• Title/Summary/Keyword: Elongated grains

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Grain Refinement and Phase Transformation of Friction Welded Carbon Steel and Copper Joints

  • Lee, W.B.;Lee, C.Y.;Yeon, Y.M.;Kim, K.K.;Jung, S.B
    • International Journal of Korean Welding Society
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    • v.3 no.2
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    • pp.46-52
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    • 2003
  • The refinement of microstructure and phase transformation near the interface of pure copper/carbon steel dissimilar metals joints with various friction welding parameters have been studied in this paper. The microstructure of copper and carbon steel joints were changed to be a finer grain compared to those of the base metals due to the frictional heat and plastic deformation. The microstructure of copper side experienced wide range of deformed region from the weld interface and divided into very fine equaxied grains and elongated grains. Especially, the microstructures near the interface on carbon steel were transformed from ferrite and pearlite dual structure to fine ferrite, grain boundary pearlite and martensite due to the welding thermal cycle and rapid cooling rate after welding. These microstructures were varied with each friction welding parameters. The recrystallization on copper side is reason for softening in copper side and martensite transformation could explain the remarkable hardening region in carbon steel side.

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R-Curve Behavior in a Gas-Pressure Sintered Silicon Nitride (가스압 소결된 질화규소의 R-Curve 거동)

  • 김상섭;김성진;백성기
    • Journal of the Korean Ceramic Society
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    • v.29 no.12
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    • pp.949-955
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    • 1992
  • R-curves, fracture resistance (KR) as a function of crack extension (Δa), of a gas-pressure sintered monolithic Si3N4 were determined by controlled flaw/strength technique. Rising R-curve behavior was observed, confirming the operation of microstructural toughening process during crack growth. The R-curve parameters, k and m in the equation, KR=k(Δa)m, were determined to 30.301 and 0.1146, respectively. Microstructural observation of growing crack revealed that the bridging in the crack wake by unbroken ligament of large elongated ${\beta}$-grains was the mechanism primarily for the rising R-curve behavior.

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The Fabrication of $Si_3N_4/SiC$ Nano-Composite ($Si_3N_4/SiC$ Nano Composite의 제조)

  • Lee, Su-Yeong;Lee, Han-Seop
    • 연구논문집
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    • s.23
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    • pp.165-171
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    • 1993
  • $Si_3N_4/Sic$. nano-composites were fabricated by hot-pressing, gas pressure sintering. The composites contained up to 50 wt. % of SiC. The mechanical properties such as strength, toughness, and hardness of the composite are compared each other. The flexural strength of the composites was improved significantly by introducing fine SiC particles into $Si_3N_4$ matrix, while the fracture toughness was not improved. The increase in flexural strength is attributed to the formation of uniformly elongated $\beta -Si_3N_4$ grains as well as the reduction of grain size.

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Effect of Annealing Conditions on Microstructures and Mechanical Properties of a 5083 Al Alloy deformed at Cryogenic Temperature (어닐링 조건이 극저온 압연 5083 Al Alloy의 미세조직 및 기계적성질에 미치는 영향)

  • 이영범;남원종
    • Transactions of Materials Processing
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    • v.13 no.5
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    • pp.449-454
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    • 2004
  • The annealing behavior of a 5083 Al alloy deformed at cryogenic temperature was investigated, focusing on the evolution of microstructures and mechanical properties. Especially, the effects of annealing temperature, $150~300^{\circ}C$, and time, 3∼60min., on microstructures and mechanical properties of the sheets received 85% reduction at cryogenic temperature were investigated. The optimization of the annealing conditions resulted in a mixture of equiaxed grains and elongated subgrains, exhibiting a good combination of uniform elongation and high strength.

