발간논문

Home > KJMM 논문 > 발간논문

Pulsed current activated sintering and spark plasma sintering enhance sinter-ability. The advantage of these processes is that they allow very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured intermetallic compounds, oxides, metal-ceramic composites, high temperature materials and hard materials were produced with a simultaneous application of pressure and a pulsed current within several minutes. The role of the current in sintering or synthesis has been the focus of several studies aimed at explaining enhanced sintering and improved properties. The role played by the current has had various interpretations, with the effect explained in terms of the fast heating rate due to Joule heating, the presence of plasma in pores separating powder particles, and the intrinsic contribution of the current to mass transport.

keyword : Pulsed current, Nanostructured materials, Sintering, Synthesis

Abstract: This study investigated the effects of plasma nitriding on the erosion behavior of AISI 403 stainless steel solid state particles at ambient temperature and 620℃ using Fe3O4 erodent particles at impact angles of 15-90°. The plasma nitriding formed a thick hardened layer which had a high microhardness of 1210 ± 15 Hv. At ambient temperature, AISI 403 suffered from severe erosion damage at low impact angles and exhibited ductile erosive behavior, while the nitrided layer experienced no significant damage at any impact angles. For AISI 403, the erosion rates at 620℃ were ~2 times higher than those obtained at ambient temperature. When eroded at 620℃, the nitrided layer exhibited a ductile erosive manner, with a maximum erosion rate occurring at a low impact angle of 30°. The results from high temperature hardness measurement and the observation of the eroded surface are discussed to explain the high-temperature erosion behavior of the nitrided AISI 403 stainless steel.

keyword : metals, surface modification, erosion, hardness test, plasma nitriding

Abstract: In this study, AZ31B Mg alloy samples were rolled in two different routes. One sample was normally rolled by maintaining the rolling direction (RD) and the other one was cross rolled by rotating the RD once by 90° around the normal direction (ND). Deformed bands, similar to shear, were observed in response to increasing deformation in the sheets. The texture was split by approximately 10-15° from the ND toward the RD in both sheets. The degree of the split texture rotation depended on the direction of the deformed bands. The grains in the deformed bands had a weak texture owing to the rotated c-axis of the hexagonal close-packed (hcp) structure. The texture affected the mechanical properties because of the easy basal slip for the observed split basal texture. Cross rolling definitely lowered the total elongation which was attributed to the frequent development of deformation bands in the cross rolled sample. It turned out that occurrence of a large number of compression twins in the cross rolled sample accelerated hardening of the AZ31B Mg alloy during the tensile deformation. The r-value of the cross-rolled sample was higher than that of the normal-rolled sample.

keyword : alloys, rolling, texture, EBSD, mechanical properties

Abstract: Effect of aging heat treatment was investigated in a the weldment of heavy-sectioned SA335 F22V pressure vessel steel for an advanced fossil power plant. Aging heat treatment was carried out at the 454 ℃ and 482 ℃ up to 10000 hours and then microstructural analysis and mechanical tests, namely tensile and impact tests, were performed. The results showed that aging heat treatment did not affect the microstructure and microhardness of the heat affected zone. Although temper embrittlement around the heat affected zone showing a decrease in toughness and an increase in transition temperature its susceptibility was comparable to that at the base material, which is due to the optimized content of alloying elements like V and P.

keyword : metals, welding, microstructure, tensile test, F22V

"Abstract: The influence of annealing treatment on the interface-correlated microstructural evolution and subsequent mechanical properties of an Al1050/AZ31B clad sheet was systematically investigated. A scanning electron microscope with an attached energy dispersive spectroscopy revealed that diffusive layer consisted of γ (Mg17Al12), and β (Mg2Al3) phases was generated and grown with increasing annealing time and temperature. Mechanical properties were evaluated by uniaxial tensile and peel tests. Improvement of bonding strength between Al1050 and AZ31B by feasible annealing seemed to be strongly related to the generation of metallurgical bonding by a diffusive intermetallic compound layer whose overall thickness was limited to below 1.4 μm."

keyword : composites, rolling, interfaces, scanning electron microscopy, SEM, mechanical properties

