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1.
The strains inherent to the martensitic transformation of austenite particles in 9Ni steel create dislocation structures in
the tempered martensite. These dislocation structures were studied by the complementary techniques of X-ray line profile analysis
and transmission electron microscopy. The energy required to form these dislocation structures affects the thermodynamics
of the transformation. We propose that changes in these dislocation structures reduce the “mechanical stability” of the austenite
particles as they grow larger during isothermal tempering. 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.
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.
LIU Dong yu BAI Bing zhe FANG Hong sheng YANG Zhi gang ZHANG Chi YAN Wen yan 《钢铁研究学报(英文版)》2002,9(1):46-49
Themicrostructureofsomehighstrengthlowalloysteelsandultrahighstrengthlowalloysteelsincommercialuseisusuallytemperedmartensite .However ,slack quenchedmixedmicrostructurecomprisingmartensiteandbainiteisgenerallyen counteredincommercialpracticewhentheselo… 相似文献
5.
An investigation into the mechanisms of tempered martensite embrittlement (TME), also know as “500°F” or “350°C” or one-step
temper embrittlement, has been made in commercial, ultra-high strength 4340 and Si-modified 4340 (300-M) alloy steels, with
particular focus given to the role of interlath films of retained austenite. Studies were performed on the variation of i)
strength and toughness, and ii) the morphology, volume fraction and thermal and mechanical stability of retained austenite,
as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing
temperature. TME was observed as a decrease in bothK
Ic and Charpy V-notch impact energy after tempering around 300°C in 4340 and 425°C in 300-M, where the mechanisms of fracture
were either interlath cleavage or largely transgranular cleavage. The embrittlement was found to be concurrent with the interlath
precipitation of cementite during temperingand the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon
in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude
of the embrittlement was found to be significantly greater in microstructures containing increasing volume fractions of retained
austenite. Specifically, in 300-M the decrease inK
Ic, due to TME, was a 5 MPa√m in oil quenched structures with less than 4 pct austenite, compared to a massive decrease of 70
MPa√m in slowly (air) cooled structures containing 25 pct austenite. A complete mechanism of tempered martensite embrittlement
is proposed involving i) precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained
austenite, and ii) subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized
by carbon depletion from carbide precipitation. The deterioration in toughness, associated with TME, is therefore ascribed
to the embrittling effect of i) interlath cementite precipitates and ii) an interlath layer of mechanically-transformed austenite,i.e., untempered martensite. The presence of residual impurity elements in prior austenite grain boundaries, having segregated
there during austenitization, may accentuate this process by providing an alternative weak path for fracture. The relative
importance of these effects is discussed.
Formerly with the Lawrence Berkeley Laboratory, University of California. 相似文献
6.
回火温度对Mn-Ni钢亚稳奥氏体形貌及其力学性能的影响 总被引:1,自引:0,他引:1
利用了X射线衍射仪(XRD)、电子背散射衍射(EBSD)和透射电子显微镜(TEM)研究了回火温度对一种Mn-Ni钢亚稳奥氏体形貌及其力学性能的影响。结果表明,随着回火温度的升高,室温亚稳奥氏体的体积分数逐渐升高。当回火温度为600和625 ℃时,亚稳奥氏体主要以片层状在回火马氏体板条间析出,且排列方向与周围的马氏体板条平行,这种片层状亚稳奥氏体分布较为均匀,尺寸较小,约为60~100 nm,且稳定性较高;当回火温度为650 ℃时,试验钢中出现尺寸较大的块状奥氏体在回火马氏体界面的交叉处不均匀析出。分析表明,块状奥氏体有利于提高钢的塑性,不利于改善钢的低温韧性;而片层状奥氏体能大幅度的改善钢的低温韧性。 相似文献
7.
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
8.
A. Bojack L. Zhao P. F. Morris J. Sietsma 《Metallurgical and Materials Transactions A》2014,45(13):5956-5967
The formation of austenite during tempering of a 13Cr6Ni2Mo supermartensitic stainless steel (X2CrNiMoV13-5-2) was investigated using an in situ thermo-magnetic technique to establish the kinetics of the martensite to austenite transformation and the stability of austenite. The austenite fraction was obtained from in situ magnetization measurements. It was found that during heating to the tempering temperature 1 to 2 vol pct of austenite, retained during quenching after the austenitization treatment, decomposed between 623 K and 753 K (350 °C and 480 °C). The activation energy for martensite to austenite transformation was found by JMAK-fitting to be 233 kJ/mol. This value is similar to the activation energy for Ni and Mn diffusion in iron and supports the assumption that partitioning of Ni and Mn to austenite are mainly rate determining for the austenite formation during tempering. This also indicates that the stability of austenite during cooling after tempering depends on these elements. With increasing tempering temperature the thermal stability of austenite is decreasing due to the lower concentrations of austenite-stabilizing elements in the increased fraction of austenite. After cooling from the tempering temperature the retained austenite was further partially decomposed during holding at room temperature. This appears to be related to previous martensite formation during cooling. 相似文献
9.
研究了一种屈服强度大于785 MPa的船板钢,测试了其动态连续冷却相变曲线(CCT),研究了试验钢经控制轧制+直接淬火+回火(DQ- T)工艺处理后的组织性能。结果表明,直接淬火(DQ)钢板组织为板条马氏体(LM),回火后铜、铌元素呈弥散析出。经500 ℃回火钢板的强度最高,冲击韧性(KV2)最低。钢板经710 ℃回火,其组织为二次马氏体(SLM)+铁素体,屈服强度(Re)为810 MPa,抗拉强度(Rm)为 1 066 MPa,伸长率(A)为17%,在-80 ℃下KV2为97 J,达到最佳强韧性匹配。 相似文献
10.
