• Korean Journal of Materials Research
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• v.30 no.9
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• pp.465-473
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• 2020

For surface hardening of a continuous casting mold component, a fundamental metallurgical investigation on dissimilar laser clads (Cu-NiCrBSi) is performed. In particular, variation behavior of microstructures and mechanical properties (hardness and wear resistance) of dissimilar clads during long-term service is clarified by performing high-temperature post-clad heat treatment (temperature range: 500 ~ 1,000 ℃ and isothermal holding time: 20 ~ 500 min). The microstructures of clad metals (as-clads) consist of fine dendrite morphologies and severe microsegregations of the alloying elements (Cr and Si); substrate material (Cu) is clearly confirmed. During the post-clad heat treatment, the microsegregations are totally homogenized, and secondary phases (Cr-based borides and carbides) precipitated during the short-term heat treatment are also almost dissolved, especially at the heat treatment conditions of 950 ℃ for 500 min. Owing to these microstructural homogenization behaviors, an opposite tendency of the surface mechanical properties can be confirmed. In other words, the wear resistance (wear rate) improves from 4.1 × 10^-2 ㎣/Nm (as-clad condition) to 1.4 × 10^-2 ㎣/Nm (heat-treated at 950 ℃ for 500 min), whereas the hardness decreases from 453 HV (as-clad condition) to 142 HV (heat-treated at 950 ℃ for 500 min).

• Bulletin of the Korean Chemical Society
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• v.32 no.7
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• pp.2369-2376
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• 2011

Effect of heat treatment of carbon nanofibers (CNF) on electrical properties and crystallization behavior of polypropylene was reported. Two types of CNFs (untreated and heat treated at 2300 $^{\circ}C$) were incorporated into polypropylene (PP) using intensive mixing. A significant drop in volume resistivity was observed with composites containing untreated 5 wt % and heat treated 3 wt % CNF. In non-isothermal crystallization studies, both untreated and heat treated CNFs acted as nucleating agents. Composites with heat treated CNFs showed a higher crystallization temperature than composites with untreated CNFs did. TEM results of CNF revealed that an irregular structure of CNFs can be converted into the continuous graphitic structure after heat treatment. Furthermore, STM showed that the higher carbonization temperature leads to the higher graphite degree which presents the larger carbon network size, suggesting that a more graphitic structure of CNFs led to a higher crystallization temperature of PP.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.3
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• pp.121-127
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• 2023

When high carbon steel is heated in an appropriate austenizing temperature range and subjected to austempering, the size and shape of lamellar structure can be controlled. The high carbon steel sheet having the pearlite structure has excellent elastic characteristics because it has strong restoring force when properly rolled, and is applied in a process known as patenting-process using lead bath. In the case of isothermal treatment using lead-medium, it is possible to quickly reach a uniform temperature due to high heat transfer characteristics, but it is difficult to replace it with process technology that requires treatment to remove harmfulness lead. In this study, we intend to develop fluidization technology using garnet powder to replace the lead medium. After heating the high-carbon steel, the cooling rate was changed by compressed air to the vicinity of the nose of the continuous cooling curve, and then maintained for 90 s and then exposed to room temperature. The microstructure of the treated specimens were analyzed and compared with the existing products treated with lead bath. The higher the flow rate of compressed air, the faster the cooling rate to the pearlite transformation temperature, so lamellar spacing decreases and the hardness tends to increase.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.33 no.6
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• pp.282-287
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• 2023

The effect of stress relief heat treatment temperature and duration time on the microstructure, residual stress and Vickers hardness of additively manufactured Ti-6Al-4V alloy using laser powder bed fusion process was clarified. As a result of stress relief heat treatment for 240 minutes at 823 K and 60 minutes or more at 873 K, residual stress was decreased less than 30 MPa without grain growth and phase transformation which causes dimensional distortion and deterioration of mechanical properties. In addition, hardness was increased with increasing heat treatment temperature and duration time. It was deduced that the refinement of acicular martensitic α' phase due to the increasing duration time of isothermal heat treatment at 773~873 K, which was not detected by XRD and phase map analysis using SEM-EBSD, probably increases the hardness.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.12S
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• pp.1278-1285
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• 2003

This study investigated the phase transformation of the REBa$_2$Cu$_3$$O_{7-x}$ (RE=Nd, Gd, Dy) superconductor, and CCT (Continuous-Cooling-Transformation) along with the TTT (Time-Temperature-Transformation) diagrams are suggested according to the isothermal and continuous cooling heat-treatments. The peritectic temperature of the 123 phases decreased approximately 3$0^{\circ}C$ when the ionic radius of the rare-earth elements was reduced. The optimum cooling rate where BC and Cu-free phases do not exist was 0.001$^{\circ}C$/s. At this cooling late, the 123 phase grew with a c-axis Perpendicular to the surface and had a well-distributed 211 phase. When the oxygen partial pressure was reduced Outing isothermal heat-treatment, the formation temperature of the 211 phase decreased. In addition, the formation temperature of the 123 phases decreased from 100$0^{\circ}C$ (Nd-123) to 9$25^{\circ}C$ (Gd-123), and finally 875$^{\circ}C$ (Dy-123) according to the decrease in the ionic radius of the tare-earth elements. Compared to Nd-123, Gd- and Dy-123 had a better texture with a well-distributed 211 phase.e.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.31 no.2
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• pp.84-88
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• 2021

