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1.
E. B. Hawbolt B. Chau J. K. Brimacombe 《Metallurgical and Materials Transactions A》1983,14(9):1803-1815
The kinetics of the austenite-to-pearlite transformation have been measured under isothermal and continuous-cooling conditions
on a eutectoid carbon (1080) steel using a diametral dilatometric technique. The isothermal transformation kinetics have been
analyzed in terms of the Avrami Equation containing the two parametersn andb; the initiation of transformation was characterized by an empirically determined transformation-start time (tAv). The parametern was found to be nearly constant; and neithern norb was dependent on the cooling rate betweenT
A1 and the test temperature. Continuous-cooling tests were performed with cooling rates ranging from 7.5 to 108 °C per second,
and the initiation of transformation was determined. Comparison of this transformation-start time for different cooling rates
with the measured slow cooling of a test coupon immersed in a salt bath indicates that, particularly at lower temperatures,
the transformation in the traditional T-T-T test specimen may not be isothermal. The additivity rule was found to predict
accurately the time taken, relative to tAv, to reach a given fraction of austenite transformed, even though there is some question that the isokinetic condition was
met above 660 °C. However, the additivity rule does not hold for the pretransformation or incubation period, as originally
proposed by Scheil, and seriously overestimates the incubation time. Application of the additivity rule to the prediction
of transformation-finish time, based on transformation start at TA1, also leads to overestimates, but these are less serious. The isothermal parameters—n (T),b (T), and tAv (T)—have been used to predict continuous-cooling transformation kinetics which are in close agreement with measurements at four
cooling rates ranging from 7.5 to 64 °C per second. 相似文献
2.
Dev Venugopalan 《Metallurgical and Materials Transactions A》1990,21(3):913-918
The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation
to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine
the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper,
the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs
at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite,
has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing
number density of graphite spheroids. Predictions of the model are compared with data available in literature. 相似文献
3.
A model has been developed to describe the microscopic behavior of phase transformation of carbon steels in the range of cooling
rate occurring in continuous casting. In the liquid-to solidphase transformation, this model simulates the phenomena of dendrite
nucleation and growth during solidification. Both δ- and γ-dendrites are involved. The nucleation and growth model has been established on the basis of published experimental
data and previous work. Also, a model of the peritectic transformation of carbon steels has been included. In the solid-to
solidphase transformation, the model considers the δ→ γ, γ→ α, and γ→ α + Fe3C phase transformations. The δ→ γ and γ→ α phase transformations have been modeled by using the Johnson-Mehl equation, also known as the Avrami equation. For the
pearlite transformation, a nucleation law, as well as the growth kinetics, has been established. Good agreement has been found
between the prediction of the model and the experimental data. 相似文献
4.
Young IL Son Young-Kook Lee Kyung-Tae Park 《Metallurgical and Materials Transactions A》2006,37(10):3161-3164
Reverse transformation characteristics of a low-carbon steel consisting of ultrafine-grained (UFG) ferrite and severely deformed
pearlite by severe plastic deformation were investigated and compared to those of the steel having coarse-grained (CG) ferrite
and undeformed pearlite by austenitization and subsequent air cooling. Coarse-grained steel exhibited two serial transformation
stages, i.e., pear-lite → austenite followed by ferrite → austenite. Contrarily, UFG steel transformed with the three serial stages, i.e., probably carbon-supersaturated ferrite → austenite, not-fully-dissolved pearlite → austenite, and ferrite → austenite transformations. 相似文献
5.
Continuous cooling transformation (CCT) diagrams for HSLA-80 and HSLA-100 steels pertaining to fusion welding with heat inputs
of 10 to 40 kJ/cm, and peak temperatures of 1000 °C to 1400 °C have been developed. The corresponding nonlinear cooling profiles
and related γ → α phase transformation start and finish temperatures for various peak temperature conditions have been taken
into account. The martensite start (M
s
) temperature for each of the grades and ambient temperature microstructures were considered for mapping the CCT diagrams.
The austenite condition and cooling rate are found to influence the phase transformation temperatures, transformation kinetics,
and morphology of the transformed products. In the fine-grain heat-affected zone (FGHAZ) of HSLA-80 steel, the transformation
during cooling begins at temperatures of 550 °C to 560 °C, and in the HSLA-100 steel at 470 °C to 490 °C. In comparison, the
transformation temperature is lower by 120 °C and 30 °C in the coarse-grain heat-affected zone (CGHAZ) of HSLA-80 steel and
HSLA-100 steel, respectively. At these temperatures, acicular ferrite (AF) and lath martensite (LM) phases are formed. While
the FGHAZ contains a greater proportion of acicular ferrite, the CGHAZ has a higher volume fraction of LM. Cooling profiles
from the same peak temperature influence the transformation kinetics with slower cooling rates producing a higher volume fraction
of acicular ferrite at the expense of LM. The CCT diagrams produced can predict the microstructure of the entire HAZ and have
overcome the limitations of the conventional CCT diagrams, primarily with respect to the CGHAZ. 相似文献
6.
