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1.
The tensile curves as well as original and selective dissoluted fracture surfaces have been studied. The function of the γ' phase on the mechanical properties of the steel has been investigated. The fracturing process consists of: formation of dimples – formation of splits in dimples – the splits in dimples join each other to form secondary cracks – formation of inner lips and tiny dimple regions. The morphology of fracture looks like ridges and peaks. It is considered that the column boundaries in ingots may split easily; this causes bedding fracture and the degradation of Z–direction properties. The formation of bedding fracture is affected markedly by heat treatment and tensile temperature. Some ways of improving mechanical properties have been discussed. 相似文献
2.
The effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel were investigated
by observing crack initiation, propagation, and fracture behaviors. Crack initiation occurred by decohesion between martensite
and ferrite. However, cracks propagated along the ferrite-martensite interface in a high temperature quenched specimen, whereas
in specimens quenched from lower temperature cracks propagated into the martensite particle. Tensile fracture behaviors were
not strongly influenced by the cooling rate. At both cooling rates of 5.6 and 0.1 °C/sec, specimens quenched from high temperature
fractured by partially brittle fracture mode, but fracture mode changed to ductile mode as the quenching temperature decreased.
The effect of transformed ferrite on the fracture mode was not substantially different from that of retained ferrite. However,
the crack initiation and propagation was influenced by the variation in martensite distribution caused by different growth
behavior of transformed ferrite. 相似文献
3.
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
4.
《Acta Metallurgica》1988,36(2):311-320
Tensile stress-strain data over the whole strain range were obtained for a range of pearlites from very coarse to relatively fine (interlamellar spacings 0.53 and 0.13 μm, respectively). Transmission electron microscopy (TEM) for pearlite subjected to various amounts of strain was performed. Coupling mechanical data with TEM examination provided a detailed picture of how pearlite yields, deforms, work hardens, and fails under uniaxial tension. It is shown that yielding and work hardening of pearlite are largely controlled by processes occurring in ferrite. The role of a cementite plate at low stresses is mainly to limit the slip distance in ferrite. It is found that the tensile fracture is determined by processes in the colonies with lamellae parallel to the tensile axis and that the stress necessary to break a cementite plate corresponds to the true U.T.S. The influence of interlamellar spacing on the yield strength, flow stress, and the true U.T.S. is quantitatively explained. 相似文献
5.
《Acta Metallurgica》1980,28(5):639-649
Several particle containing aluminum alloys have been heavily cold worked by wire drawing and torsion, and their stress-strain behaviour monitored. The shape of the resulting stress-strain curve is shown to be independent of the mode of deformation and consists of an initial region of high work hardening rate which gradually decreases to a reasonably constant work hardening region at large strains. This terminal work hardening rate increases with increasing solute, which inhibits dynamic recovery mechanisms.Two of the alloys containing higher volume fractions of particles exhibit work softening in addition to work hardening. The results are discussed in terms of some of the current work hardening models. 相似文献
6.
Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures 总被引:1,自引:0,他引:1
Wei Tong Hong Tao Nian Zhang Xiquan Jiang Manuel P. Marya Louis G. HectorJr. Xiaohong Q. Gayden 《Metallurgical and Materials Transactions A》2005,36(10):2651-2669
Plastic deformation of miniature tensile bars generated from dual-phase steel weld microstructures (i.e., fusion zone, heat-affected zone, and base material) was investigated up to final rupture failure. Uniaxial tensile true
stress-strain curves beyond diffuse necking were obtained with a novel strain-mapping technique based on digital image correlation
(DIC). Key microstructural features (including defects) in each of these three metallurgical zones were examined to explore
the material influence on the plastic deformation and failure behavior. For weld fusion zones with minimal defects, diffuse
necking was found to begin at 6 pct strain and continue up to 55 to 80 pct strain. The flow stresses of the weld fusion zones
were at least twice those of the base material, and fracture strains exceeded 100 pct for both materials. The heat-affected
zones exhibited a range of complex deformation behaviors, as expected from their microstructural variety. Only those fusion
zones with substantial defects (e.g., shrinkage voids, cracks, and contaminants) failed prematurely by edge cracking, as signaled by their highly irregular strain
maps. 相似文献
7.
