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1.
Sun -Keun Hwang Sungho Jin J. W. Morris 《Metallurgical and Materials Transactions A》1975,6(11):2015-2021
The behavior of retained austenite in a fine-grained Fe-12Ni-0.25Ti cryogenic alloy system was investigated. When the alloy was held at a temperature within the two phase (α + γ) field austenite reversion occurred by a diffusion-controlled process. The redistribution of solute elements appeared to control the stability of the reverted austenite. The microstructural appearance of the retained austenite was examined using transmission electron microscopy. A preferential distribution of the austenite phase along martensite lath boundaries was observed. A precipitate-correlated austenite was also found to occur. The beneficial effect of introducing retained austenite appeared in the improvement in the tensile elongation as well as in the Charpy impact toughness at low temperatures. The retained austenite in this system did not improveK IC at ?196°C. On the contrary, samples containing retained austenite showed increased susceptibility to unstable crack propagation in low temperature fracture toughness tests. 相似文献
2.
The precipitation kinetics in the aging temperature range of 713 to 813 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9
pct Ni-4.1 pct Mo-1.9 pct Ti, mass pct) has been studied. Study on microstructure and mechanical properties showed that a
great deal of Ni3Ti and a type of unknown spheroidal precipitates both with average diameter of 2 to 3 nm are formed in the early aging stage
at 713 K, which results in a high strength and a relatively low fracture toughness. Ni3Ti precipitates grow into needle or rod shape and become the main precipitation as the aging time is prolonged. Strength increases
and fracture toughness (K
IC
) decreases with growth of the precipitates. The ultra-high strength of the maraging steel subjected to long-time aging at
713 K is attributed to the high resistance to coarsening of the precipitates. The strengthening in the underaged condition
at 713 K is a combination of dislocations cutting through precipitates and the Orowan mechanisms. Aged at 813 K, the size
of Ni3Ti precipitates is seriously nonuniform at the early stage and a small amount of interlath reverted austenite is formed. Thereafter,
Ni3Ti precipitates coarsen sharply accompanied with the embrittlement. Intralath reverted austenite appears subsequently. In
the later stage of aging, the coarsened Ni3Ti precipitates dissolve into the striplike intralath reverted austenite that is disorderly embedded in the matrix. All of
these result in a low strength and low fracture toughness under overaging condition. Analysis shows that the formation of
reverted austenite contains the diffusion and Kudjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) shear mechanisms. 相似文献
3.
Transmission electron microscopy examination of hardening and toughening phenomena in Aermet 100 总被引:9,自引:0,他引:9
The effect of tempering on the microstructure and mechanical properties of ultrahigh strength Aermet 100 steel was examined.
In the as-quenched condition, the steel contained a dispersion of relatively fine, undissolved, (CrTiFeMo)C and (CrFeMo)23C6 carbides in a martensitic matrix. Upon tempering at 427 °C, the martensite decomposed to form a high density of cementite
particles concomitant with a significant drop in toughness. Tempering at 454 °C resulted in peak strength (yield strength
∼ 1756 MPa) due to the precipitation of coherent zones of fine carbides. The peak in toughness (170 MPa√m), attained at a
tempering temperature of 482 °C, was attributed to both the absence of cementite and the formation of reverted, stable austenite.
Tempering at higher temperatures resulted in loss of both strength and toughness, which was suggested to be the result of
precipitate coarsening and formation of unstable austenite, respectively. The details of the electron microscopy studies and
mechanism of strengthening and toughening are discussed in light of the current understanding of this subject. 相似文献
4.
Chang WANG Wen-quan CAO Yun HAN Cun-yu WANG Chong-xiang HUANG Han DONG 《钢铁研究学报(英文版)》2015,22(1):42-47
A duplex ultrafine microstructure in a medium manganese steel(0.2C-5Mn)was produced by austenite reverted transformation annealing(ART-annealing).The microstructural evolution during annealing was examined by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD).Based on the microstructure examination,it was found that some M3 C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing.But with further increasing annealing time,these carbides would be dissolved and finally disappeared.Meanwhile,the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increased with the increase of the annealing time,which resulted in a duplex microstructure consisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing.The mechanical property examinations by uniaxial tensile tests showed that ART-annealing(6h,650 ℃)resulted in a superhigh product of strength to elongation up to 42GPa·%. 相似文献
5.
