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1.
The low-temperature toughness of C-Mn weld steel with different grain sizes was investigated with notched and precracked specimens.
The results indicated that the fine grain steel, evaluated by notched specimens (Charpy V-notch and 4 point bending specimens),
is tougher than that of the coarse grain steel over a temperature range from -196 °C to -30 °C. On the other hand, the coarse
grain steel, evaluated with precracked specimens, has a remarkably greater plane strain fracture toughness compared to the
fine grain steel. The microstructural analysis revealed that the fracture toughness of both the fine grain and the coarse
grain steel is not directly related to the distance of the fracture initiation site from the precrack tip or the size of the
ferrite grain. The behavioral discrepancy can be explained in terms of the ratio of local fracture stress to yield stress,i.e., σ
f
f/σ
y
. The fine grain steel had a higherσ
f
/σ
y
in the notched specimens but a lower value in the precracked specimens compared to the coarse grain steel. The scatter of
toughness data can be mainly attributed to the probabilistic distribution of the weakest particle. We suggested thatσ
f
/σ
y
may be a useful parameter for the engineering evaluation of toughness. 相似文献
2.
The fracture behaviors of low alloy steels with similar grain sizes but different sizes of carbide particles were investigated
using precracked and notched specimens. The results indicate that in precracked specimens (COD), steel with coarser carbide
particles has a lower toughness than steel with finer carbide particles over a temperature range from –196 °C to – 90 °C.
However, in notched specimens (four-point bending (4PB) and Charpy V), these two steels shows similar toughness at low temperature
where specimens are fractured by cleavage without fibrous cracking. In the transition temperature range, the steel with coarser
carbide particles conversely shows a little higher toughness due to the longer extension length of the fibrous crack. This
phenomenon indicates that in precracked specimens, the second-phase particles play a leading role in cleavage fracture, while
in notched specimens, the grain size dominates the fracture behavior. 相似文献
3.
The microscopic fracture behaviors of a C-Mn steel plate and of two types of weld metal in Charpy V, crack tip opening displacement
(COD), and precracked impact specimens have been studied by observations of the size distribution of the cleavage-initiating
particles, the morphology of the fibrous crack zones, the microstructure in the region initiating the cleavage crack, and
the size distribution of microcracks remaining in the specimens. It has been found that the different values of the microscopic
cleavage fracture stress, σf, measured in notched and precracked specimens of the same metal result from the change in the critical event (critical length)
in the fracture processes. The critical event in the Charpy V-notched specimens at —45 °C to —65 °C is the propagation of
a ferrite grain-sized microcrack into the ferrite matrix; however, in the precracked COD specimens at —110 °C, it is the propagation
of a second-phase particle-sized microcrack into the neighboring ferrite grain. The change of the critical event is considered
to be related to the difference of effective shear stress ahead of a notch root or a crack tip. The different sensitivities
to the notch acuity of the base and weld metal result from their micro-structures. Finally, the prerequisite for establishing
a correlation between Charpy V and COD tests is analyzed. 相似文献
4.
Four-point bend (4PB) tests of notched specimens loaded at various loading rates, for low alloy steel with different grain
sizes, were done, and the microscopic observation and finite-element method (FEM) calculations were carried out. It was found
that for the coarse-grained (CG) microstructure, an appreciable drop in notch toughness with a loading rate of around 60 mm/min
appeared, and further increasing the loading rate leads to a slight additional decrease in notch toughness. For the fine-grained
(FG) microstructure, the effect of loading rate was not apparent. The change in toughness resulted from a change of the critical
event controlling the cleavage fracture with increasing loading rate. For the CG microstructure with a lower cleavage-fracture
stress (σ
f
), with an increasing loading rate, the critical event of cleavage fracture can be changed from the propagation of a pearlite
colony-sized crack or a ferrite grain-sized crack, through the mixed critical events of crack propagation and crack nucleation,
then to crack nucleation. This change deteriorates the toughness. For the FG microstructure with a higher cleavage-fracture
stress, the critical event of cleavage fracture is the crack propagation and does not change in the loading-rate range from
120 to 500 mm/min. The measured σ
f
values do not change with loading rate, as long as the critical event of cleavage fracture does not change. The higher notch
toughness of the FG microstructure arises from its higher σ
f
and the critical plastic strain (ε
pc
) for initiating a crack nucleus, and the fracture behavior of this FG steel is not sensitive to loading rate in the range
of this work. 相似文献
5.