Microstructure and Polytype of in situ-Toughened Silicon Carbide

  • Young Wook Kim;Mamoru Mitomo;Hideki Hirotsuru
    • The Korean Journal of Ceramics
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    • v.2 no.3
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    • pp.152-156
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    • 1996
  • Fine (~0.09 $\mu$m) $\beta$-SiC Powders with 3.3wt% of large (~0.44$\mu$m) $\alpha$-SiC of $\beta$-SiC particles (seeds) added were hotpressed at 175$0^{\circ}C$ using $Y_2O_3$, $Al_2O_3$ and CaO as sintering aids and then annealed at 185$0^{\circ}C$ for 4 h to enhance grain growth. The resultant microstructure and polytypes were analyzed by high resolution electron microscopy (HREM).Growth of $\beta$-SiC with high density of microtwins and formation of ${\alpha}/{\beta}$ composite grains consisting of $\alpha$-SiC domain sandwiched between $\beta$-SiC domains were found in both specimens. When large $\alpha$-SiC (mostly 6H) seeds were added, the $\beta$-SiC transformend preferentially to the 6H polytype. In contrast, when large $\beta$-SiC (3C) seeds were added, the fine $\beta$-SiC transformed preferentially to the 4H polytype. Such results suggested that the polytype formation in SiC was influenced by crystalline form of seeds added as well as the chemistry of sintering aids. The ${\alpha}/{\beta}$ interface played and important role in the formation of elongated grains as evidenced by presence of ${\alpha}/{\beta}$ composite grains with high aspect ratio.

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Occurrences of Asbestos within Gapyeong Serpentinite Mines and Characteristics of Host Rocks (가평 사문암 광산의 석면 산출의 산상과 모암의 특성)

  • Song, Suckhwan;Kang, Joongu
    • Journal of Korean Society of Occupational and Environmental Hygiene
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    • v.26 no.3
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    • pp.253-266
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    • 2016
  • Objectives: This study is for characteristics of asbestos occurrence (NOA, naturally occurring asbestos) from the Gapyeong area and its host rocks, serpentinites. Methods: Representative samples are collected from the serpentinite bodies, following degrees of hydrothermal alteration and metamorphism, after about 2 year field trips. Mineralogical, morphological and optical characteristics of the asbestos and host rocks are confirmed by PLM, XRD EPMA and SEM results. Results: The serpentinites are dunites and harzburgites, and host asbestos, including chrysotile, tremolite and actinolite. The asbestos chrysotiles are found as veins ranging from several millimeters to several centimeters in thickness, while asbestos-tremolite and -actinolite occur along cracks and fractures ranging up to ten centimeters in thickness. The chryostiles occur mainly as cross and slip fibers, while the amphibole asbestos is found as vein, slip and oblique fibers. More tremolitic grains are colorless and commonly show elongated or fiber shapes, whereas the magnesio hornblende grains mainly show light green and occur as subhedral to euhedral diamond grains. Conclusions: Overall characteristics of serpentinites from the Gapyeong area are similar to worldwide orogenic-related Alpine type ultramafic rocks serpentinized and serpentinites in South Chungcheong-do Province, Korea, and occurrences of asbestos are similar to those of the ultramafic bodies in South Chungcheong-do Province.

Microstructure Development of Spark Plasma Sintered Silicon Carbide with Al-B-C (Al-B-C 첨가 탄화규소의 스파크 플라즈마 소결에 의한 미세구조 발달)

  • Cho, Kyeong-Sik;Lee, Kwang-Soon;Lee, Hyun-Kwuon;Lee, Sang-Jin;Choi, Heon-Jin
    • Journal of the Korean Ceramic Society
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    • v.42 no.8 s.279
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    • pp.567-574
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    • 2005
  • Densification of SiC powder with additives of total amount of2, 4, 8 $wt\%$ Al-B-C was carried out by Spark Plasma Sintering (SPS). The unique features of the process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. The heating rate and applied pressure were kept at $100^{\circ}C/min$ and 40 MPa, while the sintering temperature and holding time varied from 1700 - $1800^{\circ}C$ for 10 - 40 min, respectively. The SPS-sintered specimens with different amount of Al-B-C at $1800^{\circ}C$ reached near-theoretical density. The $3C{\rightarrow}6H,\;15R{\rightarrow}4H$ phase transformation of SiC was enhanced by increasing the additive amount. The microstructure of SiC sintered up to $1750^{\circ}C$ consisted of fine equiaxed grains. In contrast, the growth of large elongated grains in small matrix grains was shown in sintered bodies at $1800^{\circ}C$, and the plate-like grains interlocking microstructure had been developed by increasing the holding time at $1800^{\circ}C$. The grain growth rate decreases with increasing amount of Al-B-C in SiC starting powder, however, the both of volume fraction and aspect ratio of large grains in sintered body increased.