The change of the photoluminescence properties of La2O3-CaF2-Al2O3-SiO2 glass-ceramics doped with rare earth material, that is used as laser and optical sensors, was analyzed according to heat treatment temperature. The heat treatment conditions for fabricating glass-ceramics were obtained through non-isothermal thermal analysis, and X-ray diffraction analysis was performed to determine the degree of crystal growth and kinds of crystal phases generated according to the heat treatment temperature. Using Scherrer's equation, it was predicted that crystals with a size of 25~40 nm would be generated inside the glass-ceramics. Photoluminescence (PL) analysis showed that the specimens heat-treated at 660℃ to 670℃ for 1 hour had the highest PL intensity. Also, from the CIE color coordinate analysis, all glass-ceramics specimens emitted red-orange light regardless of the heat treatment condition.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.2
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• pp.49-58
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• 1993

Isothermal transformation behavior during patenting and variations of microstructure and tensile strength of patented wires were investigated in Si-added high carbon steel. The TTT curves of the steels were made for two different austenitizing temperature. As the salt bath temperature was increased, the observed microstructures were bainite at $450^{\circ}C$, the mixture of bainite and pearlite at $500^{\circ}C$, and to pearlite at $600^{\circ}C$, The tensile strength of patented wire exhibited the highest value when the structure was pearlite. while the bainitic structure showed the lowest.

• Journal of the Korean Ceramic Society
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• v.33 no.8
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• pp.909-914
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• 1996

In this research [RCOOH]-$\varepsilon$-Caprolactam-Montmorillonite was synthesized by reaction between Na-Mont-morillonite (10-Carboxy-n-decyl)dimethylethylammonium (=RCCOH) ion and $\varepsilon$-Caprolactame-Motmorillonite ob-tained was 3.62$\AA$ After heat treatment of this intercalations complex at 37$^{\circ}C$ in high vacuum for 12 h 24 h, 40 h and 90 h the basal spacing was reduced to 35.8$\AA$, 34.2, 17.5 $\AA$ and 16.6$\AA$ respectively. The calculated amount of free $\varepsilon$-caprolactame included int he intercaltions complex and that of $\varepsilon$-caprolactame which is interca-lated into the interlayer space of montomorillonite and still remained after heat treatment are 4.6~4.9 and 0.5 molecules per unite cell of montmorillonite respectively.

• Journal of the Korean Society for Heat Treatment
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• v.9 no.1
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• pp.27-33
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• 1996

Metallographic observation was carried out by scanning and transmission electron microscopy to evaluate microstructural characteristics of partially austenitized and isothermally transformed 1.0C-1.5Cr bearing steel. It was observed that lower bainite formed in the local region of specimen partially austenitized and isothermally held at $250^{\circ}C$ for 1/3 hr and formed in almost all area of the specimen isothermally held at $250^{\circ}C$ for 2 hrs. Lower bainitic carbides with midrib was also observed in the specimen partially austenitized and isothermally held at $250^{\circ}C$ for 4 hrs. Midrib was nearly carbide-free region and thicker in the vicinity of spherical carbides than the other region. Lengthening speed of lower bainitic carbides was remarkabey increased at isothermal holding time ranging from 2 hrs to 4 hrs.

• Journal of the Korean Society for Heat Treatment
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• v.2 no.2
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• pp.68-83
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• 1989

Physical metallurgy aspects of gaseous ferritic nitrocarburising are reviewed in the light of basic studies undertaken since 1975 which have illustrated inconsistencies between the iron-carbon-nitrogen ternary phase diagram at $570^{\circ}C$ and the experimental observation of the co-existence of the ${\varepsilon}$ carbonitride phase and ferrite. Thermodynamic investigations by Xu and Li together with those by Slycke et al are reviewed to illustrate compatability between a modified isothermal section of the Fe-C-N system and the formation and growth of a monophased ${\varepsilon}$ structure under a variety of processing conditions. The implications of the modified diagram in terms of innovations in industrial ferritic nitrocarburising practice are discussed, together with limitations on the control of the process. The importance of the developing technology of black nitrocarburising for enhanced wear, fatigue, and corrosion resistance is emphasised. Basic studies and industrial status of austenitic nitrocarburising treatments are also reviewed, which highlight the importance of substrate strengthening for high load bearing applications of anti-scuff thermochemical treatments.