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1.
Manfred Schirra 《国际钢铁研究》1993,64(6):322-330
Martensitic CrMoVNb 1.4914 type steel, which is at present being tested as a material for fuel element wrapper tubes, was subjected to tests in order to find out the impact on the original hardening and tempering strength of brief temperature rises up to 975°C. T-transients in the range between 800 and 900°C (20–36 min > Ac1b) do not exert a pronounced influence on creep-rupture strength; merely the times up to ≤ 1 % creep strain are clearly reduced, as is indicated by creep-rupture tests at 650°C. There is a more pronounced influence on creep rupture and creep behaviour if the transient extends into the 975°C region and is subsequently held in the range of 600–750°C, where transformation to the pearlite stage occurs. High creep stability is seen at holding temperatures of < 600 and > 400°C. The explanation is furnished by the findings obtained in isothermal creep-rupture tests above Ac1b (800–925°C). Extensive metallographic examination confirms the structural changes expected from the IT-diagram. 相似文献
2.
Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb 总被引:1,自引:0,他引:1
T. Fujita K. Asakura T. Sawada T. Takamatsu Y. Otoguro 《Metallurgical and Materials Transactions A》1981,12(6):1071-1079
The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and
good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels
and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The
optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05
pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during
initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep
rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation
during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the
precipitation of Fe2Mo. 相似文献
3.
In an effort to understand the effects of long-term aging at high temperatures and those of the superimposed creep stresses
on the microstructural variations in a 0.50Cr-0.50Mo-0.25V steel, the shoulder and gage portions of the specimens subjected
to stress rupture tests at 500 °C and 540 °C have been studied by transmission electron microscopy. The optical microstructure,
in the normalized and tempered condition, consists of about 95 pct ferrite with the remainder bainite. The ferrite has an
extremely fine precipitation of vanadium carbide (VC) as interphase precipitate as well as in random distribution. In the
bainitic area, M3C is present in the globular as well as plate-like morphology. Specimens ruptured at 500 °C and 540 °C were selected for transmission
electron microscopic study. Both the interphase precipitates and randomly distrib- uted precipitates of VC coarsen considerably
due to prolonged exposure at elevated tempera- tures, and ultimately fine M2C platelets nucleate and grow on either side of these precipitates, resulting in the formation of the so-called H-carbides.
The superimposed creep stress has a marginal effect on the coarsening kinetics of VC but has enhanced its precipitation along
dis- locations and has promoted the formation of H-carbides. 相似文献
4.
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
5.
In this study two different heat treatments were conducted on an X 37 Cr Mo V 5‐1 hot‐work tool steel, resulting either in a tempered fully martensitic matrix or a matrix almost consisting of tempered bainite. Short‐term creep tests were performed at a high stress level of 800 MPa and at temperatures in the range from 450 °C to 500 °C. Creep specimens consisting of a tempered fully martensitic microstructure exhibited a three times longer creep‐to‐rupture time, than those consisting of a tempered almost bainitic microstructure. Microstructural investigations of creep specimens were performed by transmission electron microscopy. Results of these investigations revealed that due to a lower cooling rate, which is necessary to form bainite, the tempered bainitic microstructure consists of large former bainitic plates, whereas tempered martensite shows fine former martensitic laths. Tempered bainite also exhibits a higher number density of large M3C, M7C3 and MC carbides than tempered martensite. Small M2C carbides appear in both microstructures in the same quantity, however, nanometer‐sized MC carbides could only be found in tempered martensite. Thus poor short‐term creep behavior of the tempered almost bainitic microstructure can be explained by the lesser amount of strengthening relevant precipitates, a smaller size‐effect due to distance of bainitic interfaces as well as lower solid solution hardening. 相似文献
6.
In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural
variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at
520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure
of the steel consists of 90 pct ferrite and 10 pct bainite, and M3C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr2N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation.
On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo2C, while the superimposed creep stress has promoted the precipitation of Mo2C needles with fine globular precipitates of M23C6. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo2C needles and ellipsoidal M23C6 carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both
cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible. 相似文献
7.
