Effect of deformation parameters on the austenite dynamic recrystallization behavior of a eutectoid pearlite rail steel

Understandings of the effect of hot deformation parameters close to the practical production line on grain refinement are crucial for enhancing both the strength and toughness of future rail steels.In this work,the austenite dynamic recrystallization(DRX)behaviors of a eutectoid pearlite rail steel were studied using a thermo-mechanical simulator with hot deformation parameters frequently employed in rail production lines.The single-pass hot deformation results reveal that the prior austenite grain sizes(PAGSs)for samples with different deformation reductions decrease initially with an increase in deformation temperature.However,once the deformation temperature is beyond a certain threshold,the PAGSs start to increase.It can be attributed to the rise in DRX volume fraction and the increase of DRX grain with deformation temperature,respectively.Three-pass hot deformation results show that the accumulated strain generated in the first and second deformation passes can increase the extent of DRX.In the case of complete DRX,PAGS is predominantly determined by the deformation temperature of the final pass.It suggests a strategic approach during industrial production where part of the deformation reduction in low temperature range can be shifted to the medium temperature range to release rolling mill loads.

ELECTRON MICROSCOPE STUDY ON δ-EUTECTOID IN HIGH SPEED STEEL

The morphology and structure of δ-eutectoid and the transformation of δ-eutectoidic austenite in high speed steel have been studied under SEM and TEM.The δ-eutectoid in as-cast high speed steel is rod-shaped eutectoidic units with different orientations and is markedly surrounded by carbide shell.The eutectoidic carbide is rod-shaped M_6C of square eross section.The eutectoidic austenite may transform into pearlite structure during cooling.

Theoretical Model of Transformation Superlastic Diffusion Bonding for Eutectoid Steel

Based on current theories of diffusion and creep cavity closure at high temperature, a theoretical analysis of phase transformation diffusion bonding for T8/T8 eutectoid steel is carried out. The diffusion bonding is mainly described as two-stage process: Ⅰ The interfacial cavity with shape change from diamond to cylinder.Ⅱ The radius of the cylindrical cavity are reduced and eliminated gradually. A new theoretical model is established for the process of transformation superplastic diffusion bonding (TSDB) on the basis of a theoretical model for isothermal superplastic diffusion. The model can predict the bonding quality which is affected by technological parameters, such as limit cycling temperature, the compressive stress, the numbers of thermal cycles and temperature cycling through the phase transformation in the thermal cycling and so on. Results show that the maximum temperature, the compressive stress, the numbers of thermal cycles and the rate of temperature changing speed in the thermal cycling have an important influence on TSDB process. Meanwhile, reasonable technological parameters chosen from theoretical analysis is in good agreement with those obtained from experimental results.

Three-dimensional multi-phase-field simulation of eutectoid alloy based on OpenCL parallel

Seeking high-performance computing methods to solve the problem of a large amount of calculation,low calculation efficiency,and small simulation scale on the traditional single central processing unit (CPU) platform is of great value to the simulation study of micro-structure.In this study,based on the three-dimensional multi-phase-field model of KKSO coupling phase-field and solute field,the open computing language (OpenCL) + graphics processing unit (GPU) heterogeneous parallel computing technology is used to simulate the eutectoid growth of Fe-C alloy and the end growth process of pearlite under pure diffusion.The effects of initial supercooling and different diffusion coefficients on the growth morphology of lamellar pearlite were investigated.The results show that ferrite and cementite are perpendicular to the front of the solid-solid interface and are coupled and coordinated to grow,and there is no leading phase under the initial supercooling degree of 20 K.With the continuous increase of the initial supercooling degree (19 K-22 K),the morphology changes of the eutectoid layer are as follows:cementite stops growing → slice amplitude increases → regular symmetric growth → oblique growth → layer merge.With the increase of the diffusion coefficient from 3×10^(-13) m^(2)·s^(-1) to 15×10^(-13) m^(2)·s^(-1),the growth rate of the microstructure of the lamellar pearlite increases linearly,and there is no obvious change in the frontal appearance of the pearlite.

