45钢等径弯曲通道变形及组织细化研究

研究了等径弯曲通道(ECAP)变形后45钢中先共析铁素体及珠光体组织的演变特征.结果表明,ECAP变形4道次后,片层状的珠光体组织演变成了超细的渗碳体颗粒均匀分布于亚微晶铁素体基体的组织.先共析铁素体由原始的平均晶粒尺寸约为30 μm演变为大角度晶界分离的、平均晶粒尺寸约为0.4μm的超细晶组织.ECAP变形后,先共析铁素体首先在其内部会形成具有薄片层界面(LBs)的板条位错胞甚至亚晶组织.进一步变形时位错胞或亚晶可继续细化.再进一步变形时通过晶界滑移和晶粒旋转的方式可以获得具有大角度晶界分离的、等轴的超细晶组织.     

铌对中碳Si-Mn系弹簧钢珠光体相变行为的影响

采用热模拟实验方法研究了铌对Si--Mn系弹簧钢相变特征的影响,分析了NbC的形变诱导析出行为.结果表明:弹簧钢中添加微量铌,推迟了珠光体转变,马氏体转变的最小冷却速率由5℃.s-1变为3℃.s-1;细化了珠光体,改变了珠光体组织形貌,渗碳体片层变薄、形状变得不规则,出现弯曲、断续;含铌弹簧钢在850℃变形时发生了NbC的形变诱导析出,NbC的析出位置为珠光体中的铁素体片层内、珠光体球团边界和位错处,析出物颗粒直径为10～15nm,形状近似球形.     

陕西省自然科学研究计划项目

 钢和铁基合金通过等径弯曲通道变形(ECAP)可获得超细晶组织，从而改善材料的性能。成功实现了C方式650 ℃时珠光体65Mn钢的等径弯曲通道变形，累积等效真应变约为5。片层状珠光体组织演变成超细的渗碳体颗粒均匀分布于铁素体基体的组织，而且铁素体基体为均匀等轴晶粒，平均晶粒尺寸约为0 3 μm。     

共析渗碳体结构与变形行为的透射电镜研究

通过大量钢铁材料的透射电子显微镜分析,研究了渗碳体结构与变形行为之间的关系,探讨了渗碳体层片厚度对珠光体强韧性影响的机理。结果显示：在变形时,厚片渗碳体常以层片堆垛的方式生长,变形时易于沿层片间被切断;薄片渗碳体则难以切断,通常以弯曲的方式来适应变形。     

PD3钢轨钢接触疲劳变形组织的TEM观察

在透射电镜下观察接触疲劳失效后不同状态的ＰＤ３钢轨钢的变形组织。结果表明，接触疲劳变形组织的主要特点是：珠光体片层间距减小，珠光体团被剪切分割成亚团，先析铁素体内形成位错胞结构；铁素体／渗碳体界面上存在高的位错密度；珠光体中渗碳体片发生变形与断裂。同时对珠光体片层间距与变形组织的关系进行了分析讨论。     

轧制变形对U75V重轨钢珠光体片层间距的影响

U75V重轨钢铸坯尺寸为280 mm×380 mm,重轨轧材经14道次粗轧(BD1)、中轧(BD2)和CCS 6道次精轧(UR₁、ER、UR₂、UR₃、EF、UF)而成。用扫描电镜测量了重轨钢各道次珠光体片层间距。结果表明,随重轨钢BD1E、BD2B、UR和UF道次变形量(面积压缩比)增加,重轨头部、腰部、底部和腿部的珠光体组织片层间距分别由0.512,0.414,0.493,0.452μm降至0.293,0.269,0.253,0.229μm,从而明显地提高重轨的力学性能。     

铁素体和珠光体含量影响变形过程的原位研究

在扫描电镜下原位观察了两种钢的拉伸变形过程,两种钢分别为以铁素体为主、含少量珠光体的纯净高强钢和以珠光体为主、含少量先共析铁素体的车轮钢.纯净钢拉伸时,不论试样厚度满足平面应变与否,均以铁素体的滑移变形为主,并最终导致韧性开裂,裂纹连续扩展,少量的珠光体对整个变形断裂过程几乎没有影响;断口呈现韧窝状.对于车轮钢,当试样厚度很薄不满足平面应变条件时,尽管先共析铁素体很少,拉伸时,仍以先共析铁素体的变形为先导过程,并在先共析铁素体与珠光体的界面处优先开裂,成为不连续微裂纹,断口呈现韧窝和准解理两种混合特征;当试样厚度满足平面应变条件时,则以珠光体中渗碳体片层的脆性开裂为主,断口呈现准解理特征.      

