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1.
A study of the correlation between crack paths and crack growth response was undertaken to define better the elemental processes
involved in gaseous hydrogen embrittlement. AISI 4340 steel fractured under sustained load in hydrogen and in hydrogen sulfide
over a range of temperatures and pressures, whose crack growth kinetics have been well characterized previously, was chosen
for study. Fractographic results showed that crack growth followed predominantly along prior-austenite grain boundaries, with
a small amount of quasi-cleavage, at low temperatures. At high temperatures, crack growth occurred primarily by microvoid
coalescence. The fracture surface morphology, which is indicative of the micromechanisms for crack growth, was essentially
the same for hydrogen and hydrogen sulfide. Changes in fracture morphology,i.e., crack paths, corresponded to changes in crack growth kinetics, both of which depended on pressure and temperature. There
was no evidence for crack nucleation in advance of the main crack, and this suggests that the fracture process zone is located
within one prior-austenite grain diameter from the crack tip. The experimental results indicate that microstructure plays
an important role in determining crack growth response. The prior-austenite grain boundaries are seen to be most susceptible
to hydrogen embrittlement, followed by the (110)α’ and (112)α’ cleavage planes. The martensite matrix, on the other hand, is relatively immune. The observed changes in crack growth rate
with temperature and pressure in the higher temperature region are explained in terms of the partitioning of hydrogen into
the different microstructural elements and the consequent changes in the micromechanisms for fracture.
Leave from the Department of Materials Science, Shanghai Jaio Tong University, Shanghai, People’s Republic of China.
Formerly Research Associate, Department of Mechanical Engineering and Mechanics. 相似文献
2.
A “hydrogen partitioning” model has been developed to account for the pressure and temperature dependence for hydrogen-assisted
crack growth. The model gives explicit recognition to the role of hydr en-microstructure interactions in determining the distribution
(or partitioning) of hydrogen among the various microstructural elements (principally between the prior-austenite grain boundaries
and the matrix) and the rate of crack growth along the elements. It also takes into account the role of various rate controlling
processes in determining the rate that hydrogen is being supplied to the fracture process (or embrittlement) zone. Quantitative
assessment of the model indicates very good agreements between the model predictions and the observed crack growth responses
for AISI 4340 and 4130 steels tested in hydrogen and for AISI 4340 steel tested in hydrogen sulfide. This model accurately
characterizes the reduction in crack growth rate and the concomitant change in fracture mode at “high” temperatures. Through
its integration with the earlier models, based on rate controlling processes, the model predicts the pressure and temperature
dependence for K-independent crack growth over the entire range of environmental conditions. 相似文献
3.
Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn 总被引:2,自引:0,他引:2
The kinetics of hydrogen-induced cracking have been studied in the Ti-5Al-2.5Sn titanium alloy having a structure of acicular
α platelets in a β matrix. It was observed that the relationship between hydrogen-induced crack growth rate and applied stress
intensity can be described by three separable regions of behavior. The crack-growth rate at low stress-intensity levels was
found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate
at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such
a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal per
mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the
fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth
at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate
stress-intensity levels the transport of hydrogen to some interaction site along the α-β boundary is the rate-controlling
mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce
a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip. 相似文献
4.
