• Title/Summary/Keyword: cementite precipitation

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Modeling of Cementite Precipitation Kinetics on Solute Carbon Content in Extra and Ultra Low Carbon Steels (극저탄소강의 고용 탄소 함량에 미치는 시멘타이트 석출 속도 모델링)

  • Choi, Jong Min;Park, Bong June;Kim, Sung Il;Lee, Kyung Sub;Lee, Kyung Jong
    • Korean Journal of Metals and Materials
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    • v.48 no.3
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    • pp.187-193
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    • 2010
  • The solute carbon content in ferrite is one of the important factors to obtain good formability in low carbon steels. Although most of the carbons are consumed by the formation of grain boundary cementite during coiling after hot-rolling, the carbon content after coiling is normally observed much more than that of equilibrium. In this study, a classical nucleation and growth model is used to simulate the precipitation kinetics of the grain boundary cementite from coiling temperature (CT) to room temperature (RT). The predicted precipitation behaviors depending on the initial carbon content and the cooling rate are compared with the reported. As a result, the lateral growth of thickening of cementite is a major factor for the sluggish reaction of grain boundary cementite. The reduction of solute carbon content after coiling is divided into three regions: a) increase due to no cementite precipitation, b) decrease due to the fast length-wise growth of cementite, c) increase due to the slow thickness-wise growth of cementite.

Effect of Cementite Precipitation on Carburizing Behavior of Vacuum Carburized AISI 4115 Steel (진공침탄에 의한 AISI 4115강의 침탄 거동에 미치는 세멘타이트 석출의 영향)

  • Gi-Hoon Kwon;Hyunjun Park;Yoon-Ho Son;Young-Kook Lee;Kyoungil Moon
    • Journal of the Korean Society for Heat Treatment
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    • v.36 no.6
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    • pp.402-411
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    • 2023
  • In order to examine the effect of cementite precipitated on the steel surface on the carburizing rate, the carburizing process was carried out at various boost times to measure the mass gain and carbon flux, phase analysis and carbon concentration analysis were performed on the surface of the carburized specimen. In the case of the only boost type, the longer the boost time, the more the mass gain by the diffused carbon follows the parabolic law and tends to increase. In particular, as the boost time increased, the depth of cementite precipitation and the average size of cementite on the steel surface increased. At a boost time of 7 min, the fraction of cementite precipitated on the surface is 7.32 vol.%, and the carburizing rate of carbon into the surface (surface-carbon flux) is about 17.4% compared to the calculated value because the area of the chemical (catalyst) where the carburization reaction takes place is reduced. The measured carbon concentration profile of the carburized specimen tended to be generally lower than the carbon concentration calculated by the model without considering precipitated cementite. On the other hand, in the pulse type, the mass gain by the diffused carbon increased according to the boost time following a linear law. At a boost time of 7 min, the fraction of cementite precipitated on the surface was 3.62 vol.%, and the surface-carbon flux decreased by about 4.1% compared to the calculated value. As a result, a model for predicting the actual carbon flux was presented by applying the carburization resistace coefficient derived from the surface cementite fraction as a variable.

Effect of Microstructure Change According to Tempering Temperature on Room Temperature Tensile Properties in Carbon Steel of SM30C (SM30C의 탄소강에서 템퍼링 온도에 따른 미세조직 변화가 상온 인장특성에 미치는 영향)

  • Yebeen Ji;Kibeom Kim;Jung jong Min;Kwonhoo Kim
    • Journal of the Korean Society for Heat Treatment
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    • v.36 no.1
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    • pp.1-6
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    • 2023
  • In order to process plastic with similar mechanical performance to metal materials, it is necessary to improve the strength and hardness of core parts of the injection equipment in extrusion system. The tempering process is a heat treatment performed to reduce brittleness and improve elongation along with improvement of dimensional defects of martensite formed after quenching. In this study, changes in microstructure and mechanical properties according to temperature were evaluated after quenching and tempering of SM30C material. As a result, the strength and hardness were gradually decreased by tempering at 250~400℃, and the decrease was greatly increased under the tempering condition at 450℃. Under the tempering condition of 200~400℃, the main structure was lath martensite, and the precipitation amount and size of needle-shaped cementite increased along the lath with the increase of the tempering temperature. Most of the shape of cementite has a needle-like structure, and the formation of some spherical cementite is observed. Under the tempering condition of 450℃, a mixed structure of ferrite and martensite was formed according to the decomposition of martensite.