高碳珠光体钢在温变形过程中的组织变化

用SEM、TEM等方法研究了一种原始组织为层片状珠光体的高碳钢在多向温变形(楔横轧)和单向温变形(压缩)条件下试样心部与表层显微组织的演变情况.结果表明,在多向温变形条件下,试样表层可获得铁素体晶粒与渗碳体颗粒尺寸均在0.3 μm以下的超微细(α θ)复相组织,心部渗碳体片虽已碎断并部分球化,但其排布形态与变形珠光体基本保持一致,而单向温变形条件下的情况则恰好相反;变形过程中试样所受应力、应变状态的不同是引起组织差异的根本原因.     

不同含碳量的帘线钢在相同冷速下的组织对比

采用光学显微镜、扫描电镜（SEM）、热膨胀仪和显微硬度计,对比研究碳含量为70级和80级的帘线钢在相同冷却速率下的组织构成、珠光体片间距、晶界渗碳体（GBC）、相变点和显微硬度。结果表明：70级帘线钢易出现先共析铁素体;80级帘线钢由于碳含量高,冷却速率较慢时,易出现晶界渗碳体,随着冷却速率的增大,相变点降低,先共析铁素体、晶界渗碳体析出减少,珠光体片间距减小,硬度增高。     

钢中渗碳体的冷变形特征及其作用

对热轧和球化退火两种状态的同一种中碳低合金结构钢冷变形前后的组织结构进行了透射电子显微镜观察。结果表明 :层片状共析渗碳体与铁素体基体具有 (10 1) α- Fe∥ (0 11) Fe3C,(12 1) α- F e∥ (10 0 ) F e3C的平行位向关系。冷变形过程中 ,当渗碳体层片方向与剪应力方向呈近 4 5°或 90°时 ,层片状渗碳体分别具有流变、扭折、剪切等变形特征 ;粒状渗碳体对铁素体基体的变形有阻碍作用且形变产生的位错以 Orowan机制绕过粒状渗碳体 ,因而材料具有较好的韧性。     

Effect of Equal Channel Angular Pressing on Lamellar Microstructures in Nickel Aluminum Bronze

A lamellar nickel aluminum bronze has been processed by equal channel angular pressing (ECAP) and its microstructure investigated. The orientation, spacing, and morphology of the lamellae after ECAP were observed to be dependent on the orientation of the lamellae before ECAP with respect to the shear plane. Two distinct microstructures were produced following a single ECAP pass. The most common type consisted of fragmented lamellae with a reduced lamellar spacing, inclined between >0 deg and <45 deg to the exit channel, and the second type comprised bent lamellae with an increased spacing and were inclined between >45 deg and <180 deg, with exceptions at 0/180 deg and 45 deg. A model describing the evolution of lamellar microstructures subjected to a single ECAP pass has been proposed.     

共析钢中片层状渗碳体冷轧后的形态变化

The pearlitic transformation and the deformation behavior of lamellar cementite after cold rolling in eutectoid steels Fe-0. 76%C-0. 137%Mn (mass fraction) were studied by means of Formastor-F (Full Automatic Transformation Testing Instrument) and field emission scanning electronic microscopy (FESEM) observation. Fine and coarse pearlite were obtained in the eutectoid steels austenitized at 900℃ for 15min, then hold at 620℃ for 90 s and 690℃ for 7 h, respectively. Thedeformation behavior of cold rolled lamellar cementite could be classified as: cleavage fracture, inhomogeneous slip, fragmentation, thinning or necking, and homogeneous bending. The cementite lamellae with the thickness of more than 100 um could be deformed plastically.     

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.     

High temperature plastic-flow behavior of mixtures of austenite,cementite, ferrite,and pearlite in plain-carbon steels

The plastic-flow behavior of ferrite + pearlite, pearlite + cementite, and austenite + cementite mixtures in plain carbon steels has been examined over the temperature range 500 to 1050 °C, strain-rate range 6 x l0⁻⁶ to 2 x l0⁻² s⁻¹, and carbon range 0.005C to 1.89C. Up to the eutectoid temperature the strength of the ferrite + pearlite mixture more than doubles as the carbon content increases from 0.005C to 0.7C, so that whereas in low-carbon steels the ferrite is weaker than the higher temperature austenite phase, in eutectoid steels the fully pearlitic structure is stronger than the fully austenitic structure. Manganese and silicon strengthen ferrite more effectively than they do austenite. A 0.17 pct phosphorus addition strengthens the ferrite + pearlite mixture across the range of microstructures from fully ferritic to fully pearlitic. Beyond the eutectoid composition, the amount of proeutectoid cementite does not significantly affect the strength of the pearlite, but above the eutectoid temperature it appreciably strengthens the austenite and cementite mixture at the strain rate of 2 X 10^-2 s^-1.     