H. F. Lopez R. Bharadwaj J. L. Albarran L. Martinez 《Metallurgical and Materials Transactions A》1999,30(9):2419-2428
In this work, the role of the microstructure in the stress sulfide cracking (SSC) resistance of an API X-80 steel was investigated
by exposure of as-received and heat-treated specimens to a H2S-saturated aqueous National Association of Corrosion Engineers (NACE) solution. It was found that for similar corrosive environments
and applied stress intensity factors of 30 to 46 MPa√m, crack growth in LEFM (linear elastic fracture mechanics) compact specimens
is strongly influenced by heat treating. In the as-received alloy, crack growth in the direction normal to rolling was controlled
by metal dissolution of the crack tip region in contact with the corrosive environment, with crack growth rates of the order
of 1/W(da/dt)∼8.3×10−4 h−1. Alternatively, crack growth in the direction parallel to the rolling direction did not show metal dissolution, but instead
hydrogen embrittlement along segregation bands. In this case, crack growth rates of the order of 1.2×10−3 h−1 were exhibited. In the martensitic condition, the rate of crack propagation was relatively fast (1/W(da/dt)∼4.5×10−2 h−1), indicating severe hydrogen embrittlement. Crack arrest events were found to occur in water-sprayed and quenched and tempered
specimens, with threshold stress intensity values (K
ISSC) of 26 and 32 MPa√m, respectively. Apparently, in the water-sprayed condition, numerous microcracks developed in the crack
tip plastic zone. Crack growth occurred by linking of microcracks, which were able to reach the main crack tip. In particular,
preferential microcrack growth occurred across carbide regions, but their growth was severely limited in the ferritic matrix.
Quenching and tempering (Q&T) resulted in a tempered martensite microstructure characterized by fine distribution carbides,
most of which were cementite. In this case, the crack path continually shifted to follow the ferrite interlath boundaries,
which contained mostly fine cementite precipitates. As a result, the crack was tortuous with numerous bifurcations along ferrite
grain boundaries. Most of the tests were carried out in NaCl-free NACE solutions; the only exception was the as-received condition
where 5 wt pct NaCl was added to the sour environment. In this case, crack growth did not occur after exposing the specimen
to the salt-free NACE solution for 30 days, but addition of 5 pct NaCl promoted crack propagation. 相似文献
5.
R. Dutton K. Nuttall M. P. Puls L. A. Simpson 《Metallurgical and Materials Transactions A》1977,8(10):1553-1562
Mechanisms which have been formulated to describe delayed hydrogen cracking in hydride-forming metals are reviewed and discussed.
Particular emphasis is placed on the commercial alloy Zr-2.5 pct Nb (Cb) which is extensively used in nuclear reactor core
components. A quantitative model for hydrogen cracking in this material is presented and compared with available experimental
data. The kinetics of crack propagation are controlled by the growth of hydrides at the stressed crack tip by the diffusive
ingress of hydrogen into this region. The driving force for the diffusion flux is provided by the local stress gradient which
interacts with both hydrogen atoms in solution and hydrogen atoms being dissolved and reprecipitated at the crack tip. The
model is developed using concepts of elastoplastic fracture mechanics. Stage I crack growth is controlled by hydrides growing
in the elastic stress gradient, while Stage II is controlled by hydride growth in the plastic zone at the crack tip. Recent
experimental observations are presented which indicate that the process occurs in an intermittent fashion; hydride clusters
accumulate at the crack tip followed by unstable crack advance and subsequent crack arrest in repeated cycles. 相似文献
6.
W. W. Gerberich Y. T. Chen C. ST. John 《Metallurgical and Materials Transactions A》1975,6(9):1485-1498
Analysis of hydrogen-stress field interactions have led to kinetic criteria for slow crack growth. Using both elastic and
plastic stress fields under opening-mode loading, criteria for stage I, II, III growth are developed in terms of the pressure
tensor gradient at the crack tip. It is proposed that stage I (stress-intensity dependent) growth kinetics are predominantly
controlled by the elastic stress field while stage II (nearly stress-intensity independent) kinetics are controlled by the
plastic stress field. Measurements of slow crack growth in cathodically-charged AISI 4340 steel verify the overall aspects
of the correlation. Detailed measurement and analysis of the increase in crack-tip radius with increasing applied stress intensity
have led to a proposed decrease in crack growth rate during stage II growth. Some experimental evidence corroborates this
later hypothesis and is consistent with long range diffusional flow of hydrogen as the controlling mechanism for crack growth
kinetics.
Partial fulfillment of the M.S. Degree at the University of Minnesota 相似文献
7.