Ludwig Schfer 《国际钢铁研究》1994,65(5):184-186
The influence of various temperature transients on the tirne-to-rupture of a martensitic chromium steel was investigated in tests in which the transient temperature and transient time, cooling rate, and test temperature were varied systematically. Above a temperature of approximately 800 °C an austenitic structure is produced which can be transformed back into martensite by rapid cooling to a temperature below approximately 200 °C. The time-to-rupture of this martensite may be reduced by a factor of five compared to the initial condition. At annealing temperatures below 780 °C (Ac1b) this loss in time-to-rupture is either less pronounced or non-existent. If austenite produced above some 800 °C is cooled slowly, pearlite of very low creep-rupture strength is generated, or the austenite remains in existence above 300 °C and, depending on the test temperature, also may have an unsatisfactory creep-rupture strength. 相似文献
8.
《Baosteel Technical Research》2010,(Z1):124
Precipitation strengthening as well as solution strengthening is key mechanism for heat resistant steels.It is very important to characterize the precipitates in 9%Cr ferrite heat resistant steels,especially to show the nanometer-sized particles.By transmission electronic microscope attached with an energy dispersive spectrometer as well as optical microscope,scanning electronic microscope,the microstructure and chemical composition of precipitates in a 9%Cr heat resistant steel after different heat treatments were investigated.It was found that the microstructure of normalized sample was martensite with fine NbC and Fe3C.The microstructure of tempered sample is tempered martensite,and there mainly were two types of precipitates,M23C6 with the size range of 50 - 300 nm and MX with the size of 10 - 100 nm.Superfine M23C6 precipitated preferably on prior austenitic grain boundaries and martensitic lath boundaries,while nanometer-sized MX precipitates were distributed randomly. After short-term creep,Laves phase formed along grain boundaries of the 9%Cr steel,and M23 C6 and MX precipitates were found to become coarser.More information about precipitates in the 9%Cr steel had been exhibited by atomic force microscopy.Thereby,distribution,size and shape of the precipitates as well as their compositions and structures were revealed. 相似文献
9.
This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation
of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing
temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture
are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve
and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries
becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However,
some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like
cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides
and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement. 相似文献
10.
Valeriy Dudko Andrey Belyakov Dmitri Molodov Rustam Kaibyshev 《Metallurgical and Materials Transactions A》2013,44(1):162-172
Structural changes in a 9 pct Cr martensitic steel during a creep test at 923 K (720 °C) under the applied stress of 118 MPa were examined. The tempered martensite lath structure (TMLS) was characterized by M23C6-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm. The M23C6 particles were located on the packet/block/lath boundaries, whereas the MX precipitates were distributed homogeneously throughout TMLS. TMLS in the grip portion of the crept specimen changed scarcely during the tests. In contrast, the structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M23C6 carbides and precipitations of Laves phase during creep. The most pronounced structural changes occurred just at the beginning of the tertiary creep regime, which was interpreted as a result of the change in the mechanism of grain boundary pinning by precipitates. 相似文献
11.
K. S. Chandravathi Kinkar Laha C. S. Sasmal P. Parameswaran M. Nandagopal H. M. Tailor M. D. Mathew T. Jayakumar E. Rajendra Kumar 《Metallurgical and Materials Transactions A》2014,45(10):4280-4292
Microstructure and mechanical properties of 9Cr-W-0.06Ta Reduced Activation Ferritic-Martensitic (RAFM) steels having various tungsten contents ranging from 1 to 2 wt pct have been investigated on subjecting the steels to isothermal heat treatments for 5 minutes at temperatures ranging from 973 K to 1473 K (700 °C to 1200 °C) (below Ac1 to above Ac3) followed by oil quenching and tempering at 1033 K (760 °C) for 60 minutes. The steels possessed tempered martensite structure at all the heat-treated conditions. Prior-austenitic grain size of the steels was found to decrease on heating in the intercritical temperature range (between Ac1 and Ac3) and at temperatures just above the Ac3 followed by increase at higher heating temperatures. All the steels suffered significant reduction in hardness, tensile, and creep strength on heating in the intercritical temperature range, and the reduction was less for steel having higher tungsten content. Strength of the steels increased on heating above Ac3 and was higher for higher tungsten content. Transmission Electron Microscopy (TEM) investigations of the steels revealed coarsening of martensitic substructure and precipitates on heating in the intercritical temperature range, and the coarsening was relatively less for higher tungsten content steel, resulting in less reduction in tensile and creep strength on intercritical heating. Tensile and creep strengths of the steels at different microstructural conditions have been rationalized based on the estimated inter-barrier spacing to dislocation motion. The study revealed the uniqueness of inter-barrier spacing to dislocation motion in determining the strength of tempered martensitic steels subjected to different heat treatments. 相似文献
12.
