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1.
Key parameters for a thermomechanically controlled processing and accelerated cooling process (TMCP-AcC) were determined for
integrated mass production to produce extra high-yield-strength microalloyed low carbon SiMnCrNiCu steel plates for offshore
structure and bulk shipbuilding. Confocal scanning microscopy was used to make in-situ observations on the austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate
the flow stress behavior, static recrystallization (SRX) of austenite, and decomposition behavior of the TMCP conditioned
austenite during continuous cooling. The Kocks–Mecking model was employed to describe the constitutive behavior, while the
Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach was used to predict the SRX kinetics. The effects of hot rolling schedule and
AcC on microstructure and properties were investigated by test-scale rolling trials. The bridging between the laboratory observations
and the process parameter determination to optimize the mass production was made by integrated industrial production trials
on a set of a 5-m heavy plate mill equipped with an accelerated cooling system. Successful production of 60- and 50-mm-thick
plates with yield strength in excess of 460 MPa and excellent toughness at low temperature (213 K (–60 °C)) in the parent
metal and the simulated coarse-grained heat affected zone (CGHAZ) provides a useful integrated database for developing advanced
high-strength steel plates via TMCP-AcC. 相似文献
2.
With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation. 相似文献
3.
The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless
steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in
301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in
the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes
of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination
of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct
yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase
in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile
deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase
transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain
and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic
transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic
deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made
great contribution to the excellent mechanical properties in UFG austenitic SS. 相似文献
4.
Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV. 相似文献
5.
Austenite formation during intercritical annealing was studied in a cold-rolled dual-phase (DP) steel based on a low-carbon
DP780 composition processed in the mill. Two heating rates, 10 and 50 K/s, and a range of annealing temperatures from 1053 K
to 1133 K (780 °C to 860 °C) were applied to study their effects on the progress of austenitization. The effect of these process
parameters on the final microstructures and mechanical properties was also investigated using a fixed cooling rate of 10 K/s
after corresponding annealing treatments. It was found that the heating rate affects the austenite formation not only during
continuous heating, but also during isothermal holding, and the effect is more pronounced at lower annealing temperatures.
Faster heating delays the recrystallization kinetics of the investigated steel. The rate of austenite formation and its distribution
are strongly influenced by the extent of overlapping of the processes of recrystallization and austenitization. It appeared
that the heating rate and temperature of intercritical annealing have a stronger effect on the final tensile strength (TS)
of the DP steel than holding time. Both higher annealing temperatures and long holding times minimize the strength difference
caused by a difference in heating rate. 相似文献
6.
This article presents a study of fatigue-crack propagation behavior in Nitinol, a 50Ni-50Ti (at. pct) superelastic/shape-memory
alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance.
Specifically, fatigue-crack growth was characterized in stable austenite (at 120 °C), superelastic austenite (at 37 °C), and
martensite (at −65 °C and − 196 °C). In general, fatigue-crack growth resistance was found to increase with decreasing temperature,
such that fatigue thresholds were higher and crack-growth rates slower in martensite compared to stable austenite and superelastic
austenite. Of note was the observation that the stress-induced transformation of the superelastic austenite structure, which
occurs readily at 37 °C during uniaxial tensile testing, could be suppressed during fatigue-crack propagation by the tensile
hydrostatic stress state ahead of a crack tip in plane strain; this effect, however, was not seen in thinner specimens, where
the constraint was relaxed due to prevailing plane-stress conditions. 相似文献
7.
Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of
the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical
behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile
testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which
these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with
increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed
at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when
deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured
in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3
joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms
(dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a
homogeneous subgrain structure. 相似文献
8.
Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced
plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental
temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures
are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent
on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that
display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using
in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between
annealing conditions help explain the observed strain hardening behaviors. 相似文献
9.
BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened
by Cu-rich precipitates and M 2C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms.
Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate
susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite
grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic
boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries
of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic
(face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility
values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility
values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic
calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high
Ni content of the BA-160 alloy. 相似文献
10.
In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure o°Curred in tensile test
at 600 °C to 700 °C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress
and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed
in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied
stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P,
S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very
significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more
brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this
embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced
both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries.
TAKATOSHI OGAWA, formerly with Kyoto University.
YOSHIFUMI OHMURA, formerly with Kyoto University.
This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville
Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy
and Heat Treatment Committees. 相似文献
11.
