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1.
The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two
high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling)
and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C
to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the
dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start
temperatures (T
s
) of 550 °C to 560 °C while cooling from a peak temperature (T
p
) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T
s
value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation
to lath martensite. In HSLA-100 steel, the T
s
value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered
below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics
appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in
complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically
all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the
final microstructure and is harder than the CGHAZ of HSLA-100 steel. 相似文献
2.
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
3.
Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding 总被引:3,自引:0,他引:3
G. Spanos R. W. Fonda R. A. Vandermeer A. Matuszeski 《Metallurgical and Materials Transactions A》1995,26(12):3277-3293
The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone
(HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld
thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region,
the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high
heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer.
The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix
phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and
precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical
microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of
the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction
between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those
predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent
upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.
A. MATUSZESKI, formerly Summer Student, Physical Metallurgy Branch, Naval Research Laboratory. 相似文献
4.
HSLA-80 and HSLA-100 steels have been subjected to weld-simulated grain-coarsened heat-affected zone (GCHAZ) and grain-refined
heat-affected zone (GRHAZ) treatments at peak temperatures of 1350 °C and 950 °C, respectively, followed by varying cooling
rates to approximate the weld heat inputs of 10 to 50 kJ/cm. Subsequent slow strain rate testing in synthetic seawater has
been employed to assess the hydrogen embrittlement (HE) propensity of the materials. It is indicated that in spite of an increase
in strength after weld simulation, further ductility deterioration, compared to the base material under similar testing conditions,
did not occur in GCHAZ HSLA-100 steel and for low heat input condition of GRHAZ HSLA-80. This has been attributed to their
HE resistant microstructures. Predominant acicular ferrite or lath martensite or a combination of both imparts resistance
to HE, as observed in the case of grain-coarsened HSLA-100 and for the low heat input grain-refined HSLA-80 steels. The deleterious
effect of bainitic-martensitic microstructure has been reflected in the ductility values of grain-coarsened HSLA-80, which
is in agreement with the observation of higher susceptibility of the as-received HSLA-100 steel having a similar structure.
However, contrary to its beneficial effect in the as-received HSLA-80, an acicular ferrite structure has shown vulnerability
toward HE for high heat input grain-refined HSLA-80. This has been attributed to the presence of polygonal ferrite and to
the development of an HE susceptible substructure on GRHAZ weld simulation. 相似文献
5.
S. W. Thompson D. J. Colvin G. Krauss 《Metallurgical and Materials Transactions A》1996,27(6):1557-1571
Decomposition of fine-grained austenite (10-μm grain size) during continuous cooling of an HSLA-80 plate steel (containing
0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning
electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms
primarily to polygonal ferrite over a wide range of cooling rates, and Widmanst?tten ferrite sideplates frequently evolve
from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite,
and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and
acicular ferrite form at temperatures slightly below those at which polygonal and Widmanst?tten ferrite form. At cooling rates
less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower
bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal
and Widmanst?tten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanst?tten
ferrite. Kinetic and microstruc-tural differences between Widmanst?tten ferrite, acicular ferrite, and bainite (both upper
and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation
products.
Formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines.
This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms
of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in
Rosemont, Illinois. 相似文献
6.
Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M23C6 carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite. 相似文献
7.
Jeremy L. Caron Sudarsanam Suresh Babu John C. Lippold 《Metallurgical and Materials Transactions A》2011,42(13):4015-4031
A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval,
blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V
(wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as
the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353 HV in the coarse-grained
HAZ (CGHAZ) to a maximum of 448 HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld,
hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation
and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation
product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input
(HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good
agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the
BM at both test temperatures of 293 K and 223 K (20 °C and –50 °C). Hardness increases were observed for multipass HAZ simulations
employing an initial CGHAZ simulation. 相似文献
8.
Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite 总被引:1,自引:0,他引:1
R. Bengochea B. López I. Gutierrez B. López I. Gutierrez 《Metallurgical and Materials Transactions A》1998,29(2):417-426
It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures
below the nonrecrystallization temperature (T
nr
). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation.
In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T
nr
for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at
different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution
of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation
is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence
becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential
nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite
coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume
is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence
the mechanical properties of the steel. 相似文献
9.
J. R. Strife M. J. Carr G. S. Ansell 《Metallurgical and Materials Transactions A》1977,8(9):1471-1484
The effect of austenite prestrain above theM
d
temperature on the structure and transformation kinetics of the martensitic transformation observed on cooling was determined
for a series of Fe-Ni-Cr-C alloys. The alloys exhibited a shift in martensite morphology in the nondeformed state from twinned
plate to lath while theM
s
temperature, carbon content, and austenite grain size were constant. The transformation behavior was observed over the temperature
range 0 to -196°C as a function of tensile prestrains performed above theM
d
temperature. A range of prestrains from 5 pct to 45 pct was investigated. It is concluded that the response of a given alloy
to austenite prestrain above theM
d
temperature can be correlated with the morphology of the martensite observed in the nondeformed, as-quenched state. For the
range of prestrains investigated, the transformation of austenite to lath martensite is much more susceptible to stabilization
by austenite prestrain above theM
d
temperature than is the transformation of austenite to plate martensite. 相似文献
10.
Influence of tempering on the microstructure and mechanical properties of HSLA-100 steel plates 总被引:5,自引:0,他引:5
The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77,
Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1
hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel
revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of
retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed
on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate
tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85
°C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal
rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and
UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068
MPa, with 88 J at −85 °C). 相似文献
11.
C. R. Das S. K. Albert A. K. Bhaduri B. S. Murty 《Metallurgical and Materials Transactions A》2013,44(5):2171-2186
The effect of initial heat treatment on microstructure and mechanical properties of boron-free and boron-containing modified 9Cr-1Mo steel (P91 and P91B, respectively) has been studied under different heat-treatment conditions. The prior austenite grains evolved in P91 steel, having different prior austenite grain sizes, were found to be similar in size after heat treatment in the range of 1073 K to 1448 K (800 °C to 1175 °C) for 5 minutes. The microstructural evolution in P91B steel having different prior austenite grain sizes appeared to be uniform when subjected to different heat-treatment temperatures with the prior austenite grain size being similar to that of initial grain size. Lath martensite was observed in P91B steel after all heat treatments. On the other hand, lath martensite was observed in P91 steel only when subjected to high-temperature heat treatment, whereas subgrain/substructure as well as coarse precipitates were observed after a lower temperature heat treatment. Large differences in the hardness/strength values between different microstructures corresponding to coarse-grained heat-affected zone (CGHAZ) and intercritical HAZ (ICHAZ) of P91 steel weldment were due to the distinct difference in these microstructures. The difference in hardness/strength values between the CGHAZ and ICHAZ was found to be insignificant in P91B steel under similar heat-treatment conditions. 相似文献
12.
John W. Elmer Joe Wong Thorsten Ressler 《Metallurgical and Materials Transactions A》2001,32(5):1175-1187
Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel
were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time
observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal
phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed
α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were
combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature
gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C
of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on
cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite;
however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld. 相似文献
13.
G. S. Ansell S. J. Donachie R. W. Messler 《Metallurgical and Materials Transactions B》1971,2(9):2443-2449
The effect of very high quench rates on the transformation kinetics of a series of Fe?C and low-alloy steels and the morphology of an Fe?14Ni-0.76C alloy was investigated. TheM S temperatures of the Fe?C and Fe?C?X alloys increased between 90° and 122°C in a sigmoidal fashion over a quench rate range from 2,750° to 24,800°C per sec. The sensitivity of theM s temperature to the quench rate from the austenitizing temperature to 315°C was shown to be related to the influence of the third alloying element on the diffusivity of carbon in austenite. Transmission electron microscopy and optical metallography showed that the morphology of an Fe?14Ni?0.76C martensite is changed from a lath structure in slow quenched samples to a plate structure in fast quenched samples. The substructure of the untransformed austenite adjacent to the martensite plates changed from planar dislocation arrays to dislocation tangles with increased quench rate. These results were explained using a model for ferrous martensite strengthening based upon the extent of carbon segregation to imperfections in the austenite during cooling. 相似文献
14.
Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics,
precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range
just below Ar
3. For cooling rates in the range 0.1 °C s−1 to 130 °C s−1, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically
with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are
decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at
850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb)
x
S
y
precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains.
Pre-existing intragranular (Ti,Nb)
x
S
y
precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain
structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels. 相似文献
15.
Asok Joarder 《国际钢铁研究》1994,65(8):345-349
The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr-Mo-V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955°C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450°C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670°C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670°C exhibited microstructures almost identical to those observed in one hour tempering. 相似文献
16.
D. Chae C. J. Young D. M. Goto D. A. Koss 《Metallurgical and Materials Transactions A》2001,32(9):2229-2237
The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has
been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the
HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences
in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the
thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses
of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible
to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ.
The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination
of localized cleavage and ductile microvoid fracture. 相似文献
17.
M. Arbab Rehan Anna Medvedeva Lars-Erik Svensson Leif Karlsson 《Metallurgical and Materials Transactions A》2017,48(11):5233-5243
Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M s ) and martensite finish (M f ) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the M s temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed. 相似文献
18.
The effect of high quench rate on theM
s
temperature, percent transformed, martensite morphology and austenite hardness has been studied for several Fe-Ni-C steels.
For these steels the quench rate was varied only in the austenite region. TheM
s
temperature was found to increase with increased quench rate for both high- and low carbon steels while the percent transformation
increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the
as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness
decreased slightly with increasing quench rate while the martensite morphology changed from lath to plate. Parallel aligned
plate structures were observed which resemble a twinned lath morphology. It was demonstrated that the actual difference between
this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes
were compared to the measured changes in martensite properties in order to identify the mechanism of the morphology shift.
It was concluded that for these alloys the morphology was controlled by the austenite shear mode.
S. J. Donachie was formerly a Graduate Assistant. 相似文献
19.
The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with Af′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing Ms‐temperature and new α′‐martensite formation upon cooling. 相似文献
20.
Divya Jain Dieter Isheim David N. Seidman 《Metallurgical and Materials Transactions A》2017,48(7):3205-3219
HSLA-115 is a newly developed Cu-bearing high-strength low-carbon martensitic steel for use in Naval structural applications. This research provides, for the first time, a comprehensive compositional analysis of carbon redistribution and associated complex phase transformations in an isothermal aging study of HSLA-115 at 823 K (550 °C). Specifically, we characterize carbon segregation at lath boundaries, grain-refining niobium carbonitrides, cementite, and secondary hardening M2C carbides, in addition to copper precipitation, by 3D atom probe tomography (APT). Segregation of carbon (3 to 6 at. pct C) is observed at martensitic lath boundaries in the as-quenched and 0.12-hour aged microstructures. On further aging, carbon redistributes itself forming cementite and M2C carbides. Niobium carbonitride precipitates do not dissolve during the austenitizing treatment and are inherited in the as-quenched and aged microstructures; these are characterized along with cementite by synchrotron X-ray diffraction and APT. Sub-nanometer-sized M2C carbide precipitates are observed after the formation of Cu precipitates, co-located with the latter, indicating heterogeneous nucleation of M2C. The temporal evolution of the composition and morphology of M2C carbides at 823 K (550 °C) is described using APT; their precipitation kinetics is intertwined with Cu precipitates, affecting the bulk mechanical properties of HSLA-115. Phase compositions determined by APT are compared with computed compositions at thermodynamic equilibrium using ThermoCalc. 相似文献