In this study, the effect of finish rolling temperature and coiling temperature on the microstructure and mechanical properties of high-Al-low-Si dual-phase (DP) steels is explored. Two different finish rolling temperatures (850 and 790°C) and three different coiling temperatures (200, 250 and 300°C) were studied. The results indicated that all the different processing conditions led to ferrite-martensite DP microstructure. With the decrease in finish rolling temperature, the volume fraction of ferrite was increased and martensite content was decreased. When the coiling temperature was increased to 300°C, autotempered martensite was obtained, which led to the softening of martensite and decrease in tensile strength and strain hardening ability, but higher post-necking elongation. Moreover, the nanoscale Nb-based carbides played a crucial role in refining the microstructure of hot-rolled high-Al-low-Si DP steel. EBSD (Electron Backscattered Diffraction) analysis revealed that the ferrite grains were fine, and decrease in finish rolling temperature and coiling temperature led to an increase in low-angle boundaries. When the finish rolling temperature was decreased to 790°C and coiling temperature was decreased to 200°C, the steel had excellent mechanical properties with tensile strength of 885?MPa, uniform and total elongation of 16.0 and 25.94%, respectively, and the product of tensile strength and total elongation was 20?264?MPa%. The improvement of strength and plasticity can be attributed to the fraction of ferrite and martensite, precipitation of NbC, fine microstructure. 相似文献
13.
The structure and mechanical properties of 35Kh12G3MVFDR steel are investigated. After normalization or quenching, the steel contains up to 35 vol % austenite and may be assigned to the martensitic–austenitic class. On heat treatment—tempering, isothermal holding, or isothermal quenching—the austenite is converted to martensite within 2 h. The martensite in 35Kh12G3MVFDR steel is more thermally stable: the first signs of its conversion to sorbitic structure are observed after 25-h isothermal quenching at 640°C, and its complete decomposition requires 50 h. The breakdown of martensite is accompanied by decrease in the high-temperature strength and hardness. Aging of the quenched and tempered 35Kh12G3MVFDR steel at 670–720°C lowers the hardness from 61–65 HRA to 55–60 HRA after 1600–3200 h and the yield point at 20°C from 1350 MPa to 750–850 MPa. Likewise, the yield point at 720°C declines from 310 MPa to 160–230 MPa after 600 h and then stops. The state of the martensitic structure of 35Kh12G3MVFDR steel determines its creep resistance at 700°C. For example, the martensite remains in the steel structure after relatively brief isothermal quenching (up to 24 h at 640°C), and consequently the creep limit σ700°C 0.1%/h is no lower than after simple quenching with subsequent high tempering: 86.2 ± 9.4 MPa and 89.3 ± 8.8 MPa, respectively. At the same time, in response to the decomposition of martensitic structure as a result of prolonged aging (1600 h at 670°C), σ700°C 0.1%/h declines to 63.9 ± 7.1 MPa. In contrast to martensite, the austenite in 35Kh12G3MVFDR steel is thermally unstable and is converted to martensite after only 1–2 h of heating, depending on the temperature. 相似文献
14.
The effect of tungsten on creep behavior and microstructural evolution was investigated for tempered martensitic 9Cr steels
with various W concentrations from 0 to 4 wt pct. The creep rupture testing was carried out at 823, 873, and 923 K for up
to 54 Ms (15,000 hours). The creep and creep rupture strength increased linearly with W concentration up to about 3 wt pct,
where the steels consisted of the single constituent of the tempered martensite. It increased only slightly above 3 wt pct,
where the matrix consisted of the tempered martensite and δ-ferrite. The minimum creep rate was described by a power law.
The apparent activation energy for the minimum creep rate showed a tendency similar to the W concentration dependence of the
creep-rupture strength and was larger than the activation energy for self-diffusion at high W concentrations above 1 wt pct.
The martensite lath microstructure with fine carbides along lath boundaries was responsible for a high resistance to creep
deformation. With increasing W con- centration, the martensite lath microstructure became stabilized, which decreased the
minimum creep rate and increased the apparent activation energy for the minimum creep rate. 相似文献
15.
