• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 춘계학술대회논문집
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• pp.196-201
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• 2000

A thermodynamic model was developed to analyze the characteristics of the heat generation during transformation of austenite in 0.186wt% and 0.458 wt%. carbon steels. The heat capacity and the heat evolved during transformation were formulated as functions of temperature and chemical composition for ferrite bainite and pearlite. in addition using the transformation dilatometer the transformation heat evolved during cooling was measured and the transformation behavior was observed. It was found that the heat capacity of ferrite was similar to those of pearlite and bainite. The heat capacity of ferrite was greater than that of bainite which was greater than that of pearlite. The molar heat of transformation to pearlite was greater than that to bainite which was greater than that to ferrite. The heats were found to be increased with decreased temperature and increasing the carbon content, It was also observed that the thermodynamic model. The heat of transformation in the higher carbon steel was greater than that in the lower carbon one. This was attributed to the lower transformation temperature and the greater amount of transformed pearlite in the higher carbon steel.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.276-281
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• 2002

A metallurgical model for bainite transformation kinetics in the coarse-grained heat affected zone(CGHAZ) on the basis of an Avrami-type equation was studied. Isothermal transformation tests were carried out to obtain the empirical equations for incubation time and Avrami kinetic constants for C-Mn-Mo-Ni steel. The effect of prior austenite grain size(PAGS) on the reaction rate of bainite was also investigated. Compared with experimental transformation behavior of bainite, the predicted behavior was in good agreement. It was also found that a smaller grain size retard the bainite reaction rate, contrary to the classical grain size effect and this is considered to be caused by constraint of grain size to bainite growth.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.11-14
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• 2002

A metallurgical model for bainite transformation kinetics in the coarse-grained heat affected zone(CGHAZ) on the basis of an Avrami-type equation was studied. Isothermal transformation tests were carried out to obtain the empirical equations for incubation time and Avrami kinetic constants for C-Mn-Mo-Ni steel. The effect of prior austenite grain size(PAGS) on the reaction rate of bainite was also investigated. Compared with experimental transformation behavior of bainite, the predicted behavior was in good agreement. It was also found that a smaller grain size retard the bainite reaction rate, contrary to the classical grain size effect and this is considered to be caused by constraint of grain size to bainite growth.

• Metals and materials international
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• 제24권6호
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• pp.1202-1212
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• 2018

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and $250^{\circ}C$), while it shows no significant difference at lower austempering temperature ($200^{\circ}C$). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of $220^{\circ}C$. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

• 한국주조공학회지
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• 제12권3호
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• pp.230-237
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• 1992

The purpose of present investigation is to obtain ductile cast iron with ferrite-bainite matrix by pearlite-bainite transformation treatment. Ductile cast irons having three kinds of Mn ampunt had been manufactured. Mn increased pearlite volume fraction iin as-cast ductile cast iron. Ductile cast irons of different pearlite fraction were austenitized at $875\;^{\circ}C$ for 230-350 sec or $925\;^{\circ}C$ for 130-170 sec followed by austempering at $300\;^{\circ}C$ or $400\;^{\circ}C$ for the various periods of time from 5 to 30 min. When specimen was austenitixed for 130 sec at $925\;^{\circ}C$ and for 230 sec at $875\;^{\circ}C$, pearlite was transformed into austenite. Bainite around graphite was found at $925^{\circ}C$ for 170 sec. Bainite in grain boundary of ferrite was happened at $875^{\circ}C$ for 350 sec. During the austempering process, acicular bainite was precipitated at $300^{\circ}C$ and lath bainite was precipitated at $400^{\circ}C$. Increment in manganese content restrained bainitic transformation. Retained austenie was of little quantity.

• 소성∙가공
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• 제28권4호
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• pp.219-225
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• 2019

This study deals with the effect of austempering temperature and time on the microstructures and mechanical properties of high-carbon nano-bainite steels. Although all the austempered specimens are mainly composed of bainite, martensite, and retained austenite, the specimens which are austempered at lower temperatures contain finer packets of bainite. As the duration for austempering increases, bainite packets are clearly seen due to larger amount of carbon atoms being redistributes into bainite and retained austenite during bainite transformation. As the austempering time increases, the hardness of the specimens gradually decreases as a result of lower martensite volume fraction, and later increases again due to the formation of nano-bainite structure. The Charpy impact test results indicate that the impact toughness of the austempered specimens can be improved if the formation of nano-bainite structure and the transformation induced plasticity effect of retained austenite are optimized at higher austempering temperature.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 추계학술대회 논문집
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• pp.456-459
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• 2005

Continuous cooling transformation behaviors were studied fur low carbon HSLA steels containing three different level $(1\~3\;wt\%)$ of Ni addition. Thermo-mechanical processing (TMP) simulations to construct continuous cooling (CCT) diagram were conducted by using Gleeble system. As cooling rate increased, pearlite, granular bainite, acicular ferrite, bainitic ferrite and lath martensite were transformed from deformed austenite. Fully bainitic microstructure were developed at all cooling rate condition in high Ni containing steel due to hardenability increasing effects of Ni. Ni also influenced the transformation kinetics. At the slowest cooling rate of $0.3^{\circ}C/s$, transformation delayed with decreasing Ni contents because of the diffusion of substitutional alloy elements. However, cooling rate slightly increased to $1^{\circ}C/s$, transformation kinetics accelerated with decreasing Ni contents because nucleation of bainite was sluggish due to hardening of residual austenite.

• 한국자동차공학회논문집
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• 제8권4호
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• pp.141-148
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• 2000

As the steel plates used for automobile safety the TRIP-assisted multiphase steels are being introduced to automobile industry with respect to their remarkable mechanical properties for the combination of high strength and large elongation. This multiphase structure is generated by two stage heat treatment (intercritical annealing & isothermal treatment) The metastable retained austenite can be transformed to martensite when plastically deformed which results in TRIP effect. Actually the microstructure of TRIP-assisted steels consist of a fine dispersite. There present discussion deals with bainite reaction kinetics of austenite in the process o f two stage heat treatment. In relation to bainite transformation the characteristics of bainite reaction is found to be influenced by the bainite tempering temperature and also by the relative rate in which carbides precipitate within residual austenite.

• 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.

• 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.