In this investigation, fracture loads, toughness values, and the lengths of fibrous cracks on fracture surfaces were measured
in Charpy V, crack tip opening displacement (COD), and precracked impact testing at various temperatures for C-Mn base steel
and C-Mn and Ti-B weld metals. The uniaxial tensile properties of these metals were measured as well. By plotting the parameters
related to toughness against the length of fibrous cracks measured, the energy absorbed by unit crack extension was estimated.
The local cleavage fracture stresses, oy, were measured in Charpy V-notched and COD precracked specimens. The results showed
σf about 600 MPa higher in the latter than in the former. Based on the results obtained, the factors controlling the toughness
were analyzed. This was explained by the brittle transition temperature of the base metal being higher than that of the weld
metal in the Charpy V test; however, it was lower in the COD test. The differences in fracture behavior between various types
of toughness specimens were analyzed. The prerequisite condition for establishing the correlation between the results of Charpy
V and COD tests was also discussed. 相似文献
6.
Tensile prestrains of various levels were applied to blank steel specimens. Four-point bend tests of notched specimens at
various temperatures revealed an appreciable drop in the notch toughness of the specimens, which experienced 3 pct tensile
prestrain. Further prestrains of up to 20 pct had a negligible effect on the notch toughness despite additional increases
in the yield strength. Microscopic analyses combined with finite element method (FEM) calculations revealed that the decrease
in toughness resulted from a change of the critical event controlling the cleavage fracture. The increase in yield strength
provided by prestraining allowed the normal tensile stress at the notch tip to exceed the local fracture stress σ
f
for propagating a just-nucleated microcrack. As a result, for the coarsegrained steel with low σ
f
tested presently, the critical event was changed from tensile stress-controlled propagation of a nucleated microcrack to
plastic strain-controlled nucleation of the microcrack at the notch tip. A reduction of toughness was induced as a result
of this. The increase in yield strength provided by decreasing the test temperature acted in the same way. 相似文献
7.
Study of mechanism of cleavage fracture at low temperature 总被引:8,自引:0,他引:8
In this investigation, a series of crack opening displacement (COD) tests were carried out at several low temperatures for
C-Mn weld steel. Some of the specimens were loaded until fracture, and the mechanical properties and microscopic parameters
on fracture surfaces were measured. Other specimens were unloaded before fracture at different applied loads. The distributions
of the elongated cavities and the cleavage microcracks ahead of fatigue crack tips were observed in detail. Based on the experimental
results, the combined criterion of a critical strainε
p ≥ εc) for initiating a crack nucleus, a critical stress triaxiality(σ
m/σ ≥ tc) for preventing it from blunting, and a critical normal stress(σ
yy/σf) for the cleavage extension was proposed again, and the critical values of εp and σm/−σ for the C-Mn weld steel were measured. The reason why the minimum COD value could not be zero is explained. The mechanism
of generation of the lower limit COD value on the lower shelf of the toughness transition curve is proposed. 相似文献
8.
The results of experimental studies of the influence of cementite particles on the fracture toughness of a number of spheroidized
carbon steels at low temperatures were analyzed in terms of current theories of crack-tip behavior. The fracture toughness
parameterK
IC was evaluated by using circumferentially notched and fatigue-cracked cylindrical specimens. The conclusions are summarized
as follows: 1) In general,K
IC decreases with increasing volume fraction and increasing size of the carbide particles. 2) Crack initiation occurs at the
carbide particles. 3) Crack propagation occurs by cleavage if the stress conditions satisfy the Ritchie, Knott and Rice criterion
that a critical cleavage stress is achieved over a minimum microstructural size scale. The critical stress is that required
to propagate a crack from a particle and the minimum size scale is of the order of 1 to 2 grain sizes. 4) Crack propagation
occurs initially by fibrous rupture if the stress intensification is insufficient to attain the critical cleavage stress.
P. Rawal was formerly affiliated. 相似文献
9.