Lakshmiprasad Maddi D. Barbadikar M. Sahare A. R. Ballal D. R. Peshwe R. K. Paretkar K. Laha M. D. Mathew 《Transactions of the Indian Institute of Metals》2015,68(2):259-266
P92 steel (9Cr–0.5Mo–1.8W) was subjected to a heat treatment of 1050 °C/30 min/air cooling/780 °C/120 min/air cooling followed by 1080 °C/30 min/air cooling/740 °C/60 min/air cooling to obtain tempered martensite microstructure, for better creep strength. Stress rupture tests carried at 600 °C in the range of 250–350 MPa resulted in rupture times in the range of 200–3000 h. Straight line plot of stress rupture curve indicated no major change in deformation mechanism. Coarsening of precipitates and substructure development were the main reasons for microstructure degradation, consequently leading to reduced hardness of the sample. Gauge and grip portions of the same sample were sectioned to comparatively evaluate the effects of stress and aging. Gauge portion of 3000 h sample showed considerable change in the microstructure in terms of boundary migration, while that of grip portion hardly evolved. The ruptured samples exhibited predominantly ductile fracture with elongated cavities at higher rupture times. 相似文献
16.
A. Kabadwal M. Tamura K. Shinozuka H. Esaka 《Metallurgical and Materials Transactions A》2010,41(2):364-379
The effect of tempering temperature and creep exposure on the microstructure of a modified 9Cr steel was investigated. Creep-interrupted
specimens, including the grip portion, were investigated precisely using mainly X-ray and inductively coupled plasma (ICP)
spectroscopy. After saturation of precipitation due to creep exposure, the amount of extracted residue decreased once and
then increased within a short period (dip). Chemical analysis showed that during the dip, the precipitates temporarily dissolved
into the matrix and precipitated again. The size of the Cr23C6 increased gradually during creep, but the growth rate was relatively small, as compared to the Ostwald ripening. The size
of the VN particles in the specimens tempered at 800 °C in the early stage of creep was very fine, approximately 20 nm, and
tended to decrease further with the progress of creep. The size variations of the precipitates and the dip were explained
from the annihilation or migration of precipitation sites, i.e., dislocations and boundaries, during creep. Transient creep for the specimens tempered at 500 °C was controlled by a reduction
of the mobile dislocation density. On the other hand, transient creep for 800 °C was due to precipitation hardening of fine
VN particles with the progress of creep, which was supported by the increase in both the lattice strain and the activation
energy with creep. 相似文献
17.
Influence of thermomechanical treatments on the microstructure and mechanical properties of HSLA-100 steel plates 总被引:2,自引:0,他引:2
The influence of thermomechanical treatment (TMT), i.e., controlled rolling and direct quenching, as a function of rolling temperature and deformation on the microstructure and
mechanical properties of HSLA-100 steel have been studied. The optical microstructure of the direct quenched (DQ) and tempered
steel rooled at lower temperatures (800 °C and 900 °C) showed elongated and deformed grains, whereas complete equiaxed grains
were visible after rolling at 1000 °C. The transmission electron microscope (TEM) microstructure of the 800 °C rooled DQ steel
showed shorter, irregular, and closer martensite laths with extremely fine Cu and Nb(C,N) precipitates after tempering at
450 °C. The precipitates coarsened somewhat after tempering at 650 °C; the degree of coarsening was, however, less compared
to that of the reheat-quenched (RQ) and tempered steel, indicating that the DQ steel was slightly more resistant to tempering.
Similar to the RQ steel, at a 450 °C tempering condition, the DQ steel exhibited peak strength with extremely poor impact
toughness. After tempering at 650 °C, the toughness of the DQ steel improved significantly, but at the expense of its strength.
In general, the strength of the DQ and tempered steel was good and comparable to that of the RQ and tempered steel, although,
its impact toughness was marginally less than the latter. The optimum combination of strength and toughness in the DQ steels
was achieved after 900 °C rolling with 50 pct deformation, followed by direct quenching and tempering at 650 °C (yield strength
(YS)=903 MPa, ultimate tensile strength (UTS)=928 MPa, and Charpy V-notch (CVN) strength=143 J at −85 °C). 相似文献
18.
Morphology and properties of low-carbon bainite 总被引:11,自引:0,他引:11
H. Ohtani S. Okaguchi Y. Fujishiro Y. Ohmori 《Metallurgical and Materials Transactions A》1990,21(3):877-888
Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous
cooling transformation is lathlike ferrite elongated in the 〈11l〉b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine
carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage
of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]f growth direction and the habit plane between (232)f and (lll)f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the
cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites
are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the
highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path
limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the
tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated
cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite.
Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength
and toughness of low-carbon steels.
This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World
Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations
Committee and the TMS Ferrous Metallurgy Committee. 相似文献
19.
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
20.
Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10?7 s?1 to 5·10?9 s?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20. 相似文献