P. C. Campbell E. B. Hawbolt J. K. Brimacombe 《Metallurgical and Materials Transactions A》1991,22(11):2779-2790
In the second part of this paper, the microstructural evolution and mechanical properties of plain-carbon steel rods which
have been subjected to known cooling conditions are described. Specifically, the isothermal phase transformation kinetics
for the decomposition of austenite into ferrite and pearlite have been determined with a diametral dilatometer and characterized
in terms of empirical coefficients in the Avrami equation. The continuous cooling transformation (CCT) start time, fraction
ferrite, ferrite grain diameter, and pearlite interlamellar spacing have been quantified and correlated with steel composition
and cooling rate. Tensile tests have been conducted to obtain yield strength (YS) and ultimate tensile strength (UTS), which,
with literature data, have been related to the microstructure and composition of the steels. These correlations, which apply
to both hypoeutectoid and eutectoid steels, have been incorporated in a mathematical model of the Stelmor process, to be described
in Part III of this article.[441]
Formerly Graduate Student, The University of British Columbia. 相似文献
7.
M. B. Kuban Ph.D. R. Jayaraman E. B. Hawbolt J. K. Brimacombe 《Metallurgical and Materials Transactions A》1986,17(9):1493-1503
The limits of applicability of the Additivity Principle, necessary for the prediction of continuous-cooling transformation
kinetics from isothermal transformation data, are clarified based on an analysis of recently measured austenite-to-pearlite
transformation kinetics in a eutectoid, plain-carbon steel. It has been found that additivity holds for the transformation
event, exclusive of the incubation period, in this steel. But the isokinetic condition defined by Avrami, and the early site-saturation
criteria postulated by Cahn as sufficient conditions for additivity are not satisfied. Thus a new condition, termed “effective
site saturation”, is proposed in which the growth of pearlite nucleated early in the transformation dominates the overall
kinetics of austenite decomposition. A criterion for effective site saturation has been established. 相似文献
8.
利用Thermecmastor-Z型热模拟试验机,结合金相显微镜(OM)、扫描电镜(SEM)、维氏硬度计等,系统研究了奥氏体区变形对50CrV4钢连续冷却相变和等温相变规律的影响。建立了试验钢动态CCT曲线。研究结果表明,奥氏体变形能促进连续冷却转变过程中铁素体-珠光体、贝氏体转变,但亦可提高奥氏体的机械稳定性,进而抑制马氏体转变,Ms点由331.6℃(奥氏体未变形)降低至291℃(950℃下变形50%+890℃下变形50%,变形速率均为5s-1,变形后冷速为20℃/s)。当轧后冷速小于0.5℃/s时,试验钢中可获得铁素体+珠光体组织。此外,在研究不同变形量对试验钢等温相变规律影响时发现,650℃等温时,试验钢中发生铁素体-珠光体相变。随着变形量的增加(由30%增加至50%),其等温相变动力学加快(相变完成时间由197.6s减小至136.5s),铁素体体晶粒尺寸、珠光体片层间距减小,硬度增加。 相似文献
9.
The eutectoid transformation of austenite in cast iron is known to proceed by both the meta-stable γ → α + Fe3C reaction common in Fe-C alloys of near eutectoid composition, and by the direct γ → α + Graphite reaction, with the graphite
phase functioning as a car-bon sink. In addition, the meta-stable cementite constituent of the pearlite can dissolve near
the graphite phase (Fe3C → α + Graphite), producing free ferrite. Isothermal trans-formation studies on a typical ductile iron (nodular cast iron)
confirmed that all of these reaction mechanisms are normally operative. The addition of 1.3 pct Mn was found to substantially
retard all stages of the transformation by retarding the onset of the eutectoid transformation, decreasing the diffusivity
of carbon in ferrite, and stabilizing the cemen-tite. Minor additions of Sb (0.08 pct) or Sn (0.12 pct) were found to inhibit
the γ →α + Graphite reaction path, as well as the Fe3C → α + Graphite dissolution step, but did not significantly affect the meta-stable γ → α + Fe3C reaction. Scanning Auger microprobe analysis indicated that Sn and Sb adsorb at the nodule/metal interphase boundaries during
solidification. This adsorbed layer acts as a barrier to the carbon flow necessary for the direct γ → α + Graphite and Fe3C → α + Graphite reactions. With the graphite phase dis-abled as a sink for the excess carbon, the metal transforms like a
nongraphitic steel. The effects of Mn, Sn, and Sb on the eutectoid transformation of ductile iron were shown to be consistent
with their behavior in malleable iron. 相似文献
10.
