共查询到20条相似文献,搜索用时 31 毫秒
1.
K. LAHA K.S. CHANDRAVATHI P. PARAMESWARAN K. BHANU SANKARA RAO S.L. MANNAN 《Metallurgical and Materials Transactions A》2007,38(1):58-68
In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge
of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed
on the base metal subjected to soaking for 5 minutes at temperatures below Ac1 to above Ac3 and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical
region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher
test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture
life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled
with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical
region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized
to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the
joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation
of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the
soft zone resulting in premature failure. 相似文献
2.
K. Laha K. S. Chandravathi P. Parameswaran Sunil Goyal M. D. Mathew 《Metallurgical and Materials Transactions A》2012,43(4):1174-1186
Evaluations of creep rupture properties of dissimilar weld joints of 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels with Alloy
800 at 823 K were carried out. The joints were fabricated by a fusion welding process employing an INCONEL 182 weld electrode.
All the joints displayed lower creep rupture strength than their respective ferritic steel base metals, and the strength reduction
was greater in the 2.25Cr-1Mo steel joint and less in the 9Cr-1Mo steel joint. Failure location in the joints was found to
shift from the ferritic steel base metal to the intercritical region of the heat-affected zone (HAZ) of the ferritic steel
(type IV cracking) with the decrease in stress. At still lower stresses, the failure in the joints occurred at the ferritic/austenitic
weld interface. The stress-life variation of the joints showed two-slope behavior and the slope change coincided with the
occurrence of ferritic/austenitic weld interface cracking. Preferential creep cavitation in the soft intercritical HAZ induced
type IV failure, whereas creep cavitation at the interfacial particles induced ferritic/austenitic weld interface cracking.
Micromechanisms of the type IV failure and the ferritic/austenitic interface cracking in the dissimilar weld joint of the
ferritic steels and relative cracking susceptibility of the joints are discussed based on microstructural investigation, mechanical
testing, and finite element analysis (FEA) of the stress state across the joint. 相似文献
3.
K. Laha K. S. Chandravathi K. Bhanu Sankara Rao S. L. Mannan D. H. Sastry 《Metallurgical and Materials Transactions A》2001,32(1):115-124
The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar
weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried
out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process
using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently
given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated
in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted
of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain
(FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo
steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp
line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while
a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly
lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared
to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the
specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred
in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint,
at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with
a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near
to the weld interface. The failure occurred by extensive intergranular creep cavity formation. 相似文献
4.
5.
P. Parameswaran K. Laha K. S. Chandravathi K. Bhanu Sankara Rao 《Transactions of the Indian Institute of Metals》2010,63(2-3):479-483
Fusion welded joints of Cr-Mo steels fail prematurely under creep condition at the heat affected zone (HAZ) close to the base metal, termed as type IV cracking. Optical metallography and hardness testing across the joint establish that the type IV cracking occurs in the soft intercritical HAZ. Based on detailed microstructural studies carried out to understand the evolution of the microstructure and its role in determining the tendency for type IV cracking, the factors that lead to deterioration of creep strength in intercritical HAZ in weld joint of Cr-Mo steels are:
- fine grained structure
- coarse M23C6 carbides at grain and sub-grain boundaries
- dissolution of M2X and MX types of intragranular precipitates.
6.
Vani Shankar K. Mariappan R. Sandhya M. D. Mathew T. Jayakumar 《Metallurgical and Materials Transactions A》2014,45(3):1390-1400
Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone. 相似文献
7.
C. R. Das S. K. Albert J. Swaminathan S. Raju A. K. Bhaduri B. S. Murty 《Metallurgical and Materials Transactions A》2012,43(10):3724-3741
The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423?K (1150?°C) normalized base metal, was found to be much higher than that prepared from 1323?K (1050?°C) normalized base metal because of the stabilization of lath martensite by fine M23C6 precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423?K (1150?°C) normalized base metal compared with that of the weldment prepared from 1323?K (1050?°C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature. 相似文献
8.
Shaju K. Albert Masayuki Kondo Masaaki Tabuchi Fuxing Yin Kota Sawada Fujio Abe 《Metallurgical and Materials Transactions A》2005,36(2):333-343
New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power
plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon
by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type
IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the
present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180
ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa.
Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in
the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal
and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the
creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels
and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed,
not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide
coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ)
in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld
joints. 相似文献
9.