Effects of Si Content and Austempering Conditions on Properties of High Carbon Cast Steel (고탄소강의 특성에 미치는 규소 함량 및 오스템퍼링 조건의 영향)

  • Kim, Won-Bae;Kim, Myung-Sik;Kim, Jong-Chul;Sohn, Ho-Sang;Ye, Byung-Joon
    • Journal of Korea Foundry Society
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    • v.25 no.2
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    • pp.95-101
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    • 2005
  • This study has been carried out to investigate the microstructure of austempered high carbon cast steel with the variation of silicon and heat treatment conditions. The results show that an lower ausferritic structure is formed at the low austemepring temperature ($250{\sim}300^{\circ}C$) and an upper ausferritic structure is formed at the high austemepring temperature ($350{\sim}400^{\circ}C$). As an austempering temperature increased, the retained austenite volume fraction increased, however hardness decreased. Also, as a silicon content increased, the precipitation of cementite was suppressed, therefore 2nd reaction of autempering transformation was delayed.

Effect of Post-Weld Heat Treatment on the Mechanical Properties and Microstructure of P-No. 1 Carbon Steels (P-No. 1 탄소강의 기계적 특성과 미세조직에 미치는 용접후열처리의 영향)

  • Lee, Seung-Gun;Kang, Yongjoon;Kim, Gi-Dong;Kang, Sung-Sik
    • Journal of Welding and Joining
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    • v.35 no.1
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    • pp.26-33
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    • 2017
  • This study aims to investigate the suitability of requirement for post-weld heat treatment(PWHT) temperature when different P-No. materials are welded, which is defined by ASME Sec. III Code. For SA-516 Gr. 60 and SA-106 Gr. B carbon steels that are typical P-No. 1 material, simulated heat treatment were conducted for 8 h at $610^{\circ}C$, $650^{\circ}C$, $690^{\circ}C$, and $730^{\circ}C$, last two temperature falls in the temperature of PWHT for P-No. 5A low-alloy steels. Tensile and Charpy impact tests were performed for the heat-treated specimens, and then microstructure was analyzed by optical microscopy and scanning electron microscopy with energy-dispersive spectrometry. The Charpy impact properties deteriorated significantly mainly due to a large amount of cementite precipitation when the temperature of simulated heat treatment was $730^{\circ}C$. Therefore, when dissimilar metal welding is carried out for P-No. 1 carbon steel and different P-No. low alloy steel, the PWHT temperature should be carefully selected to avoid significant deterioration of impact properties for P-No. 1 carbon steel.

Comparison of Microstructure & Mechanical Properties between Mn-Mo-Ni and Ni-Mo-Cr Low Alloy Steels for Reactor Pressure Vessels (원자로 압력용기용 Mn-Mo-Ni계 및 Ni-Mo-Cr계 저합금강의 미세조직과 기계적 특성 비교)