Howard G. Nelson Dell P. Williams Alan S. Tetelman 《Metallurgical and Materials Transactions B》1971,2(4):953-959
Gaseous hydrogen embrittlement of quenched and tempered 4130 steel was studied as a function of temperature from −42° to 164°C
in a partially dissociated hydrogen environment at low molecular hydrogen pressures (≈8 × 10−3 torr). Atomic hydrogen was created by dissociation of molecular hydrogen on a hot tungsten filament located near a crack
opening. The presence of atomic hydrogen was found to increase the rate of hydrogen-induced, slow crack growth by several
orders of magnitude and to significantly alter the temperature dependence of embrittlement from what is observed in the presence
of molecular hydrogen alone. Based on a previous study, these observations are interpreted in terms of a difference between
the hydrogen-transport reaction step controlling hydrogen-induced, slow crack growth in the molecular hydrogen and the atomic-molecular
hydrogen environments. Finally, a comparison is made between the kinetics of hydrogen-induced, slow crack growth observed
in the presence of atomic-molecular hydrogen and the kinetics of known, possible hydrogen-transport reactions in an effort
to identify the reaction step controlling hydrogen embrittlement in the presence of atomic hydrogen. 相似文献
8.
The conditions of cathodic charging, gaseous hydrogen environment, and loading for which a TRIP steel may or may not be susceptible
to hydrogen embrittlement were investigated. In the austenitic state, the TRIP steel appeared to be relatively immune to hydrogen
embrittlement. It was shown that it is the strain-induced martensitic phase, α′, which is embrittled. In TRIP steel single-edge-notch specimens under fixed loads in gaseous hydrogen, slow crack growth
occurs when the stress intensity level exceeds a threshold level of about 25 ksi-in.1/2 and the growth rate varies approximately as the 2.5 power of the stress intensity level. The activation energy for this slow
crack growth was found to be about 10,000 cal/g-atom, the approximate activation for hydrogen diffusion in martensite. Thus
it was concluded that slow crack growth in TRIP steel loaded in gaseous hydrogen involves the diffusion of hydrogen through
the α′ phase.
Formerly with the Lawrence Berkeley Laboratory, Berkeley, Calif. 相似文献
9.
Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment
enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel
(tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined.
A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found
to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5
± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for
these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen
and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation
energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen
responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation
energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously
identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the
influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation
of alloying and residual impurity elements on crack growth was also considered. 相似文献
10.
The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel
alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged
at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air
at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration
was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated.
In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing
temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage
II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing
41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture
mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content
favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these
phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility
in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with
that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth
is reached at a location that is very near the crack tip.
Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign. 相似文献
11.
Hideya Anzai Jiro Kuniya Isao Masaoka 《Metallurgical and Materials Transactions A》1992,23(4):1291-1298
In this report, hydrogen-assisted cracking (HAC) behavior of low-alloy steel was evaluated using a constant elongation rate
tensile test, and the temperature and crack tip strain rate effects were observed. It was found that temperature dependence
of the threshold condition (C
σm
c
) of HAC above about 100 °C followed the relation C
σm
c
= Kexp(−41,300/Rr) whereK is a constant andT is absolute temperature. The relationship between HAC growth rate and crack tip strain rate was established as almost linear,
irrespective of temperature and hydrogen concentration at the crack tip. Hydrogen heat release tests were also performed.
From these tests, formation and growth of microcracks which are trap sites of hydrogen were thought to be the mechanism of
HAC in the steel. From this mechanism, HAC behavior of the low-alloy steel could be qualitatively explained. 相似文献
12.
A comprehensive study has been carried out on a 7075-T651 alloy to examine the influence of water vapor on fatigue crack growth.
The kinetics of fatigue crack growth were determined as a function of water vapor pressure at room temperature and at 353
K. Detailed fractographic analyses and surface chemistry studies were carried out to identify the micromechanisms and to quantify
the chemical interactions for corrosion fatigue crack growth in this alloy. Experiments were also carried out in ultra-high
vacuum and in oxygen to provide for comparisons. Two regions of fatigue crack growth response were identified. In the low
pressure region (below 67 Pa at 5 Hz), crack growth is controlled by the rate of transport of water vapor to the crack tip,
and the response can be described by a model for transport controlled crack growth. At pressures above 67 Pa, additional increases
in crack growth rate occurred, which are attributed to the further reactions of water vapor with segregated magnesium in this
alloy. Different micromechanisms for crack growth have been identified for vacuum, oxygen, and water vapor. These micromechanisms
are considered in relation to the environmental parameters through a modified superposition model for corrosion fatigue. 相似文献
13.
