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1.
The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress. 相似文献
2.
The role of the multiaxial state of stress on creep deformation and rupture behavior of 304HCu austenitic stainless steel was assessed by performing creep rupture tests on both smooth and notched specimens of the steel. The multiaxial state of stress was introduced by incorporating circumferential U-notches of different root radii ranging from 0.25 to 5.00 mm on the smooth specimens of the steel. Creep tests were carried out at 973 K over the stress range of 140 to 220 MPa. In the presence of notch, the creep rupture strength of the steel was found to increase with the associated decrease in rupture ductility. Over the investigated stress range and notch sharpness, the strengthening was found to increase drastically with notch sharpness and tended toward saturation. The fractographic studies revealed the mixed mode of failure consisting of transgranular dimples and intergranular creep cavitation for shallow notches, whereas the failure was predominantly intergranular for relatively sharper notches. Detailed finite element analysis of stress distribution across the notch throat plane on creep exposure was carried out to assess the creep failure of the material in the presence of notch. The reduction in von-Mises stress across the notch throat plane, which was greater for sharper notches, increased the creep rupture strength of the material. The variation in fracture behavior of the material in the presence of notch was elucidated based on the von-Mises, maximum principal, and hydrostatic stresses. Electron backscatter diffraction analysis of creep strain distribution across the notch revealed localized creep straining at the notch root for sharper notches. A master curve for predicting creep rupture life under the multiaxial state of stress was generated considering the representative stress having contributions from both the von-Mises and principal stress components of the stress field in the notch throat plane. Rupture ductility was also predicted based on the multiaxial state of stress. 相似文献
3.
Type 347 austenitic stainless steel (18Cr-12Ni-Nb) was alloyed with copper (3 wt pct), boron (0.01 to 0.06 wt pct), and cerium
(0.01 wt pct) with an aim to increase the creep rupture strength of the steel through the improved deformation and cavitation
resistance. Short-term creep rupture strength was found to increase with the addition of copper in the 347 steel, but the
long-term strength was inferior. Extensive creep cavitation deprived the steel of the beneficial effect of creep deformation
resistance induced by nano-size copper particles. Boron and cerium additions in the copper-containing steel increased its
creep rupture strength and ductility, which were more for higher boron content. Creep deformation, grain boundary sliding,
and creep cavity nucleation and growth in the steel were found to be suppressed by microalloying the copper-containing steel
with boron and cerium, and the suppression was more for higher boron content. An auger electron spectroscopic study revealed
the segregation of boron instead of sulfur on the cavity surface of the boron- and cerium-microalloyed steel. Cerium acted
as a scavenger for soluble sulfur in the steels through the precipitation of cerium sulfide (CeS). This inhibited the segregation
of sulfur and facilitated the segregation of boron on cavity surface. Boron segregation on the nucleated cavity surface reduced
its growth rate. Microalloying the copper-containing 347 steel with boron and cerium thus enabled to use the full extent of
creep deformation resistance rendered by copper nano-size particle by increase in creep rupture strength and ductility. 相似文献
4.
The influence of prior cold work (PCW) on low-cycle fatigue (LCF) behavior of type 304 stainless steel has been studied at
300, 823, 923, and 1023 K by conducting total axial strain-controlled tests in solution annealed (SA, 0 pct PCW) condition
and on specimens having three levels of PCW, namely, 10, 20, and 30 pct. A triangular waveform with a constant frequency of
0.1 Hz was employed for all of the tests performed over strain amplitudes in the range of ±0.25 to ± 1.25 pct. These studies
have revealed that fatigue life is strongly dependent on PCW, temperature, and strain amplitude employed in testing. The SA
material generally displayed better endurance in terms of total and plastic strain amplitudes than the material in 10, 20,
and 30 pct PCW conditions at all of the temperatures. However, at 300 K at very low strain amplitudes, PCW material exhibited
better total strain fatigue resistance. At 823 K, LCF life decreased with increasing PCW, whereas at 923 K, 10 pct PCW displayed
the lowest life. An improvement in life occurred for prior deformations exceeding 10 pct at all strain amplitudes at 923 K.
Fatigue life showed a noticeable decrease with increasing temperature up to 1023 K in PCW state. On the other hand, SA material
displayed a minimum in fatigue life at 923 K. The fatigue life results of SA as well as all of the PCW conditions obeyed the
Basquin and Coffin-Manson relationships at 300, 823, and 923 K. The constants and exponents in these equations were found
to depend on the test temperature and prior metallurgical state of the material. A study is made of cyclic stress-strain behavior
in SA and PCW states and the relationship between the cyclic strain-hardening exponent and fatigue behavior at different temperatures
has been explored. The influence of environment on fatigue crack initiation and propagation behavior has been examined. 相似文献
5.
