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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.
Sunil Goyal K. Laha K. S. Chandravathi K. Bhanu Sankara Rao 《Transactions of the Indian Institute of Metals》2010,63(2-3):461-466
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
4.
5.
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
6.
Divya Jain David N. Seidman Erin J. Barrick John N. DuPont 《Metallurgical and Materials Transactions A》2018,49(4):1031-1043
Newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels rely on the Ni-enriched, thermally stable austenite [formed via multistep intercritical Quench-Lamellarization-Tempering (QLT)-treatment] for their superior mechanical properties, specifically ballistic resistance. Critical to the thermal stability of austenite is its composition, which can be severely affected in the weld heat-affected zones (HAZs) and thus needs investigations. This article represents the first study of the nanoscale redistributions of C, Ni, and Mn in single-pass HAZ microstructures of QLT-treated 10 wt pct Ni steels. Local compositions of Ni-rich regions (representative of austenite compositions) in the HAZs are determined using site-specific 3-D atom-probe tomography (APT). Martensite-start temperatures are then calculated for these compositions, employing the Ghosh-Olson thermodynamic and kinetics approach. These calculations predict that austenite (present at high temperatures) in the HAZs is susceptible to a martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT-treated base-metal. While C in the QLT-treated base-metal is consumed primarily in MC and M2C-type carbide precipitates (M is Mo, Cr, V), its higher concentration in the Ni-rich regions in the HAZs indicates the dissolution of carbide precipitates, particularly M2C carbide precipitates. The role of M2C carbide precipitates and austenite stability is discussed in relation to the increase in microhardness values observed in the HAZs, relative to the QLT-treated base-metal. Insights gained from this research on austenite stability and carbide precipitation in the single-pass HAZ microstructures will assist in designing multiple weld cycles for these novel 10 wt pct Ni steels. 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.
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
9.
《Acta Metallurgica》1987,35(2):341-352
The behaviour of carbo-nitride particles in HSLA steels when subjected to a weld thermal cycle corresponding to heat inputs of 2, 5, 10 or 20 kJ/mm has been studied by analytical electron microscopy. Three types of response were observed:
- 1.(a) complete dissolution followed by reprecipitation on cooling (in a Nb-V containing steel),
- 2.(b) partial dissolution accompanied by precipitate coarsening (in two Ti bearing steels)
- 3.(c) dissolution followed by reprecipitation at the peak temperature of the thermal cycle (in Ti-Nb bearing steels).
10.
S. D. Mann Ph.D. Student D. G. McCulloch B. C. Muddle 《Metallurgical and Materials Transactions A》1995,26(3):509-520
Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M5C2 carbides, although some were M3C. The M5C2 carbide, also known as the Hägg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M5C2 rods appeared to nucleate heterogeneously on M2C carbides and persist in ferrite regions from which the needlelike M2C carbides had disappeared. This suggests that the M5C2 carbide is more stable thermodynamically than M2C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components. 相似文献
11.
Yuan Liu Susumu Tsukamoto Kota Sawada Fujio Abe 《Metallurgical and Materials Transactions A》2014,45(3):1306-1314
Microstructure evolution of newly developed 9Cr-3W-3Co-V, Nb steel with boron addition (B steel) has been analyzed during HAZ thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ) and post-weld heat treatment (PWHT) to elucidate the prevention mechanism of type IV failure by boron addition. It was found that enhancement of the boundary strengthening by precipitates is the main reason for prevention of type IV failure by boron addition. In B steel HAZ, original austenite is reconstituted through martensitic α to γ reverse transformation during the heating and original martensite is reconstituted through martensitic transformation during cooling of the Ac3 HAZ thermal cycle. This process allows M23C6 carbides to precipitate at the prior austenite grain (PAG) and block boundaries during PWHT even if the chemical segregation of carbide forming elements exists. The effect of boundary strengthening on the creep property has also been investigated. Microstructure evolution during creep was compared among Gr.92 with different Ac3 HAZ microstructures prepared by three kinds of heat treatments and B steel. The results revealed that both the boundary length and kernel average misorientation value decreased in all samples during creep. However, this process occurred very rapidly in Ac3 HAZ simulated Gr.92, whereas it was significantly retarded in the other samples with sufficient boundary strengthening by precipitates. This result confirms that the precipitates formed at PAG and block boundaries play the most important role to stabilize the microstructure of Ac3 HAZ simulated samples during creep and prolong the creep life. 相似文献
12.
