• Corrosion Science and Technology
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• 제2권5호
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• pp.225-232
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• 2003

High temperature oxidation of SUS430 and SUS304 stainless steels in 16.7 kPa $O_2$ - 20.3 kPa $H_2O$ - balanced N2 atmosphere at 1273 K was studied focused on the scale spalling during cooling after an isothermal oxidation. Spalling of the oxide scale during cooling occurred only for SUS304 stainless steel. The oxide scale was composed of two layers and they detached at the interface between them. The reason for the spalling could not be explained only by thermal stresses applied to the specimen during heating and cooling. A new mechanism for scale spalling was proposed based on combination of thermal stresses and thermal shock caused by a fast Martensite transformation of substrate metal.

• 한국주조공학회지
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• 제11권5호
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• pp.406-413
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• 1991

Ductile cast iron(DCI) with a multi-phase(ferrite-bainite-martensite) structures was produced by various special heat treatment. Intercritical heat treatment(I. C.), intermediate heat treatment(I. M.) and step quenching(S. Q.) were used to strengthen and toughen the fracture initiation sites such as graphite-marix interfaces and eutectic cell boundaries in DCI. The purpose of this study was to investigate of DCI by the special heat treatment. (I. C., I. M., and S. Q.) At a result, bainite nucleation rate at higher temperature was higher than that of at lower temperature, and shapes of bainite and martensite became bar /spheroidal type with increase of isothermal transformation time.

• 열처리공학회지
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• 제29권1호
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• pp.8-14
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• 2016

This article presents a study on the tensile properties of hypoeutectoid steels with different ferrite-pearlite microstructures. Nine kinds of hypoeutectoid steel specimens were fabricated by varying carbon content and isothermal transformation temperature. The microstructural factors such as ferrite & pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured and then tensile tests were carried out on the specimens in order to investigate the correlation of the microstructural factors with strength and ductility. The pearlite volume fraction usually increased with decreasing transformation temperature, while the pearlite interlamellar spacing and cementite thickness decreased mostly with decreasing transformation temperature, irrespective of carbon content. The tensile test results showed that the yield and tensile strengths of all the steel specimens increased and their ductility was also improved as the transformation temperature decreased. For the steel specimens investigated, the difference in the transformation temperature dependence of strength and ductility could be explained by the fact that the variation in pearlite fraction with transformation temperature noticeably affected various microstructural factors such as pearlite interlamellar spacing and cementite thickness associated with pearlite fracture mechanism such as void initiation, cementite necking, and cracking.

• 한국재료학회지
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• 제27권4호
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• pp.184-191
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• 2017

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of low-carbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

• 한국분말재료학회지
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• 제16권1호
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• pp.9-15
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• 2009

Amorphization and crystallization behaviors of $Ti_{50}Cu_{50}Ni_{20}Al_{10}$ powders during high-energy ball milling and subsequent heat treatment were studied. Full amorphization obtained after milling for 30 h was confirmed by X-ray diffraction and transmission electron microscope. The morphology of powders prepared using different milling times was observed by field-emission scanning electron microscope. The powders developed a fine, layered, homogeneous structure with prolonged milling. The crystallization behavior showed that the glass transition, $T_g$, onset crystallization, $T_x$, and super cooled liquid range ${\Delta}T=T_x-T_g$ were 691,771 and 80 K, respectively. The isothermal transformation kinetics was analyzed by the John-Mehn-Avrami equation. The Avrami exponent was close to 2.5, which corresponds to the transformation process with a diffusion-controlled type at nearly constant nucleation rate. The activation energy of crystallization for the alloy in the isothermal annealing process calculated using an Arrhenius plot was 345 kJ/mol.

• 한국재료학회지
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• 제6권8호
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• pp.841-851
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• 1996

최근의 연구에 의하면 직접 소입강에서 미량의 베이나이트상의 생성이 확인되었다. 마르텐사이트 변태전에 생성된 베이나이트상은 마르텐사이트의 패킷을 미세화시키고 입도 미세화로 기계적 성질을 향상시킨다고 한다. 본 연구에서는 미량 합금 원소로 Mo, B 등을 첨가한 강을 $1200^{\circ}C$에서 단조하여 물에 직접 소입한 후, 베이나이트상의 분율을 조절하기 위해 베이나이트 변태 온도에서 일정시간 유지하는 열처리 과정을 거쳐 생성된 베이나이트상의 분율과 기계적 성질간의 관계를 고찰하였다. 이로써 마르텐사이트와 베이나이트 혼합 조직의 강도와 인성을 증가시키는 효과적인 베이나이트 분율을 조사한 후에, 직접 소입시에 이와 같은 분율의 베이나이트 함량으로 조절할 수 있는 방법을 제시하여 고강도 고인성형 직접 소입강의 개발에 활용하고자 한다.