Simplified Thermodynamic Model for Pro-Eutectoid Ferrite Formation in Multicomponent Structural Steel

By introducing aparameter of difference in ferrite formation temperature between binary Fe-C and multicomponent system,and referring to the thermodynamic model for Fe-C binary system,a simplified thermodynamic model for pro-eutectoid ferrite formation in Fe-ΣXiC multicomponent structural steels(Xi=Mn,Si,Mo,Cr,Ni or Ti,etc)was suggested.The comparison of the calculated Ae3 temperatures with the measured data of steels 42 shows that the relative standard deviation and root-mean-square(RMS)error between them are only 0.71% and 8.92 K,respectively.However,the deviations between the same measured data and the values calculated from the superelement model are as high as 1.86% and 23.83 K,respectively.It can be concluded that the simplified thermodynamic model for pro-eutectoid ferrite formation in multicomponent structural steels is acceptable and the calculated Ae3 temperatures are in good agreement with the experimental data.

EUTECTOID STRUCTURES IN NICKEL-ALUMINIUM BRONZES

This paper discusses the microstructure of nickel-aluminium bronzes con-taining（9-10）wt% Al and（0.1～5）wt% Ni,in which three eutectoids may exist.Themorphologies of α+γ2 and α+β’2 are similar but the latter is finer.In the α+β’2+γ2eutectoid structure β’2 is mainly concentrated at the α/β boundaries,while γ2 is distrib-uted inside prior B regions.Both K-S and N-W orientation relationships exist betweenα and β’2（or γ2）in alloys consisting of α+β’2 or α+γ2.However,if the alloy consists ofα+β’2+γ2 eutectoid,the lattices of β’2 and γ2 are parallel,both of them have the N-Worientation relationship with α.

Microstructures of Thermomechanically Treated Eutectoid Zn-Al Alloy

A continuously cast eutectoid Zn-Al alloy was extruded at about 250℃ and the complex microstructures of three phases (α. ε and η'E) were observed in the extruded eutectoid Zn-Al alloy.The fcc Al-rich α phase appeared as isolated particles with clear boundaries instead of the α phase occurring with diffuse boundary within the dendrites of the as-cast structure. A new unstable phase η'E was determined to have hexagonal close packed crystal structure. The decomposed β's, phase comprised the matrix. The new phase η'E decompoeed and a four phase transformation α+ε T'+η occurred afterwards during the isothermal holding. The extruded alloy tended to be stable after these two phase transformations. The early shrinkage and the following increase in dimension were related to the decomposition of the new phase η'E and the four phase transformation.

Kinetics Analysis of Pre-Eutectoid Transformation of Austenite in Carbon-Manganese Clean Steel with Rare Earths

By means of rapid cool dilatometry, the influence of rare earths on kinetics of pre-eutectoid transformation of austenite of carbon-manganese clean steel was studied. While the content of rare earths were proper, the nucleation of pre-eutectoid ferrite on the grain boundary of austenite was accelerated and the incubation period of pre-eutectoid transformation was shortened. But while rare earths were excessive, opposite actions appeared. Rare earths decreased transformation velocity constant in the process of transformation from austenite to ferrite, which meant that the growth of per-eutectoid ferrite became slow. The reason above varieties was related to effect of rare earth on grain boundary constitution of austenite and diffusion of carbon element.