  • Kim, Min-Chul;Park, Sang Gyu;Lee, Bong-Sang
    • Korean Journal of Metals and Materials
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    • v.48 no.3
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    • pp.194-202
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    • 2010
  • Application of a stronger and more durable material for reactor pressure vessels (RPVs) might be an effective way to insure the integrity and increase the efficiency of nuclear power plants. A series of research projects to apply the SA508 Gr.4 steel in ASME code to RPVs are in progress because of its excellent strength and durability compared to commercial RPV steel (SA508 Gr.3 steel). In this study, the microstructural characteristics and mechanical properties of SA508 Gr.3 Mn-Mo-Ni low alloy steel and SA508 Gr.4N Ni-Mo-Cr low alloy steel were investigated. The differences in the stable phases between these two low alloy steels were evaluated by means of a thermodynamic calculation using ThermoCalc. They were then compared to microstructural features and correlated with mechanical properties. Mn-Mo-Ni low alloy steel shows the upper bainite structure that has coarse cementite in the lath boundaries. However, Ni-Mo-Cr low alloy steel shows the mixture of lower bainite and tempered martensite structure that homogeneously precipitates the small carbides such as $M_{23}C_6$ and $M_7C_3$ due to an increase of hardenability and Cr addition. In the mechanical properties, Ni-Mo-Cr low alloy steel has higher strength and toughness than Mn-Mo-Ni low alloy steel. Ni and Cr additions increase the strength by solid solution hardening. In addition, microstructural changes from upper bainite to tempered martensite improve the strength of the low alloy steel by grain refining effect, and the changes in the precipitation behavior by Cr addition improve the ductile-brittle transition behavior along with a toughening effect of Ni addition.

Investigation of Changes in Structural Characteristics and Chemical Composition after Heat Treatment Process of JIS-SUJ2 Bearing Steel (JIS-SUJ2 베어링강의 열처리 이후 표면 및 심부에서 나타나는 구조적, 화학적 물성 변화)

  • Donghee Lee;Kyun Taek Cho;Hyeonmin Yim;Seung-Hwan Oh;Tae Bum Kim;Woo-Byoung Kim
    • Korean Journal of Materials Research
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    • v.33 no.12
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    • pp.558-564
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    • 2023
  • In this study, we designed and manufactured a large angular contact ball bearing (LACBB) with low deformation using JIS-SUJ2 steel and analyzed changes in its structural characteristics and chemical composition upon heat treatment. The bearing was produced by hot forging and heat treatment including a quenching and tempering (Q/T) process, and its properties were analyzed using 4 mm thick specimens. A difference in the size distribution of the carbide in the outer and inner parts of the bearing was observed and it was confirmed that large and non-uniform carbide was distributed in the inner part of the bearing. After heat treatment, the hardness value of the outer part increased from 13.4 HRC to 61 HRC and the inner part increased from 8.0 HRC to 59.7 HRC. As a result of X-ray diffraction (XRD) measurements, the volume fraction of the retained austenite contained in the outer part was calculated to be 3.5~4.8 % and the inner part was calculated to be 3.6~5.0 %. The surface chemical composition and the content of chemical bonds were quantified through X-ray photoelectron spectroscopy (XPS), and a decrease in C=C bonds and an increase in Fe-C bonds were confirmed.

Synthesis of Super Iron Carbide from Hematite Fines with $CO-H_2$ Gas Mixture (Hematite系 微粉鑛石을 사용한 $CO-H_2$ 混合 Gas에 의한 高炭化鐵의 合成)

  • Chung, Uoo-Chang
    • Resources Recycling
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    • v.13 no.5
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    • pp.45-50
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    • 2004
  • To investigate the characteristics of phases formed in iron carbides, super iron carbide was synthesized from hematite fines with $CO-H_2$ gas mixture after reduction under $H_2$ gas at $600^{\circ}C$. Before carburization, the surface of iron powder reduced was pre-treated in the atmosphere of 0.05 vol% $NH_3$-Ar. The synthesized iron carbides were comprehensively explored by C/S analyzer(Low C/S determinator), M$\"{o}$ssbauer spectroscopy, X-ray diffraction patterns(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), and Raman spectroscopy at various reaction time of 5, 10, 15, 20, 25, 30, and 35 min, respectively. By adding a small amount of $NH_3$ gas, the super iron carbides containing 10 wt% carbon were synthesized, and its addition stabilized iron carbides. It was found that the $NH_3$ treatment played a major role in the formation of iron carbide without decomposition($Fe_3C{\to}$3Fe+C) of iron carbides and precipitation of free carbon. It also succeed to synthesize super iron carbide, $Fe_5C_2$, as a stable single phase without involving Fe and $Fe_3C$ phases.