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1.
Dependence of fracture toughness of austempered ductile iron on austempering temperature 总被引:1,自引:0,他引:1
Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at
260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture
toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise
in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural
features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that
fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite
containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture
toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained
austenite content. A plot of K
IC
2
against σ
y
(X
γ, C
γ)1/2 resulted in a straight line, as predicted by the model. 相似文献
2.
In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical
stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature
was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy
(TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering
and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased
with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature
stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens,
austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C,
no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were
observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the
crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening
mechanisms were also discussed.
Formerly Graduate Student, Seoul National University. 相似文献
3.
Toshio Takahashi Toshihiko Abe Shuji Tada 《Metallurgical and Materials Transactions A》1996,27(6):1589-1598
Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal
graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI.
Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also
less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel
has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties
and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering
temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although
it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards
the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched
austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness.
This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms
of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in
Rosemont, Illinois. 相似文献
4.
Application of Rietveld Refinement to Investigate the High Chromium White Cast Iron Austempered at Different Temperatures 总被引:1,自引:0,他引:1
The effect of austempering temperature on the microstructure and properties of a high chromium white cast iron was investigated with the Rietveld refinement method. The result shows that the upper bainite exists in the sample austempered at 623 K and the martensite, lower bainite, M7C3, and retained austenite exist in the samples austempered at 563 K and 593 K. The relative content of the retained austenite increases with increasing the austempering temperature from 563 K to 623 K. The higher hardness, impact toughness and impact abrasive wear resistance can be obtained for the specimen austempered at 593 K. 相似文献
5.
Two medium carbon low-alloy MnSiCrB cast steels containing different Cu contents (0.01 wt pct and 0.62 wt pct) were designed,
and the effect of Cu on the mechanical properties and corrosion–abrasion wear behavior of the cast steels was studied. The
results showed that the low-alloy MnSiCrB cast steels obtained excellent hardenability by a cheap alloying scheme. The microstructure
of the MnSiCrB cast steels after water quenching from 1123 K (850 °C) consists of lath martensite and retained austenite.
After tempering at 503 K (230 °C), carbides precipitated, and the hardness of the cast steels reached 51 to 52 HRC. The addition
of Cu was detrimental to the ductility and impact toughness but was beneficial to the wear resistance in a corrosion–abrasion
wear test. The MnSiCrB cast steel with Cu by the simple alloying scheme and heat treatment has the advantages of being high
performance, low cost, and environmentally friendly. It is a potential, advanced wear-resistant cast steel for corrosion–abrasion
wear conditions. 相似文献
6.
Chongmin Kim A. Richard Johnson William F. Hosford 《Metallurgical and Materials Transactions A》1982,13(9):1595-1605
The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4
pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated.
In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is
softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature
is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing
the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit
2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter
of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the
final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened
MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There
is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples
is a maximum. The above described results were explained through changes in the microstructure and the fracture modes.
CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI. 相似文献
7.
8.
Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet 总被引:1,自引:0,他引:1
The effect of phosphorus and silicon on the formation of retained austenite has been investigated in a low-carbon steel cold
rolled, intercritically annealed, and isothermally held in a temperature range of bainitic transformation followed by air
cooling. The steel sheet containing phosphorus after final heat-treatment consisted of ferrite, retained austenite, and bainite
or martensite. Phosphorus, especially in the presence of silicon, in steel was useful to assist the formation of retained
austenite. Mechanical properties, such as tensile strength, uniform elongation, and the combination of tensile strength/ductility,
were improved when phosphorus was increased up to 0.07 pct in 0.5 pct Si steel. This could be attributed to the strain-induced
transformation of retained austenite during tensile deformation. Furthermore, two types of retained austenite were observed
in P-containing steel. One is larger than about 1 μm in size and usually exists adjacent to bainite; the other one is of submicron
size and usually exists in a ferrite matrix. High phosphorus content promotes the formation of stable (small size) austenites
which are considered to be stabilized mainly by their small size effect and have a different formation mechanism from the
coarser retained austenite in the lower P steels. The retained austenites of submicron size showed mechanical stability even
after 10 pct deformation, suggesting that these small austenites have little effect on ductility. The 0.07 pct P-0.5 pct Si-1.5
pct Mn-0.12 pct C steel showed a high strength of 730 MPa and a total elongation of 36 pct. 相似文献
9.
