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1.
Y. -M. Pan D. S. Dunn G. A. Cragnolino N. Sridhar 《Metallurgical and Materials Transactions A》2000,31(4):1163-1173
Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal
stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the
mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from
600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated
to grainboundary chemistry analyses. Sensitized microstructures were found to contain M23C6-type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 °C for 100 hours
resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with
time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent
chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used
to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons
between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key
model parameters. 相似文献
2.
T. M. Devine 《Metallurgical and Materials Transactions A》1980,11(5):791-800
The influence of ferrite morphology and carbon content on the intergranular corrosion behavior of 308 stainless steel was
investigated using four wrought alloys and six weld deposited alloys. The four wrought alloys were heat treated at four different
annealing temperatures to introduce four different amounts of ferrite. The annealed samples along with the weld deposited
alloys were aged at temperatures ranging from 480 to 700°C for times varying between 15 min and 1000 h and then tested for
intergranular corrosion susceptibility in acidified copper-copper sulfate solution. For a given carbon content there exists
a critical amount and distribution of α-γ boundary area above which the alloy is immune and below which it is susceptible
to intergranular corrosion. For amounts and distributions of α-γ boundary area less than the critical value two types of sensitization
behavior are possible. First, there may be a sufficient amount and distribution of α-γ boundary area to insure rapid healing
of the sensitized microstructure. Second, there may be an inadequate amount or distribution of α-γ boundary area to produce
either immunity or rapid healing and the alloy behaves as a fully austenitic alloy regardless of the amount of ferrite present.
A model is presented which describes as a function of carbon content the critical amounts and distributions of α-γ boundary
area required for rapid healing and immunity to sensitization. 相似文献
3.
Unfavorable morphology of precipitates and inclusions has been thought to be the cause of severe intergranular stress corrosion
cracking (IGSCC) in double aged INCONEL* X-750 alloy used in reactor water environments. A single step aging treatment of
200 hours at 811 °C followed by furnace cooling after solution treating for 2 hours at 1075 °C has been found to provide an
improved combination of strength, ductility, and resistance to SCC under simulated PWR test conditions. In this single aged
condition a reprecipitated secondary carbide, together with γ′ was produced at the grain boundary which resulted in a mixed
fracture mode comprising dimple rupture and microvoid coalescence compared with a predominantly intergranular mode for the
fully age hardened specimens. This improvement has been explained in terms of the morphology of the second phase precipitates
which are produced in these heat treatment regimes.
INCONEL is a trademark of the INCO family of companies. 相似文献
4.
C. F. Willis R. Gronsky T. M. Devine 《Metallurgical and Materials Transactions A》1991,22(12):2889-2902
Transmission electron microscopy (TEM) and microanalytical chemistry were performed on sensitized samples of duplex welds
that exhibited both skeletal ferrite microstructures and lath ferrite microstructures. The objective was to understand why
welds with lath ferrite, contrary to a theoretical prediction, are not immune to sensitization. Most of the ferrite-austenite
(α-γ) interphase boundaries in the welds with skeletal ferrite were curved and incoherent, while those in welds with lath
ferrite were predominantly planar and semicoherent. The density of carbide precipitation on incoherent boundaries was much
greater than that on semicoherent boundaries. Carbide precipitates on incoherent boundaries were typically equiaxed, while
those on semicoherent boundaries had very high aspect ratios and appeared to form along ledges in the interphase boundary.
During sensitizing heat treatments, the chromium-depleted zone on the ferrite side of the interphase region transformed to
austenite, causing the α-γ interphase boundary to move into the ferrite region. This markedly increased the width of the chromium-depleted
zone in the austenite phase and extended the time of heat treatment required to replenish the zone with chromium. It is proposed
that migration of the α-γ interphase boundary, which occurs to a much greater extent in the welds with lath ferrite, is responsible
for their unexpected susceptibility to sensitization at 550°C. 相似文献
5.
6.
H. Sidhom T. Amadou H. Sahlaoui C. Braham 《Metallurgical and Materials Transactions A》2007,38(6):1269-1280
7.