The present study describes the changes in the creep properties associated with microstructural evolution during thermal exposures to near service temperatures in indigenously developed reduced activation ferritic-martensitic steels with varying tungsten (1 and 1.4 wt pct W) contents. The creep behavior has been studied employing impression creep (IC) test, and the changes in impression creep behavior with tungsten content have been correlated with the observed microstructures. The results of IC test showed that an increase in 0.4 pct W decreases the creep rate to nearly half the value. Creep strength of 1.4 pct W steel showed an increase in steels aged for short durations which decreased as aging time increased. The microstructural changes include coarsening of precipitates, reduction in dislocation density, changes in microchemistry, and formation of new phases. The formation of various phases and their volume fractions have been predicted using the JMatPro software for the two steels and validated by experimental methods. Detailed transmission electron microscopy analysis shows coarsening of precipitates and formation of a discontinuous network of Laves phase in 1.4 W steel aged for 10,000 hours at 823 K (550 °C) which is in agreement with the JMatPro simulation results. 相似文献
6.
Small punch creep (SPC) studies have been carried out to evaluate the creep properties of 316LN stainless steel (SS) at 923 K (650 °C) at various stress levels. The results have been compared with uniaxial creep rupture data obtained from conventional creep tests. The minimum deflection rate was found to obey Norton power law. SPC rupture life was correlated with uniaxial creep rupture life. The influence of nitrogen content on the creep rupture properties of 316LN SS was investigated in the range of 0.07 to 0.14 wt pct. SPC rupture life increased and the minimum deflection rate decreased with the increase in nitrogen content. The trends were found to be in agreement with the results obtained from uniaxial creep rupture tests. These studies have established that SPC is a fast and reliable technique to screen creep properties of different experimental heats of materials for optimizing the chemical composition for developing creep-resistant materials. 相似文献
7.
Nitrogen-alloyed 316LN stainless steel is used as a structural material for high temperature fast breeder reactor components. With a view to increase the design life of the components up to 60 years and beyond, studies are being carried out to develop nitrogen alloyed 316LN stainless steel with superior tensile, creep and low cycle fatigue properties. This paper presents the results from studies on the influence of nitrogen on the high temperature creep properties of this material. The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt%. Creep tests were carried out at 923 K at stress levels 140, 175, 200 and 225 MPa. Creep rupture strength increased substantially with increase in nitrogen content. The variation of steady state creep rate with stress showed a power law relationship. The power law exponent varied between 6.4 and 13.7 depending upon the nitrogen content. Rupture ductility was generally above 40% at all the test conditions and for all the nitrogen contents. It was observed that the internal creep damage and surface damage decreased with increase in nitrogen content. Fracture mode was found to generally shift from intergranular failure to transgranular failure with increasing nitrogen content. 相似文献
8.
A program to study the effect of Sb, P, Sn and B on creep properties of four normalized and tempered 1.25 Cr-0.5 Mo steels
at 538°C (1000°F) has been completed. Results show that even a combined addition of large amounts of Sb, P and Sn does not
affect short time creep strength or ductility of the steel at 538°C (1000°F). Addition of B resulted in an increase or decrease
of creep strength depending on the nature of the impurity species present, presumably due to B-impurity interactions. Regardless
of the effect on creep strength, B additions caused sharp reductions in rupture ductility in all cases. Comparison of the
present results on the four laboratory steels (100 pct bainite) with results of a previous study on a commercial steel (60
pct bainite + 40 pct ferrite) show that the effect of microstructure becomes negligible and rupture strength values of the
various steels at 538°C (1000°F) approach each other at rupture times in excess of 10 4 h. 相似文献
9.
Creep behavior and degradation of subgrain structures and precipitates of Gr. 122 type xCr-2W-0.4Mo-1Cu-VNb ( x = 5, 7, 9, 10.5, and 12 pct) steels were evaluated during short-term and long-term static aging and creep with regard to
the Cr content of steel. Creep rupture life increased from 5 to 12 pct Cr in the short-term creep region, whereas in the long-term
creep region, it increased up to 9 pct Cr and then decreased with the addition of Cr from 9 to 12 pct. Behavior of creep rupture
life was attributed to the size of elongated subgrains. In the short-term creep region, subgrain size decreased from 5 to
12 pct Cr, corresponding to the longer creep strength. However, in the long-term creep region after 10 4 hours, subgrain size increased up to 9 pct Cr and then decreased from 9 to 12 pct, corresponding to the behavior of creep
rupture life. M 23C 6 and MX precipitates had the highest number fraction among all of the precipitates present in the studied steels. Cr concentration
dependence of spacing of M 23C 6 and MX precipitates exhibited a V-like shape during short-term as well as long-term aging at 923 K (650 °C), and the minimum
spacing of precipitates belonged to 9 pct Cr steel, corresponding to the lowest recovery speed of subgrain structures. In
the short-term creep region, subgrain coarsening during creep was controlled by strain and proceeded slower with the addition
of Cr, whereas in long-term creep region, subgrain coarsening was controlled by the stability of precipitates rather than
due to the creep plastic deformation and took place faster from 9 to 12 pct and 9 to 5 pct Cr. However, M 23C 6 precipitates played a more important role than MX precipitates in the control of subgrain coarsening, and there was a closer
correlation between spacing of M 23C 6 precipitates and subgrain size during static aging and long-term creep region. 相似文献
10.
