• 대한금속재료학회지
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• 제48권3호
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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.

• 열처리공학회지
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• 제36권6호
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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.

• 열처리공학회지
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• 제36권1호
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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.

• 한국주조공학회지
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• 제25권2호
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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.

• Journal of Welding and Joining
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• 제35권1호
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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.

• 대한금속재료학회지
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• 제48권3호
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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.

• 한국재료학회지
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• 제33권12호
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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.

• 자원리싸이클링
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• 제13권5호
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• pp.45-50
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• 2004

탄화철을 합성하는 공정을 수소($H_2$) 환원과 $CO-H_2$ 혼합가스에 의한 탄화의 2단계 과정으로 나누어서 수행했다. 환원종료 후 미량의 암모니아 가스를 첨가하여 환원철의 표면을 개질한 후, 탄화시간 경과별 탄화상태를 C/S 분석기(Low C/S determinator), 뫼스바우어 분광 분석기(Mossbauer spectroscopy), XRD(X-ray diffraction patterns), SEM(Scanning electron microscopy), TEM(Transmission electron microscopy), XPS(Photoelectron spectroscopy), 및 라만분광기 (Raman spectroscopy)를 사용하여 조사하였다. 연구결과, 미량의 암모니아가스로 환원철 표면을 개질함으로써 탄화철의 분해 및 유리탄소의 석출을 방지할 수 있을 뿐만아니라 6.68wt% 이상 10wt% 까지 탄소가 과고용된 상태에서도 분해되거나 유리탄소를 석출하지 않고 안정상태를 유지하였다. 이러한 결과로부터 철(Fe)과 세멘타이트(cementite, $Fe_3C$)가 혼합되지 않고 고탄화철(SIC, super iron carbide)인 Fe5C2 상태의 안정한 단일상을 얻는데 성공하였다.