발간논문

Home > KJMM 논문 > 발간논문

Effects of boron on the microstructure and high temperature tensile strength of 9Cr-1.5Mo ferritic steels were investigated. Three alloys having different boron content were manufactured by vacuum induction melting and electro-slag remelting. Boron was segregated in prior austenite grain boundary during normalizing treatment. And this boron were slowly diffused into the grain interior during tempering and aging at 700℃. Major precipitates in these ferritic steels are M23C6 and MX. However, FeMo2B2 was also precipitated in alloy containing 196 ppm. boron. FeMO2B2 phase was formed at prior austenite grain boundary and martensite lath boundary after 1,000 hours aging at 700℃. It was expected that the formation of FeMO2B2 are closely related to the redistribution of boron atoms. The growth rate of FeMO2B2 was higher than those of M23C6 and MX phase. The tensile strength in 9Cr-1.5Mo ferritic steels were increased by increase of boron contents in tempered condition. However, the tensile strength for alloy containing 196 ppm. boron was lower than that for alloy containing 144 ppm. boron after 3,000 hours aging at 700℃. This is due to the formation of FeMO2B2 and its rapid growth rate in alloy containing 196 ppm. boron.

keyword : 9Cr-1.5Mo ferritic steels, Precipitation behavior, Boron effect, FeMo2B2 phase, Tensile strength

The CU50Zr43Al7 monolithic bulk amorphous alloy was prepared to investigate the mechanism of the deformation-induced nanocrystallization. The sample tested under quasistatic uniaxial compression showed a strength of ~2 GPa along with large plastic strains of ~7%. High resolution transmission electron microscopy (HRTEM) conducted on the fractured sample revealed abundant nanocrystallites that were precipitated homogeneously in the amorphous matrix. The deformation-induced nanocrystallization behavior observed from the monolithic amorphous alloy was explained based on the thermodynamic and kinetic viewpoints.

keyword : Deformation-induced nanocrystallite, Uniaxial compression, Amorphous alloy

The effects of intercritical heat treatment(IHT) on microstructures and mechanical properties of weld heat-affected zone(HAZ) in Mn-Mo-Ni low alloy steels were investigated using the simulated HAZ specimens prepared by a gleeble thermal simulator system. The microstructures and mechanical properties of coarse-grained HAZ(CGHAZ) and fine-grained HAZ(FGHAZ) were not influenced by the heat treatment condition before welding, whereas they showed a significant increase in impact toughness after post-weld heat treatment(PWHT). In case of tempered HAZ(THAZ), carbide sizes were slightly increased after welding and PWHT process. However the microstructural improvements by IHT, such as precipitate refinement and spherodization were maintained during the welding and PWHT process, and consequently THAZ with IHT showed better ductile-brittle transition behavior than those with conventional heat treatment(CHT). In conclusion, the application of IHT shows beneficial effects on mechanical properties of both base metal and THAZ.

keyword : Intercritical heat treatment, IHT, Low alloy steel, Heat-affected zone, HAZ, Impact toughness, Ductile-brittle transition

The effects of carbon content on the microstructures and mechanical properties of 1.5Mn-0.3Si1.5Al-0.5Cu TRIP-aided cold-rolled steel sheets with different C contents (0.1, 0.12 and 0.15 wt%) were investigated. The cold-rolled TRIP-aided steel sheets were fabricated via two-step heat treatment route composed of intercritical annealing and isothermal heat treatment. The triple phase microstructures generated during heat treatment, focusing on the retained austenite, were assessed using optical microscopy and X-ray diffraction. The tensile and yield strengths obtained from uniaxial tensile test were increased but elongation was decreased with increasing carbon content, which were attributed to the remarkable increase in the volume fraction of second phases mainly consisted of bainite and retained austenite. The tensile strength and ductility balance has a good correlation with the amount of retained austenite and its characteristic can be explained by means of strain dependency of incremental strain hardening exponent.