A program to study the effect of Sb, P, Sn and B on creep properties of four normalized and tempered 1.25 Cr−0.5 Mo steels
at 538°C (1000°F) has been completed. Results show that even a combined addition of large amounts of Sb, P and Sn does not
affect short time creep strength or ductility of the steel at 538°C (1000°F). Addition of B resulted in an increase or decrease
of creep strength depending on the nature of the impurity species present, presumably due to B-impurity interactions. Regardless
of the effect on creep strength, B additions caused sharp reductions in rupture ductility in all cases. Comparison of the
present results on the four laboratory steels (100 pct bainite) with results of a previous study on a commercial steel (60
pct bainite + 40 pct ferrite) show that the effect of microstructure becomes negligible and rupture strength values of the
various steels at 538°C (1000°F) approach each other at rupture times in excess of 10 4 h. 相似文献
12.
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 Fe 2Mo. 相似文献
13.
A study of the relationship between structure and mechanical properties of thin TD-NiCr sheet indicated that the elevated
temperature tensile, stress-rupture, and creep strength properties are dependent on grain aspect ratio and sheet thickness.
In general, the strength properties increase with increasing grain aspect ratio and sheet thickness. Tensile testing revealed
an absence of ductility at elevated temperatures (T ≥ 1144 K). Significant creep damage, as determined by subsequent tensile testing at room temperature, occurs after very small
amounts (< 0.1 pct) of prior creep deformation at elevated temperatures (1144 ≤ T ≤ 1477 K). A threshold stress for creep appears to exist. Creep exposure below the threshold stress at T ≥ 1366 K results in almost full retention of room temperature tensile properties. 相似文献
14.
The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in
vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically
20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases
the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only
in heavily necked parts of the samples. In addition to Ni 5Zr and ZrO 2 inclusions, a Zr 4C 2S 2 carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20
wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation
by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and
the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates
in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor
alloying additions in constructing and using fracture mechanism maps. 相似文献
15.
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
16.
The relationship between cyclic deformation, slip-mode and dislocation structures is investigated in 316LN stainless steel (with 0.07–0.22 wt% Nitrogen) subjected to low cycle fatigue at temperatures in the range 300–873 K and at a 0.6 % strain amplitude. Irrespective of the nitrogen content, cyclic softening/saturation occupied a large fraction of fatigue life at temperatures <773 K. The end-of-life dislocation structures (e.g. dislocation cells, planar slip-bands) characterizing the cyclic softening/saturation belong to wavy/mixed/planar slip-modes of deformation. On the other hand at temperatures ≥773 K, similar dislocation structures are noticed to be associated with significant cyclic strengthening with limited softening. The differences in the above deformation behavior is found to be controlled not by the nature of slip-mode but by the consequences of dynamic strain aging occurrence (e.g. significant cyclic strengthening and pronounced serrations) which are noticed to vary in the temperature range 573–873 K. Maximum fatigue life is observed at 0.11–0.14 wt% N that induced mixed mode of deformation. 相似文献
17.
Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint. 相似文献
18.
Creep deformation in electron-beam-melted polycrystalline rhenium sheet was evaluated at 2200° to 4200°F (1477° to 2588°K) and 4 to 40 ksi (28 to 276 MN per sq m). Comparisons were made with powder metallurgy rhenium under similar conditions. Changes in creep-rupture behavior resulting from electron beam melting of rhenium were greater ductility, higher primary creep rate, and longer rupture life, especially at lower temperatures. The activation energy for creep was 72 kcal per mole for electron-beam-melted rhenium and 64 kcal per mole for powder metallurgy rhenium. 相似文献
19.
Creep and rupture tests have been conducted on NiTaC-13, an advanced TaC fiber strengthened composite. A simple equation is
developed to describe the creep behavior in argon for strains up to about 1 pct at temperatures between 871 and 1093°C. This
equation may readily be incorporated in a nonlinear analysis of the deformation of a body subjected to nonsteady and nonuniform
stresses and temperatures. The creep rates in air show a progressive increase relative to those in argon due to a loss in
cross-section resulting from oxidation. The Larson-Miller parameter is shown to be unreliable for either correlation or extrapolation
of rupture data. This is especially true for air tests. A modified parameter is, however, shown to give a good correlation
with all the data. Although metallurgical instabilities are present, they have no clear effect on rupture strength and are
not uniquely linked with the parametric representation. There is a systematic increase in ductility with increase in temperature
and the generally high level of ductility is reflected in pronounced notch strengthening. Some load relaxation tests indicate
that fiber failure occurs in excess of 1 pct composite strain. It is suggested, therefore, that 1 pct could be an appropriate
design limiting strain for this class of material. 相似文献
20.
Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P)
to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure
of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with
high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility
was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K
(450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted
in complete softening depending on the development of recrystallization. Therefore, the temperature of approximately 923 K
(650 °C) can be considered as a critical recrystallization temperature for UFG IF steel. The annealing at 873 K (600 °C) for
different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense
of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength,
uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing
the better combination of strength and ductility compared with cold-rolled samples. 相似文献
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