• Elastomers and Composites
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• 제53권2호
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• pp.52-56
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• 2018

The present study examines the transformation of dynamic DSC data into the equivalent isothermal data for the formation kinetics of a polyurethane elastomer. The reaction of 2'-dichloro-4,4'-methylenedianiline (MOCA) with a PTMG/TDI-based isocyanate prepolymer was evaluated. DSC measurement was performed in the dynamic scanning mode with several different heating rates to obtain the reaction thermograms. Then, the data was transformed into the isothermal data through a procedure based on Ozawa analysis. The main feature of this procedure was the transformation of $({\alpha}-T)_{\beta}$ curves from dynamic DSC into $({\alpha}-t)_T$ curves using the isoconversional $(t-T)_{\alpha}$ diagram. Validity was discussed for the relationship between the dynamic DSC data and the transformed isothermal results.

• 열처리공학회지
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• 제27권2호
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• pp.72-78
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• 2014

Isothermal transformation behavior of a 10% Cr heat resistant steel fabricated by centrifugal casting process was investigated. Normalized specimen at $1100^{\circ}C$ for 1 hour was isothermally annealed at temperature range between $600^{\circ}C$ and $700^{\circ}C$ with various time. The annealed specimen had eutectoid structure which was generated along austenitic grain boundary during isothermal annealing. Areal fraction of eutectoid structure increased up to 25% after holding at $700^{\circ}C$ for 20 hours. It was observed that austenitic matrix was transformed to ferrite structure and fine $M_{23}C_6$ carbides with increase of annealing time. Time-temperature-transformation diagram of the centrifugally cast 10% Cr steel with 0.18 wt% C was plotted based on the results of isothermal transformation behavior.

• 한국재료학회지
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• 제12권5호
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• pp.375-381
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• 2002

N-type ${\beta}-FeSi_2$ with a nominal composition of $Fe_{0.98}Co_{0.02}Si_2$ powders has been produced by mechanical alloying process and consolidated by vacuum hot pressing. As-milled powders were of metastable state and fully transformed to ${\beta}-FeSi_2$ phase by subsequent isothermal annealing. However, as-consolidated $Fe_{0.98}Co_{0.02}Si_2$ consisted of untransformed mixture of ${\alpha}-Fe_2Si_ 5$ and $\varepsilon$-FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting ${\beta}-FeSi_2$ phase. The transformation behavior of ${\beta}-FeSi_2$ was investigated by utilizing DTA, a modified TGA under magnetic field, SEM, and XRD analyses. Isothermal annealing at $830^{\circ}C$ in vacuum led to the thermoelectric semiconducting ${\beta}-FeSi_2$ phase transformation, but some residual metallic $\alpha$ and $\varepsilon$ phases were unavoidable even after prolonged annealing. Thermoelectric properties were remarkably improved by isothermal annealing due to the transformation from metallic $\alpha$ and $\varepsilon$ phases to semiconducting phases.

• 한국세라믹학회지
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• 제27권8호
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• pp.971-978
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• 1990

The ifluence of transformability and stabilized effects in tetragonal phase on the isothermal phase transformation of Y-TZP at low temperature were investigated. The transformability of Y-TZP powders were gradually increased with calcination temeprature and reached maximum at critical temperature, but when the Y-TZP powders were calcined above critical temperature, transformability of Y-TZP were gradually decreased with increasing calcination temperature. It was concluded that maximum transformability was appeared because particle size effects decreased and constrain effects increased with calcined temperature. The isothermal phase transformation during aging at 25$0^{\circ}C$ only observed in Y-TZP stabilized by constrain effects and the amounts of transformation during aging at 25$0^{\circ}C$ only observed in Y-TZP stabilized by constrain effects and the amount of transformed monoclinic phase during aging decreased with increasing constrain effects. From these results, the mechanism of isothermal phase transformation and degradation behaviors at low temperature in Y-TZP was concluded that occurred by decreasing of constrain effects due to stress relaxation at grian boundary.

• 한국재료학회지
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• 제11권12호
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• pp.1068-1073
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• 2001

An n-type iron$silicide(Fe_{0.98}Co_{0.02}Si_2)$has been produced by mechanical alloying process and consolidated by vacuum hot pressing. Although as-milled powders after 120 hours of milling did not show an alloying progress,${\beta}-FeSi_2$phase transformation was induced by isothermal annealing at$830{\circ}C$for 1 hour, and the fully transformed${\beta}-FeSi_2$phase was obtained after 4 hours of annealing. Near fully dense specimen was obtained after vacuum hot pressing at$ 1100{\circ}C$with a stress of 60MPa. However, as-consolidated iron silicides were consisted of untransformed mixture of ${\Alpha}-Fe_2Si_5$and ${\varepsilon-FeSi$phases. Thus, isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting${\beta}-FeSi_2$phase. The condition for${\beta}-FeSi_2$transformation was investigated by utilizing DTA, SEM, and XRD analysis. The phase transformation was shown to be taken place by a vacuum isothermal annealing at$830{\circ}C$and the transformation behaviour was investigated as a function of annealing time. The mechanical properties of${\beta}-FeSi_2$materials before and after isothermal annealing were characterized in this study.