The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range
(ΔK = 31 MPa-m1/2) in deaerated deionized water and 0.3 M Na2SO4 solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions
of bare steel surfaces with the deaerated 0.3 M Na2SO4 solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions
in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments
appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction
rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge
transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in
the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent
activation energy of about 40 kJ/M. 相似文献
14.
Robert O. Ritchie 《Metallurgical and Materials Transactions A》1977,8(7):1131-1140
Interactions between hydrogen embrittlement and temper embrittlement have been examined in a study of fracture and low growth
rate (near-threshold) fatigue crack propagation in 300-M high strength steel, tested in humid air. The steel was investigated
in an unembrittled condition (oil quenched after tempering at 650°C) and temper embrittled condition (step-cooled after tempering
at 650°C). Step-cooling resulted in a severe loss of toughness (approximately 50 pct reduction), without loss in strength,
concurrent with a change in fracture mode from micr ovoid coalescence to inter granular. Using Auger spectroscopy analysis,
the embrittlement was attributed to the cosegregation of alloying elements (Ni and Mn) and impurity elements (P and Si) to
prior austenite grain boundaries. Prior temper embrittlement gave rise to a substantial reduction in resistance to fatigue
crack propagation, particularly at lower stress intensities approaching the threshold for crack growth(x0394;K
o). At intermediate growth rates (10-5 to 10-3 mmJcycle), propagation rates in both unembrittled and embrittled material were largely similar, and only weakly dependent
on the load ratio, consistent with the striation mechanism of growth observed. At near-threshold growth rates (<10−5 to 10−6 mmJcycle), embrittled material exhibited significantly higher growth rates, 30 pct reduction in threshold ΔKo values and intergranular facets on fatigue fracture surfaces. Near-threshold propagation rates (and ΔKo values) were also found to be strongly dependent on the load ratio. The results are discussed in terms of the combined influence
of segregated impurity atoms (temper embrittlement) and hydrogen atoms, evolved from crack tip surface reactions with water
vapor in the moist air environment (hydrogen embrittlement). The significance of crack closure concepts on this model is briefly
described. ntmis]formerly with the Lawrence Berkeley Laboratory, University of California in Berkeley.
Formerly with the Lawrence Berkeley Laboratery, University of California in Berkeley. 相似文献
15.
G. F. Pittinato 《Metallurgical and Materials Transactions B》1972,3(1):235-243
The effects of a hydrogen environment on the fatigue crack growth rates in Ti-6A1-4V ELI (STA) and weld material were determined
in the temperature range of ambient to -200°F. The hydrogen environment resulted in an acceleration of the crack growth rate
and a change in the fracture mode for both materials in the temperature range of ambient to -100°F. At -200°F, there was no
significant difference between the crack growth rates obtained in helium and hydrogen gas. The degree of hydrogén-enhanced
crack growth was found to be dependent on the crack tip stress-intensity range, temperature, and microstructure of the material.
The data is consistent with an embrittlement mechanism involving hydrogen diffusing ahead of the crack front. 相似文献
16.
The kinetics of the initial stages of hydrogen attack in a commercial 0.3 pct C steel (grade A516) were investigated using
anin situ dilatometer. The time, temperature and hydrogen pressure dependences of the rate of sample expansion were measured at hydrogen
pressures from 1 to 20 MPa, and temperatures from 350 to 475 °C for sample strains of 10-6 to 10-3. Sample expansion began shortly after hydrogen exposure and proceeded at a nearly constant rate throughout the “incubation
period” preceding rapid attack. At high temperatures and low pressures, this rate was proportional to PH
2
1.9±0.2 and had an apparent activation energy of 115 ± 9 kJ. At high pressures and low temperatures, the rate was proportional to
PPH
2
1.0.62±0.07
and showed an apparent activation energy of 210 ± 13 kJ. This suggests that bubble growth during the incubation period occurs
predominantly by grain boundary diffusion and is driven by near-equilibrium internal methane pressures. Sample expansion in
the subsequent stages of accelerating growth probably is controlled by creep and methane generation.
