钢中珠光体相变机制的研究进展

简要总结了钢中珠光体相变的机制，特别是珠光体形核的晶体学和长大的动力学。基于形核时的最小形核能和渗碳体与铁素体“合作”生长的要求，珠光体通常在奥氏体晶界形核并只向某一晶粒内生长，因而晶界珠光体与相邻晶粒存在特定取向关系而与生长进入的晶粒无特定取向关系。晶体学研究表明，珠光体形核的领先相与碳浓度及渗碳体与铁素体间的取向关系(分别为Pitsch-Petch、Bagaryatsky或lsaichev)无关。最近的研究发现，具Bl结构的非共格杂质粒子界面是过共析钢中晶内珠光体形核的有效位置，初步的结果表明，晶内珠光体形核的原因在于杂质粒子生长造成的局部区域碳贫化及低能的珠光体，杂质界面取代高能的奥氏体，杂质界面。特定过冷度下的层片间距与正向界面推移速率的关系是珠光体生长动力学的主要问题。基于珠光体与奥氏体间的无序界面假设和稳态扩散方程分别发展了以体扩散控制和以界面扩散控制为主的动力学理论。两种理论均与实验结果符合较好，但后者似更合理。最近的实验在珠光体中发现了生长台阶与结构台阶，表明生长动力学方程需要修正。     

Computer-aided three-dimensional visualization of twisted cementite lamellae in eutectoid steel

The three-dimensional morphology and crystallography of cementite lamellae in a 0.8 mass% C–Fe alloy were examined by a combination of scanning electron microscopy using a double-beam field emission gun, electron back-scattered diffraction, and transmission electron microscopy. The cementite lamellae were twisted in three-dimensions from changes in both the ferrite/cementite habit plane and ferrite orientation, but they maintained their orientation relationship. The cementite lamellae were straight while they grew along the colony/austenite boundary. As the colony/austenite boundary migrated sideways, isolating the cementite from the austenite, the cementite lamellae curved. A possible mechanism for the cementite twisting is proposed.     

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.     

TEM STUDY OF AUSTENITE FORMATION IN A LOW CARBON STEEL

<正> 低碳铁素体加马氏体双相钢的强度和塑性取决于马氏体的数量和分布,这与亚温区加热过程中奥氏体形成的特点有关。由于奥氏体的形成是在高温下进行,速度快,难于直接获得有关形核的信息。本文研究低碳钢中奥氏体形成的部位,以及预先冷轧的影响。 试验用低碳1.5Mn钢,其化学成分为(wt-％):C 0.08,Mn 1.45,si 0.21,Al 0.045,N 0.005。经高频感应电炉熔炼成45kg的钢锭后,热轧成2.5mm厚的板材。在真空炉中经过1200℃均匀化退火24h。然后,在盐炉中900℃加热15min后空冷,获得铁素体加珠光体型的正火原始组织。另将一部分正火的坯料进行厚度压下量     

奥氏体形成机制

以T8等钢为例研究并综合分析了珠光体逆共析转变为奥氏体的过程和机制,指出,奥氏体是碳或各种化学元素溶入γ-Fe中所形成的固溶体。奥氏体在铁素体／渗碳体相界面形核,也可在珠光体领域交界处和原奥氏体晶界上形核。奥氏体以体扩散形式长大。奥氏体形核及长大过程中可同时吞噬铁素体和渗碳体,或只吞噬铁素体片长大,或吞噬铁素体较快,而吞噬渗碳体较慢,完成逆共析转变,最后剩下部分渗碳体。为减少相变积累的畸变和缓和应变能,而调整生长方向,形成了孪晶。奥氏体中存在未溶碳化物,需继续溶解和均匀化。     

Pearlite to austenite transformation in an Fe–2.6Cr–1C alloy

The mechanism of austenite formation, the kinetics of cementite lamellae dissolution and the crystallography of the austenitization from pearlite have been studied in an Fe–2.6 wt% Cr–0.96 wt% C alloy. Austenite grains nucleate after rather long incubation time both at pearlite colony boundaries and at the ferrite/cementite interfaces within a pearlite. Characteristic morphologies of transformation products were observed at various stages of transformation. Particular attention was paid to the structural evolution close to the α/γ interfaces. No indications of the diffusion ahead of the α/γ interface were found. The kinetics of austenite growth is controlled initially by carbon diffusion but in later stages by chromium diffusion. The results are discussed by assuming local equilibrium at the moving interfaces with the software and database ThermoCalc. The method involves the driving force determination for the diffusion of carbon and substitutional element during austenitization. The orientation relationships between ferrite, martensite and cementite were also determined. The possible austenite orientations were evaluated assuming the α/θ, α/γ and γ/θ orientation relationships so far obtained.     

