• Journal of Korea Foundry Society
• /
• v.18 no.6
• /
• pp.541-549
• /
• 1998

Microstructural characteristics of hypereutectic Al-Si alloys during reheating at semi-solid temperature have been investigated. The size and morphology of primary Si particles in wedge-type mold-cast ingot has been compared with hot-rolled sheet and Si particulate reinforced Al composite. Effects of P and Sr addition on the morphological changes of primary Si particles have been also investigated. Observation of the solidification microstructures of the wedge-type mold-cast ingot at different cooling rates showed that alloying elements such as P and Sr affect the morphology of Si particles, especially in the area solidified at a slow cooling rate. Negligible change in the size of primary crystals was observed after reheating experiment, but ${\alpha}-halo$ formed around the Si particles and fine particles of Si precipitated in the surrounding area of the Si particles. In addition, there seemed to be no coarsening with increasing of holding time and the region of ${\alpha}-halo$ being decreased. Nucleation and recrystallization was accelerated with addition of alloying elements during hot rolling resulting in a decrease of primary Si particle size. In the case of extruded specimens, morphological change of primary Si particles was not observed after reheating. No ${\alpha}-halo$ formation was observed in Si reinforced Al composite because of the oxide film formed on the Si particles which acted as a diffusion barrier between substrate and the primary Si particles.

• Korean Journal of Materials Research
• /
• v.13 no.9
• /
• pp.581-586
• /
• 2003

3%C-18%Cr-1%Mo-2%Ni-1%Mn high chromium cast iron was casted and destabilized at temperatures of 900, 1000 and $1100^{\circ}C$ for 1, 2, 4 and 8hr under $N_2$atmosphere to observe the effect of destabilization temperature and time on the carbide and matrix structures. In as-cast condition, the microstructure consisted of $M_{ 7}$$C_3$ carbides and matrix structures which were composed of 91.50% austenite and 8.50% martensite. Numerous fine secondary carbides were observed in the specimens destabilized at $900^{\circ}C$ for 1, 2, 4 and 8hr. But, the number of secondary carbides were much reduced with the increased destabilization temperature. More austenite was formed in the matrix with the higher destabilization temperature. The amounts of austenite in the matrix were 4.23% at $900^{\circ}C$, 29.68% at $1000^{\circ}C$ and 66.51% at$ 1100^{\circ}C$, respectively. However, the effect of destabilization time on the secondary carbide and matrix was very weak compared with that of destabilization temperature. The ranges of the amount of austenite in the matrix from 1hr to 8hr destabilization heat treatment were: 3.95%-4.35% at $900^{\circ}C$, 28.89%-30.15% at $1000^{\circ}C$ and 65.13%-67.12% at $1100^{\circ}C$, respectively. The variation ranges were very narrow. The equilibrium concentration of C and Cr in austenite was already reached within 1hr during destabilization heat treatment. After an attainment of the equilibrium concentration of C and Cr in austenite, no more secondary carbide was precipitated from the matrix.

• Journal of Advanced Marine Engineering and Technology
• /
• v.36 no.1
• /
• pp.78-84
• /
• 2012

Mold may experience deterioration of molding quality as the abrasion of mold occurring due to friction between mold and molded product works as allowance in the course of press process. Therefore, to improve the wear-resistance of mold, methods like nitriding, carburizing, flame and induction surface hardening treatment etc have been applied. However, as such methods are accompanied by problems like shape limitation or product deformation etc, laser surface treatment technology is under review as surface treatment method that can solve such problems. Therefore, in this study, mold material cast iron was surface-treated by using high power diode laser. In previous report 1 and report 2, the heat treatment characteristics were compared by the differences of die materials and shapes, then this paper observed microstructure by using optical microscope and scanning electronic microscope to analyze the structural difference of hardened zone, interface area and base metal after heat treatment. And the structural condition was grasped through EDS. As a result of microstructure, hardened zone showed formation of acicular martensite.