In-situ spatially resolved X-ray diffraction (SRXRD) experiments were performed during gas tung-sten arc (GTA) welding of AISI 1045
C-Mn steel. Ferrite (α) and austenite (γ) phases were identified and quantified in the weld heat-affected zone (HAZ) from
the real time SRXRD data. The results were compiled with weld temperatures calculated using a coupled thermal fluids model
to create a phase map of the HAZ. Kinetics of the α → γ transformation during weld heating and the reverse γ → α transformation
during weld cooling were extracted from the map. Superheating as high as 250 °C above the A3 temperature was observed for
the α → γ phase transformation to reach completion at locations near the fusion zone (FZ) boundary. The SRXRD experiments
revealed that the newly created γ phase exists with two distinct lattice parameters, resulting from the inhomogeneous distribution
of carbon and manganese in the starting pearlitic/ferritic microstructure. During cooling, the reverse γ → α phase transformation
was shown to depend on the HAZ location. In the fine-grained region of the HAZ, the γ → α transformation begins near the A3
temperature and ends near the A1 temperature. In this region, where the cooling rates are below 40 °C/s, the transformation
occurs by nucleation and growth of pearlite. In the coarse-grained region of the HAZ, the γ → α transformation requires 200
°C of undercooling for completion. This high degree of undercooling is caused by the large grains coupled with cooling rates
in excess of 50 °C/s that result in a bainitic transformation mechanism. 相似文献
11.
Ph. Maitrepierre D. Thivellier R. Tricot 《Metallurgical and Materials Transactions A》1975,6(1):287-301
The influence of boron on the isothermal decomposition of Fe-Ni6-C0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron
slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history.
Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied.
Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates
easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation
was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates
were identified as Fe23(B, C)6 (FCC structure with a ≈ 10.6?). The grain boundary Fe23(B, C)6 were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe23(B, C)6 precipitates with respect to the γ → proeutectoid ferrite reaction is discussed. 相似文献
12.
S. Darzens D. R. Mumm D. R. Clarke A. G. Evans 《Metallurgical and Materials Transactions A》2003,34(3):511-522
Observations and measurements have been performed on a thermal barrier system comprising a Ptaluminide bond coat and a thermal
barrier coating deposited by electron beam evaporation. Past research has highlighted a displacement instability in the thermally
grown oxide (TGO), as it affects the failure mechanism in the thermal barrier coating (TBC). Phase transformations in the
bond coat have also been identified, with a proposed role in the TGO instability. The present study assesses this influence
by characterizing the transformations as well as their spatial correlation with the instability sites. Both the isothermal
transformation from β→γ′ and the martensite transformation in the β have been addressed. Toward the end of life, the instabilities are preferentially located in the β phase, between neighboring domains of γ′. After cycling, the composition of the β grains is spatially uniform. Within the γ′, there are Ni and Al composition gradients in narrow layers near the interfaces with the β phase and the TGO. An evaluation suggests that the primary influence of transformation on the cyclic displacement of the
TGO is to create a local misfit between the growing γ′ domains and the volume strain accompanying the martensite transformation in the intervening β phase, upon cooling and reheating. 相似文献
13.
14.
Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels 总被引:1,自引:0,他引:1
A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon
steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind
the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all
nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which
can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted
by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates
the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified
by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between
nucleation site distribution and the nucleation rate is demonstrated by computer simulation. 相似文献
15.
C. Capdevila F. G. Caballero C. García de Andrés 《Metallurgical and Materials Transactions A》2001,32(3):661-669
The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite
in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution
of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid
temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast,
since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided,
and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected,
and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite
with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent
agreement (higher than 93 pct in R
2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of
temperature studied. 相似文献
16.
Ferrite and bainite in alloy steels 总被引:1,自引:0,他引:1
The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite.
The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and
precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized.
Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic
factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property
relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy
steels by direct transformation of austenite is explored. 相似文献
17.
The influence of long time intercritical heat-treatments (720 to 750 °C) on the impact and tensile properties of ferrite/pearlite
steels has been examined. Intercritical annealing enables Mn to diffuse to the α/ γ boundaries and refine the grain-boundary carbides on cooling to room temperature. When the resulting microstructure formed
on cooling to room temperature is ferrite/pearlite, this treatment can result in a significant improvement in impact behavior
compared to a normalizing treatment. On heating to the intercritical annealing temperature, Mn enrichment of the γ combined
with a fast enough cooling rate results in martensite formation. This may be beneficial to both strength and impact behavior
at low volume fractions of martensite, but a marked deterioration in impact behavior occurs at high volume fractions, even
though grain-boundary carbides remain fine. 相似文献
18.
Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI
1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating
cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the
γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows
the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C.
This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal
pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures,
carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original
austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300
°C and above, depending on the weld input power. 相似文献
19.
Sandeep Sangal Naresh C. Goel Kris Tangri 《Metallurgical and Materials Transactions A》1985,16(11):2023-2029
The role of metastable retained austenite(γ
R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of
‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived
in Part I of this paper. It has been found that for a given volume fraction ofγ
R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ
R does not contribute either to strength or ductility of the steel, highly unstableγ
R which causes an increase in the strength is detrimental to ductility. Aγ
R which is moderately stable and undergoesγ
R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ
R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ
R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels. 相似文献
20.
J.Y. Hwang S. Nag A.R.P. Singh R. Srinivasan J. Tiley G.B. Viswanathan H.L. Fraser R. Banerjee 《Metallurgical and Materials Transactions A》2009,40(13):3059-3068
The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has
been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy
(EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale
secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr
while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary
precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary
γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process. 相似文献