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1.
This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which
were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures
that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K
Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of
plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation
fields ahead of cracks, suggesting thatK
Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The
trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation
of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating
the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels
studied can be explained quantitatively using different fracture models. 相似文献
2.
James P. Materkowski George Krauss 《Metallurgical and Materials Transactions A》1979,10(11):1643-1651
The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch
(CVN) impact and compact tension plane strain fracture toughness (K
1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered
martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures
between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was
always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage
across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture,
apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated
with TME in the high P steel.K
IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of
a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture
zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C). 相似文献
3.
The effect of heat treatment on microstructure and cryogenic fracture properties in 5Ni and 9Ni steel 总被引:2,自引:0,他引:2
Heat treatments were utilized in 5Ni and 9Ni steel which resulted in the development of tempered microstructures which contained
either no measurable retained austenite (<0.5 pct) or approximately 4 to 5 pct retained austenite as determined by X-ray diffraction.
Microstructural observations coupled with the results of tensile testing indicated that the formation of retained austenite
correlated with a decrease in carbon content of the matrix. Relative values ofK
IC
at 77 K were estimated from slow bend precracked Charpy data using both the COD and equivalent energy measurements. In addition,
Charpy impact properties at 77 K were determined. In the 9Ni alloy, optimum fracture toughness was achieved in specimens which
contained retained austenite. This was attributed to changes in yield and work hardening behavior which accompanied the microstructural
changes. In the 5Ni alloy, fracture toughness equivalent to that observed in the 9Ni alloy was developed in grain refined
and tempered microstructures containing <0.5 pct retained austenite. A decrease in fracture toughness was observed in grain
refined 5Ni specimens containing 3.8 pct retained austenite due to the premature onset of unstable cracking. This was attributed
to the transformation of retained austenite to brittle martensite during deformation. It was concluded that the formation
of thermally stable retained austenite is beneficial to the fracture toughness of Ni steels at 77 K as a result of austenite
gettering carbon from the matrix during tempering. However, it was also concluded that the mechanical stability of the retained
austenite is critical in achieving a favorable enhancement of cryogenic fracture toughness properties.
Formerly with Union Carbide Corporation, Tarrytown, NY 相似文献
4.
The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔKTH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness KIC, showing that for high m values KIC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK. 相似文献
5.
A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI
4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested
in previous work, a systematic trend ofincreasing plane strain fracture toughness(K)
Ic
anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected.
This effect is seen under both static <slowbend> and dynamic (impact) loading conditions, and is rationalized in terms of
a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius
on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence)
mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture,
based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease
in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures,
consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness,
on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution
of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are
discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement
of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance
of this work on commonly-used Charpy/KIc empirical correlations is briefly discussed.
formerly with Lawrence Berkeley Laboratory, Berkeley, CA 相似文献
6.
Yoshiyuki Tomita 《Metallurgical and Materials Transactions A》1987,18(8):1495-1501
A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (K
IC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified
heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which
appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593
K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead
bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved
using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly
oil-quenched 4340 steel increased theK
IC values by 15 to 20 MPa • m1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,K
IC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichK
IC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340
steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorK
IC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inK
IC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the
lower bainite just ahead of the current crack. 相似文献
7.
Yoshiyuki Tomita 《Metallurgical and Materials Transactions A》1988,19(10):2513-2521
Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain
fracture toughness (K
IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different
mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron
microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows:
in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural
factor controllingK
IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet
is composed of well-defined blocks, and the block size controls theK
IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase
bainite have a significant effect on theK
IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite,
dramatically improves theK
IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the
parent martensite is associated with tempered martensite, it significantly lowers the KIC. 相似文献
8.
An NM400 wear-resistant steel was hot rolled and then the plates were heat-treated by direct quenching and tempering (DQT) and reheat quenching and tempering (RQT) techniques, respectively. The Charpy impact test was carried out with an instrumented Charpy impact tester. The microstructure and fracture surface were investigated by a combination of optical microscopy, transmission electron microscopy and scanning electron microscopy methods. It was found that the impact toughness of DQT specimen was much higher than that of RQT specimen. The microstructure of both DQT and RQT specimens was characterized by a mixture of tempered lath martensite and lower bainite. The lower bainite in DQT specimen extended into prior austenite grains and the content was higher than that in RQT specimen. The lower bainite in DQT specimen improved the impact toughness by increasing the proportion of large-angle boundaries and relieving the stress concentration at the crack tip. A number of fine and dispersed carbides in DQT specimen also contributed to the improvement of the impact toughness. 相似文献
9.