V. D. Vijayanand J. Ganesh Kumar P. K. Parida V. Ganesan K. Laha 《Metallurgical and Materials Transactions A》2017,48(2):706-721
Effect of electrode size on creep deformation and rupture behavior has been assessed by carrying out creep tests at 923 K (650 °C) over the stress range 140 to 225 MPa on 316LN stainless steel weld joints fabricated employing 2.5 and 4 mm diameter electrodes. The multi-pass welding technique not only changes the morphology of delta ferrite from vermicular to globular in the previous weld bead region near to the weld bead interface, but also subjects the region to thermo-mechanical heat treatment to generate appreciable strength gradient. Electron backscatter diffraction analysis revealed significant localized strain gradients in regions adjoining the weld pass interface for the joint fabricated with large electrode size. Larger electrode diameter joint exhibited higher creep rupture strength than the smaller diameter electrode joint. However, both the joints had lower creep rupture strength than the base metal. Failure in the joints was associated with microstructural instability in the fusion zone, and the vermicular delta ferrite zone was more prone to creep cavitation. Larger electrode diameter joint was found to be more resistant to failure caused by creep cavitation than the smaller diameter electrode joint. This has been attributed to the larger strength gradient between the beads and significant separation between the cavity prone vermicular delta ferrite zones which hindered the cavity growth. Close proximity of cavitated zones in smaller electrode joint facilitated their faster coalescence leading to more reduction in creep rupture strength. Failure location in the joints was found to depend on the electrode size and applied stress. The change in failure location has been assessed on performing finite element analysis of stress distribution across the joint on incorporating tensile and creep strengths of different constituents of joints, estimated by ball indentation and impression creep testing techniques. 相似文献
10.
Hiromichi Hongo Masaaki Tabuchi Takashi Watanabe 《Metallurgical and Materials Transactions A》2012,43(4):1163-1173
Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical
(USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as
a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The
current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these
welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and
923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time
steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured
quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which
coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an
area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with
the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both
the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution
of Type IV creep damage. 相似文献
11.
12.
W. H. Miller R. T. Chen E. A. Starke 《Metallurgical and Materials Transactions A》1987,18(8):1451-1468
The effects of microstructure, temperature, and stress level on the creep response of Ti-6211 have been investigated. A variety of microstructures simulating the heat affected zone of a weld (HAZ), as well as the as-received structure, were tested in a temperature range of 298 K to 873 K. At stress levels below the tensile yield strength, creep curves saturate in the ambient temperature regime. The colony type Widmanstätten alpha + beta as-received structure exhibited the highest creep strains at ambient temperatures. Long slip lengths associated with the large colony size, and sliding along various interfaces account for the relatively high creep strains to saturation. Planar arrays of straight dislocations operating on a single slip system were observed for samples crept at 298 K while thermally activated cross slip was observed for samples crept at 453 K. Beta-annealed martensitic micro-structures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along G. B. alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed. 相似文献
13.
Yu-Hsian Hsiao Hongyan Zhang Glenn S. Daehn 《Metallurgical and Materials Transactions A》1996,27(4):891-900
The difference in creep strength between a base metal and a weld metal always produces a multiaxial stress state even if the
macroscopic loading is uniaxial. In this study, weldments were formed between wrought 316 stainless steel and two types of
316 weld metals with slightly different creep properties and chemical compositions. Full-size 316 weldments, including base
metal, heat-affected zone (HAZ) and welds, were creep tested at 650 °C. The multiaxial stress distributions in full-size 316
weldments were simulated by the finite element method (FEM). Three stress parameters, namely, the maximum principal stress
(MPS), the yon Mises effective stress (VMS), and the principal facet stress (PFS), were used to correlate the local multiaxial
stresses with local creep damage distributions and failure lifetime. Metallographic examination and creep rupture data showed
that the PFS parameter gave the best prediction of the creep damage distribution caused by the multiaxial stresses in 316
weldments. This approach may have application in the design, life prediction, and in-service evaluation of weldments.
Performed this work at Ohio State
This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” sympsosium
as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow
and Fracture Committee. 相似文献
14.
V. D. Vijayanand S. D. Yadav P. Parameswaran K. Laha P. K. Parida G. V. P. Reddy 《Metallurgical and Materials Transactions A》2018,49(10):4409-4412
The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal. 相似文献
15.