Y. Liu S. Tsukamoto T. Shirane F. Abe 《Metallurgical and Materials Transactions A》2013,44(10):4626-4633
The mechanism of type IV failure has been investigated by using a conventional 9Cr ferritic heat-resistant steel Gr.92. In order to clarify the main cause of type IV failure, different heat treatments were performed on the base metal in order to change the prior austenite grain (PAG) size and precipitate distribution after applying the heat-affected zone (HAZ) simulated thermal cycle at the peak temperature of around A c3 (A c3 HAZ thermal cycle) and postweld heat treatment (PWHT). The microstructural evolution during the A c3 HAZ thermal cycle and PWHT was investigated by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). It was found that M23C6 carbides were scarcely precipitated at the newly formed fine PAG, block, and lath boundaries in A c3 HAZ-simulated Gr.92, because the carbide forming elements such as Cr and C were segregated at the former PAG and block boundaries of the base metal. On the other hand, if all the boundaries were covered by sufficient M23C6 carbides by homogenization of the alloying elements prior to applying the HAZ thermal cycle, the creep strength was much improved even if the fine PAG was formed. From these results, it is concluded that fine-grained microstructure cannot account for the occurrence of type IV failure, and it only has a small effect during long-term creep. The most important factor is the precipitate formation behavior at various boundaries. Without sufficient boundary strengthening by precipitates, the microstructure of A c3 HAZ undergoes severe changes even during PWHT and causes premature failure during creep. 相似文献
13.
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
14.
《钢铁冶炼》2013,40(3):170-175
AbstractThere is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M23C6 carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M2N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M23C6 grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking. 相似文献
15.
Shigemitsu Kihara John B. Newkirk Akira Ohtomo Yoshinori Saiga 《Metallurgical and Materials Transactions A》1980,11(6):1019-1031
Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5
MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective
in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate
from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained
in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute
atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary
carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference
of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start
to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration.
5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix. 相似文献
16.
Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO2 laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of BL + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility. 相似文献
17.
A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi-steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)-reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules. 相似文献
18.
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
19.
Sabine M. Schlogl Erik Van Der Giessen Yvonne Van Leeuwen 《Metallurgical and Materials Transactions A》2000,31(1):125-137
Low-carbon, low-alloy Cr-Mo steels may fail by hydrogen attack when they are exposed to high hydrogen pressures at elevated
temperatures. During this process, the dissolved hydrogen reacts with the carbides of the steel to form methane in grain boundary
cavities. The methane pressure inside these cavities depends on the microstructure of the used steel, which consists of a
ferritic matrix and alloy carbides such as M7C3, M23C6, M6C, and M2C. The different phases in the multicomponent system Fe-Cr-Mo-V-C are modeled with the sublattice model. Their Gibbs energies
are then used to calculate the equilibrium methane pressure as a function of the microstructure. Driven by the methane pressure,
the cavities grow due to grain boundary diffusion and dislocation creep, which is described by analytical relations. This
leads to progressive development of damage inside the material but, at the same time, to a decrease of the carbon content
in the steel. This reduction depends on, among other factors, the methane pressure and the damage state. As the carbon content
also affects the creep parameters, this process of decarburization may accelerate the cavity growth. Model calculations are
used to obtain insight into the influence of this decarburization process on damage evolution and the final lifetime. 相似文献
20.
The microstructure of chromium-tungsten steels 总被引:1,自引:0,他引:1
Chromium-tungsten steels are being developed to replace the Cr-Mo steels for fusion-reactor applications. Eight experimental
steels were produced and examined by optical and electron microscopy. Chromium concentrations of 2.25, 5, 9 and 12 pct were
used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten
and vanadium, three other 2.25Cr steels were produced as follows: an alloy with 2 pct W and 0 pct V and alloys with 0 and
1 pct W and 0.25 pct V. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta also was studied. For all alloys, carbon
was maintained at 0.1 pct. Two pct tungsten was required in the 2.25Cr steels to produce 100 pct bainite (no polygonal ferrite).
The 5Cr and 9Cr steels were 100 pct martensite, but the 12Cr steel contained about 25 pct delta-ferrite. Precipitate morphology
and precipitate types varied, depending on the chromium content. For the 2.25Cr steels, M3C and M7C3 were the primary precipitates; for the 9Cr and 12Cr steels, M23C6 was the primary precipitate. The 5Cr steel contained M7C3 and M23C6. All of the steels with vanadium also contained MC. 相似文献