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1.
Ryo Saito Nobuo Nakada Shouhei Yabu Kohtaro Hayashi 《Metallurgical and Materials Transactions A》2018,49(12):6001-6009
Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface?>?pearlite block?>?colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels. 相似文献
2.
G. R. Speich V. A. Demarest R. L. Miller 《Metallurgical and Materials Transactions A》1981,12(8):1419-1428
The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of
1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of
most dual-phase steels.
Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution
is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at
high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures
(~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion
in austenite. Diffusion models for the various steps were analyzed and compared with experimental results. 相似文献
3.
Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel 总被引:1,自引:0,他引:1
D. Z. Yang E. L. Brown D. K. Matlock G. Krauss 《Metallurgical and Materials Transactions A》1985,16(8):1385-1392
The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si
steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between
650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation
initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix.
The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence
the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries
of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries.
Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were
the sites for later austenite formation. 相似文献
4.
K. Han G. D. W. Smith D. V. Edmonds 《Metallurgical and Materials Transactions A》1995,26(7):1617-1631
Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase
transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar
products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products
comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of
vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually
decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain
boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1
to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects
of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions,
an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the
rate of interphase precipitation of vanadium carbides.
Formerly Graduate Student, Department of Materials, Oxford University
Formerly University Lecturer, Department of Materials, Oxford University 相似文献
5.
A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed. 相似文献
6.
A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries
in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction
technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary;
the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed
an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a
lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation
relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced
by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into
the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared
with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed. 相似文献
7.
Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels 总被引:1,自引:0,他引:1
M. Tokizane N. Matsumura K. Tsuzaki T. Maki I. Tamura 《Metallurgical and Materials Transactions A》1982,13(8):1379-1388
The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low
carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath
martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched
specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite
grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath
martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially
at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries.
On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly
progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly
fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation
proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain
structure was obtained most effectively. 相似文献
8.
J. R. Bradley H. I. Aaronson K. C. Russell W. C. Johnson 《Metallurgical and Materials Transactions A》1977,8(12):1955-1961
Austenitizing an Fe-0.23 pct C alloy at 1300°C and further at 900°C prior to isothermal transformation was found to increase
the growth kinetics of grain boundary ferrite allotriomorphs while decreasing their rate of nucleation. A scanning Auger microprobe
was used to establish that sulfur segregates to the austenite grain boundaries and does so increasingly with decreasing austenitizing
temperature. A binding free energy of sulfur to these boundaries of approximately 13 kcal/mole (54.4 kj/mole) was calculated
from theMcLean adsorption isotherm. The kinetic results were explained in terms of preferential reduction of the austenite
grain boundary energy decreasig nucleation kinetics, and adsorption of sulfur at α:γ boundaries increasing the carbon concentration
gradient in austenite driving growth. 相似文献
9.
10.
Gregory N. Haidemenopoulos 《国际钢铁研究》1996,67(3):93-99
Tr ansformation i nduced p lasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Si steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels. 相似文献
11.
M. M. Souza J. R. C. Guimarães K. K. Chawla 《Metallurgical and Materials Transactions A》1982,13(4):575-579
With the introduction of dual phase steels, it is increasingly becoming important to obtain a thorough understanding of intercritical
austenitization phenomena. Quantitative microscopy techniques were used to study the process of intercritical austenitization
(740°C) of two Fe-Mn-C steels, one of them being microalloyed with Nb. The two steels showed essentially the same kinetics,viz., three stages of intercritical austenitization: (i) austenite growth into pearlite until complete pearlite dissolution, (ii)
growth of austenite into ferrite, and (iii) equilibration of ferrite and austenite. However, compared to data published by
other researchers, the maximum amount of austenite, in our case, was reached much faster. Ferrite-ferrite interface processes
and preferred nucleation at particles in the ferrite boundaries accelerated the austenite growth. Austenite growth out of
pearlite colonies was asymmetric due to the fast ferrite-ferrite interface processes. 相似文献
12.
Ming-Chun Zhao Toshihiro Hanamura Hai Qiu Ke Yang 《Metallurgical and Materials Transactions A》2006,37(5):1657-1664
The microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels
with 0.15 pct carbon content and 0.45 pct carbon content upon annealing below the austenized temperature was investigated.
The average grain sizes of the ferrites with a normal density and with a high density of cementite particles were plotted,
respectively, as a function of the annealed temperature and time, and exhibited different coarsening behaviors. The average
grain sizes of the ferrites with a normal density of cementite particles gradually coarsened by increasing the annealing temperature
or time, while those with a high density of cementite particles hardly changed at first, and then coarsened after reaching
a certain annealing condition. The coarsening of the ferrite grain size in the steel with 0.15 pct carbon content occurred
much more readily than that in the steel with 0.45 pct carbon content upon annealing. The spacing and the critical spacing
of cementite particle were measured and hypothetically calculated, respectively. The size and the distribution of cementite
particles was one of the critical factors affecting the microstructural evolution in this type of cementite particle spherodized
steels. Most of the coarsening of the ferrite grain size occurred after the cementite particle spacing reached the required
critical value. 相似文献
13.
The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that
had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite
and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final
microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures.
The results of this study have shown that the 1.5 pct Mn has lowered both the AC3 and AC1, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were
not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more
rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures. 相似文献
14.