Formerly a Graduate Student at Ohio State University 相似文献
17.
Predicting the kinetics of hydrogen generation at the tips of corrosion fatigue cracks 总被引:1,自引:0,他引:1
A. Turnbull M. Saenz Santa de Maria 《Metallurgical and Materials Transactions A》1988,19(7):1795-1806
A model has been developed to predict the rate of generation of hydrogen atoms at the tips of fatigue cracks for steel cathodically
protected in marine environments. The model incorporates crack-tip chemistry, scraping electrode measurements, and crack-tip
deformation. The current density for generation of hydrogen atoms by reduction of water at the crack tip has been calculated
for a range of electrochemical and mechanical variables (electrode potential, cyclic frequency, waveform, ΔK, and R value).
The crack-tip current density is always greater than on adjacent crack walls and tends to increase with decreasing (more negative)
potential. However, at potentials more negative than about-900 mV (SCE), at a cyclic frequency of 0.1 Hz, cathodic reduction
of water on the external surface of the steel is predicted to be the dominant source of hydrogen atoms. Decreasing the frequency
reduces the crack-tip current density and further emphasizes the dominance of bulk charging. There is little difference in
hydrogen charging current densities at the crack tip for sinusoidal, triangular, or positive sawtooth waveforms, but square-wave
loading provides a greater charging current. Increasing ΔK and R value have only a small effect on crack-tip current density
but increase the area of the active surface and thus lead to more significant charging. Hydrogen-atom concentration profiles
in fracture mechanics specimens and in tubular sections have been calculated for conditions in which bulk charging of the
steel with hydrogen is dominant. To ensure that crack growth rates are “steady-state” values, test times have to be long enough
to establish steady conditions of hydrogen charging. Crack growth data from fracture mechanics specimens may not be directly
relevant to cracking in tubular sections because of hydrogen concentration gradients in the latter. 相似文献
18.
Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy 总被引:3,自引:0,他引:3
R. P. Wei P. S. Pao R. G. Hart T. W. Weir G. W. Simmons 《Metallurgical and Materials Transactions A》1980,11(1):151-158
Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water
vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851
aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency
of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this
threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum
crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution
on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).
The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue
crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for
crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs
through a “hydrogen embrittle ment” mechanism. 相似文献
19.
Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn 总被引:1,自引:0,他引:1
The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn
containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they
increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence
on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately
by cleavage ofα grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip
can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed
thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification
of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied
stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in
terms of the effective stress intensity,K
eff. From Arrhenius plots of crack growth rates for variousK
eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through theβ phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence
of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem. 相似文献
20.
The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high purity hydrogen are examined using
crack-tip stress intensity,K, as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly definedK-independent stage (Stage II). Crack growth rates in an 18 Ni (250) maraging steel are examined for temperatures from -60°C
to 100°C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures
the activation energy for Stage II crack growth was found to be 16.7 ± 3.3 kJ/mole. Temperature and hydrogen partial pressure
are shown to interact in a complex manner to determine the apparentK
th
and the crack growth behavior. Comparison of results on ‘250’ and ‘300’ grades of 18 Ni maraging steel indicate a significant
influence of alloy composition and/or strength level on the crack growth behavior. These phenomenological observations are
discussed in terms of possible underlying controlling processes.
Formerly a Graduate Student and Research Assistant.
Based on a thesis submitted in partial fulfillment of requirements for the M.S. degree in Metallurgy and Materials Science,
Lehigh University, Bethlehem, PA. 相似文献