsurface observations of the failed coated steels. Many micropits were formed on the creater surface of wear track of the coated steels in the wedge forming mode. Subsurface cracks were initiated at the relatively large oxide inclusions and propagated in the intergranular fracture mode. It was also found in the surface of wear track after the substrate deformation during repeated rolling contact that Carbon and Oxygen elements of the coating were retained. It was suggested on the basis of the experimental results that failures during rolling contact of the steels were delayed or protected by carbon nitride coating of the steels with smaller oxide inclusions and finer grains.

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Coatings were deposited at 200℃ on S45C carbon steel using unbalanced magnetron sputtering (UBMS) of an fcc STS 316L (UNS S31603) target of 9㎝ in diameter. The total deposition pressure in the active Ar gas is 8×10^(-3)Torr. Coatings were deposited at various target power densities and bias voltages. Chemical compositions of metallic elements in coatings were measured by energy dispersive X-rays spectroscopy (EDS). X-Ray diffraction (XRD) measurements were performed to evaluate the crystal structure of the stainless steel films. Corrosion properties of the coated specimens were examined using electrochemical polarization measurements. The porosity rate was obtained from a comparison of the dc polarization resistance of the uncoated and coated substrate. The residual stress of the film was calculated from the curvature measured by stylus method. EDS results indicated that chemical compositions of coatings were similar to those of the target material that was a commercial grade STS 316L. XRD results showed that an austenitic phase could be achieved by using above 25W/㎠ power density in UBMS system. Potentiodynamic polarization curves indicated that the values of the corrosion potential of coatings are approximately at the potential of between those of bulk STS 316L and S45C. The increase of a power density caused the high residual tensile stress of coating, while, the increase of a bias voltage led to a substantial decrease in porosity. Finally, an optimized deposition condition for corrosion protection was found for 25W/㎠ , -200V coating.

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Four Iron(Fe)-Zinc(Zn) intermetallic phases were investigated using Mo¨ssbauer spectroscopy and XRD, in order to characterize the Mo¨ssbauer properties. Mo¨ssbauer analysis showed that the Mo¨ssbauer spectra for them were uniquely different from each other. The Gamma and Gamma-1 phases were composed of four subspectra, while the Delta and Zeta phases were consisted of three and one subspectrum, respectively. The C site of Gamma phase and the D site of Gamma-1 phase were included into their Mo¨ssbauer characterization. The D site of Gamma phase, which has not been previously reported, was also added for its Mo¨ssbauer characterization in this study. The Mo¨ssbauer properties can be used for the phase formation and transformation studies as a function of the synthetic conditions.

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