8.
Charles G. Schmidt Robert D. Caligiuri Lawrence E. Eiselstein Sharon S. Wing Daniel Cubicciotti 《Metallurgical and Materials Transactions A》1987,18(8):1483-1493
Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which
low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects
of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was
determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with
constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions
were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted
in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O2 or 180 °C/1.0 ppm O2), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower
temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from
LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries.
Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility. 相似文献
9.
Heon-Young Ha Tae-Ho Lee Sung-Joon Kim 《Metallurgical and Materials Transactions A》2012,43(9):2999-3005
The effects of carbon fraction on various corrosion properties of Fe18Cr10MnNC alloys were investigated. The alloys contained 0.6?wt pct of nitrogen and carbon, and the carbon fraction varied from 0.03 to 0.47. With increasing the carbon fraction, corrosion potential raised, critical dissolution rate decreased, and pitting potential increased. The high carbon fraction was responsible for high resistance against intergranular corrosion of the alloys aged at 1123?K (850?°C) for 100?seconds. But after aging at 1123?K (850?°C) for 600?seconds, the intergranular corrosion accelerated with increasing the carbon fraction. 相似文献
10.
This paper compares the effects of deformation which induces martensite in austenitic stainless steel with deformation which
does not on the sensitization and corrosion susceptibility of these alloys. We show that deformation which induces martensite
causes rapid sensitization at temperatures below 600 °C, leads to extensive transgranular corrosion, and can produce rapid
healing. The martensite is also an area of extensive carbide precipitation. Deformation alone noticeably increases the kinetics
of sensitization only at temperatures where undeformed samples are readily sensitized. Without the presence of martensite,
intergranular corrosion is always the predominant corrosion path, rapid healing is not observed, and most carbides precipitate
along the grain boundaries. 相似文献
11.
Ercan Balikci A. Raman R. A. Mirshams 《Metallurgical and Materials Transactions A》1997,28(10):1993-2003
IN738LC is a modern, nickel-based superalloy utilized at high temperatures in aggressive environments. Durability of this
superalloy is dependent on the strengthening of γ′ precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120
°C/2 h/accelerated air-cooled (AAC) solution treatment, given in the literature, already produces a bimodal precipitate microstructure,
which is, thus, not an adequate solutionizing procedure to yield a single-phase solid solution in the alloy at the outset.
However, the 1235 °C/4 h/water quenched (WQ) solution treatment does produce the single-phase condition. A microstructure
with fine precipitates develops if solutionizing is carried out under 1200 °C/4 h/AAC conditions. Agings at lower temperatures
after 1200 °C/4 h/AAC or 1250 °C/4 h/AAC or WQ conditions yield analogous microstructures. Agings below ∼950 °C for 24 hours
yield nearly spheroidal precipitates, and single aging for 24 hours at 1050 °C or 1120 °C produces cuboidal precipitates.
Two different γ′ precipitate growth processes are observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size
microstructures) and growth through solute absorption from the matrix. Average activation energies for the precipitate growth
processes are 191 and 350 kJ/mol in the ranges of 850 °C to 1050 °C and 1050 °C to 1120 °C, respectively, calculated using
the precipitate sizes from microstructures in the WQ condition, and 150 and 298 kJ/mol in the analogous temperature ranges,
calculated from precipitate sizes in the microstructures in the slow furnace-cooled condition. 相似文献
12.
S. DowneyII K. Han P.N. Kalu K. Yang Z.M. Du 《Metallurgical and Materials Transactions A》2010,41(4):881-887
This article reports our efforts in characterization of an ultralow carbon 316LN-type stainless steel. The carbon content
in the material is one-third that in a conventional 316LN, which further inhibits the formation of grain boundary carbides
and therefore sensitizations. Our primary effort is focused on characterization of submicron size precipitates in the materials
with the electron backscatter diffraction (EBSD) technique complemented by Auger electron spectroscopy (AES). Thermodynamic
calculations suggested that several precipitates, such as M23C6, Chi, Sigma, and Cr2N, can form in a low carbon 316LN. In the steels heat treated at 973 K (700 °C) for 100 hours, a combination of EBSD and AES
conclusively identified the grain boundary precipitates (≥100 nm) as Cr2N, which has a hexagonal closed-packed crystallographic structure. Increases of the nitrogen content promote formation of
large size Cr2N precipitates. Therefore, prolonged heat treatment at relatively high temperatures of ultralow carbon 316LN steels may result
in a sensitization. 相似文献
13.