Three low carbon structural steels of different plate thickness have been investigated for hydrogen assisted cold cracking by the IRC weldability test at different restraint intensities. At diffusible hydrogen levels of 10–15 N ml/100 g Fe (ISO 3690), cracking decreases at increasing heat inputs due to a drop in restraint stress and hardness as well as an increase in hydrogen diffusion times. Critical heat inputs for crack prevention range from 0.95 to 1.4 kJmm?1. Higher restraints enforce higher cracking stresses as well as final stresses of uncracked test welds. Higher restraints and lower heat inputs also induce faster stress increase during cooling which, for the steels containing Ni and Cu, shift the location of cracking from the HAZ to the weld metal. The steel without Ni and lower maximum HAZ hardness reveals weld metal cracking only, regardless of welding conditions. It can be concluded that for weld metal cracking, the relation between stress increase- and hydrogen effusion rates but also the relation between weld metal and HAZ microstructure and mechanical properties are responsible. 相似文献
16.
W. A. Baeslack W. F. Savage D. J. Duquette 《Metallurgical and Materials Transactions A》1979,10(10):1429-1435
The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex
austeno-ferritic Type 304 weld metal in boiling MgCl2 was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals
is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite
interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking
in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may
be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these
regions preferentially susceptible to dissolution.
W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York 相似文献
17.
Vani Shankar M. Valsan K. Bhanu Sankara Rao S. D. Pathak 《Transactions of the Indian Institute of Metals》2010,63(2-3):622-627
The aim of the present paper is to study the low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel weld joint. Total axial strain controlled continuous cycling tests were conducted between 773 K and 873 K and at strain amplitudes ±0.25%, ±0.4%, ±0.6% and ±1%. Hold tests were also conducted at +0.6% and 823 K and 873 K temperatures to study the creep-fatigue interaction behavior of the weld joint. The alloy exhibited cyclic softening from first cycle onwards irrespective of the loading conditions. Failure location in the weld joint was correlated to the test parameters. Detailed replica study conducted on all the failed specimens revealed that most of the failures occurred in one side of the heat affected zone (HAZ) of the weld joint. Strain localization in the soft zone of the HAZ and subsurface creep cavity formation in this region and their linkage had caused enhanced crack propagation that translated into lower fatigue life of the weld joint at high temperatures. Type IV mode of failure was identified to be operative under tensile hold and high temperatures. The alloy was also found to be compressive dwell sensitive and it was ascertained that the lower life under compression hold compared to tension hold was due to the deleterious effect of oxidation. 相似文献
18.
核能已经逐渐取代化石能源成为新一代能源,作为重要构件的高温气冷堆中间换热器得到了广泛关注。由于GH3128合金具有较好的焊接性、较高的高温抗氧化性能和组织稳定性,有望成为超高温气冷堆中间换热器的候选材料,但基于换热器结构复杂性以及密封性的要求,焊接是其生产和制造的关键成形手段。采用脉冲钨极氩弧焊(GTAW)对GH3128合金2 mm板材进行对接焊,研究了热处理对焊接焊接接头显微组织以及应力的影响。结果表明,在优化焊接工艺参数下,固溶态板材接头表现出最高的强塑性,室温及高温拉伸断裂位置均为母材。由于热轧态与固溶态板材接头热影响区在焊接过程中产生残余应力,导致该区硬化,在高温变形过程中残余应力诱发热影响区μ相析出,对接头持久、蠕变性能造成不利影响。焊后热处理消除了接头热影响区的残余应力,减少了持久、蠕变过程中μ相的析出,接头持久寿命得以改善。在1 200 ℃下,残余应力可为焊后热处理过程中静态再结晶提供激活能,接头热影响区发生再结晶,硬度下降,接头塑性变形能力不协调,导致室温拉伸与950 ℃拉伸断裂位置均为焊接接头。对固溶态板材试样进行不同的焊后热处理,EBSD扫描结果分析发现,接头经过1 100 ℃×10 min热处理后,残余应力明显消失,温度升高至1 140 ℃后,热影响区开始发生再结晶。 相似文献
19.
R. K. Singh Raman 《Metallurgical and Materials Transactions A》1995,26(7):1847-1858
In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions
with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected
zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain
bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic
examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested
that M23C6 and M7C3 pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo2C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution
of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural
variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents
were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies
of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary
ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests
monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the
SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales
formed over the various regions is proposed to arise from the difference in microstructural features in different regions
of the weldments.
Formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India 相似文献
20.
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