Effect of $\alpha-SiC $seed on microstructure and fracture toughness of pressureless-sintered $\beta-SiC$ ($\alpha-SiC $seed의 첨가가 상압소결된 $\beta-SiC$의 미세구조와 파괴인성에 미치는 영향)

  • Young-Wook Kim;Won-Joong Kim;Kyeong-Sik Cho;Heon -Jin Choi
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.7 no.1
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    • pp.18-26
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    • 1997
  • $\beta-SiC $powder with or without the addition of 1 wt% of $\alpha-SiC$ particles (seeds) was pressureless-sintered at $1950^{\circ}C$ for 0.5, 2 and 4 h using $Y_3Al_5O_{12}$ (yttrium aluminum garnet, YAG) as a sintering aid. The introduction of $\alpha-SiC$ seeds into $\beta-SiC$ accelerated :he grain growth of elongated large grains during sintering, resulting in the coarser microstructure. The fracture toughnesses of materials with $\alpha$-SiC seeds and without $\alpha-SiC$ seeds sintered for 4 h were 7.5 and 6.1 $MPa\cdot \textrm m^{1/2}$, respectively. Higher fracture toughness of the material with seeds was due to the enhanced bridging by elongated grains, resulting from coarser microstructure.

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Crack Path Behavior of SiC Based Tools for Spectacle Lens Cutting (렌즈절삭용 탄화규소계 공구의 크랙전파 거동)

  • Lee, Young-Il
    • Journal of Korean Ophthalmic Optics Society
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    • v.11 no.2
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    • pp.85-89
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    • 2006
  • To research of the improved mechanical properties of materials for spectacle lens cutting, SiC and TiC were used as the main powder. Also, $Al_2O_3$ and $Y_2O_3$ was included as a sintering additive. The weight ratio of the alumina($Al_2O_3$) to yttria($Y_2O_3$) was set to 1:1. The materials for spectacle lens cutting were fabricated by hot-pressing at $1810^{\circ}C$ for 1h and subsequently annealed at $1860^{\circ}C$ for 3, 6 and 12h to initiated grain growth. The longer annealing time is, the bigger the grain size is. The microstructures were observed by scanning electron microscopy (SEM). The SEM images were quantitatively analyzed by image analysis (Image-Pro Plus, Media Cybernetics, Maryland, U.S.A.). Crack deflection by elongated SiC grains was most frequently observed as the dominant toughening mechanism. Crack deflection was generally observed for elongated SiC grains with aspect ratio(AR) > 2.5 and grain thickness < $2.3{\mu}m$. Crack bridging was also observed as one of the operating toughness mechanism.

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Effect of Electrical Resistance Welding on Microstructure and Mechanical Properties of API X70 Linepipe Steel (ERW 용접 전후 API X70 라인파이프강의 미세조직과 기계적 특성 변화)

  • Oh, Dong-Kyu;Choi, Ye-Won;Shin, Seung-Hyeok;Jeong, Han-Gil;Kwack, Jin-Sub;Hwang, Byoungchul
    • Journal of the Korean Society for Heat Treatment
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    • v.35 no.4
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    • pp.185-192
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    • 2022
  • Variations in the microstructure and mechanical properties of API X70 steel processed by piping, electrical resistance welding (ERW), and post seam annealing (PSA) are investigated in this study. In the welding zone, some elongated pearlites are formed and grains coarsening occurs due to extra heat caused by the ERW and PSA processes. After the piping, the base metal shows continuous yielding behavior and a decrease in yield and impact strengths because mobile dislocation and back stress are introduced during the piping process. On the other hand, the ERW and PSA processes additionally decreased the impact strength of welding zone at room and low temperatures because some elongated pearlites easily act as crack initiation site and coarse ferrite grains facilitate crack propagation. As a result, the fracture surface of the welding zone specimen tested at low temperature revealed mostly cleavage fracture unlike the base metal specimen.