• Journal of the Korean Society for Heat Treatment
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• v.26 no.6
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• pp.300-305
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• 2013

The effects of solute carbon atoms on the thermal expansion coefficients of ferrite and austenite as well as austenitization behavior were investigated by comparing carbon-free ferrite and carbon-containing ferrite. The thermal expansion coefficients and austenitization start and finish temperatures were measured using a dilatometer. Solute carbon atoms at elevated temperatures above the cementite dissolution temperature (650 K) decreased the thermal expansion coefficients of both ferrite and austenite. In addition, minute amount of carbon atoms dissolved in ferrite stimulated austenite nucleation during continuous heating, resulting in the lower starting temperature of austenitization.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05b
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• pp.72-77
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• 1998

액체금속로 노심재료로 사용되고 있는 HT9강의 열처리에 따른 미세조직 변화를 관찰하였다. 열간압연상태에서의 조직은 마르덴사이트, $\delta$-페라이트, 잔류 오스테나이트, 그리고 탄화물로 이루어져 있다. 잔류 오스테나이트는 austenitization하여도 그 함량은 감소하지만 완전히 제거되지는 않았으나, tempering후에는 잔류하지 않았다. 오스테나이트 입자크기는 austenitization 온도가 증가함에 따라 성장하여 11005$^{\circ}C$에서는 약 42$\mu\textrm{m}$였다. Austenitization 온도 증가에 따라 탄화물의 평균 크기 및 평균 면적 분율은 크게 감소하고 있었다. 50$0^{\circ}C$에서 tempering한 경우에는 Fe-rich 탄화물이 석출하였으나, tempering 온도가 증가하면 Cr-rich M$_2$$_3$C$_{6}$ 탄화물이 석출하고 있었다. Austenitization 상태에서는 100$0^{\circ}C$ 까지는 탄화물의 분해가 일어나 경도값이 증가하나 100$0^{\circ}C$ 이상에서는 일정한 경도값을 나타내었다. Tempering한 시편에서는 탄화물의 석출과 전위의 회복으로 인해 tempering 온도가 증가함에 따라 경도값이 급격히 감소하였다.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.3
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• pp.207-215
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• 1998

13%Cr martensitic stainless steel was microalloyed with 0~1.5%Mo, and the mechanical properties were tested at the various heat treated conditions. Mo addition increased austenitization temperature(Ac1), and had little influence on the hardness and tensile properties at the annealed condition. The higher the austenitizing temperature, the higher the hardness and tensile strength, but Mo addition decreased those properties. The impact energy after austenitization increased with addition of Mo. The decrease of mechanical properties and increase of impact energy of Mo-alloyed steel after austenitization are thought to be caused by formation of ductile ${\delta}$-ferrite phase in the microstructure.

• Journal of the Korean Society for Heat Treatment
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• v.16 no.2
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• pp.83-89
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• 2003

As a research for developing fine-grained high strength low carbon steels, the effects of austenitization temperature and hot deformation on microstructure was investigated in 0.15 wt.% carbon steels with microalloying elements such as Nb and Ti. When the steels were reheated at $1250^{\circ}C$, Nb containing steel showed very coarse austenite grain size of $200{\mu}m$ whereas Nb-Ti steel did fine one of $70{\mu}m$ because Ti carbonitrides could suppress the austenite grain growth. In case of 50% reduction at $850^{\circ}C$, the austenite grains in the Nb steel partially recrystallized while those in the Nb-Ti steel fully recrystallized probably due to finer prior austenite grains.For the Nb-Ti steel, ferrite grain size was not sensitively changed with austenitization temperature and compression strain and, severe deformation of 80% reduction was not essentially necessary to refine ferrite grains to about $3{\mu}m$ which could be obtained through lighter deformation of 40% reduction.

• Journal of Welding and Joining
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• v.24 no.1
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• pp.77-87
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• 2006

Considering ferrite grain size in the base metal, the prediction model for $A_{c3}$ temperature and prior austenite grain size at just above $A_{c3}$ temperature was proposed. In order to predict $A_{c3}$ temperature, the Avrami equation was modified with the variation of ferrite grain size, and its kinetic parameters were measured from non-isothermal data during continuous heating. From calculation using a proposed model, $A_{c3}$ temperatures increased with increasing ferrite grain size and heating rate. Meanwhile, by converting the phase transformation kinetic model that predicts the ferrite grain size from austenite grain size during cooling, a prediction model for prior austenite grain size at just above the $A_{c3}$ temperature during heating was developed.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.2
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• pp.65-71
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• 2007

