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T. Karthikeyan Manmath Kumar Dash S. Saroja M. Vijayalakshmi 《Metallurgical and Materials Transactions A》2013,44(4):1673-1685
The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite. 相似文献
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针状铁素体是一种具有大角度晶界、高位错密度的板条状中温转变组织,该组织能有效细化晶粒并具有良好的强韧性匹配.因此,通常在低合金高强度钢焊缝和粗晶区中,利用细小的夹杂物来诱导针状铁素体形成,形成有效晶粒尺寸细小的针状铁素体联锁组织或者针状铁素体和贝氏体的复合组织,使其具有良好的韧性.然而,相关研究对针状铁素体组织的形成机理和控制原理的解释并不十分清楚,对于针状铁素体的定义和理解也存在差异.总结了针状铁素体的本质、相变、形核、形态、晶体学取向关系、长大行为、细化机理和力学性能等方面的特征,归纳了奥氏体晶粒尺寸、转变温度、冷却速度、夹杂物类型和尺寸等对针状铁素体形成的影响,提出了针状铁素体组织形态和转变机理方面几个仍需深入研究的问题和方向. 相似文献
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Inrecentyears ,thedemandofhigherweldingqualityisincreasingwiththedevelopmentofhighstrengthsteel.Thestrengthandtoughnessofweldmetalsareimportantforuseperformanceofweldingstructure ,especiallyforlowalloyhighstrengthsteelwhosetensilestrengthσb ≥ 10 0 0MPa ,and… 相似文献
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Copper precipitation during continuous cooling and isothermal aging of a710-type steels 总被引:11,自引:0,他引:11
Precipitation in copper-containing A710 (also referred to as HSLA-80) and modified-A710 steels was investigated by transmission electron microscopy. Isothermal aging of as-quenched specimens at 675 °C produced e-copper precipitates located primarily at α-iron matrix dislocations. The precipitates exhibited multiple variants of an orientation relationship (OR) consistent with that reported by Kurdjumov and Sachs, fine fault formation, and associated streaking in electron diffraction patterns. For reaustenitized and continuously cooled specimens, the primary precipitation event was associated with interphase precipitation of copper at ferrite/austenite interfaces. Interphase precipitates frequently displayed ORs other than that reported by Kurdjumov and Sachs, although a unique crystallographic variant was observed within any one region of interphase precipitation, faults were observed infrequently, and streaking was not observed in diffraction patterns. At high temperatures during cooling, precipitate-free ferrite formed, whereas at lower temperatures, nucleation of copper precipitates occurred at ferrite/austenite interfaces for crystals of polygonal ferrite and Widmanstätten ferrite. This latter feature precludes the formation of Widmanstätten ferritevia a displacive mechanism. Interphase precipitation was not observed for granular ferrite or acicular ferrite. Less-common precipitation events during continuous cooling included the formation of AIN and CuS. 相似文献
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Minju Kang Hyunmin Kim Sunghak Lee Sang Yong Shin 《Metallurgical and Materials Transactions A》2014,45(2):682-697
In this study, drop weight tear tests (DWTT) were conducted on API X70 and X80 linepipe steels fabricated with various compositions and rolling and cooling conditions in order to correlate the strain hardening with the abnormal cleavage fracture occurring in the hammer-impacted area. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, Charpy impact energy, and strain hardening. All the steels consisted of fine acicular ferrite, together with some bainitic ferrite, granular bainite, and martensite-austenite constituent. As the volume fraction of acicular ferrite increased, the area fraction of DWTT abnormal cleavage fracture decreased because the toughness of acicular ferrite was higher than other microstructures. The area fraction of abnormal cleavage fracture was weakly related with strain hardening exponents obtained from the quasi-static tensile and compressive tests, but showed better correlation with those obtained from the dynamic compressive test. This tendency could be more clearly observed when steels having similar Charpy impact energy levels were grouped. Since the DWTT was performed under a dynamic loading condition, thus, the abnormal cleavage fracture behavior should be related with the strain hardening analyzed under a dynamic loading condition. 相似文献
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Yongjin Kim Sang Yong Shin Yang Gon Kim Nack J. Kim Sunghak Lee 《Metallurgical and Materials Transactions A》2010,41(8):1961-1972
This study aimed at investigating effects of strain rate and test temperature on deformation and fracture behavior of three
API X70 and X80 linepipe steels fabricated by varying alloying elements and hot-rolling conditions. Quasi-static and dynamic
torsional tests were conducted on these steels having different grain sizes and volume fractions of acicular ferrite and polygonal
ferrite, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and adiabatic shear band formation. The dynamic torsional test results indicated that
the steels rolled in the single-phase region had the higher maximum shear stress than the steel rolled in the two-phase region,
because their microstructures were composed mainly of acicular ferrites. Particularly in the API X80 steel rolled in the single-phase
region, increased dynamic torsional properties could be explained by the decrease in the overall effective grain size due
to the presence of acicular ferrite having smaller effective grain size. The possibility of the adiabatic shear band formation
at low temperatures was also analyzed by the energy required for void initiation and difference in effective grain size. 相似文献
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Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS 总被引:3,自引:0,他引:3
I. Madariaga Ph.D. J. L. Romero I. Gutiérrez 《Metallurgical and Materials Transactions A》1998,29(3):1003-1015
The isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated
using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine
microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase.