keyword : TRIP-aided steel, Retained austenite, Microstructure, Mechanical property

The purpose of this study is evaluation of the effect of flow velocity and oxygen concentration on the corrosion behavior of carbon steel by electrochemical analysis. Electrochemical corrosion behavior of SA106Gr.C steel in neutral water at 150℃ was carried out by employing a high temperature rotating cylinder electrode (HTRCE) and a water chemistry control system. While corrosion current density could be partly controlled by flow velocity below dissolved oxygen concentration of 50 ppb, effect of flow on corrosion rate was diminished above oxygen concentration of 100 ppb. Based on the ECP profile with dissolved oxygen concentration, when the oxygen concentration was over 50 ppb, magnetite oxidized into hematite which has a very low solubility and electric conductivity and a different protective layer formed which can markedly increase in polarization resistance resulting in reduction of corrosion current density.

keyword : Flow accelerated corrosion, Carbon steel, Rotating cylinder electrode, Dissolved oxygen, High temperature corrosion

An improved fluoride ion cleaning (FIC) process, in Which thermal decomposition of teflon powder was employed, was developed for removal of oxide on narrow deep cracks and applied to the oxide on cracks in gas turbine blades. The process is very effective in removal of oxide because teflon in a fluidized form is directly filled upto the cracks. The effectiveness of this improved process was discussed by comparing the results obtained with and without filling the cracks when applied in a conventional FIC reactor using teflon powder.

keyword : Nickel alloy, Oxide, Etching

The majority of magnesium components is generally used in the automotive and electronic industries by conventional casting process. However, there is a strong need to develop high strength casting alloy for wide-spread application of Mg alloys. Grain refinement is most commonly carried out through the addition of master alloy to the melt in order to achieve premium mechanical properties. Thus, the present study was carried out to investigate the influence of Al-3%Ti-0.15%C and Al-5%Ti-1%B master alloys on the grain structure of Mg-9Al-1Zn and Mg-6Al-0.35Mn alloy. It has been found that grain refinement was successful when using small addition of Al-3%Ti-0.15%C and Al-5%Ti-1%B master alloy on the AZ91D and AM60B alloy. The tensile strength and yield strength of AZ91D and AM60B alloys were increased with Al3%Ti-0.15%C and Al-5%Ti-1%B master additions.

keyword : Mg alloy, AZ91D. AM60B, Grain refinement, Al-Ti-C, Al-Ti-B

As the demand for flip chip products increases, the need for high volume, low cost manufacturing processes also increases. Currently solder paste printing is the solder deposition method of choice for device pitches down to 200 ㎛. However limitations in print quality and solder paste volume mean that this technology is not likely to move below this pitch. This paper provides transformation method of flip chip solder bump manufacturing using Si carrier. 150 ㎛ solder bump was formed by this method.

keyword : Flip chip, Transformation, Si carrier

Electroleaching of 53.3%Fe-22.1%Ni-19.2%Cu-5.4%Co alloy has been investigated in sulfuric acid solutions. Leached metal content, except copper, was increased linearly in proportion to its alloying concentration during electroleaching. The surface of leached alloy was covered with porous sludge layer. The results of ICP and XRD analyses verified that the main component of the sludge layer was copper oxides. The current efficiency of electroleaching at 75 mAcm^(-2) and 50℃ was 68.9% in 0.5 M sulfuric acid solution. In contrast, addition of chloride ion in sulfuric acid solution enhances the leaching of copper and prevents the formation of sludge layer. The leaching behavior of copper in the alloy may be changed from copper oxide formation to copper chloride formation and dissolution by the addition of chloride ion in sulfuric acid solutions. The current efficiency at 75 mAcm^(-2) and 50℃ was increased to 88.9% in 0.5M sulfuric acid solution with 0.2 M sodium chloride.

keyword : Fe-Ni-Cu-Co alloy, Electroleaching, Sulfuric acid, Chloride, Polarization