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In this study, we tried to improve the oxidation resistance of Nb-12Si (wt%) alloys at 1200 °C or higher through pack cementation coatings. Nb-12Si (wt%) alloys were prepared by arc-melting under Ar atmosphere. When the alloys were coated using pack powder mixtures composed of Si, Al₂O₃ and NaF, two silicide layers composed of NbSi₂ and Nb₅Si₃ phases were successfully produced on the substrate. The Si-pack coatings were performed with various heat treatment temperatures and time conditions. The microstructures and thickness changes of the coating layers were analyzed to determine the growth behaviors of the coating layer. The growth constant of 8.4×10^-9 cm²/sec was obtained with a diffusion growth mode. In addition, in order to examine the resistance of the Si-pack coated alloys, isothermal static oxidation tests were performed at 1200 °C and higher temperatures. As a result, the oxidation resistance of the alloys was determined by protecting the surface of the alloys with silicide oxide layers formed by the silicide coatings. The uncoated specimens exhibited an abnormal weight increase due to the formation of Nb oxide. The coated specimen showed excellent oxidation resistance at 1200 °C for up to 12 hrs, while the previous reports on the same alloy verified oxidation resistance only up to 1100 °C. It appears that the excellent oxidation resistance is closely related to the NbSi₂ coating layer thickness. The oxidation behaviors of the coating layers after the oxidation tests were discussed in terms of microstructural and phase analyses. (Received May 14, 2020; Accepted June 24, 2020)

keyword : Nb alloys, pack cementation coatings, high temperature, oxidation behaviors

A method for evaluating residual stress using an instrumented indentation test was developed some decades ago. More recently, another method was developed, using a Knoop indenter. The conversion factor ratio, which is one of the key factors in the evaluation algorithm, has been taken to be 0.34, although this value comes from an experimental result and its physical meaning has not been examined. Here we examine the physical meaning of this conversion factor from the previous residual stress model, and calculate its ratio using analytical model of the stress field beneath the indenter. In this process, we assumed that the conversion factor ratio was the ratio of the projected area of the plastic zone generated during the Knoop indentation test. An analysis of the stress field beneath the indenter was performed by FE simulation. Actual nanoindentation was conducted after Knoop indentation testing, using the interface-bonding technique, to identify the plastic zone. In addition, the conversion factor ratio was also calculated for the case where residual stress was present, and the geometric ratio of the Knoop indenter was different. A comparison of our results with those from previous studies showed that the conversion factor ratio obtained using our assumption was in good agreement with previous studies. (Received May 20, 2020; Accepted June 15, 2020)

keyword : instrumented indentation, knoop indenter, metallic material, residual stress, plastic behavior

The small punch (SP) test is one of the small specimen test techniques, and standardization of the SP test method for evaluating the mechanical properties of metallic materials is in progress. In this study, the impact transition temperature of reactor pressure vessel steels (RPV) in nuclear power plants was estimated using the draft standard SP test method. The SP fracture energy (ESP) and normalized SP fracture energy (ENSP) of the RPV steels were evaluated at various temperatures, and their transition curves were derived and compared to the transition curve in the Charpy V notch (CVN) test. The SP transition region appeared at a much lower temperature range than that of the CVN owing to the size and notch effect. Ductile brittle transition temperature (DBTT) in the SP transition curve showed a linear relationship with DBTT and T41J in the CVN transition curve. The ductile to brittle transition behaviors of SP specimens were analyzed using fractographs and compared with the transition curves in E_SP and E_NSP. E_NSP started to decrease at the temperature at which the SP ductile to brittle transition behavior occurred, and this means that the E_NSP transition curves were in good agreement with transition behavior in the SP test. However, the E_SP transition curves did not match transition behavior. Using DBTT in the E_NSP transition curve is appropriate to estimate the CVN_DBTT and T_41J. (Received May 21, 2020; Accepted June 24, 2020)

keyword : reactor pressure vessel, small punch test, ASTM WK61832, charpy impact test, transition temperature

In this study, consolidated WC-Fe compacts as a function of Fe contents were fabricated by a spark plasma sintering method, following the mixed by the ball-milled powder. Fe among the metallic binders was added to WC enhance not only the driving force of sintering process but also suppressing the grain growth. WC-Fe mixed powder samples were fabricated with 5, 10 and 15 wt.% Fe contents, and the particle sizes of the mixed powders were determined to be 2.15 to 3.15 μm, respectively. The WC-Fe mixed powders were processed by spark plasma sintering, at a sintering temperature of 1300 oC. Consequently, the relative densities of the WC-5, 10 and 15 wt.% Fe sintered-bodies were about 99.2, 99.5 and 100%, respectively. The grain sizes of the WC-5, 10 and 15 wt.% Fe sintered-bodies were about 0.92, 0.98 and 1.02 μm, respectively. The Fe particles penetrated into the WC particles by dissolving and re-precipitation, and the final sintered bodies were completely densified. The mechanical properties of the WC-Fe sintered-bodies exhibited a hardness up to 1934 kg·mm² and a fracture toughness above 6.88 MPa·m^1/2. The microstructure behavior of the WC-Fe sintered-bodies was investigated in terms of mechanical properties to examine their properties for various Fe contents. In addition, the mechanical and physical properties were compared with the reported values for other sintering-processes, i.e. HFIHS, HIP, etc. (Received January 20, 2020; Accepted June 24, 2020)

keyword : WC-Fe, spark plasma sintering method, microstructure, grain size, mechanical properties