• 열처리공학회지
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• 제29권3호
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• pp.103-108
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• 2016

In recent years, TRIP steels which are composed of ferrite, bainite, and retained austenite have drawn much attention for automotive sheets due to excellent combination of strength and ductility. The effect of two-step isothermal heat treatment of bainitic transformation on microstructures, especially retained austenites and tensile properties in the conventional TRIP steel was investigated. A two-step isothermal heat treatment, in which 50% bainitic transformation occurred at high temperature, followed by bainitic transformation at low temperature, improves tensile properties, resulting from enhanced mechanical stability of retained austenite against external plastic deformation due to refinement of retained austenites, compared to single-step isothermal heat treatment.

• Journal of Welding and Joining
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• 제35권1호
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• pp.89-94
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• 2017

The microstructure and dilatation for 0.15C steels were investigated to define the phase transformation during the quenching and partitioning (Q&P) process. For the one step Q&P dilatation, the isothermal martensite/bainite transformation occurred because the holding temperature was between $M_s$ and $M_f$. The isothermally transformed martensite/bainite and the athermally transformed martensite were produced by a loss of retained austenite. As the holding time increased, new martensite-start ($M_s$) temperature produced from the final quenching process decreased due to the carbon partitioning from the martensite to the retained austenite. This was the direct evidence of increment for the retained austenite stability. For the two step Q&P dilatation, the isothermal bainitic transformation occurred because the partitioning temperature was larger than the $M_s$ and new $M_s$. The partitioning at $400^{\circ}C$ indicated the short incubation period for the bainite transformation than the $350^{\circ}C$ partitioning because the partitioning at $400^{\circ}C$ should acquire the larger thermal driving force for carbon partitioning than the $350^{\circ}C$ partitioning. A quick drop of $M_s$ and short period of bainite incubation for the $400^{\circ}C$ partitioning steel were also the direct evidence of significant effects of carbon partitioning on the stability of retained austenite.

• 열처리공학회지
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• 제32권1호
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• pp.17-28
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• 2019

Jominy hardenability curves of low alloy steel containing less than 5 wt.% of alloying elements in total were calculated by applying Scheil's rule of additivity to pre-calculated isothermal transformation curve. Isothermal transformation curve for each phase in steel was approximated as a simple mathematical equation by using Kirkaldy's approach and all coefficients in the equation were estimated from experimental temperature-time-transformation (TTT) and/or continuous cooling transformation (CCT) data in the literature. Then jominy test with simple boundary conditions was performed in computer by applying the finite difference scheme. The resultant cooling curves at each location along a longitudinal direction of Jominy bar were applied to calculate phase fractions as well as mechanical properties such as micro Vickers hardness. The simulated results were compared with experimental CCT data and Jominy curves in the literature.

• 한국세라믹학회지
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• 제33권6호
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• pp.637-644
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• 1996

$Zr(i-OC_3H_7)_4$로부터 가수분해법으로 Y-TZP 분말을 제조한 다음, 하소온도를 달리하여 정방정상의 상온 안정화와 25$0^{\circ}C$에서의 등온상전이 거동을 고찰하였다. 그 결과, 정방정상 지르코니아 입자의 상온 안정화 효과는 하소온도가 낮을 수록, 고용된 $Y_2O_3$의 양이 클 수록 증가하였다. 또한 25$0^{\circ}C$에서 aging 하면서 Y-TZP 분말의 등온상전이를 관찰한 결과, 입자크기 효과 및 고용효과에 의해 상온 안정화된 정방정상 지르코니아 입자에서는 aging 시 등온상전이가 일어나지 않고, 반면에 구속효과에 의하여 상온 안정화된 정방정상 입자에서는 등온상전이가 활발히 진행되었다. 등온상전이로 생성된 단사정상 입자의 내부에는 많은 쌍정과 미세균열들이 존재하였는데 이는 상전이시 변형에너지를 감소시키기 위한 것으로 사료되었다.

• 한국분말재료학회지
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• 제17권2호
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• pp.154-159
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• 2010

In this study, the sintering behavior of (Nd, Dy)-Fe-B powder which fabricated by strip-casting was investigated with various sintering temperatures and holding times. The relative density over 99% could be obtained by both sintering at $1070^{\circ}C$ for 1h and sintering at $970^{\circ}C$ for 20h. The grain growth was observed in sintered specimen at $1050^{\circ}C$ compared to one at $970^{\circ}C$. The isothermal sintering process below $1000^{\circ}C$ led to suppress grain growth showing the improved magnetic properties. The phase transformation of Nd-rich was confirmed by X-ray diffraction pattern.