Microstructural changes and hardness variations in STS 304 steel have been investigated during the processes of carbide dispersion (CD) carburization; carburization, austenitization, subzero treatment and tempering. The carbon content of the surface layer increased up to maximum 4.0% after carburization, and the content was homogenized with the value of 2.3% to the $95{\mu}m$ from the surface after austenitization. The carbide appeared during CD carburization process was $Cr_7C_3$ type, which was composed network carbides along the austenite grain boundaries, square type carbides in the interior of the grain and fine nano-sized carbides. The fine nano-sized carbides precipitated at the austenitization stage and possibly subzero treatment stage were coarsened after tempering at $200^{\circ}C$, resulting the hardness decrease. The tempered steel without subzero treatment increased hardness with increasing time due to the continuous precipitation of fine carbides during tempering. The nano-sized carbide appeared square type morphology.

• Korean Journal of Materials Research
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• v.31 no.1
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• pp.29-37
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• 2021

Martensitic stainless steel is commonly used in the medical implant instrument. The alloy has drawbacks in terms of strength and wear properties when applied to instruments with sharp parts. 440C STS alloy, with improved durability, is an alternative to replace 420 J2 STS. In the present study, the carbide precipitation, and mechanical and corrosion properties of STS 440C alloy are studied as a function of different heat treatments. The STS 440C alloy is first austenitized at different temperatures; this is immediately followed by oil quenching and sub-zero treatment. After sub-zero treatment, the alloy is tempered at low temperatures. The microstructures of the heat treated STS 440C alloy consist of martensite and retained austenite and carbides. Using EDX and SADP with a TEM, the precipitated carbides are identified as a Cr23C6 carbide with a size of 1 to 2 ㎛. The hardness of STS 440C alloy is improved by austenitization at 1,100 ℃ with sub-zero treatment and tempering at 200 ℃. The values of Ecorr and Icorr for STS 440C increase with austenitization temperature. Results can be explained by the dissolution of Cr-carbide and the increase in the retained austenite. Sub-zero treatment followed by tempering shows a little difference in the properties of potentiodynamic polarizations.

• Proceedings of the KWS Conference
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• 2005.06a
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• pp.129-131
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• 2005

• Korean Journal of Materials Research
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• v.25 no.9
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• pp.497-502
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• 2015

The hardenability of boron steel specimens with different molybdenum and chromium contents was investigated using dilatometry and microstructural observations, and then was quantitatively measured at a critical cooling rate corresponding to 90 % martensite hardness obtained from a hardness distribution plotted as a function of cooling rate. Based on the results, the effect of an austenitizing temperature on the hardenability and tensile properties was discussed in terms of segregation and precipitation behavior of boron atoms at austenite grain boundaries. The molybdenum addition completely suppressed the formation of pro-eutectoid ferrite even at the slowest cooling rate of $0.2^{\circ}C/s$, while the chromium addition did at the cooling rates above $3^{\circ}C/s$. On the other hand, the hardenability of the molybdenum-added boron steel specimens decreased with an increasing austenitizing temperature. This is associated with the preferred precipitation of boron atoms since a considerable number of boron atoms could be concentrated along austenite grain boundaries by a non-equilibrium segregation mechanism. The secondary ion mass spectroscopy results showed that boron atoms were mostly segregated at austenite grain boundaries without noticeable precipitation at higher austenitization temperatures, while they formed as precipitates at lower austenitization temperatures, particularly in the molybdenum-added boron steel specimens.

• Korean Journal of Materials Research
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• v.26 no.11
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• pp.583-589
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• 2016

In this study, recrystallization behaviors in the two-phase (${\alpha}+{\gamma}$) region of micro-alloyed steels such as Base, Nb, TiNbV and CAlN were investigated in terms of flow stress, microstructure and associated grain boundary characteristics. The flow stress of all specimens reached peak stress and gradually decreased, which means that recrystallization or recovery of proeutectoid deformed ferrite and recovery or transformation to ferrite of deformed austenite occurred by thermal activation. The precipitation of carbide or nitride via the addition of micro-alloying elements, because it reduced prior austenite grain size upon austenitization, promoted transformation of austenite to ferrite and increased flow stress. The strain-induced precipitation under deformation in the two-phase region, on the other hand, increased the flow stress when the micro-alloying elements were dissolved during austenitization. The recrystallization of the Nb specimen was more effectively retarded than that of the TiNbV specimen during deformation in the two-phase region.