Abstract: Sulfur-containing species are one of the major impurities in the secondary side of PWR (Pressurized Water Reactor) nuclear power plants. These come from feed water, resin resources, cooling water leakage, and condenser leakage. In the operating conditions of a steam generator, SO4 2. ions can be reduced to S2., S2O3 2., S4O6 2. by hydrazine. This work focused on stress corrosion cracking (SCC) of mill annealed, sensitized, and thermally treated Alloy 600 in a Na2S4O6 solution. Analysis of carbide distribution, intergranular corrosion tests, including an electrochemical method, and electrochemical noise measurement during an SCC test were performed to determine SCC susceptibility of Alloy 600. Alloy 600 MA did not reveal SCC but Alloy 600 SE+ and Alloy 600 TT showed severe IGSCC. Sensitization and thermal treatment to Alloy 600 MA degraded the intergranular corrosion resistance and thus induced intergranular stress corrosion cracking in the Na2S4O6 solution and its SCC mechanism seems to be a pre-existing active path model.

keyword : Metals, Annealing, SCC, Electrochemistry, optical microscopy

Abstract: Nanocrystalline materials have received much attention as advanced engineering materials due to improved physical and mechanical properties. As nanomaterials undoubtedly possess high strength and hardness, as well as excellent ductility and toughness, more attention has been paid to the application of nanomaterials. Nanopowders of Mo, W and Si were made by high-energy ball milling. A dense nanostructured (W,Mo)Si2 compound was sintered by the pulsed current activated combustion method within 2 minutes from mechanically activated powders of Mo, W and Si. A highly dense (Mo,W)Si2 compound was produced under simultaneous application of a 80 MPa pressure and the pulsed current. Mechanical properties and microstructure were investigated. The hardness and fracture toughness of (Mo,W)Si2 were 1242 kg/mm2 and 4.3 MPa.m1/2, respectively. The mechanical properties were better than monolithic MoSi2 or WSi2.

keyword : Compounds, Nanostructured materials, Sintering, Mechanical properties

Abstract: This study investigated the effect of the aging time of weldment of 24Cr-3.5Mo duplex stainless steel on the microstructure and corrosion behavior. After performing FCAW, we carried out heat treatments at varying times at 850 ℃ and performed observation of microstructure, potentio dynamic test, SEM-EDS analysis, and X-ray diffraction analysis. As the aging time increased, the fraction of δ-ferrite decreased sharply, but the fraction of γ slightly increased. The σ phase was generated at a non-metallic inclusion along the grain boundaries of δ-ferrite and γ, while the χ phase was generated in the structure of δ-ferrite. As the intermetallic compounds increased, the critical pitting potential fell sharply, and PREN of the surrounding structure decreased by 5 due to precipitation of the σ phase in 3.5% NaCl at 60 ℃. Pitting occurred intensively under a multi-pass line which relatively had more intermetallic compounds, and the precipitation of the σ phase caused the formation of Cr carbide.

keyword : duplex stainless steel, welding, intermetallic compound, EBSD

Abstract: The variations in characteristics of deformation and microstructures along the thickness direction of a Cu-Fe-P copper alloy by an unidirectional shear rolling (USR) were investigated in detail. The copper alloy sample with a thickness of 3 mm was rolled to 50% reduction at ambient temperature without lubrication at a differential speed ratio of 2.0:1, and then rolled again to 50% reduction in a different rolling direction. The conventional rolling was also performed at the same rolling conditions for the same initial material for comparison. The shear strain showed positive values at all positions for the samples rolled by the USR. The effects of USR on microstructure and mechanical properties of the as-rolled materials are discussed, comparing them to those of the conventional rolling.

keyword : differential speed rolling, copper alloy, strain, microstructure, mechanical property

The effects of densification on the magnetic properties were studied in Fe-based amorphous powder cores mixed with pure iron powder. To normalize the effect of powder size on magnetic properties, the rapidly solidified amorphous powders were sieved and classified. The amorphous powders of .150 to +270 mesh were mixed with the pure iron powder of 3 μm in diameter up to 20 wt%. The powder composites were mixed with 2 wt% water glass diluted by DI Water at a ratio of 1:2. For the consolidation, the powder composites were compacted into a cylindrical die with lubricant to form toroidal cores, which were subsequently heat-treated at 430-460 ℃ for 10 minutes. The soft magnetic properties of the powder cores were investigated using a B-H curve analyzer and a flux meter at the frequency range of 10-100 kHz. The microstructure of the mixed powders and the compacted cores were observed using FE-SEM. It was notable that the density of the powder composites increased with an increase of the pure iron powders. As a result, the permeability and the core loss of the composite cores (especially, mixed with ~10 wt% pure iron powders) were greatly improved over those of conventional amorphous powder cores.

keyword : amorphous, magnetic meterials, powder processing, magnetic properties, scanning electron microscopy, SEM