Effects of Microalloying on the Impact Toughness of Ultrahigh-Strength TRIP-Aided Martensitic Steels
Junya Kobayashi Daiki Ina Yuji Nakajima Koh-ichi Sugimoto 《Metallurgical and Materials Transactions A》2013,44(11):5006-5017
The effects of the addition of Cr, Mo, and/or Ni on the Charpy impact toughness of a 0.2 pct C-1.5 pct Si-1.5 pct Mn-0.05 pct Nb transformation-induced plasticity (TRIP)-aided steel with a lath-martensite structure matrix (i.e., a TRIP-aided martensitic steel or TM steel) were investigated with the aim of using the steel in automotive applications. In addition, the relationship between the toughness of the various alloyed steels and their metallurgical characteristics was determined. When Cr, Cr-Mo, or Cr-Mo-Ni was added to the base steel, the TM steel exhibited a high upper-shelf Charpy impact absorbed value that ranged from 100 to 120 J/cm2 and a low ductile–brittle fracture appearance transition temperature that ranged from 123 K to 143 K (?150 °C to ?130 °C), while also exhibiting a tensile strength of about 1.5 GPa. This impact toughness of the alloyed steels was far superior to that of conventional martensitic steel and was caused by the presence of (i) a softened wide lath-martensite matrix, which contained only a small amount of carbide and hence had a lower carbon concentration, (ii) a large amount of finely dispersed martensite-retained austenite complex phase, and (iii) a metastable retained austenite phase of 2 to 4 vol pct in the complex phase, which led to plastic relaxation via strain-induced transformation and played an important role in the suppression of the initiation and propagation of voids and/or cleavage cracks. 相似文献
10.
Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower
bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines
such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties
were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength
occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel
was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed
to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation
mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical
properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation
upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss
of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel.
B. V. N. RAO, formerly Senior Research Engineer, Analytical Chemistry Department, General Motors Research Laboratories 相似文献
11.
Sung-Joon Kim Chang Gjl Lee Ildong Choi Sunghak Lee 《Metallurgical and Materials Transactions A》2001,32(3):505-514
The main emphasis of this study has been placed on understanding the effects of manganese and silicon additions and of heat-treatment
(intercritical annealing and isothermal treatment) conditions on the microstructures and mechanical properties of 0.15 wt
pct C transformation-induced plasticity (TRIP)-aided cold-rolled steel sheets. The steel sheets were intercritically annealed
and isothermally treated at the bainitic region. Microstructural observation and tensile tests were conducted, and volume
fractions of retained austenite were measured. Steels having a high manganese content had higher retained austenite fractions
than the steels having a low manganese content, but showed characteristics of a dual-phase steel such as continuous yielding
behavior, high tensile strength over 1000 MPa, and a low elongation of about 20 pct. The retained austenite fractions and
mechanical properties varied with the heat-treatment conditions. In particular, the retained austenite fractions increased
with decreasing intercritical annealing and isothermal treatment temperatures, thereby resulting in the improvement of the
elongation and strength-ductility balance without a serious decrease in the yield or tensile strength. These findings suggested
that the intercritical annealing and isothermal treatment conditions should be established in consideration of the stability
of austenite and the solubility of alloying elements in the austenite formed during the intercritical annealing. 相似文献
12.
Dieter Isheim Allen H. Hunter Xian J. Zhang David N. Seidman 《Metallurgical and Materials Transactions A》2013,44(7):3046-3059
Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M2C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations. 相似文献
13.
14.
Bingxu Wang Gary C. Barber Chuanlin Tao Xue Han Xichen Sun 《Metallurgical and Materials Transactions B》2018,49(5):2261-2269
Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications. 相似文献
15.
