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1.
It has been generally believed that pearlite transformation in hypoeutectoid steels starts when the average carbon concentration in untransformed austenite reaches the A cm line after the formation of proeutectoid ferrite. To test this concept experimentally, volume fractions of proeutectoid ferrite/pearlite and carbon contents in the austenite being transformed into pearlite were measured for the Fe-2Mn-0.3C alloy isothermally transformed in the temperature range 848 K to 898 K (575 °C to 625 °C). It was found that lamellar pearlite can form even when the average carbon content in untransformed austenite is much lower than the A cm line. This peculiar observation is probably due to the two-dimensional diffusion of carbon, i.e., parallel to and normal to the austenite/pearlite interface, which enables lamellar cementite to grow continuously by supplying carbon atoms to its growth front. This results in proeutectoid ferrite fractions with respect to pearlite being much lower than those predicted by the lever rule. With decreasing prior austenite grain size, proeutectoid ferrite fractions with respect to pearlite were found to increase, but the thickness of proeutectoid ferrite was constant within the range of grain size investigated. This is due to the existence of the critical α/ γ interface velocity only below which pearlite (actually cementite) can be nucleated at the migrating α/ γ interface. Furthermore, the upper limit temperatures for pearlite formation in the Fe-1Mn-0.33C and Fe-2Mn-0.3C alloys were found to be well between the PLE/NPLE and PE A e1 temperatures. 相似文献
2.
Austenitization kinetics of pearlite and ferrite aggregates, in a low carbon steel containing 0.15 wt pct C. were studied
in the temperature range of 800 to 870 °C. The transformation was monitored by estimating the volume fraction of austenite
and the surface area per unit volume of the austenite-ferrite interfaces by quantitative microscopy. It is concluded that
the austenite growth geometry is two dimensional. Between 800 and 840 °C, the two dimensional radial growth of the austenite
particles can be described by a simple parabolic law. This suggests a diffusion controlled growth mechanism. However, at 870
°C, the kinetics of the process are different from those at the lower temperatures studied, and the radial growth rate approaches
a constant value at long times. 相似文献
3.
Partitioning of chromium between cementite and ferrite during the austenite to pearlite transformation in a eutectoid steel
containing 1.29 pct chromium has been studied using analytical electron microscopy. No partitioning occurred at the austenite-pearlite
interface below 703°C (the no-partition temperature), while above this temperature chromium partitioned preferentially to
cementite at the transformation front. Chromium segregation to cementite occurred at all transformation temperatures after
pearlite had formed. Measurements of pearlite growth rate and interlamellar spacing have been made for a range of transformation
temperatures, and used to examine the rate controlling process for pearlite growth below the no-partition temperature. Growth
rates calculated assuming volume diffusion of carbon to be rate controlling were in reasonable agreement with measured growth
rates, although the discrepancies between the rates could be accounted for by the partial involvement of interfacial diffusion.
Formerly affiliated. 相似文献
4.
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
5.
Continuous cooling transformation diagrams were determined for molybdenum-boron steels containing 0.24, 0.4, and 0.66 pct Mo with 0.1 pct C, and also 0.4 pct Mo with 0.2 pct C, after thermomechanically working by compressive deformation to 12, 25, and 50 pct reduction at 830°C (1525°F), as well as for the steels in the underformed condition. In underformed specimens, higher carbon or molybdenum decreased the limiting cooling rate for the avoidance of polygonal ferrite formation. The same was true for deformed specimens, although increased deformation raised the limiting cooling rates of all compositions. The limiting cooling rate for polygonal ferrite formation increased exponentially with austenite, deformation, as measured by true strain. Thermomechanical working also raised bainite start temperatures at fast cooling rates and caused small increases in martensite start temperatures. 相似文献
6.
Substitutional alloying effects on reversion kinetics from pearlite structure at 1073 K (800 °C) in an Fe-0.6 mass pct C binary
alloy and Fe-0.6C-1 or 2 mass pct M (M = Mn, Si, Cr) ternary alloys were studied. Reverse transformation in the Fe-0.6C binary
alloy at 1073 K (800 °C) was finished after holding for approximately 5.5 seconds. The reversion kinetics was accelerated
slightly by the addition of Mn but retarded by the addition of Si or Cr. The difference of acceleration effects by the addition
of the 1 and 2 mass pct Mn is small, whereas the retardation effect becomes more significant by increasing the amount of addition
of Si or Cr. It is clarified from the thermodynamic viewpoint of carbon diffusion that austenite can grow without partitioning
of Mn or Si in the Mn- or Si-added alloys. On the one hand, austenite growth is controlled by the carbon diffusion, whereas
the addition of them affects carbon activity gradient, resulting in changes in reversion kinetics. On the other hand, thermodynamic
calculation implies that the long-range diffusion of Cr is necessary for austenite growth in the Cr-added alloys. It is proposed
that austenite growth from pearlite in the Cr-added alloys is controlled by the diffusion of Cr along austenite/pearlite interface. 相似文献
7.