基于温变形耦合逆相变细化铁素体制备

采用Gleeble-3500热模拟试验机进行650℃多道次温变形以研究某中碳钢奥氏体及铁素体晶粒的细化，并利用OM、TEM和EBSD等检测手段进行表征。结果表明，650℃下6道次的快速压缩变形(总应变为2.6)后有逆转变发生，所得奥氏体晶粒平均尺寸为3.4μm;再在10℃/s的速率冷却到600℃,可以形成平均晶粒尺寸为0.64μm的铁素体，且渗碳体呈粒状分布于基体之上。     

Effect of copper additions on the isothermal bainitic transformation in hypereutectoid copper and copper-nickel steels

During the isothermal bainitic transformation in hypereutectoid steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation not only within tempered bainitic ferrite in both types of steel using low tempering temperatures, but also within the tempered bainitic cementite of the copper steel at higher tempering temperatures. The interpretation of these experimental data strongly supports the theory that bainite formation is promoted through a shear controlled type of mechanism.     

Effects of transformation temperature on variant pairing of bainitic ferrite in low carbon steel

The effects of transformation temperature on the variant pairing tendency of bainitic ferrite (BF) and the orientation relationship (OR) between BF and austenite matrix are investigated quantitatively in low-carbon, low-alloy steel by means of electron backscatter diffraction analysis. By numerical reconstruction of the austenite orientation it was found that BF holds a near Kurdjumov-Sachs OR with respect to the austenite matrix. The angular deviation of the close-packed planes (CPPs) becomes smaller with decreasing transformation temperature, while the small misorientation angle between the close-packed directions (CPDs) remains nearly the same. BF variants with small misorientations are formed side by side at high transformation temperatures by sharing the same Bain correspondence. In contrast, BF variants are formed adjacently at lower transformation temperatures by sharing the same parallel relations for CPP. In particular, pairing of twin-related variants is dominant in this case. Such a variant pairing tendency with respect to transformation temperature is discussed from the viewpoint of self-accommodation of transformation strain and inter-variant boundary energy.     

Crystallography of grain boundary cementite dendrites

The crystallography and morphology of proeutectoid grain boundary cementite precipitates were studied in an isothermally transformed Fe-1.3%C-12%Mn steel. Grain boundary cementite precipitates develop with complex variations of a morphology that can be described as fern-like dendrites growing preferentially within and along austenite grain boundaries. The dendritic morphology was made most readily apparent by deep etching specimens of the alloy whose growth had been arrested after relatively short times during the solid state austenite→cementite transformation. Trace analysis revealed that these precipitates usually have no crystallographically preferred primary or secondary dendrite arm growth directions. Electron Backscattered Diffraction pattern analysis showed that most of the grain boundary cementite precipitates approximated one of the known cementite–austenite crystallographic orientation relationships (OR) with at least one of its adjacent austenite grains. Occasionally, the grain boundary cementite exhibited an OR that achieved ‘best fit’ directions between the cementite and austenite in both adjacent austenite grains.     

关于60Si2MnA钢奥氏体中温转变的研究

60Si2MnA钢过冷奥氏体的中温转变产物为贝氏体铁素体和残余奥氏体,贝氏体铁索体的形成与自催化效应密切相关。在回火过程中,贝氏体中的奥氏体以扩散转变方式分解为铁素体和渗碳体。从而说明奥氏体的等温转变与回火转变是本质不同的转变。     

Dynamic Phase Transformation and Spheroidization of Cementite of Hypereutectoid Steel Containing Aluminum during Deformation

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了合金元素Al对过共析钢缓冷相变和 过冷奥氏体动态相变组织的影响. 结果表明: 在缓冷相变时, Al的加入抑制网状渗碳体形成, 细化珠光体 片层间距; 在过冷奥氏体形变过程中, 动态转变经历动态相变和相变所得珠光体中渗碳体球化及铁素体动 态再结晶等过程. 在动态相变过程中, 没有形成晶界网状渗碳体, 而直接产生珠光体. Al的加入使动态相变过程中奥氏体的稳定性提高、珠光体转变推迟, 进一步细化了珠光体片层间距. 在相变所得珠光体中渗碳体球化及铁素体动态再结晶的过程中, Al阻碍渗碳体粗化, 使渗碳体颗粒和铁素体晶粒尺寸细化.     

Microstructure and crystallography of massive cementite layers on ferrite substrates

Upon gaseous nitrocarburizing, massive cementite layers were grown on ferrite substrates. By means of electron backscatter diffraction, it was shown that the Bagaryatsky orientation relationship holds between the cementite grains of the compound layer and the ferrite grains of the substrate. The experimental data exhibit a preference for specific variants of this orientation relationship: those variants corresponding to relatively low cementite–ferrite misfit-strain energy are favoured. Furthermore, the orientation relationship of neighboring cementite grains grown on the same ferrite grain allows the establishment of low-energy grain boundaries between these cementite grains. In contrast, at ferrite-grain boundaries intersecting the substrate surface, high-energy cementite-grain boundaries occur between cementite grains on adjacent ferrite grains. The latter cementite-grain boundaries promote carbon grain-boundary diffusion through the cementite, which is experimentally supported by the observation of relatively large cementite-layer thicknesses at these locations.     