The effect of aging heat treatment conditions on the mechanical properties and microstructure of the base and weld metal of Alloy 282 superalloy was investigated. The aging heat treatment conditions employed in this study were as follows: two-steps (1010 °C for 2 hours plus 788 °C for 8 hours) and one-step at 788 °C, 738 °C, and 688 °C for 4 hours. The base metal with the one-step aged condition exhibited lower hardness but longer creep life than that treated with the two-step aged condition. The base metal subjected to the two-step aging exhibited the highest hardness and the shortest creep life, mainly due to the precipitation of Mo-rich M₆C-type carbides and coarse γ' (Ni₃(Al,Ti)) phase. For the weld metal, regardless of aging heat treatment conditions, creep elongation and life decreased significantly compared to the base metal, due to the coarse effective grain size and inhomogeneous distribution of γ' precipitates. (Received May 27, 2020; Accepted July 01, 2020)

keyword : alloy 282, aging heat treatment, creep, γ

Recently, various worldwide studies have been conducted to improve converter operation by modifying the lance nozzle. In this study, a numerical analysis of the lance nozzle was conducted to improve the dephosphorization efficiency, and to reduce blowing time. The lance nozzle was designed in the form of an abnormal expansion type nozzle capable of increasing the oxygen supply rate. ANSYS FLUENT, a commercial flow analysis program, was used to verify the flow characteristics of the supersonic jet. The nozzle shape was designed according to the ratio of exit diameter to throat diameter, and an analysis was carried out based on the change in inlet flow rate, to confirm the influence of the oxygen supply flow rate. The velocity of the oxygen jet was the fastest for the normal expansion type nozzle and decreased with the abnormal expansion type nozzle. In addition, the oxygen jet velocity tended to increase with the oxygen flow rate. (Received March 23, 2020; Accepted June 17, 2020)

keyword : converter operation, soft blowing, lance nozzle, abnormal expansion nozzle, mathematical model

This study investigated the influence of laser shock peening without coating (LSPw/oC) on the degradation of copper electrical contacts. A theoretical calculation of the plastic-affected depth (PAD) induced by LSPw/oC was performed, based on the laser-induced plasma pressure along with the Hugoniot elastic limit of our LSPw/oC experimental conditions. The theoretical PAD was obtained approximately 650 μm from the surface for the LSPw/oC at the laser energy density of 5.3 GW/cm2. Various characterization methods such as the Vicker’s hardness test, residual stress test, and electron backscattered diffraction (EBSD) mapping indicated the PAD may play a significant role in laser induced effective depth for LSPw/oC. At a laser energy density of 5.3 GW/cm², the laser shock-peened copper showed approximately double the surface hardness as compared to the pure copper. This was attributed to grain refinement, which was confirmed by measuring average grain sizes, and by observing mechanical twin structures from the EBSD analysis. Additionally, a compressive residual stress was induced down to the PAD but gradually switched to a tensile residual stress below PAD. The surface hardening effect conferred by LSPw/oC to the pure copper surface resulted in excellent wear resistance, i.e., a low coefficient of friction and wear loss. As a result, the contact exhibited lower electrical resistance following the fretting friction test compared to pure copper; this would result in a significant delay in electrical contact failure. (Received May 25, 2020; Accepted July 01, 2020)

keyword : laser shock peening without coating, copper, wear resistance, surface hardening, residual stress, contact failure

Assuming that many of the materials produced in a non-equilibrium state remain unchanged, an extended space-energy conservation law was proposed based on the existing energy conservation law. In the present study, by analyzing the well developed equilibrium binary phase diagram of iron (Fe) - carbon (C), we show that energy non-equilibrium microstructures can appear as a part of the equilibrium between the space energy and the mass energy. The correlation between these two energies is objectively and logically explained via (1) one-to-one correspondences between the equilibrium and non-equilibrium phases based on the binary Fe-C phase diagram and (2) the heat-treated Fe-C phases with the spatial energy represented by temperature. Additionally, we found that the morphological and microstructural changes in non-equilibrium states could be consistently explained using the extended energy law as a major premise. This suggests that material factors such as size, distribution, and the shape of materials, which appear to have no energy transfer, are all formed to balance the energy equilibrium with the spatial energy surrounding the materials. Thus, an extended energy conservation law, which can control mass through space or vice versa, can provide a comprehensive logical framework for analyzing various unsolved physicochemical phenomena. (Received May 28, 2020; Accepted June 17, 2020)

keyword : energy equilibrium, phase diagram, mass energy, space energy

Al-Si coated ultra-high strength steels (UHSS) are widely used in automotive applications because of their high strength-to-weight ratio, corrosion protection and good weldability. The hot stamping (also called hot press forming) process is an effective and suitable technique for producing automotive parts from Al-Si coated UHSS. However, critical issues, such as the transfer of the coating materials and the build-up of these materials on the tool surface, have been encountered. These defects affect tool life and product quality. The wear behaviour of Al-Si coated 22MnB5 and Cr coated SKD11 tool was investigated using a high temperature friction test which mimicked the actual hot stamping environment. Two kinds of Al-Si coated 22MnB5 with ~19.6 (AlSi-A) and ~29.3 (AlSi-B) μm coating thicknesses were used in this study. After the friction test the coated layer of AlSi-A was found to be worn up to the diffusion layer, while for AlSi-B, the coating layer mostly remained after the friction test. Adhesive wear predominantly occurred on the tool surface in both cases, but the wear increased significantly in case of AlSi-B. This suggests that the coating thickness and the associated surface roughness are critical factors affecting wear behaviour. (Received June 9, 2020; Accepted June 25, 2020)

keyword : friction, wear, hot stamping, Al-Si coating, ultra-high strength steel