Effect of Hot Deformation on Pearlite Transformation of 86CrMoV7 Steel

The continuous cooling transformation (CCT) diagrams of 86CrMoV7 steel samples including hot deformed and not hot deformed were constructed by dilatometry, metallography and transmission electron microscopy (TEM). The results showed that hot deformation accelerated pearlite transformation and fine pearlite microstructure. Moreover, the undissolved carbides became the nucleating sites of pearlite, accelerated pearlite formation and fine pearlite if the steel had been deformed at high temperature. In contrast, undissolved carbides did not make any influence on pearlite transformation if the steel had not been deformed at high temperature.     

热变形参数对TA15合金组织的影响及其相变动力学

采用Gleeble-1500热模拟机单道次热压缩变形实验和元胞自动机方法观察模拟了TA15在变形温度为750~1100℃、应变速率为0.01~1s-1范围内的微观组织变化,研究了不同变形参数对等温热压缩过程中微观组织和性能的影响;根据元胞自动机模拟结果,计算其相变动力学。热压缩实验结果表明,该合金属于温度敏感型和应变速率敏感型合金,随着温度的升高或应变速率的降低,β相含量增加;元胞自动机模拟结果表明,相变动力学Avrami指数n在2.0~3.0之间。     

热变形对超纯净管线钢组织的影响

研究了成分和热变形对三种低碳微合金管线钢的连续冷却转变(CCT曲线)和组织的影响．结果表明：在含碳量为0．025％的低碳微合金钢中加入0．3％的Mo能推迟铁素体、珠光体转变，扩大针状铁素体(Acicular fenite)形成的冷却速度范围；高碳含量使针状铁素体向板条铁素体(Lath ferrite)转化．热变形使针状铁素体的形成温度区间从400～500℃扩大到450～700℃，显著加速相变过程，使CCT曲线明显向左上方移动，获得针状铁素体的临界冷却速度增加，抑制板条铁素体的形成，有利于获得细的针状铁素体组织，并细化岛状组织，但对残余奥氏体量影响不大。     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

Influence of austenitising temperature on kinetics of phase transformations in medium carbon TRIP steel

Abstract

Comprehensive, dilatometric studies of the kinetic transformations of undercooled austenite in medium carbon TRansformation Induced Plasticity (TRIP) steel during cooling from the temperature range of 750–1030°C were carried out. Selection of austenitising temperatures was based on the knowledge of the critical temperatures. The influence of the austenitising temperature on the formation ranges of new phases and microstructural constituents during continuous cooling was determined. It was also shown that the annealing of the investigated steel at the lowest temperature resulted in the strongest lowering of Ms temperature as well as in the lowering hardenability. Based on the analysis of the stated relationships, modifications of the annealing, adapted to the investigated steel chemical composition, were proposed.     

Constitutive modeling for flow stress of 55SiMnMo bainite steel at hot working conditions

The hot deformation behavior of 55SiMnMo bainite steel was studied through isothermal hot compression tests conducted using a Gleeble 3500 at 950–1100 °C, with strain rates of 0.01 s⁻¹ to 10 s⁻¹. A constitutive equation was established using the experimental results to describe the stress–strain relationship based on the dislocation density variation, considering the influence of the dynamic softening mechanism. When dynamic recovery is the only softening mechanism, a constitutive equation for flow stress was obtained from the variation of the dislocation density during hot deformation based on work hardening and dynamic recovery. When dynamic recrystallization occurs, the relationship between the dislocation density and the volume fraction of dynamic recrystallization was used to predict the flow stress after the peak. The reliability of the model was verified through a comparison between the predicted flow stress curves from the model and the experimental data.     

In situ synchrotron X-ray study of bainite transformation kinetics in a low-carbon Si-containing steel

The bainite transformation in a low-carbon Si-containing steel has been studied in situ by synchrotron X-rays. While the austenite is homogeneous prior to transformation, the carbon distribution becomes nonuniform as bainite plates form. This is because of the different degrees of physical isolation of films and blocks of residual austenite. The method for converting dilatational strain into bainite volume fraction, using lattice strain as a reference, during isothermal transformation was found to overestimate it. The bainitic and martensitic ferrite did not exhibit a tetragonal unit cell due to the low-carbon content of the steel and the high transformation temperature.     