Hao Qu Gary M. Michal Arthur H. Heuer 《Metallurgical and Materials Transactions A》2014,45(6):2741-2749
In excess of 30 vol. pct austenite can be retained in 0.3C-4.0Mn steels subjected to a dual stabilization heat treatment (DSHT) schedule—a five stage precisely controlled cooling schedule that is a variant of the quench and partition process. The temperature of the second quench (stage III) in the DSHT process plays an essential role in the retained austenite contents produced at carbon-partitioning temperatures of 723 K or 748 K (450° C or 475 °C) (stage IV). A thermodynamic model successfully predicted the retained austenite contents in heat-treated steels, particularly for a completely austenitized material. The microstructure and mechanical behavior of two heat-treated steels with similar levels of retained austenite (~30 vol. pct) were studied. Optimum properties—tensile strengths up to 1650 MPa and ~20 pct total elongation—were observed in a steel containing 0.3C-4.0Mn-2.1Si, 1.5 Al, and 0.5 Cr. 相似文献
16.
M. Nili Ahmadabadi 《Metallurgical and Materials Transactions A》1998,29(9):2297-2306
A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the
bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional
austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature)
and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following
austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water.
The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function
of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering
temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature
encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition
of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component
of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis. 相似文献
17.
An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered
ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently
austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through
optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated
with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results
in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon
content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture
toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve
fracture toughness. 相似文献
18.
R. Taillard P. Verrier T. Maurickx J. Foct 《Metallurgical and Materials Transactions A》1995,26(2):447-457
The aim of this article is to present the beneficial effect of a reduction of silicon content on coarse-grained heat-affected
zone (CGHAZ) toughness. This study was achieved with experi-mental and industrial E355 structural steels. These 0.09 wt pct
C steels were Ti-microalloyed with silicon contents ranging from 0.05 to 0.5 wt pct. First, we demonstrate that the CGHAZ
toughness is predominantly affected by the volume fraction of retained austenite (γr). Second, we show that the existence
of retained austenite pertains only to its carbon enrichment. This enrichment is promoted essentially by an increase of the
silicon level due to the retarding action of silicon on the formation of carbides in ferrite as well as in austenite. In the
same way, the increase of silicon content slows down the decomposition of retained austenite into pearlite. The reduction
of the silicon content of the steel greatly increases the ductility of the CGHAZ through the decrease of the volume fraction
of retained austenite.
Formerly Graduate Students, Physical Metallurgy Laboratory, University of Lille. 相似文献
19.
P. W. Hochanadel G. R. Edwards C. V. Robino M. J. Cieslak 《Metallurgical and Materials Transactions A》1994,25(4):789-798
The microstructure of investment cast PH 13-8 Mo stainless steel heat-treated to various conditions was studied using light and electron microscopy, electron probe microanalysis, and Mössbauer spectroscopy. The mechanical properties were investigated by using uniaxial tensile testing, hardness testing, and Charpy impact testing. TheΒ-NiAl strengthening precipitates, though detectable by electron diffraction, were difficult to resolve by transmission electron microscopy (TEM) in specimens aged at low temperatures (566 °C and below). A high dislocation density was observed in the lath martensitic structure. The higher strength and lower ductility observed at low aging temperatures was attributed to both the high dislocation density and the precipitation ofΒ-NiAl. When samples were aged at high temperatures (> 566 °C), a lower dislocation density and a reverted austenite fraction on the order of 15 pct were observed. SphericalΒ-NiAl precipitates were observed in the overaged condition. The decrease in strength and corresponding increase in ductility observed in samples aged at temperatures above 566 °C were attributed to the reverted austenite and recovery. Mechanical properties were improved when the homogenizing temperature and time were increased. Electron probe microanalysis quantified the increased homogeneity realized by increasing homogenizing temperature and time. Elimination of the refrigeration step, which normally follows the solution treatment, did not degrade the mechanical properties. Mössbauer spectroscopy showed only minor decreases in the fraction of retained austenite when refrigeration followed the solution treatment. 相似文献
20.
Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel 总被引:7,自引:0,他引:7
Koh-Ichi Sugimoto Mitsuyuki Kobayashi Shun-Ichi Hashimoto 《Metallurgical and Materials Transactions A》1992,23(11):3085-3091
The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained
austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15
vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS),
about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening
rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100
°C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite
transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced
TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior
and retained aus- tenite stability. 相似文献