15.
The nucleation of intragranular ferrite from austenite in Fe-Ni-P alloys was investigated in order to understand the development of the Widmanstätten pattern in iron meteorites. Alloys containing 5 to 10 wt pct Ni and 0 to 1 wt pct P were used to simulate iron meteorite compositions. In the isothermal and controlled cooling experiments the reaction path γ → α+ γ serves only to nucleate ferrite along austenite grain boundaries. It is necessary for (FeNi)3P to be present within y grains in order to nucleate intragranular ferrite. The reaction path γ → γ+ phosphide → α + γ + phosphide yields rod shaped ferrite nuclei that bear a near Kurdjumov-Sachs orientation relationship with the surrounding matrix. The precipitation of ferrite, both along grain boundaries and within the austenite grains, is suppressed in the absence of P. 相似文献
16.
T. Karthikeyan Manmath Kumar Dash S. Saroja M. Vijayalakshmi 《Metallurgical and Materials Transactions A》2013,44(4):1673-1685
The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite. 相似文献
17.
Bin Zhu Yisheng Zhang Chao Wang Pei Xing Liu Wei Kang Liang Jian Li 《Metallurgical and Materials Transactions A》2014,45(7):3161-3171
A model for simulating the austenitization of ultra-high strength steel during hot stamping is developed using a cellular automata approach. The microstructure state before quenching can be predicted, including grain size, volume fraction of austenite, and distribution of carbon concentration. In this model, a real initial microstructure is used as an input to simulate austenitization, and the intrinsic chemical difference is utilized to describe the ferrite and pearlite phases. The kinetics of austenitization is simulated by simultaneously considering continuous nucleation, grain growth, and grain coarsening. The UHSS is reduced to a Fe-Mn-C ternary system to calculate the driving force during extent growth in ferrite. The simulation results show that the transformation of ferrite to austenite can be divided into three stages in the condition of a heating rate of 10 K (?263 °C)/s. The transformation rate is determined by two factors, carbon concentration and temperature. The carbon concentration plays a major role at the early stages, as well as the temperature is the main factor at the later stages. The A c3 calculated is about 1073 K (800 °C) close to the measured value [1067.1 K (794.1 °C)]. Austenite grain coarsening was calculated by a curvature-driven model. The simulated morphology of the microstructure agrees well with the experimental result. Most of the dihedrals of the grain boundaries at the triple junctions are close to 120 deg. Finally, tensile tests were implied, as dwelling time increased from 3 to 10 minutes, the austenite grain size increased from 6.95 to 9.44 μm while the tensile strength decreased from 276.4 to 258.3 MPa. 相似文献
18.
High purity iron alloys containing molybdenum and carbon have been isothermally transformed in the range 600° to 900°C and the structures examined optically and in the electron microscope. The decomposition of the austenite commences at the grain boundaries forming two different ferrite/carbide morphologies. Predominantly, fine fibers of Mo2C, 300 to 500Å diam, grow in association with ferrite and in contact with the austenite, forming characteristic nodules. Increasing the carbon content from 0.2 to 1.0 wt pct results in a change in structure during transformation from Mo2C fibers/ferrite to a typical coarse pearlite reaction. In addition, Mo2C nucleates repeatedly at the moving λ-α phase boundary to form sheets of particles about 200 to 400Å apart, the individual acicular particles being about 50Å diam in the early stages of precipitation. 相似文献
19.
Dong Jun Mun Eun Joo Shin Kyung Chul Cho Jae Sang Lee Yang Mo Koo 《Metallurgical and Materials Transactions A》2012,43(5):1639-1648
The behavior of boron (B) segregation to austenite grain boundaries in low carbon steel was studied using particle tracking
autoradiography (PTA) and secondary ion mass spectroscopy (SIMS). An effective time method was used to compare the cooling
rate (CR) dependence of this segregation during continuous cooling and its time dependence during isothermal holding. Comparison
of these segregation behaviors has confirmed that the CR dependence of B segregation agrees well with its time dependence
and is mainly a result of the phenomenon of nonequilibrium segregation. Based on the CR dependence of B segregation, the continuous
cooling transformation behavior of B-bearing steel as compared with B-free steel was also investigated using dilatometry and
microstructural observations. The addition of a small amount of B to low carbon steel retarded significantly the austenite-to-ferrite
transformation and finally expanded the range of cooling programs that result in the formation of bainitic microstructures.
Analysis of the B distribution has confirmed that this retardation effect of B on ferrite transformation is attributed to
the CR dependence of B segregation to austenite grain boundaries during cooling after austenitization. 相似文献
20.
G. Spanos J. D. Ayers C. L. Vold I. E. Locci 《Metallurgical and Materials Transactions A》1993,24(4):809-818
The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified
Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen
consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-μm long and 0.1-and 0.5 -μm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for
10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates.
The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation
relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 μm in thickness and can be as
long as 0.5 μm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a
view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching
the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds,
and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect
to one another and typically 0.5 μm or less in both length and thickness and cementite particles 0.4 μm or less in size. (Occasionally,
longer martensite plates were observed; but they never exceeded 1 μm in length.) The microstructures produced here offer the
potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with
the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided
the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing
similar microstructures which will also exhibit enhanced high-temperature stability. 相似文献