The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel 总被引:2,自引:0,他引:2
G. Y. Lai W. E. Wood R. A. Clark V. F. Zackay E. R. Parker 《Metallurgical and Materials Transactions B》1974,5(7):1663-1670
The effect of austenitizing temperature on both the plane strain fracture toughness,K IC , and the microstructure of AISI 4340 was studied. Austenitizing temperatures of 870 and 1200°C were employed. All specimens austenitized at 1200°C were furnace cooled from the higher austenitizing temperature and then oil quenched from 870°C. Transmission electron microscopy revealed an apparent large increase in the amount of retained austen-ite present in the specimens austenitized at the higher temperature. Austenitizing at 870°C resulted in virtually no retained austenite; only minor amounts were found sparsely scat-tered in those areas examined. A considerably altered microstructure was observed in specimens austenitized at 1200°C. Fairly continuous 100 to 200Å thick films of retained austenite were observed between the martensite laths throughout most of the area exam-ined. Additionally, specimens austenitized at 870°C contained twinned martensite plates while those austenitized at 1200°C showed no twinning. Plane strain fracture toughness measurements exhibited an approximate 80 pct increase in toughness for specimens austen-itized at 1200°C compared to those austenitized at 870°C. The yield strength was unaffected by austenitizing temperature. The possible role of retained austenite and the elimination of twinned martensite in the enhancement of the fracture toughness of those specimens austen-itized at the higher temperature will be discussed. 相似文献
6.
E. L. Brown T. A. Whipple R. L. Tobler 《Metallurgical and Materials Transactions A》1983,14(5):1179-1188
The first fracture toughness measurements for CF8 stainless steel castings in liquid helium at 4 K are reported. Single-phase (austenite) and duplex (austenite + δ-ferrite) castings were tested. On the basis of estimates from J-integral data, the plane-strain fracture toughness (Klc) of castings containing 3.2 to 14.5 pct δ-ferrite ranged from 84 to 179 MPa · ml/2 at 4 K. In contrast, a fully austenitic casting (0 pct δ-ferrite) exhibited a Klc, value of 331 MPa · ml/2, which is nearly equivalent to the toughness of a wrought AISI 304 stainless steel of a similar strength. Light and scanning electron microscopy studies indicate that the inferior toughness of castings containing δ-ferrite may be attributed to the brittleness of this body-centered-cubic phase at cryogenic temperatures and its distribution in the microstructure. The relative stability of the austenitic phase with respect to martensitic phase transformation may also play a significant role. 相似文献
7.
S. Sankaran G. Malakondaiah Gouthama S. Sangal K. A. Padmanabhan 《Metallurgical and Materials Transactions A》2006,37(4):1191-1200
A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon
microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile,
Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of
ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of
the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were
inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture
toughness value (KIC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (KIDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the
surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile
and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness
(KIC), and conditional dynamic fracture toughness (KIDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties
were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures. 相似文献
8.
N. Isasti D. Jorge-Badiola M. L. Taheri B. López P. Uranga 《Metallurgical and Materials Transactions A》2011,42(12):3729-3742
Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior
to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both
strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure.
Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source
of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation
dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions
(0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite.
Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved
through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered
diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges
to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite
accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However,
an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary
units. 相似文献
9.
Y. Waku M. Suzuki Y. Oda Y. Kohtoku 《Metallurgical and Materials Transactions A》1996,27(10):3307-3317
The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling
process and three-point flexural strength and fracture toughness of A12O3 composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with
A12O3 powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The
microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these
Mo particles in the A12O3 matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture
toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing
Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural
strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The
microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to
inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness
may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small
spherical shape. 相似文献
10.
An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered
ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently
austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through
optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated
with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results
in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon
content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture
toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve
fracture toughness. 相似文献
11.
The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without
cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at
673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties
were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence
of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of
interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of
transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness
of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which
had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised Ms but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at
673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably
Mo2C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated
the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions. 相似文献
12.