S. Sankaran Gouthama S. Sangal K. A. Padmanabhan 《Metallurgical and Materials Transactions A》2006,37(11):3259-3273
A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium-carbon microalloyed (MA) steel through two routes,
namely, low-temperature finish forging and rolling, followed by a two-step cooling (TSC) and annealing. Transmission electron
microscopy (TEM) was employed to study the microstructural evolution in control forged and rolled material after TSC followed
by annealing (TSCA). A TEM investigation was also carried out on samples low-cycle fatigue (LCF) tested at low and high total
strain amplitudes of 0.4 and 0.7 pct in case of the forged steel (F-B-M(F)TSCA) and 0.55 and 0.8 pct for the rolled steel
(F-B-M(R)TSCA), respectively. Microstructural changes accompanying the LCF testing were identified. The two-step cooled microstructure
processed through forging (F-B-M(F)TSC) as well as rolling (F-B-M(R)TSC) revealed a complex multiphase microstructure, along
with films and blocks of retained austenite. In both microstructural conditions, vanadium carbide precipitates were too fine
to be identified after the TSC treatment. Annealing after TSC produced a stress-free microstructure. The F-B-M(F)TSCA microstructure
predominantly consisted of granular/lower bainite, lath martensite, and polygonal ferrite with interlath films as well as
blocks of retained austenite, while the F-B-M(R)TSCA microstructure predominantly consisted of lath martensite, granular/lower
bainite, and polygonal ferrite with interlath strips/films of retained austenite. Lath martensite content was higher in the
F-B-M(R)TSCA condition than in the F-B-M(R)TSCA condition. In both conditions, vanadium carbide precipitates could be seen
after annealing. Fatigue-tested F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.4 pct and F-B-M(F)TSCA microstructure
up to a total strain amplitude of 0.55 pct were stable. Lath martensite did not undergo deformation and in both microstructural
conditions dislocation cell structures were not observed in the ferrite or bainite regions. The interlath retained austenite
strips/films played a significant role in preventing the softening during fatigue loading. First, it was stable up to a total
strain amplitude of 0.4 and 0.55 pct in the respective microstructures. Second, it underwent heavy deformation during fatigue
loading at high total strain amplitudes, thereby accommodating the strain. Fatigue-tested F-B-M(F)TSCA microstructure at a
total strain amplitude of 0.7 pct and F-B-M(R)TSCA microstructure at a total strain amplitude of 0.8 pct revealed deformed
bainite/martensite laths, dislocation cells, and slip bands in the ferrite regions, which are characteristic features of cyclic
softening. The retained austenite transformed to martensite through a strain-induced transformation mechanism and, at that
stage, the microstructure contained in addition dislocation-rich bainite and ferrite. 相似文献
10.
Shen-Chih Lee Cheng-Hsun Hsu Hui-Ping Feng Chao-Chia Chang 《Metallurgical and Materials Transactions A》1998,29(10):2511-2521
Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in
the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment
processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the
as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering
heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables
on the plane strain fracture toughness (K
IC
) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM),
and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation. 相似文献
11.
Hyo Kyung Sung Seok Su Sohn Sang Yong Shin Kyung Shik Oh Sunghak Lee 《Metallurgical and Materials Transactions A》2014,45(7):3036-3050
This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O2 to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O2, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (?20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics. 相似文献
12.
Massive electrodeposits of nickel and Ni?Co alloys ranging up to 43 pct Co were examined microstructurally and tested to determine tensile properties and static and dynamic fracture toughness. Specimens were also tested after being annealed at 575 K. Annealing increased grain size, decreased yield, and ultimate strengths, and increased ductility and dynamic toughness. The as-plated Ni-43 Co was the only material to exhibit validK IC values, averaging about 38 MN/m3/2. In instrumented dynamic tests on precracked Charpy bars, the same material exhibited aK Id of 50 MN/m3/2. The yield strength of the Ni-43 Co alloy was 1154 MN/m2. All the materials tested showed dimpled, ductile rupture fracture surfaces. The Hall-Petch behavior of the nickel indicated that it is much easier to initiate flow in normal grain boundary structures than in structures composed of dislocation cell walls. 相似文献
13.
J. M. Beswick 《Metallurgical and Materials Transactions A》1989,20(10):1961-1973
Good toughness in hardened 52100 ball bearing steel is important in order to prevent premature fracture during mounting or
service of bearing elements. Steel cleanliness, residual copper content, and carbon content effects have been investigated
in relation to fracture mechanics properties, and it was observed that only the carbon content has any relevance for the range
of compositions investigated. The effect of hardening and tempering temperatures for conventional furnace-hardening techniques
on toughness was investigated, theK
lcbeing generally much less sensitive to these parameters than blunt notch toughness testing. Cold deformation of the material
prior to martensitic hardening significantly increased the blunt notch toughness. Thermal grain refining treatments did not
give the same improved blunt notch toughness as observed for prior cold deformation. Short austenitization cycles (ten seconds)
for martensitic hardening resulted in microstructures with high retained austenite contents. This microstructure resulted
in higher fracture toughness and retardation of the crack growth rates, the mechanism being associated with transformation
toughening in the plastic zone. Inductive tempering of martensitic-hardened 52100 was observed to result in similar blunt
notch toughnesses as compared to furnace tempered material of the same hardness. A poor correlation between fracture toughness
and blunt notch toughness was observed, particularly for the unstable structures,i.e., microstructures with high levels of retained austenite. Fracture toughness does not represent the intrinsic toughness of
high carbon martensite with related high contents of retained austenite. 相似文献
14.