• 대한기계학회논문집
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• 제15권2호
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• pp.630-643
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• 1991

The hydrodynamic stability equations are formulated for buoyancy-induced flows adjacent to a vertical, planar, isothermal surface in cold pure water. The resulting stability equations, when reduced to ordinary differential equation by a similarity transformation, constitute a two-point boundary-value(eigenvalue) problem, which was numerically solved for various values of the density extremum parameter R=( $T_{m}$ - $T_.inf./) / ( $T_{o}$ - $T_.inf./). These stability equations have been solved using a computer code designed to accurately solve two-point boundary-value problems. The present numerical study includes neutral stability results for the region of the flows corresponding to 0.0.leq. R. leq.0.15, where the outside buoyancy force reversals arise. The results show that a small amount of outside buoyancy force reversal causes the critical Grashof number $G^*/ to increase significantly. A further increase of the outside buoyancy force reversal causes the critical Grashof number to decrease. But the dimensionless frequency parameter $B^*/ at $G^*/ is systematically decreased. When the stability results of the present work are compared to the experimental data, the numerical results agree in a qualitative way with the experimental data.erimental data.

• 한국재료학회지
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• 제26권5호
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• pp.235-240
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• 2016

Appropriate thermo-mechanical properties of nickel-based superalloys are achieved by heat treatment, which induces precipitation and solid solution hardening; thus, information on the temperature ranges of precipitation and dissolution of the precipitates is essential for the determination of the heat treatment condition. In this study, thermal analyses of nickel-based superalloys were performed by differential scanning calorimetry method under conditions of various heating rates of 5, 10, 20, or 40K/min in a temperature range of 298~1573K. Precipitation and dissolution temperatures were determined by measuring peak temperatures, constructing trend lines, and extrapolating those lines to the zero heating rate to find the exact temperature under isothermal condition. Determined temperatures for the precipitation reactions were 813, 952, and 1062K. Determined onset, peak, and offset temperatures of the first dissolution reaction were 1302, 1388, and 1406K, respectively, and those values of the second dissolution reaction were 1405, 1414, and 1462K. Determined solvus temperature was 1462K. The study showed that it was possible to use a simple method to obtain accurate phase transition temperatures under isothermal condition.

• Carbon letters
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• 제12권4호
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• pp.229-235
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• 2011

Isotactic polyacrylonitrile (PAN) with triad isotacticity of 0.53, which was determined by $^{13}C$ NMR, using dialkylmagnesium as an initiator, was successfully synthesized. Isothermal treatment of iso-PAN was conducted in air at 200, 220, 250 and $280^{\circ}C$. Structural evolutions and chemical changes were studied with Fourier transformation infrared and wide-angle X-ray diffraction during stabilization. A new parameter $CNF={I_{2240cm}}^{-1}/ ({I_{1595cm}}^{-1}+f^*{I_{1595cm}}^{-1})$ was defined to evaluate residual nitrile groups. Crystallinity and crystal size were calculated with X-ray diffraction dates. The results indicated that the nitrile groups had partly converted into a ladder structure as stabilization proceeded. The rate of reaction increased with treatment temperature; crystallinity and crystal size decreased proportionally to pyrolysis temperature. The iso-conversional method coupled with the Kissinger and Flynn-Wall-Ozawa methods were used to determine kinetic parameters via differential scanning calorimetry analysis with different heating rates. The active energy of the reaction was 171.1 and 169.1 kJ/mol, calculated with the two methods respectively and implied the sensitivity of the reaction with temperature.

• 소성∙가공
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• 제15권2호
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• pp.112-117
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• 2006

Sliding wear mechanism of a high nitrogen austenitic 18Cr-18Mn-2Mo-0.9N steel has been investigated. Dry sliding wear tests of the steel were carried out at various loads of IN-10N under a constant sliding speed condition of 0.15m/s against AISI 52100 bearing steel balls. Solution ($1050^{\circ}C$) and isothermal aging ($900^{\circ}C$) heat treatments were performed on the steel and the effect of the heat treatments on the wear was investigated. Wear rates of the solution-treated steel specimen remained low until 5N load, and then increased abruptly at loads above 5N. The rates of isothermally aged specimens were low and increased gradually with the applied load. Worn surfaces, their cross sections, and wear debris of the steel specimens were examined with a scanning electron microscopy. Phases of the heat-treated specimen and the wear debris were identified using XRD. The transformed phase underneath a sliding track was investigated and analyzed using a TEM. Effects of the phase transformation during the wear and $Cr_{2}N$ precipitates formed during the isothermal aging on the wear of the austenitic steel were discussed.