U. K. Viswanathan G. K. Dey M. K. Asundi 《Metallurgical and Materials Transactions A》1993,24(11):2429-2442
Evolution of microstructure in a 350 grade commercial maraging steel has been examined. In the earlier stages of aging, the
strengthening phases are formed by the heterogeneous precipitation, and these phases have been identified as intermetallic
compounds of the Ni3 (Ti, Mo) and Fe2Mo types. The kinetics of precipitation are studied in terms of the activation energy by carrying out isothermal hardness
measurements of aged material. The mechanical properties in the peak-aged and overaged conditions were evaluated and the flow
behavior examined. The overaging behavior of the steel has been studied and the formation of austenite of different morphologies
identified. The crystallography of the austenite has been examined in detail. From the microstructural examination of peak-aged
and deformed samples, it could be inferred that the dislocation-precipitate interaction is by precipitate shearing. Increased
work hardening of the material in the overaged condition was suggestive of looping of precipitates by dislocations. 相似文献
13.
中碳贝氏体钢由亚微米贝氏体铁素体板条和残余奥氏体组成,对韧性起主要作用的为残余奥氏体,通过细化块状组织能显著提高贝氏体钢的韧性。为了探究块状组织细化对断裂行为的影响,采用两步贝氏体等温工艺对中碳(碳质量分数为0.3%)贝氏体钢中块状组织进行细化,对拉伸和冲击性能及其裂纹扩展行为变化进行了研究。利用光学、扫描电子(SEM)、透射电子(TEM)显微镜、X射线衍射(XRD)等对试验钢的显微组织类型和尺寸、拉伸和冲击性能及断口形貌进行表征和分析。结果表明,与一步贝氏体工艺相比,两步贝氏体工艺中新形成的贝氏体铁素体分割细化块状马氏体+残余奥氏体,随着真应变的增加,加工硬化的效果更好;断裂形式为韧性断裂,且韧窝的数量、深度更优于一步贝氏体转变,塑韧性更佳。 相似文献
14.
15.
P. W. Hochanadel G. R. Edwards C. V. Robino M. J. Cieslak 《Metallurgical and Materials Transactions A》1994,25(4):789-798
The microstructure of investment cast PH 13-8 Mo stainless steel heat-treated to various conditions was studied using light and electron microscopy, electron probe microanalysis, and Mössbauer spectroscopy. The mechanical properties were investigated by using uniaxial tensile testing, hardness testing, and Charpy impact testing. TheΒ-NiAl strengthening precipitates, though detectable by electron diffraction, were difficult to resolve by transmission electron microscopy (TEM) in specimens aged at low temperatures (566 °C and below). A high dislocation density was observed in the lath martensitic structure. The higher strength and lower ductility observed at low aging temperatures was attributed to both the high dislocation density and the precipitation ofΒ-NiAl. When samples were aged at high temperatures (> 566 °C), a lower dislocation density and a reverted austenite fraction on the order of 15 pct were observed. SphericalΒ-NiAl precipitates were observed in the overaged condition. The decrease in strength and corresponding increase in ductility observed in samples aged at temperatures above 566 °C were attributed to the reverted austenite and recovery. Mechanical properties were improved when the homogenizing temperature and time were increased. Electron probe microanalysis quantified the increased homogeneity realized by increasing homogenizing temperature and time. Elimination of the refrigeration step, which normally follows the solution treatment, did not degrade the mechanical properties. Mössbauer spectroscopy showed only minor decreases in the fraction of retained austenite when refrigeration followed the solution treatment. 相似文献
16.