8.
The processes of ductile and brittle fracture in fully pearlitic steel and their relation to both the scale of the microstructure
and the presence of substitutional alloy elements have been investigated at room temperature using smooth tensile and over
a range of temperatures using V-notched Charpy impact specimens. The results show that the early stages of cracking, revealed
in both types of specimen, are largely the result of shear cracking of the pearlite lamellae. These cracks grow and can reach
a size when they impinge upon the prior austenite boundary; afterward the character of fracture can be either microvoid coalescence
or cleavage, depending on test conditions and metallurgical variables. Further, the carbide plates of the pearlite lamellae
can act as barriers to the movement of dislocations as is the case normally with grain boundaries. For pearlite an optimum
spacing of approximately 0.2 μm resulting from a balance between carbide plate thickness and interlamellar spacing was found
to enhance toughness, although such changes are much smaller than corresponding changes due to varying alloy elements. Specific
alloy elements used herein strengthened the lamellar ferrite in pearlite, inhibiting the movement of dislocations while also
usually decreasing the lamellar cementite plate thickness for the same spacing. This dual behavior results in enhanced resistance
to the initiation and propagation of microcracks leading to an improvement in strength, ductility, and toughness. The most
effective alloy elements for the composition ranges studied in fully pearlitic steels are Si and Ni for strength improvement,
and Ni and Mn for toughness. 相似文献
9.
M. Dollar I. M. Bernstein M. Daeubler A. W. Thompson 《Metallurgical and Materials Transactions A》1989,20(3):447-451
The fatigue behavior and associated crack inititation of pearlite has been correlated with developing dislocation configurations
in a cyclically deformed fully pearlitic steel. Under fatigue conditions at low stress amplitudes, dislocations are found
to be generated and largely confined to the cementite/ferrite interfaces, most likely due to the development of elastic incompatibility
stresses between the cementite and ferrite. This deformation mode encourages fatigue crack initiation parallel to the cementite
lamellae. The fatigue limit of pearlite appears not to be influenced by the interlamellar spacing, a result basically different
from that found in monotonic deformation where yielding and flow are strongly affected by the spacing. The apparent differences
are discussed in terms of the different dislocation configurations formed during monotonic and cyclic deformation. 相似文献
10.
《Baosteel Technical Research》2010,4(3):53-57
The tensile behavior and properties of cold formed low-carbon microalloyed steel with its microstructure of all ferrite and pearlite (F+P) were investigated.Bending and flattening deformations were carried out in the laboratory on hot-rolled sheets in order to simulate the cold forming process of steel sheets during pipe fabrication and sampling of high frequency straight bead welding pipes.A comparison of the tensile behavior and properties of the material made before and after cold forming indicates that cold deformation alters the tensile behavior and properties of the material to a certain degree depending on the manner of the cold deformation and the degree.The research on the Bauschinger effect indicates that for the steels investigated,when the plastic strain is small,the back stress increases rapidly with the increase of the plastic strain and then rapidly tends to saturation.The finite element analysis indicates that the change in the properties of the steel sheets due to cold forming is a result of the Bauschinger effect and work hardening.The mechanism of the change in the properties is also given in this study. 相似文献
11.
The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel 总被引:3,自引:0,他引:3
The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness
of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results
demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent
ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified
as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact
specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics
analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using
thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the
cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage
facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained
by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic
structures.
Y.J. PARK, formerly with Carnegie-Mellon University, Pittsburgh, PA. 相似文献
12.
13.
通过对国内某厂生产的700 MPa级别汽车大梁钢热轧板进行力学性能检测,发现拉伸断口产生分层现象,利用金相显微镜、扫描电镜、透射电镜与能谱分析等手段研究其断后分层现象并分析其产生的原因。试验钢的平均屈服强度与抗拉强度分别为766.9 、839.0 MPa,平均断后伸长率为19.2%,皆达到性能指标的要求。研究结果表明,铸坯与轧材中的夹杂物与析出物分布情况较好,不是产生分层现象的主要原因,热轧板厚度方向中部的贝氏体与马奥岛成带状偏聚,造成厚度方向上明显的塑性差异,直接导致了分层现象产生。试验钢产生拉伸分层的原因被解析出,为实际生产提供了改进方向。 相似文献
14.