Investigation into the Deep Drawability of 0.1%C Eutectoid Steel

The phenomenon of anisotropism in most rolled products necessitates that the rolling direction that enhances desirable mechanical property is established. In this paper, the comparative deep drawability of as-received and annealed mild steel containing about 0.1%C was investigated. The flat steel sample was divided into two and classified as as-received and heat treated respectively. The heat treated sample was obtained by annealing at 950°C after been soaked for 5 hours and deep drawn at ambient temperatures (35°C - 42°C). From both samples, circular specimens were machine- blanked parallel to the rolling directions inclined at 0°, 45° and 90° respectively and were prepared for deep drawability test while rectangular specimens were prepared for tensile test. Both specimens, as-received and annealed were then sub- jected to tensile, cupping and microstructural analyses. Results show that the contribution to increased formability at 90° rolling direction seems to have come from the spheroid-like pearlite grains induced during annealing while the sta- bility of spread observed was achieved through a modest increase in strength. Thus, the resistance of annealed eutectoid steel to cupping is quite minimal at 90° to the rolling direction. The desirable drawability characteristics developed by the annealed eutectoid steel specimen are: cup-height, 30 mm maximum and ear, 6.4% maximum.

Effect of hot-rolling deformation on the microstructure of pearlite of eutectoid steel

The effect of austenite deformation on the pearlite microstructure of eutectoid steel w as investigated by hot uniaxial compression tests using a Gleeble 1500 thermo-mechanical simulator. The results show ed that the deformation of the austenite phase accelerated the transformation of pearlite,leading to a smaller colony size and a smaller interlamellar space of pearlite; meanw hile,the orientation of pearlite lamellae became more heterogeneous,and the lamellar thickness decreased. Larger strain of austenite w as observed to accelerate the spheroidization process of carbides w ithin pearlite colonies.

Effects of deformation parameters on formation of pro-eutectoid cementite in hypereutectoid steels

Brittle pro-eutectoid cementite that forms along prior-austenite in hypereutectoid steels is deleterious to mechanical properties. The optimum process parameters which suppress the formation of pro-eutectoid cementite in hypereutectoid steels with carbon content in the range of 0.8%-1.3% in mass fraction, were investigated. Pro-eutectoid cementite formation is effectively hindered by increasing the deformation temperature and decreasing the amount of strain. Transformation at lower temperatures close to the nose of the cooling-transformation diagram also reduces the tendency of the formation of pro-eutectoid cementite. Control of prior-austenite grain size and grain boundary conditions is important. Due to larger number of nucleation sites, finer prior-austenite grain size results in the acceleration of transformation to pro-eutectoid cementite. However, large prior-austenite and straight boundaries lead to less nucleation sites of pro-eutectoid cementite. The cooling rate and carbon content should be reduced as much as possible. The transformation temperature below 660 °C and the strain of 0.5 at deformation temperature of 850 °C are suggested.

Microstructural evolution of lamellar cementite in eutectoid steels by cold rolling

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. The deformation 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 nm could be deformed plastically.

热轧Q460C板剪切分层原因分析

本文对剪切过程中分层的热轧20mm Q460C板进行了硫印、冷蚀低倍试验,结果显示分层部位存在硫化物的连续、条状偏析带,且偏析带对应部位已开裂。然后采用金相显微技术对硫印、酸洗试验中发现的心部分层开裂部位进行了微观观察和分析,结果显示分层开裂部位的硫化物偏析带对应的硫化物呈网状、密集状态分布,个别硫化物边缘有开裂现象,同时局部析出2~10um网状分布的氮化钛颗粒。4%硝酸酒精侵蚀后,发现网状、密集分部的硫化物、氮化钛颗粒分布于网状铁素体晶粒内,心部开裂部位组织为贝氏体+共析网状铁素体+先共析断网状铁素体,裂纹沿共析网状铁素体和网状硫化物扩展。

Influence of macroscopic graphite particulates on the damping properties of Zn-Al eutectoid alloy