A study of microstructure,transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel

Differently heat treated samples of a low alloyed TRIP steel have been investigated using electron diffraction techniques in SEM and TEM. Aim was, first, to discriminate the microstructure constituents, austenite, ferrite, bainite, and martensite, second to gain information on the γ–α phase transformation mechanisms and third to correlate the mechanical properties and the microstructure of the samples. Bainite always occurs in conjunction with an orientation gradient in the surrounding ferrite matrix. It consists of fine lamellae of ferrite and austenite which show a sharp Kurdjumov–Sachs orientation relationship with each other. This was interpreted in terms of a displacive bainite formation mechanism. The microstructure is formed by growth of γ-grains during intercritical annealing and shrinking of these grains during the subsequent cooling without nucleation of new α-grains. The transformation first occurs reconstructively into ferrite and then, at lower temperature, displacively into bainite. The mechanical properties of differently heat treated samples are most strongly influenced by the amount and distribution of carbon in the retained austenite and by the degree of recovery in bainite and austenite.     

Formation mechanism of vermicular and lacy ferrite in austenitic stainless steel weld metals

Abstract

Solidification and subsequent transformation of austenitic stainless steel weld metals that solidified in the ferritic–austenitic mode were investigated from the viewpoint of crystallography. The formation mechanisms for the vermicular and lacy ferrite observed in the weld metals were clarified. The ferrite morphology is determined by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite. In particular, for the formation of continuous lacy ferrite, it is necessary that the ferrite continues to grow with the Kurdjumov–Sachs orientation relationship with austenite that is established at the stage of ferrite nucleation.     

马氏体温变形超细化过共析钢

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了含Al和不含Al两种过共析钢马氏体温变形和等温回火过程中的组织超细球化演变及超细球化组织的力学性能. 结果表明: 与马氏体等温回火相比, 马氏体温变形加快马氏体的分解动力学, 在较短的时间 内即获得超细化 (α+θ)复相组织. 温变形过程中的组织超细化演变主要经历渗碳体粒子的析出与粗化及铁素体基体的动态回复和动态再结晶; 而在等温回火过程中, 铁素体主要发生静态回复和晶粒长大, 并没有再结晶现象发生. 合金元素Al的加入在等温回火和温变形过程中均抑制马氏体的分解, 阻碍渗碳体粒子的粗化和铁素体晶粒的长大, 导致复相组织的细化. 同时, Al的加入使马氏体温变形和等温回火后所得超细化 (α+θ) 复相组织在不降低总延伸率的前提下, 强度得以明显提高.     

强磁场对硅锰铸钢等温珠光体粒化的影响

对硅锰铸钢在相变温度(A1)附近进行等温处理的研究表明,经磁场处理后的试样组织中存在更多的粒状珠光体组织。此粒状珠光体由两部分构成:一部分来自块状铁素体中直接析出粒状的不连续渗碳体;一部分来自等温过程中片状渗碳体的熔断。磁场通过促进铁素体的形核与长大,使较多的碳原子"陷落"在铁素体内以沉淀析出方式形成粒状渗碳体,处理温度越低,此作用越明显。此外,磁场下珠光体组成相磁致伸缩率存在差异,引起珠光体两组成相之间的应变能变化,有利于渗碳体以球状析出于奥氏体或铁素体中,也对粒状珠光体分数的增加作出贡献。     

The influence of lattice strain on pearlite formation in Fe–C

The effect of stress and strain on the transformation kinetics of pearlite is investigated by phase-field simulation. Strain is considered in terms of expansion/contraction during transformation and due to concentration gradients in austenite. It is demonstrated that due to the concentration dependence of the eigenstrain, an inhomogeneous stress distribution ahead of the transformation front enhances diffusion in the austenitic phase and reduces chemical supersaturation in both austenite and ferrite. The main result of the investigation is that transformation strain inhibits the cooperative growth mode of cementite and ferrite, as considered by the Zener–Hillert model, and provokes the salient growth of cementite needles ahead of the ferrite front, which we call “staggered growth”. The predicted growth velocities give the right order of magnitude compared to the experiment and close the gap between theoretical models based on diffusion only, and experimental observations.     

Crystallography of intragranular ferrite formed on (MnS+V(C,N)) complex precipitate in austenite

Crystallography of intragranular ferrite formed on incoherent MnS+V(C,N) complex precipitate in austenite was studied in an Fe–Mn–C hypoeutectoid alloy. As transformation temperature decreases, intragranular ferrite morphology changes from idiomorph to acicular ferrite. Idiomorphic ferrite has no specific orientation relationship with austenite although acicular one holds near Kurdjumov–Sachs relationship.     

MORPHOLOGY AND PHASE TRANSFORMATION OF GRANULAR BAINITE AND GRANULAR STRUCTURE

本文研究了低碳合金钢奥氏体在连续冷却或等温转变过程中形成的组织形态和转变机理。结果表明在中温贝氏体区得到“粒状贝氏体”组织,在先共析转变区得到“粒状组织”。两者都有“铁素体基体+小岛”形貌。但前者有表面浮凸现象,铁素体呈长条状,与母相维持K-S关系,惯习面为{111}_γ,小岛也多星长条状平行排列。后者无表面浮凸,铁素体呈无规则形状,与母相无严格位向关系,小岛亦呈无规则排列。然而这两种组织都是由扩散型相变机制形成的。最后,提出了形成这两种组织的相变模型。