The deformation behaviors of retained ferrite and transformed ferrite in a dual-phase steel were studied by observing slip
lines, strain development, and inhomogeneity of strain in each phase. The retained ferrite was more deformable and had a lower
degree of the strain inhomogeneity than the transformed ferrite. The degrees of strain inhomogeneity of both ferrites became
larger with increasing tensile strain. Variations in the distribution, amount, and morphology of martensite caused the difference
in the deformation behavior between the two types of ferrite by changing the degree of constraint for ferrite deformation. 相似文献
15.
采用恒应力强度因子K=33 MPa·m1/2的加载方法,利用直流电压降方法在线监测核辅管道316L不锈钢在高纯水中应力腐蚀裂纹扩展速率.对比200、250、280和325℃温度下,氩气除氧和含有2 mg·L-1溶解氧的水化学环境中材料的裂纹扩展速率发现:溶解氧为2 mg·L-1时的裂纹扩展速率明显比氩气除氧时的裂纹扩展速率高.氩气除氧时,裂纹扩展速率在250℃时有一个最高点;溶解氧为2 mg·L-1的条件下,裂纹扩展速率随温度的升高而升高. 相似文献
16.
《Acta Metallurgica》1985,33(5):913-922
The strengths of heavily deformed two phase materials are much in excess of those expected on the bases of the strain hardening behavior of the phases comprising such a composite. In this paper a model is developed to explain the strengths of this class of materials and the predictions of it are compared to results obtained in several systems. The basis for the model is that additional (geometrically necessary) dislocations are generated during deformation of two, as compared to single, phase materials as a result of the inherently greater strain incompatibility between adjacent grains in a two phase material. Thus the model predicts that, other factors being the same, the greater the disparity between the flow curves of the composite constituents, the greater the excess strength generated. The strain hardening behaviour of the individual phases influences also the strengths obtained in a composite. Composites comprised of materials for which dynamic recovery processes are particularly effective do not display large incremental strengths as these processes eliminate both geometrically necessary and statistical dislocations. Conversely, composites containing materials which do not dynamically recover (e.g. iron which work hardens linearly) display rather impressive excess strengths as a result of the complementary interaction between statistical and geometrical dislocations. The agreement between the model developed and experimental results is good. The two adjustable parameters of the model (one concerning the partitioning of the geometrical dislocations between the phases and the other a measure of the inherent strain incompatibility between them) have physically plausible numerical values. 相似文献
17.
18.
19.
Fatigue crack growth rates (FCGR) in AISI 301 and 302 austenitic stainless steel alloys have been measured in controlled load
cycles withR = 0.05. Both annealed and cold rolled conditions were examined. The austenite phase of the AISI 301 alloy was unstable under
stress and transformed martensitically to α′ to a much greater extent than the AISI 302 alloy. At low values of mean stress
the unstable alloy had a lower FCGR than the more stable 302 alloy. The FCGR increased with mean stress until values of mean
stress ⪞70 MPa, where the FCGR was independent of mean stress and was the same for both alloys. Various metallographic and
macroscopic measurements were made to try to understand this behavior. It was concluded that residual compressive stress due
to transformation at the crack tip was responsible for the lower crack growth rates of the unstable 301 alloy. Cold worked
specimens had significantly lower crack growth rates than the annealed specimens, and both alloys behaved identically.
Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign 相似文献
20.
F. Wetscher R. Pippan S. Sturm F. Kauffmann C. Scheu G. Dehm 《Metallurgical and Materials Transactions A》2006,37(6):1963-1968
A fully pearlitic steel was deformed by high-pressure torsion up to very high strains, and the changes in the microstructure
were determined by analytic and conventional transmission electron microscopy. The imposed strain leads to a fragmentation
and an alignment of the cementite lamellae parallel to the shear plane. The electron energy-loss near-edge-fine structures
of the Fe-L2,3-edge of the iron matrix and the cementite lamellae were measured with high spatial resolution. The results indicated that
after high-pressure torsion, the iron matrix contains finely dispersed carbon-rich areas that do not show the electronic fingerprint
of cementite. However, the refinement in microstructure leads to an enormous increase in mechanical strength. 相似文献