K. Banerjee N. L. Richards M. C. Chaturvedi 《Metallurgical and Materials Transactions A》2005,36(7):1881-1890
The effect of filler alloys C-263, RENé-41, IN-718, and FM-92 on heat-affected zone (HAZ) cracking susceptibility of cast
IN-738 LC, which is a high-temperature Ni-based superalloy used at temperatures up to 980 °C and is precipitation hardened
by the γ′ (Ni3Al,Ti) phase, by gas-tungsten-arc (GTA) welding was studied. In addition, autogenous welds were also made on the IN-738 parent
material. The preweld treatments consisted of the standard solution treatment at 1120 °C for 2 hours followed by air cooling,
and a new heat treatment, which was developed to improve the HAZ cracking resistance of IN-738 LC. This heat treatment consisted
of solution treating at 1120 °C followed by air cooling then aging at 1025 °C for 16 hours followed by water quenching. Welds
were observed to suffer intergranular HAZ cracking, regardless of the filler alloy; however, the autogenous welds were most
susceptible to HAZ cracking. In general, the cracking tendency for both heat treatments was maximum for C-263 and RENE-41
fillers and decreased with the use of FM-92 and IN-718 filler alloys. The HAZ cracking was associated mainly with constitutional
liquation of γ′ and MC carbides. On some cracks, liquated low melting point containing Zr-carbosulfide and Cr-Mo borides were also observed
to be present. The cooling portion of the weld thermal cycle induced precipitation hardening via γ′ phase in the γ matrix of the weld metal. The HAZ cracking increased as the weld metal lattice mismatch between γ′ precipitates and γ matrix of the weld and its hardness (Ti + Al) increased. However, the weld-metal solidus and solidification temperature range,
determined by high-temperature differential scanning calorimetry, did not correlate with the HAZ cracking susceptibility.
It is suggested that the use of filler alloys with small γ′-γ lattice mismatch and slow age-hardening response would reduce the HAZ cracking in IN-738 LC superalloy welds. 相似文献
14.
Krystyna Stiller Jan-Olof Nilsson Kjell Norring 《Metallurgical and Materials Transactions A》1996,27(2):327-341
The microstructure in six commercial batches of alloys 600 and 690 has been investigated using scanning electron microscopy
(SEM), analytical transmission electron microscopy (ATEM), atom probe field ion microscopy (APFIM), and secondary ion mass
spectroscopy (SIMS). The materials were also tested with respect to their resistance to intergranular stress corrosion cracking
(IGSCC) in high-purity water at 365 °. Applied microanalytical techniques allowed direct measurement of carbon concentration
in the matrix together with determination of grain boundary micro structure and microchemistry in all material conditions.
The distribution of oxygen near a crack in material tested with respect to IGSCC was also investigated. The role of carbon
and chromium and intergranular precipitates on IGSCC is discussed. 相似文献
15.
The influence of thermal treatment on the chemistry and structure of grain boundaries in inconel 600
G. S. Was H. H. Tischner R. M. Latanision 《Metallurgical and Materials Transactions A》1981,12(8):1397-1408
Although it is widely accepted that certain heat treatments result in carbide precipitation accompanied by chromium depletion
at the grain boundaries, no direct evidence of this phenomenon exists for Inconel 600. Using the Scanning Transmission Electron
Microscope (STEM), the extent of grain boundary chromium depletion is quantitatively determined as a function of thermal treatment
time at 700 °C following a 30 min solution anneal at 1100 °C. Results confirm the presence of grain boundary chromium depletion
that varies in extent with time at temperature, the chromium concentration falling to values as low as 3 wt pct. The chromium
depletion volume is characterized by a depletion parameter which is correlated with intergranular corrosion test results to
determine a self-healing (desensitization) chromium concentration of 9 wt pct. Trace element segregation at grain boundaries
is measured by Auger Electron Spectroscopy (AES) as a function of aging treatment. Results show that after thermally treating
samples for various times at 700 °C, phosphorus is always present at the grain boundaries. Intergranular corrosion behavior
as a function of thermal treatment appears to be governed more strongly by chromium depletion than trace element segregation.