• International Journal of Advanced Culture Technology
• /
• v.3 no.2
• /
• pp.132-137
• /
• 2015

The purpose of this paper was to investigate the effects of solution treatment time and Sr-modification on the microstructure and property of the Al-Si piston alloy. It was found that as-cast microstructures of unmodified and Sr-modified Al-Si alloys consisted of a coarse acicular plate of eutectic Si, $Cu_3NiAl_6$ and $Mg_2Si$ phases in the ${\alpha}$-Al matrix but different in size and morphology. Both size and inter-particle spacing of Si particles were significantly changed by increasing of the solution treatment time. After a short solution treatment, the coarse acicular plate of the eutectic Si appears to be fragmented. Fully modified microstructure of Sr-modified alloy can reduce the solution treatment time to shorter compared to unmodified alloy. The maximum of a peak hardness value is found in the very short solution treatment of both Al-Si piston alloys. Compared to 10 h solution treatment, the solution treatment of 2-4 h is sufficient to achieve appropriate microstructures and hardness. The short solution treatment is very useful to increase the productivity and to reduce the manufacturing cost of the Al-Si piston alloys.

• Journal of the Korean Society for Heat Treatment
• /
• v.31 no.5
• /
• pp.220-230
• /
• 2018

The objective of this study is to investigate the effect of solution treatment on the microstructure and corrosion behavior of cast AZ91-4%RE magnesium alloy. In the as-cast state, microstructure of the AZ91-4%RE alloy was characterized by intermetallic ${\beta}(Mg_{17}Al_{12})$, $Al_{11}RE_3$ and $Al_2RE$ phase particles distributed in ${\alpha}-(Mg)$ matrix. After solution treatment, the ${\beta}$ particles with low melting point dissolved into the matrix, but Al-RE phases still remained due to their high thermal stabilities. It was found from the immersion and potentiodynamic polarization tests that corrosion rate of the AZ91-4%RE alloy increased after the solution treatment. On the contrary, EIS tests and EDS compositional analyses on the surface corrosion products indicated that the stability of the corrosion product was improved after the solution treatment. Examinations on the corroded microstructures for the ascast and solution-treated samples revealed that dissolution of the ${\beta}$ particles which play a beneficial role in suppressing corrosion propagation, would be responsible for the deterioration of corrosion resistance after the solution treatment. This result implies that the microstructural features such as amount, size and distribution of secondary phases that determine corrosion mechanism, are more influential on the corrosion rate in comparison with the stability of surface corrosion product.

• Journal of the Korean Magnetics Society
• /
• v.14 no.5
• /
• pp.196-200
• /
• 2004

In order to improve dendritic formation of Nd$_2$Fe$\_$14/B phase in the strip-cast Nd-Fe-B alloys that are frequently used for production of high energy sintered magnets, effect of small substitutional additives such as Cu (0.3～1.0 at. %) and Co (0.5～1.5 at.%) on the phase formation and microstructures of the strip-cast alloys based on Nd$\_$14/Fe$\_$80/B$\_$6/ were investigated. As the amount of Cu addition increased, formation of Nd$_2$Fe$\_$14/B dendrites along the direction normal to the strip surface was suppressed with the reduction of the strip thickness mainly due to the increase of fluidity of the melt. However, both the dendrites and their <001> preferred orientation along the direction normal to the strip surface were improved with the increase of the strip thickness as the amount of Co addition increased. The dendrites became finer after small amount of Cu or Co was added. While small copper additions tended to stabilize the formation of primary Fe, small cobalt additions suppressed it. When small amount of Zr was added, however, the dendrite structures were totally collapsed with excessive grain growth of Nd$_2$Fe$\_$14/B.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.28 no.12
• /
• pp.1968-1973
• /
• 2004