In this work, a hot compression test was carried out at 1173 K to 1473 K (900°C to 1200 °C), with a strain rate of 0.01 to
1 s −1 up to ~50 pct height reduction on functionally graded steel (FGS) specimens comprised of ferritic, bainitic, austenitic,
and martensitic layers ( αβγM γ). The stress-strain curves are strongly dependent on temperature and strain rate. Compressive flow stress varied from 40
to 105 MPa depending on the applied temperature and strain rates. Variation in steady-state flow stress with temperature and
strain rates was studied. The strain-rate-sensitivity exponent ( m) and deformation activation energy ( Q) for the αβγM γ composite under studied condition were 0.106 and 354.8 KJ mol −1, respectively, which are within the values of boundary layers of ferrite (304.9 KJ mol −1) and austenite (454.8 KJ mol −1) layers. Given the alternative microstructure of the αβγM γ FGS, a range of deformation mechanisms from dynamic recovery to dynamic recrystallization maybe prevails, where the intensity
of each mechanism depends on temperature and strain rates. In accordance with the experimental results, an empirical power-law
equation was developed over the range of temperatures and strain rates investigated. The equation accurately describes temperature
and strain-rate dependence of the flow stress. 相似文献
12.
In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure occurred in tensile test
at 600 °C to 700 °C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered
under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular
fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes
to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused
by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress
was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering,
the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle.
(4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was
considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain
boundaries.
Formerly with Kyoto University
Formerly with Kyoto University
This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville
Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy
and Heat Treatment Committees. 相似文献
13.
The effect of high-pressure torsion (HPT) and annealing on hydrogen embrittlement (HE) of a type 304 stainless steel was studied
by metallographic characterization and tensile test after hydrogen gas charging. A volume fraction of ~78 pct of the austenite
transformed to α′ martensite by the HPT processing at an equivalent strain of ~30. Annealing the HPT-processed specimen at a temperature of
873 K (600 °C) for 0.5 hours decreased the α′ martensite to ~31 pct with the average grain size reduced to ~0.43 μm through the reverse austenitic transformation. Hydrogen charge into the HPT-processed and the HPT+annealed specimens in
the hydrogen content of ~10 to 20 ppm led to no severe HE but appeared in the solution-treated specimen. Especially the 873 K
(600 °C) annealed specimen had the ~1.4 GPa tensile strength and the ~50 pct reduction of area (RA) despite the hydrogenation. 相似文献
14.
In-situ X-ray diffraction (XRD) measurements using high energy synchrotron radiation were performed to monitor in real time the formation
of delta ferrite in a martensitic 9 wt pct chromium steel under simulated weld thermal cycles. Volume fractions of martensite,
austenite, and delta ferrite were measured as a function of temperature at a 10 K/s heating rate to 1573 K (1300 °C) and subsequent
cooling. At the peak temperature, the delta ferrite concentration rose to 19 pct, of which 17 pct transformed back to austenite
on subsequent cooling. 相似文献
16.
The hot tensile test was used to investigate the effects of certain key variables on the hot ductility of low carbon steels. A transition from high to low ductility occurred at about 2200°F (1204°C) during continuous cooling of both wrought and cast specimens of low carbon steel after relatively brief exposure to temperatures above 2400°F (1316°C). The observed loss in ductility on cooling below 2200°F (1204°C): (a) increased with decreasing manganese-sulfur ratios, (b) was minimized by appropriate variations in thermal history. Metallographic and fractographic examination of the tensile specimens after thermal cycling indicated that this low ductility below 2200°F (1204°C) resulted from microcracking associated with (Mn, Fe)S precipitates found at the austenite grain boundaries. The results of this investigation help explain why different levels of hot ductility are observed in low carbon steels and what steps can be taken to improve this ductility. 相似文献
17.
The hot tensile test was used to investigate the effects of certain key variables on the hot ductility of low carbon steels.
A transition from high to low ductility occurred at about 2200°F (1204°C) during continuous cooling of both wrought and cast
specimens of low carbon steel after relatively brief exposure to temperatures above 2400°F (1316°C). The observed loss in
ductility on cooling below 2200°F (1204°C): (a) increased with decreasing manganese-sulfur ratios, (b) was minimized by appropriate
variations in thermal history. Metallographic and fractographic examination of the tensile specimens after thermal cycling
indicated that this low ductility below 2200°F (1204°C) resulted from microcracking associated with (Mn, Fe)S precipitates
found at the austenite grain boundaries. The results of this investigation help explain why different levels of hot ductility
are observed in low carbon steels and what steps can be taken to improve this ductility. 相似文献
18.
Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds −1 to 1 second −1 to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation
at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds −1 to 0.1 seconds −1, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction
or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame
for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore,
at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile
dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16
for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate
and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The
hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels.
However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental
data. 相似文献
19.
The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T)
process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C
to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay
law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower
than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at
473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel
further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the
maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization
reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed
fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained
austenite. 相似文献
20.
Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel.
The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures
from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K
and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M 3C and M 23C 6 carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously
at temperatures below 923 K (650 °C). One is lamellae of ferrite and M 3C carbide, and the other is lamellae of ferrite and M 23C 6 carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M 23C 6. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately. 相似文献
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