Investigation of Orientation Gradients in Pearlite in Hypoeutectoid Steel by use of Orientation Imaging Microscopy

The microstructure of pearlite in hypoeutectoid steel was investigated using high resolution orientation imaging microscopy. Systematic orientation gradients were observed along the longitudinal direction within the pearlitic ferrite lamellae. Corresponding orientation gradients seemed to occur also in the cementite within the same pearlite colony. The orientation gradients in the ferrite strongly correlated with the local topographical curvature of the pearlitic colony. The orientation relationship between the ferrite and the cementite lamellae was constant even in areas with strong orientation gradients.     

Cyclic deformation of pearlitic eutectoid rail steel

Cyclic deformation behavior in pearlitic eutectoid steel strongly depends on the interlamellar spacing with cyclic softening in fine pearlite, cyclic hardening in coarse pearlite, and both cyclic softening and hardening depending on the strain amplitude in medium pearlite. Dislocations in cyclically softened specimens were uniformly distributed, while dislocation cells were observed with cyclic hardening. The cell size decreased with increasing strain amplitude. Using the cell size to interlamellar spacing ratios, conditions for cell formation were quantified. Based on dislocation structure observations, mechanisms for cyclic softening and hardening were proposed. Both monotonic and cyclic yield stresses follow Hall-Petch type relations when plotted against interlamellar spacing. Surface fatigue microcrack initiation usually occurred in the ferrite matrix associated with extrusions and intrusions. Most microcracks were almost parallel to the cementite lamellae and oriented between 30 and 90 deg with respect to the tensile axis. Little influence of MnS inclusions on microcrack initiation was noticed.     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

Behavior of pearlite of various morphologies during cyclic tension

The structural evolution of hypereutectoid U10 steel with a pearlitic structure of various types (fine lamellar or partly spheroidized pearlite) is studied during fatigue loading. The fracture of these structures is considered using fractography data. The specific features of the structural transformations and the changes in the dislocation structure of the U10 steel are revealed during cyclic tension in the high-cycle fatigue region at a significant distance (10 mm) from a fatigue fracture surface. Substantial structural changes are shown to occur in U10 steel with various pearlitic structures during high-cycle fatigue tests (tension at a stress amplitude in a cycle lower than the macroscopic yield strength). These are the fragmentation, partial dissolution, and spheroidization of cementite lamellae and the polygonization of the ferrite component. The relationship between the type of fracture surface and the type of structure formed upon fatigue loading is found.     

Deformation of pearlite

Pearlite with its lamellae oriented mainly parallel to the longitudinal direction was prepared by Bolling's method of transformation in a steep temperature gradient. The Fe-0.7 pct Mn-0.9 pct C pearlite was drawn into wire and also into strip in dies designed to minimize macroscopically nonuniform deformation. Cross sections of the drawn wires and strip were examined by conventional and high-voltage transmission electron microscopy and were analyzed by quantitative metallography for a) average interlamellar spacing, b) distribution of interlamellar spacings, and c) orientation relationship between the cementite lamellae and the slip systems in the ferrite. The strength of pearlite is proportional to the reciprocal square root of the average interlamellar spacing, and the proportionality constant analogous to the Hall-Petch constant (k) is related to the strength of the cementite lamellae. If the stress for the propagation of slip through the cementite is assumed constant, a Hall-Petch type of equation can be derived for the strengthening of the pearlite against slip in the ferrite by piled-up groups of dislocations. Evidence for the plastic deformability of cementite is presented; sufficiently thin cementite plates were fully plastic. The exponential strain hardening of drawn pearlitic wires and of rolled pearlite is explained in terms of locally inhomogenous deformation revealed by the lack of fragmentation of the lamellae. This paper is based on a presentation made at a symposium on “Mechanical-Thermal Processing and Dislocation Substructure Strengthening,” held at the Annual Meeting in Las Vegas, Nevada, on February 23, 1976, under the sponsorship of the TMS/IMD Heat Treating Committee.