10.
Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture
mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that
intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended
to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted
as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen
hardness. It is argued that the gap between the cleavage fracture stress (σ
F
CL
) and that of intergranular fracture (σ
F
IG
) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σ
F
CL
-σ
F
IG
) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between
temper embrittled CrMo and NiCr steels is discussed. 相似文献
11.
Jun Kameda 《Metallurgical and Materials Transactions A》1981,12(12):2039-2048
A study has been made of the influence of intergranular solute segregation on fracture toughness K1c in a series of Ni-Cr steels individually doped with Sb, Sn, and P. By means of toughness measurements in steels having two
different intergranular Sb distributions, of measurements of acoustic emissions and of scanning electron micrographs of a
load-interrupted and post-test-fatigued specimen, the values of K1c, computed from the “pop-in” load of the loadvs clip gauge displacement curves, are found to represent the formation of many patches of contiguous intergranular microcracks
ahead of the precrack. The present experiments demonstrate that in the early stage of solute segregation, K1c decreases more substantially than does the strength of grain boundaries σ* (measured in the notched bar tests), although
the embrittlement effects of metalloid elements are the same order for both K1c and σ*. A proposed model for the stress-gradient-control of brittle fracture supports the finding that the measurements of
K1c give a distorted view of the progress of intergranular embrittlement. 相似文献
12.
H. Couque R. J. Asaro J. Duffy S. H. Lee 《Metallurgical and Materials Transactions A》1988,19(9):2179-2206
An investigation was conducted into the effects of temperature, loading rate, and various micro-structural parameters on the
initiation of plane strain fracture of a plain carbon AISI 1020 steel. Ferrite and prior austenite grain sizes were chosen
as the principal microstructural features to be in-vestigated. The microstructural variations were accomplished by changing
the austenitizing tempera-ture and by altering the cooling rate during normalization. Fracture toughness tests were conducted
using precracked notched round bars loaded in tension to produce two stress intensity rates,viz.,K
1 = 1 MPa √m s-1 andK
1
= 2 × 106 MPa √m s-1. In addition, Charpy impact tests along with quasistatic and high rate plasticity tests were conducted. The plasticity tests
were done in torsion at shear strain rates of
. Testing temperatures covered the range from -150 °C to 150 °C which encompassed fracture initiation modes involving transgranular
cleavage to fully ductile fracture. Micromechanical processes involved in void and cleavage micro-crack formation were identified
and quantified. For these purposes notched round tensile tests and subsequent metallographic observations along with TEM and
SEM observations of the plane strain fracture toughness specimens were performed. The experimental results and quantitative
micro-modeling using simple fracture models provide a means of correlating both quasistatic and dynamic fracture toughness
with microstructures. 相似文献
13.
Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel 总被引:1,自引:0,他引:1
In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out
in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded
prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities
of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations
of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the
ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature
region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered
by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying
a combined criterion composed of three items, i.e., ε
p
≥ ε
pc
for initiating a crack nucleus; σ
m
√σ ≥ T
c
for preventing the crack nucleus from blunting; and σ
yy
≥ σ
f
for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for
initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension
of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size
of an “active zone” on the initiation of the brittle cleavage fracture. 相似文献
14.
L. P. Zhang C. L. Davis M. Strangwood 《Metallurgical and Materials Transactions A》1999,30(8):2089-2096
Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby
improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures
(T
p
) and three cooling times (Δ∼
8/5) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip
opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 μm) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present
in all three steels. Large prior austenite grain size caused by higher T
p
decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence.
Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture
initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermodynamics
have been used to rationalize the influence of Ti content on TiN formation and its size. 相似文献
15.
The effects of changes in test temperature (−196 °C to 25 °C) and grain size (40 to 165 μm) on the dynamic cleavage fracture toughness (K
ID
) and Charpy impact toughness of polycrystalline niobium (Nb) have been investigated. The ductile-to-brittle transition was
found to be affected by both changes in grain size and the severity of stress concentration (i.e., notch vs fatigue-precrack). In addition to conducting impact tests on notched and fatigue-precracked Charpy specimens, extensive fracture
surface analyses have been performed in order to determine the location of apparent cleavage nucleation sites and to rationalize
the effects of changes in microstructure and experimental variables on fracture toughness. Existing finite element analyses
and the stress field distributions ahead of stress concentrators are used to compare the experimental observations with the
predictions of various fracture models. The dynamic cleavage fracture toughness, K
ID
, was shown to be 37±4 MPa√m and relatively independent of grain size (i.e., 40 to 105 μm) and test temperature over the range −196 °C to 25 °C. 相似文献
16.