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
15.
16.
The mechanical properties and microstructure of two heats of AF1410 steel were compared. The first heat, heat 811, contained
a titanium addition of 0.02 wt pct, while the second heat, heat 91, contained no titanium, manganese, or other strong sulfide
formers. The sulfur in heat 811 was gettered as titanium carbosulfide, while in heat 91 the sulfides were chromium sulfide.
The toughness of heat 811 was found to be much enhanced compared to heat 91, with Charpy impact energies of 176 J and 79 J
and KIC fracture toughness values of 235 MPa.m1/2 and 170 MPa.m1/2, respectively. This significant difference in fracture toughness is attributed to the fact that titanium carbosulfide particles
are more resistant to void nucleation than the chromium sulfide particles, which appear to nucleate voids at the onset of
plastic strain. In addition to altering the sulfide particle type, the titanium addition also results in the presence of undissolved
MC carbides in the titanium-modified steel in addition to the M2C carbides found in heat 91. These carbides act as grain growth inhibitors, resulting in a finer prior austenite grain size
and martensite packet size in heat 811. 相似文献
17.
为了研究T_(8/5)时间对X80管线钢热影响区粗晶区组织和性能的影响,在Gleeble 3500热模拟试验机上对其分别进行4个焊接工艺(T_(8/5)时间分别为21、27、33和40s)的加热冷却后,对粗晶区夏比冲击性能进行测试,并对其显微组织和冲击后断口形貌进行分析。结果表明,T_(8/5)时间为21s的焊接工艺下热影响区粗晶区具有较稳定且优异的低温夏比冲击性能;随着T_(8/5)时间由21延长至40s,热影响区粗晶区的低温冲击韧性下降,断裂方式由韧性断裂向脆性断裂转变,-20℃下冲击断裂方式由部分韧性断裂转变为完全脆性断裂;延长T_(8/5)时间促进了热影响区粗晶区近熔合线侧长条大块状M/A组元的形成,使得贝氏体板条间距变大。 相似文献
18.
《Acta Metallurgica Materialia》1994,42(6):1887-1891
An analytical model for predicting fracture toughness KIC is proposed based on a stress-modified critical strain criterion, that reflects the effect of stress triaxiality on ductile fracture. For KIC modelling, the notch-tip strain and stress state are given by introducing asingular field for the case of power-law-hardening materials. Notch fracture toughness is interpreted in terms of the notch-root radius (ϱ): KIC is predicted to increase with increasing ϱ, but has a minimum at a small ϱ. The microstructuralls characteristics distance and the reference critical strain can be estimated by fitting the KIC vs ϱ data on the model equation. Finally, previous notch fracture-toughness data are re-analyzed with the proposed model: the current analysis explains well the interaction effect between the notch-tip strain field and the local-fracture-controlling microstructure even in the small ϱ range. 相似文献
19.
以低成本、高附加值Q690E级调质板开发为目标,研究了亚温热处理与调质热处理工艺参数对试验钢显微组织与力学性能的影响。结果表明:在进行810℃亚温淬火处理的前躯体中存在大块状的铁素体时,易导致试验钢的低温冲击韧性恶化;以板条马氏体为前躯体经相同亚温淬火后,显微结构为更加细小的马氏体和以条状形态呈平行趋势分布在马氏体之间的铁素体两相混合组织,试验钢的-40℃冲击功值高达247J,但强度较低;常规调质热处理后的试验钢具有良好的综合力学性能,采用修正后的工艺参数,工业试制6~60mm规格产品的强韧性能均明显超过相关标准要求。 相似文献
20.
Guang-Jun Cai Hans-Olof Andrén Lars-Erik Svensson 《Metallurgical and Materials Transactions A》1997,28(7):1417-1428
The microstructure of three 12 pct cr steel weld metals with different nickel and nitrogen contents was studied in as-welded
condition and after postweld heat treatment with and without intercooling. Tensile strength and impact toughness of the weld
metals were investigated in different postweld heat treatment conditions. In weld metals heat treated without intercooling,
austenite decomposed by a eutectoid reaction that resulted in M23C6 aggregates around retained δ-ferrite. Two morphologies of M2N and MN precipitates were found in a low-dislocation α-ferrite. It was concluded that these phases were also transformed from austenite. In weld metals heat treated with intercooling,
M23C6 precipitates were smaller and more homogeneously distributed. Different MN precipitates were found in the tempered martensite.
The fracture mode of the weld metals at room temperature was mainly transgranular cleavage with some fibrous fracture. Intercooling
treatment improved Charpy impact toughness of the 12 pct Cr steel weld metals substantially. It was found that the important
microstructural factors affecting the impact toughness of the weld metals which were heat treated without intercooling were
the sizes of the α-ferrite grains, nonmetallic inclusions, and M23C6 aggregates. For the weld metals heat treated with intercooling, the factors which affect the toughness of the weld metals
were the sizes of martensite packets and nonmetallic inclusions. 相似文献