Partitioning of chromium between ferrite and cementite during the isothermal decomposition of austenite to pearlitic or pearlitic/bainitic
decomposition products has been studied in a 1.4 wt pct Cr eutectoid steel using analytical electron microscopy on two-stage
extraction replicas. Chromium was observed to segregate preferentially to cementite at the pearlite reaction front for temperatures
in the range 730 to 550 °C. Although the extent of partitioning decreased with decreasing reaction temperature, a no-partition
temperature could not be identified for the steel. It is clear that previous studies on thin foils have underestimated the
temperature range over which partitioning of chromium can occur. At high reaction temperatures measured values of pearlite
growth rates were found to be in excellent agreement with those calculated, using the assumption that phase boundary diffusion
of chromium was rate controlling. At lower reaction temperatures models based on volume diffusion of carbon and on phase boundary
diffusion of chromium both gave reasonable predictions of measured growth rates. However, it seems likely that solute drag
effects influence pearlite growth at temperatures in the austenite bay region which the chromium addition produces in the
T.T.T. diagram. Measurements made on upper bainite which co-existed with pearlite following transformation at 500 and 550
°C showed that preferential partitioning of chromium to cementite did not occur during this reaction.
Formerly Graduate Student, University of Manchester 相似文献
8.
The plastic-flow behavior of ferrite + pearlite, pearlite + cementite, and austenite + cementite mixtures in plain carbon
steels has been examined over the temperature range 500 to 1050 °C, strain-rate range 6 x l0 −6 to 2 x l0 −2 s −1, and carbon range 0.005C to 1.89C. Up to the eutectoid temperature the strength of the ferrite + pearlite mixture more than
doubles as the carbon content increases from 0.005C to 0.7C, so that whereas in low-carbon steels the ferrite is weaker than
the higher temperature austenite phase, in eutectoid steels the fully pearlitic structure is stronger than the fully austenitic
structure. Manganese and silicon strengthen ferrite more effectively than they do austenite. A 0.17 pct phosphorus addition
strengthens the ferrite + pearlite mixture across the range of microstructures from fully ferritic to fully pearlitic. Beyond
the eutectoid composition, the amount of proeutectoid cementite does not significantly affect the strength of the pearlite,
but above the eutectoid temperature it appreciably strengthens the austenite and cementite mixture at the strain rate of 2
X 10 -2 s -1. 相似文献
9.
Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties. 相似文献
10.
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
11.
Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy
at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21
and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as
a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite
transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits
similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus
density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during
the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination
of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation
kinetics during heating, i.e., austenite formation from ferrite and pearlite. 相似文献
12.
The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been
examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms
on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms
by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free
in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In
the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed
at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include
lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed
at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect
of low austenite carbon content. 相似文献
13.
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
14.
Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It
has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted
of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate
relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary
(superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should
be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were
found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse
grained, condition.
BRUNO WALSER, formerly with Department of Materials Science and Engineering, Stanford University 相似文献
15.
The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr
plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams
of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally
in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate
(bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation
and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free)
0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel
transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion
of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are
present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum
into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures
and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action,
molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel
and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation,
thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial
effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are
reflected in terms of higher hardenability. 相似文献
16.
In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s−1) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form. 相似文献
17.
Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the mechanical
properties of pearlitic steels being developed for wire rod production. Mechanical test results demonstrate that the alloy
additions are beneficial to the mechanical properties of the steels, especially the tensile strength. Silicon strengthens
pearlite mainly by solid-solution strengthening of the ferrite phase. Vanadium increases the strength of pearlite mainly by
precipitation strengthening of the pearlitic ferrite. When added separately, these elements produce relatively greater strengthening
at higher transformation temperatures. When added in combination the behavior is different, and substantial strength increments
are produced at all transformation temperatures studied (550 °C to 650 °C). The addition of silicon and vanadium to very-high-carbon
steels (>0.8 wt pct C) also suppresses the formation of a network of continuous grain-boundary cementite, so that these hypereutectoid
materials have high strength coupled with adequate ductility for cold drawing. A wire-drawing trial showed that total drawing
reductions in area of 90 pct could be obtained, leading to final tensile strengths of up to 2540 MPa in 3.3-mm-diameter wires. 相似文献
18.
The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen. 相似文献
19.
In excess of 30 vol. pct austenite can be retained in 0.3C-4.0Mn steels subjected to a dual stabilization heat treatment (DSHT) schedule—a five stage precisely controlled cooling schedule that is a variant of the quench and partition process. The temperature of the second quench (stage III) in the DSHT process plays an essential role in the retained austenite contents produced at carbon-partitioning temperatures of 723 K or 748 K (450° C or 475 °C) (stage IV). A thermodynamic model successfully predicted the retained austenite contents in heat-treated steels, particularly for a completely austenitized material. The microstructure and mechanical behavior of two heat-treated steels with similar levels of retained austenite (~30 vol. pct) were studied. Optimum properties—tensile strengths up to 1650 MPa and ~20 pct total elongation—were observed in a steel containing 0.3C-4.0Mn-2.1Si, 1.5 Al, and 0.5 Cr. 相似文献
20.
The isothermal austenite decomposition kinetics is studied in 0.004 wt pct C ultralow carbon (ULC) and 0.11 wt pct C low-carbon (LC) steel using high-speed quenching dilatometry. Standard samples of these steels are heated to austenitization temperatures of 1223 K and 1373 K (950 °C and 1100 °C) and then quenched to testing temperatures between 1163 K and 933 K (890 °C and 660 °C). The measured and calculated austenite-to-ferrite phase fractions are compared with dilatation values to analyze the ferrite nucleation and growth conditions during austenite decomposition. Ferrite evolution profiles are assessed to investigate the underlying growth kinetics. The analysis in ULC steel shows regimes of partitionless, partitioning, and two-stage transformation kinetics. In contrast, LC steel shows only diffusion-controlled transformation kinetics. The experimental results are well reproduced with thermokinetic calculations, thus supporting our interpretation of governing mechanisms during transformation. 相似文献
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