Environment-Assisted Cracking in Custom 465 Stainless Steel 总被引:1,自引:0,他引:1
E. U. Lee R. Goswami M. Jones A. K. Vasudevan 《Metallurgical and Materials Transactions A》2011,42(2):415-423
The influence of cold work and aging on the environment-assisted cracking (EAC) behavior and mechanical properties of Custom
465 stainless steel (SS) was studied. Four sets of specimens were made and tested. All specimens were initially solution annealed,
rapidly cooled, and refrigerated (SAR condition). The first specimen set was steel in the SAR condition. The second specimen
set was aged to the H1000 condition. The third specimen set was 60 pct cold worked, and the fourth specimen set was 60 pct
cold worked and aged at temperatures ranging from 755 K to 825 K (482 °C to 552 °C) for 4 hours in air. The specimens were
subsequently subjected to EAC and mechanical testing. The EAC testing was conducted, using the rising step load (RSL) technique,
in aqueous solutions of NaCl of pH 7.3 with concentrations ranging from 0.0035 to 3.5 pct at room temperature. The microstructure,
dislocation substructure, and crack paths, resulting from the cold work, aging, or subsequent EAC testing, were examined by
optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The aging of the cold-worked
specimens induced carbide precipitation within the martensite lath, but not at the lath or packet boundaries. In the aged
specimens, as aging temperature rose, the threshold stress intensity for EAC (KIEAC), elongation, and fracture toughness increased, but the strength and hardness decreased. The KIEAC also decreased with increasing yield strength and NaCl concentration. In the SAR and H1000 specimens, the EAC propagated
along the prior austenite grain boundary, while in the cold-worked and cold-worked and aged specimens, the EAC propagated
along the martensite lath, and its packet and prior austenite grain boundaries. The controlling mechanism for the observed
EAC was identified to be hydrogen embrittlement. 相似文献
17.
The microstructure of an Fe-31.4 pet Ni-0.3 pet C alloy was examined via transmission electron microscopy as a function of
thermomechanical treatment. The effects of prior deformation, rapid reversion to austenite and thermal cycling on the microstructure
were investigated, and operative strengthening mechanisms under various conditions were correlated to observed structures.
When midrib twinned, plate martensite of this alloy was deformed at room temperature, dislocation glide was the operating
mode, and the midrib twins and plate like structure were completely dissolved after 80 pet cold rolling. The microstructure
of reverted austenite without prior deformation was composed of sheared plates, but became finely equiaxed with prior deformation
of the martensite. The superior strength of reverted austenite in comparison to annealed austenite was due to a grain size
refinement and a higher dislocation density. However, the strengthening observed in reverted austenite with prior deformation
in comparison to reverted austenite without prior deformation was due to a grain size effect alone. Repeated thermal cyclings
increased the strength of reverted austenite. This was due to increases in the dislocation density since the grain structure
was principally dictated by the first martensite transformationreversion cycle. 相似文献
18.
E. L. Brown T. A. Whipple R. L. Tobler 《Metallurgical and Materials Transactions A》1983,14(6):1179-1188
The first fracture toughness measurements for CF8 stainless steel castings in liquid helium at 4 K are reported. Single-phase (austenite) and duplex (austenite + δ-ferrite) castings were tested. On the basis of estimates from .J-integral data, the plane-strain fracture toughness(K lc) of castings containing 3.2 to 14.5 pct δ-ferrite ranged from 84 to 179 MPa · m1/2 at 4 K. In contrast, a fully austenitic casting (0 pct (δ-ferrite) exhibited aK lc value of 331 MPa · m-2, which is nearly equivalent to the toughness of a wrought AISI 304 stainless steel of a similar strength. Light and scanning electron microscopy studies indicate that the inferior toughness of castings containing ·-ferrite may be attributed to the brittleness of this body-centered-cubic phase at cryogenic temperatures and its distribution in the microstructure. The relative stability of the austenitic phase with respect to martensitic phase transformation may also play a significant role. 相似文献
19.
C. S. Carter 《Metallurgical and Materials Transactions B》1970,1(6):1551-1559
The heat treatment response of a 350-grade maraging steel, with the nominal composition 18.5 Ni, 12 Co, 4.6 Mo, 1.4 Ti, balance
Fe, has been determined in billet and bar form. When aged at temperatures below 900°F, the material was very susceptible to
subcritical crack growth, and premature brittle fracture occurred in unnotched tension specimens loaded at a slow strain rate
in laboratory air. Fracture mechanics was used to interpret this behavior. The introduction of reverted austenite significantly
decreased the strength level but had little effect on fracture toughness. The resistance to brittle fracture of this material
is contrasted with that of high-strength steels currently used by the airplane industry. 相似文献
20.
Yi He Ph.D. Ke Yang Kai Liu Ph.D. Wei Sha Zhanli Guo 《Metallurgical and Materials Transactions A》2006,37(4):1107-1116
The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8
∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties
showed that a high number of Ni3Ti and Fe2(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness.
Ni3Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic
calculations showed that the equilibrium phases are Ni3Ti, Fe2(Mo,Ti), ferrite, and austenite. 相似文献