An elastic-plastic finite element method (FEM) was used to calculate the stress and strain distributions ahead of notches
with various root radii in a bending specimen of C-Mn steel with grain sizes of 10 and 30 μm. By accurately measuring the
distance of the cleavage initiation site from the root of the notch, the local cleavage stress σ
ƒ
*
was measured. When the notch radius increased from 0.25 to 1.0 mm, the distribution of high stress had a definite variation
but the σ
ƒ
*
remained relatively constant. In notch specimens with different root radii, the critical fracture event is identical,i.e., propagation of a ferrite grain-sized crack into the neighboring matrix. Therefore, the σ
ƒ
*
is mainly determined by the length of the critical microcrack, here, the size of ferrite grain instead of the high stress
volume for finding an eligible brittle particle. The critical strain for initiating a crack was about 1 pct. The cleavage
site ahead of a notch was related to the relative distributions of stress and strain and the random distribution of the weakest
grains. The higher fracture load of the fine-grain material can be attributed to its higher value of σ
ƒ
*
/σo as compared with the coarse-grain. The σ
ƒ
*
/σo is a potential engineering parameter for toughness assessment in notch specimens. 相似文献
17.
Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel 总被引:1,自引:0,他引:1
The microstructural parameter(s) controlling the critical cleavage fracture stress, σF, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,S
p, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured
on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased
with decreasingS
p, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and
notched-bar geometry independent values for σF, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values
for σF over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak
normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than
the predicted peak stress location. The calculated values for σF were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence
does not necessarily change σF in fully pearlitic microstructures.
formerly Graduate Student, Carnegie Mellon University 相似文献
18.
19.
Effect of strontium on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy 总被引:1,自引:0,他引:1
Xiaoqin Zeng Yingxin Wang Ph.D. Wenjiang Ding Alan A. Luo Anil K. Sachdev 《Metallurgical and Materials Transactions A》2006,37(4):1333-1341
The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and
its sensitivity to cooling rate are investigated. Three phases—blocky-shaped Mg17Al12, acicular Mg20Al20Mn5Sr, and insular Mg16(Al,Zn)2Sr—are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped Mg17Al12 phase increases, the acicular Mg20Al20Mn5Sr phase diminishes, and the insular Mg16(Al,Zn)2Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given
composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a
given cooling rate. The yield strength (σ
y
) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ
y
=35.88+279.13d
−1/2 according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with
increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy
to mixed features of quasicleavage and microvoid coalescence fracture. 相似文献
20.
Fracture toughness of the lean duplex stainless steel LDX 2101 总被引:1,自引:0,他引:1
Henrik Sieurin Rolf Sandström Elin M. Westin 《Metallurgical and Materials Transactions A》2006,37(10):2975-2981
Fracture toughness testing was performed on the recently developed lean duplex stainless steel LDX 2101 (EN 1.4162, UNS S32101).
The results were evaluated by master curve analysis, including deriving a reference temperature. The master curve approach,
originally developed for ferritic steels, has been used successfully. The reference temperature corresponds to a fracture
toughness of 100 MPa√m, which characterizes the onset of cleavage cracking at elastic or elastic-plastic instabilities. The
reference temperature, T
0, was −112 °C and −92 °C for the base and weld materials, respectively. In addition, the fracture toughness is compared with
impact toughness results. Complementary crack tip opening displacements (CTODs) have also been calculated. The toughness properties
found in traditional duplex stainless steels (DSS) are generally good. The current study verifies a high fracture toughness
for both base and weld materials and for the low alloyed grade LDX 2101. Even though the fracture toughness was somewhat lower
than for duplex stainless steel 2205, it is still sufficiently high for most low-temperature applications. 相似文献