G. S. WAS, formerly Research Assistant, Nuclear Engineering Dept., Massachusetts Institute of Technology
H. H. TISCHNER, formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute
of Technology 相似文献
16.
V. Kain P. Sengupta P. K. De S. Banerjee 《Metallurgical and Materials Transactions A》2005,36(5):1075-1084
This article describes the corrosion behavior of special austenitic alloys for waste management applications. The special
stainless steels have controlled levels of alloying and impurity elements and inclusion levels. It is shown that “active”
inclusions and segregation of chromium along flow lines accelerated IGC of nonsensitized stainless steels. Concentration of
Cr+6 ions in the grooves of dissolved inclusions increased the potential to the transpassive region of the material, leading to
accelerated attack. It is shown that a combination of cold working and controlled solution annealing resulted in a microstructure
that resisted corrosion even after a sensitization heat treatment. This imparted extra resistance to corrosion by increasing
the fraction of “random” grain boundaries above a threshold value. Randomization of grain boundaries made the stainless steels
resistant to sensitization, IGC, and intergranular stress corrosion cracking (IGSCC) in even hot chloride environments. The
increased corrosion resistance has been attributed to connectivity of random grain boundaries. The reaction mechanism between
the molten glass and the material for process pot, alloy 690, during the vitrification process has been shown to result in
depletion of chromium from the reacting surfaces. A comparison is drawn between the electrochemical behavior of alloys 33
and 22 in 1 M HCl at 65 °C. It is shown that a secondary phase formed during welding of alloy 33 impaired corrosion properties
in the HCl environment.
This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste
Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion
and Environmental Effects and Nuclear Materials Committees. 相似文献
17.
The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with
carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30
seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature,
were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with
the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures
above 350 °C and a transition carbide ({100}
α
habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large
increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on
the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature,
particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of
silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed.
This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual
AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee. 相似文献
18.
J.Y. Hwang S. Nag A.R.P. Singh R. Srinivasan J. Tiley G.B. Viswanathan H.L. Fraser R. Banerjee 《Metallurgical and Materials Transactions A》2009,40(13):3059-3068
The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has
been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy
(EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale
secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr
while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary
precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary
γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process. 相似文献
19.
The sensitization behavior of types 308 and 316 stainless steel weld metals as internal overlays for reactor pressure vessels
(RPVs) was studied with respect to the effects of postweld heat treatment (PWHT) at about 600 °C during RPV fabrication and
low-temperature aging during operation. For the study, a criterion for the rate of intergranular corrosion (IGC) for detecting
the susceptibility to intergranular stress corrosion cracking (IGSCC) in high-temperature oxygenated pure water was established
by quantitatively evaluating the results from a modified ASTM A262E test. A criterion for expecting satisfactory resistance
to IGSCC was found to be an IGC rate of about 1 μm/h. Type 308 weld metal can be sensitized as indicated by an IGC rate >1 μm/h, and can be healed, as indicated by an IGC rate <1 μm/h, depending on the length of PWHT. However, this healed weld metal can be resensitized by exposing it to 500 °C×24 h; in
this condition, it shows a relatively high susceptibility to IGSCC. On the contrary, type 316 weld metal was almost immune
to sensitization under the same heat treatments. By transmission electron microscopy (TEM), its excellent resistance was attributable
to carbon fixation by molybdenum carbide precipitation within ferrite phases during PWHT. Reheat embrittlement of type 316
weld metal was also examined. 相似文献