The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between
the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship
into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular
ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite
is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment
times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of
upper bainite formation. 相似文献
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I. Madariaga Ph.D. J. L. Romero I. Gutiérrez 《Metallurgical and Materials Transactions A》1998,29(13):1003-1015
The isothermal transformation vs time of a medium-carbon microalloyed steel at 450°C, following austenitization at 1250°C for 45 minutes, has been investigated
using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine
microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase.
The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between
the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship
into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular
ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite
is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment
times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of
upper bainite formation. 相似文献
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Ti、Zr的复合氧化物可以有效诱导针状铁素体形核,从而细化晶粒。为了研究Ti–Zr处理钢中针状铁素体转变机理,使用25 kg真空感应炉中熔炼试验所需钢种,向低合金钢中添加了质量分数为0.038%钛和0.008%锆。利用高温激光共聚焦显微镜原位观察了奥氏体化温度对针状铁素体转变行为的变化,使用扫描电镜观察了Ti–Zr处理钢种的夹杂物成分和针状铁素体在夹杂物表面形核,使用光学显微镜观察不同奥氏体化温度下的微观组织变化差异。结果表明,随着奥氏体化温度从1250 ℃增加至1400 ℃,奥氏体晶粒尺寸从125.6 μm 增加至279.8 μm,针状铁素体开始转变温度和侧板条铁素体开始转变温度先增加,在1350 ℃条件下达到最大值,后又降低,针状铁素体的体积分数由39.6%增加至83.6%;Ti–Zr处理钢中核心为Zr–Ti–O,外部为Al–Ti–Zr–O的氧化物为核心表面析出MnS的复合氧化物主要集中在1.5~3 μm,可以有效促进针状铁素体形核,贫Mn区和夹杂物与铁素体之间的良好晶格关系为该型夹杂物能够促进针状铁素体形核机理。奥氏体晶粒尺寸的增加导致多边形铁素形核位点的减少和针状铁素体的形核空间的增加,钛锆复合处理形成大量的有效诱发针状铁素体形核的夹杂物,这共同导致了针状铁素体体积分数增加。 相似文献
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为了解奥氏体在连续冷却过程中的组织演变规律,更好地控制管线钢室温下的组织形态,对X70管线钢进行了静态及动态热模拟试验,绘制出了相应的连续冷却转变曲线(CCT曲线),观察其组织,分析变形和冷却速度等因素对管线钢组织的影响。同时对X70管线钢的入精轧温度、终轧温度等因素控轧控冷工艺进行模拟研究。认为提高变形后的冷却速度能获得针状铁素体组织;在同一冷却速度下,动态连续冷却转变得到的组织更细密;降低入精轧温度、终轧温度,增加冷却速度能细化组织。 相似文献
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在变形温度为1000和1100℃,应变速率为01s-1的条件下,利用MMS-200热模拟试验机,对S32750超级双相不锈钢进行了高温压缩试验。利用电子背散射衍射(EBSD)分析了其晶体取向和微观组织。研究结果表明,铁素体在两种试验条件下均可形成<001>和<111>∥压缩轴织构,在变形温度为1100℃时,<001>织构要强一些;奥氏体在两种变形温度下均形成了<001>织构,强度很弱。在变形温度为1100℃条件下,奥氏体中存在的以Σ3为主的CSL特殊晶界数量更多。两种试验条件下,S32750超级双相不锈钢中铁素体和奥氏体均发生了动态再结晶,降低变形温度有利于细化晶粒。在铁素体向奥氏体转变过程中,奥氏体可以在铁素体晶界处生成,也可以在铁素体晶粒内部形成。 相似文献
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The fraction and microstructure of spontaneous and deformation‐induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In‐situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex‐situ X‐ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous α'‐martensite, while deformation‐induced α'‐martensite mainly formed within parallel shear‐bands. The α'‐martensite formed has an orientation relationship near the Kurdjumov‐Sachs (K‐S) relation with the parent austenite phase even at high plastic strains, and adjacent α'‐martensite variants were mainly twin related (<111> 60° or Σ3). 相似文献