Formation mechanism of the reversed austenite of Cr15 super martensitic stainless steel (SMSS) alloyed with copper after high temperature tempering was investigated by means of thermo‐calc software, transmission electron microscope (TEM), and X‐ray diffraction (XRD). The mechanical properties of the SMSS were also tested. The experimental results show that the reversed austenite with low dislocation density is formed at high temperature tempering processing. The transformation of the martensite to reversed austenite is a diffused phase transformation, and the growth of the reversed austenite is closely related to the diffusion process of Ni. The bulk reversed austenite with large amount of stacking faults is formed with the increase of the tempering temperature. The volume fraction of reversed austenite increases at first and then decreases with increasing tempering temperature, and the maximum amount of the reversed austenite is obtained at 650°C. The reversed austenite is unstable at the tempering temperature above 650°C and the martensitic phase transformation will occur at the following cooling process. The mechanical properties of Cr15 super martensitic stainless steel are significantly influenced by the volume fraction of reversed austenite. 相似文献
11.
介绍了E550钢板的主要生产工艺和技术难点,通过理论分析设计了E550的成分体系,采用Thermal-Calc和经验公式,获得了其热力学相图和相变点温度等热力学数据。根据E550的热力学特性,设计了两阶段轧制工艺,精轧的终轧温度控制在再结晶温度附近,利用奥氏体再结晶充分细化晶粒。淬火奥氏体化温度选择为920℃,回火温度设计为630℃,利用碳化物的析出强化效果和缺陷密度变化的位错强化获得良好的强韧性匹配。50 mm厚钢板的淬火态1/4厚度处的微观组织为马氏体,中心的组织为马氏体和少量贝氏体的混合物。回火热处理后,马氏体板条界面减少,碳化物在马氏体板条界面析出,钢板1/4到中心的组织均匀化。30和50 mm厚E550钢板的力学性能达到了船级社标准要求,并有较大的富裕量。热输入能量为15和50 kJ/cm焊接后,钢板具有良好的强度性能,熔合线和热影响区的冲击功较高。 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.
The scavenging effect of precipitated austenite in a low carbon, commercial Fe-5.5Ni cryogenic alloy was investigated through
observation of the dissolution of cementite precipitates during intercritical tempering and study of the associated change
in Charpy impact toughness. Cementite precipitates initially located along prior austenite grain boundaries were gradually
dissolved into reverted austenite as the intercritical tempering proceeded. The austenite tends to form at or around the carbide
particles and may be catalyzed by their presence. The Charpy impact energy is changed through both a decrease in the ductile-brittle
transition temperature and an increase in the upper shelf energy. The latter effect is specifically associated with the dissolution
of the carbides which act as preferential void nucleation sites in the untempered alloy. 相似文献
14.
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
15.
摘要:采用拉伸、冲击、金相、电子背散射衍射、透射电镜、X射线衍射等试验手段,研究了在线直接淬火+回火(DQT)与离线再加热淬火+回火(RQT)工艺对马氏体高强钢组织性能的影响。结果表明,2种试验钢组织均为板条马氏体,RQT试验钢原奥氏体晶粒及板条束呈等轴状,板条块较短,板条较宽,DQT试验钢原奥氏体晶粒呈扁平状,板条束贯穿整个晶粒,板条块呈细长状,板条宽度较小;位错强化是DQT试验钢强度较RQT高的主要原因;板条束为控制DQT和RQT试验钢韧性的最小单元;DQT试验钢大角晶界比例较低,其具有较大的马氏体板条束尺寸以及更高的位错密度,断裂应力较低,低温韧性较差。 相似文献
16.
This work was undertaken to test the influence of precipitated austenite on transgranular hydrogen embrittlement in 5.5Ni
steel. Prior work has shown that the mechanism of transgranular hydrogen embrittlement in this steel is interlath separation.
Since the austenite that forms during the tempering of 5.5Ni steel precipitates on the martenite lath boundaries, it was hypothesized
that the austenite might have a beneficial effect. The experimental results show, however, that the precipitated austenite
decreases the toughness in the presence of hydrogen. The apparent mechanism is straightforward. The precipitated austenite
transforms to martensite in the strain field ahead of the crack tip. Interlath cracks appear at the periphery of the fresh
martensite particles. They are apparently caused by the volume change that accompanies the martensite transformation, which
imposes a tension across the lath boundary. The interlath cracks link together to form the macroscopic fracture. 相似文献
17.
为了调整COST-FB2转子钢的强韧性,采用OM、SEM和TEM等手段研究了回火温度对COST-FB2转子钢的析出相类型与力学性能的影响。结果表明,随着回火温度由350 ℃升高到750 ℃,试验钢的强度、硬度不断下降,塑性和冲击功上升;试验钢350 ℃和570 ℃回火后的高强低韧性可通过再次在700 ℃回火改善。淬火后COST-FB2转子钢中的残余奥氏体,可通过在570 ℃回火消除;在350 ℃和570 ℃回火后马氏体板条内部有大量针状的M3C,700 ℃回火后的显微组织中M3C消失,M23C6在原奥氏体晶界和马氏体板条界上析出,750 ℃回火后晶界上的M23C6有聚集粗化的现象,部分马氏体板条存在回复现象。 相似文献
18.
A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been
carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these
steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure
and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the
as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath
boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower
Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties
above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase
in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission
electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite
films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical
properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on
the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness
at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments
of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior
to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness
can be obtained with as-quenched martensitic steels. 相似文献
19.
The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, Ms =-105?C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, Ms = -170?C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory. 相似文献
20.
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