Structural degradations are often experienced on the components of nuclear power plants in reactor pressure vessels (RPV) and steam generators (SG) when these components are exposed to high temperature and high pressure for a long period of time. Such conditions result in the change of microstructures and of mechanical properties of materials, which requires an evaluation of the safeguards related to structural integrity. In a primary reactor cooling system (RCS), a dissimilar weld zone exists between cast stainless steel (CF8M) in a pipe and low-alloy steel (SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time under the operating temperature between 290 and 33$0^{\circ}C$. Under the same conditions, it is well known that degradation is not observed in low alloy steel. An investigation of the effect of thermal aging on the various mechanical properties of the dissimilar weld zone is required. The purpose of the present investigation is to find the effect of thermal aging on the dissimilar weld zone. The specimens are prepared by an artificially accelerated aging technique maintained for various times at 43$0^{\circ}C$, respectively. Then, The various mechanical test for the dissimilar welds are performed.

• Journal of Korea Foundry Society
• /
• v.34 no.1
• /
• pp.6-13
• /
• 2014

The mechanical properties of low-pressure die cast (LPDC) A356-T6 automotive road wheels are evaluated and correlated with their corresponding microstructures. In this study, two types of alloy wheels processed using different LPDC gating system are investigated. The yield stress, tensile stress, and elongation values tested at room temperature are correlated with the secondary dendrite arm spacing (SDAS) with respect to the gating system, and are also compared with similar studies. The SDAS and precipitates are examined using optical microscopy, scanning electron microscopy (SEM) and energy-dispersive spectroscopy. The phase information is also investigated using X-ray diffraction. Charpy impact tests are also performed from $-100^{\circ}C$ to $200^{\circ}C$, and the fracture surfaces are examined using SEM. The impact energy is demonstrated to increase with increasing temperatures without exhibiting specific transition behaviors as in other nonferrous alloys. The fracture toughness is also evaluated using three point bend test with single-edged bend specimens. The obtained fracture toughness values are in good agreement with those in similar studies.

• Journal of the Korean Society for Heat Treatment
• /
• v.11 no.1
• /
• pp.46-53
• /
• 1998

The variation of the mechanical properties, microstructures and the formation of retained austenite with heat treatment conditions in austempered ductile cast iron has been investigated. In the case of austempered ductile cast iron below 25mm diameter, it has been found that a pearlite structure are not obtained under a super cooled condition at range of $0.05^{\circ}C/sec{\sim}10^{\circ}C/sec$, and the matrix is precipitated in graphite, bainite and retained austenite. After austempering treatment the retained austenite is increased with decreasing cooling rate. The elongation increases with decreasing super cooling rate, and the optimum result has been shown to be the elongation of 15.6% at super cooling rate of $0.05^{\circ}C/sec$. The optimum result has been shown to be the tensile strength-elongation balance of $1656kgf/mm^2.%$ and it is more than doubled to as the casting state and continuous cooling condition.

• Journal of Power System Engineering
• /
• v.6 no.1
• /
• pp.74-81
• /
• 2002

The characteristics of corrosion resistance for the surface of ductile cast iron(GCD40) by plasma nitriding process have been studied in terms of electrochemical polarization behaviors including corrosion potential($E_{corr}$), anodic polarization trends, polarization resistance($R_p$), and also have been studied microstructures, hardness and specific wear of nitrided layer Nitrided layer showed an enhanced hardness values in all the plasma nitriding condition investigated. In the result of wear test, specific wear of nitrided specimens were much decreased than that of non-treated specimens. In the results of XRD, ${\gamma}'phase\;and\;{\varepsilon}$ phase were detected in nitrided surface. And it was found that ${\varepsilon}$ phase was decreased and ${\gamma}'phase$ was increased respectively, as the nitriding time became longer. In the test of corrosion resistance, natural potentials in all the nitrided specimens were towards noble directions than in the case of non-treated specimens. The measurement of electrode potentials revealed that corrosion resistivity of plasma nitrided specimens were higher than in the case of the non-treated specimens.