The paper presents in detail the effects of macroscopic graphite (Gr) particulates on the damping behavior of Zn-Al eutectoid alloy (Zn-Al). Macroscopic defects are graphite particulates with sizes of the order of a millimeter (0.5 mm and 1.0 mm). Macroscopic graphite particulate-reinforced Zn-Al eutectoid alloys were prepared by the air pressure infiltration process. The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The internal friction (IF), as well as the relative dynamic modulus, was measured at different frequencies over the temperature range of 20 to 400℃. The damping capacity of the Zn-Al/Gr, with two different volume fractions of macroscopic graphite particulates, was compared with that of bulk Zn-Al eutectoid alloy. The damping capacity of the materials is shown to increase with increasing volume fraction of macroscopic graphite particulates. Two IF peaks were found in the IF-temperature curves. The first is a grain boundary peak, which is associated with the diffusive flux on a boundary between like phases, Al/Al. Its activation energy has been calculated to be 1.13±0.03 eV and the pre-exponential factor is 10?14 s in IF measurements. The second is a phase transition peak, which results from the transformation of Zn-Al eutectoid. In light of internal friction measurements and differential scanning calorimetry (DSC) experiments, its activation energy has been calculated to be 2.36±0.08 eV.

Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.

Atom Probe and Mssbauer Spectroscopy Investigations of Cementite Dissolution in a Cold Drawn Eutectoid Steel

Mssbauer spectrum and three dimensional atom probes（3DAP） were combined to investigate the mechanism of cementite dissolution in a cold-drawn eutectoid steel at a true strain of 2.89.The experimental results suggest that the dislocations play an important role in the dissolution of the cementite by sweeping across the nano-scaled cementite,and transferring carbon from cementite to ferrite inducing cementite decomposition.The mechanism of cementite dissolution in the steel is discussed in association with the investigation of nonstoichiometric cementite structure after heavy deformation.

Effect of dissolved niobium on eutectoid transformation behavior

The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied.Dissolved niobium is important in the eutectoid transformation behavior.It increases the eutectoid carbon content significantly（by^0.0477% per 0.00001% dissolved niobium）,increases the hardenability of steel markedly,yields finer,more uniform,polygonal proeutectoid ferrite,induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal,and increases the misorientation angle of pearlite colonies from being focused near 0°to near 60°.

Al−10Nb−3Ce−2.5B孕育剂对Zn−Al共析合金显微组织、阻尼和拉伸力学性能的影响

制备一种新型Al−10Nb−3Ce−2.5B孕育剂,并系统研究其对锌铝共析(ZA22)合金显微组织、阻尼和拉伸力学性能的影响。结果表明,该孕育剂对ZA22合金中的α相具有显著的细化作用(α相最低可被细化至约16μm),并可使得α相的形貌从粗大枝晶状转变为细小花瓣状。通过边−边匹配晶体学模型(E2EM)建立孕育剂中CeB_(6)和NbB_(2)颗粒与α相之间的位向关系(ORs),并基于该E2EM模型揭示孕育剂对ZA22合金的细化机理。与未细化ZA22合金相比,孕育细化ZA22合金的高温阻尼性能,特别是逆共析转变阻尼峰得到显著提升。此外,孕育细化ZA22合金的室温拉伸力学性能也得到了显著提高,具有最佳细化效果的ZA22合金的抗拉强度和伸长率分别比未细化ZA22合金的高18.56%和119.04%。对相关机理进行了讨论。

Active Eutectoid Decomposition of α → γ + τ _(1) and the Morphological Evolution in a Ru-Containing TiAl Alloy

The active eutectoid decomposition and its morphological evolution during slow cooling and isothermal holding were investigated in a Ru-containing Ti Al alloy. The fine τ 1/γ active eutectoid nodules precipitated at α grain boundary and interior during water quenching. The active eutectoid microstructure evolved from sheaf-like, granular to bulky net-like sluggish eutectoid morphology gradually with the decrease in quenching/holding temperatures. The active eutectoid reaction occurs from 1220 to 1290 ℃, while the beginning temperature of sluggish eutectoid locates at 1305 ± 5 ℃. The combination of the intact τ_(1) phase and immature nano-sized γ laths suggests a short incubation period of τ_(1) phase in the active type. Furtherly, the semiquantitative estimation shows that the growth velocity of active eutectoid is about ninety times higher than sluggish type. In addition, a representative feature of γ phase abruptly ripening along {111} crystallographic plane was also observed in the active eutectoid.