奥氏体变形对22CrSH齿轮钢连续冷却相变的影响

为了研究奥氏体变形对22CrSH齿轮钢连续冷却相变的影响,在Gleeble 1500热模拟机上,将22CrSH钢在950℃变形0.4及未变形处理,然后连续冷却.利用膨胀曲线、光学显微镜、透射显微镜,结合各种腐蚀方法,分析了22CrSH钢相变行为及相变组织.研究表明:奥氏体变形使多边形铁素体加珠光体混合组织的临界冷速增大;当奥氏体变形及降低冷速时,大量的晶界仿晶型铁素体占据了奥氏体晶界,贝氏体相变向针状铁素体相变转变;变形使奥氏体在中温相变区稳定性增加,室温组织中M/A岛的数量增多.     

Constitutive modeling for hot deformation behavior of T24 ferritic steel

Hot compression tests of T24 ferritic steel were carried out using Gleeble-3500 thermo mechanical simulator in the temperature range of 1323-1473 K with the strain rate of 0.01-10 s⁻¹ and the height reduction of 60%. The flow behavior of T24 ferritic steel was characterized based on analysis of the true stress-strain curves. Constitutive equations incorporating the effects of temperature, strain rate and strain have been developed to model the hot deformation behavior of T24 ferritic steel. Material constants α, n, ln A and activation energy Q in the constitutive equations were calculated as a function of strain. The flow stress values of T24 ferritic steel predicted by the proposed constitutive equations show a good agreement with experimental results, which indicated that the developed constitutive equations could give an accurate and precise prediction for the flow stress of T24 ferritic steel.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

V-Ti钢热变形奥氏体的连续冷却转变行为

为研究V—Ti微合金钢热变形奥氏体的连续冷却转变行为,在对V—Ti徽合金钷进行Gleeble3800热模拟后,建立了连续冷却转变曲线（CCT图）,探讨了变形量和高温停留对CCT曲线和相变组织的影响．研究表明：变形量的增大,促进了先共析铁素体和珠光体转变,使其孕育期缩短,一定程度上也促进了高冷速下的贝氏体转变,但使低冷速下的贝氏体转变受阻;马氏体转变温度的降低说明变形量的增大在一定程度上使马氏体相变受阻;变形后若高温停留,组织发生静态回复,对扩散型相变的先共析铁素体和珠光体转变以及半扩散型相变的贝氏体转变均不利,使先共析铁素体转变量和珠光体转变量都有相对减少,而对马氏体转变影响不大．     

Finite element analysis and experimental study of microstructure evolution of nitrogen alloyed ultralow carbon stainless steel during hot deformation

Abstract

Hot compression experiments of a nitrogen alloyed ultralow carbon stainless steel were performed in the temperature range of 1223–1423 K, at strain rates of 0.001–1 s^?1, and with deformation amounts of 30–70% on a Gleeble-3500 thermal-simulator. Based on the results from thermo-physical simulation experiments and metallographic analyses, a physically-based constitutive model and a dynamic recrystallisation (DRX) model of the studied steel were derived, and the developed models were further embedded into a finite element method (FEM) software. The microstructure evolution of the studied steel under various hot deformation conditions was simulated by FEM, and the effects of deformation amount, strain rate and temperature on the microstructure evolution were clarified. The results obtained from the finite element analysis were verified by the experiments. The finding confirms that the thermal-mechanical FEM coupled with the developed constitutive model and DRX model can be used to accurately predict the microstructure evolution of the studied steel during hot deformation.     

On the polar reciprocity model for hot deformation characteristics of materials

Studies on the models of hot deformation of materials indicate that there is a linear relationship between the dissipation efficiency parameter of the dynamic materials model, and the intrinsic hot workability parameter of the polar reciprocity model, and both models need a stability criterion for delineating the regimes of flow instabilities in the processing maps.     

Forming behavior and workability of Hastelloy X superalloy during hot deformation

The hot deformation behavior of Hastelloy X superalloy has been characterized using hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during hot working. Hot workability of this alloy has been analyzed by calculating flow localization parameter and construction of workability map for occurrence of shear band. In addition, on the basis of flow stress data obtained as a function of temperature and strain rate in compression, power dissipation map and instability map for hot working have been developed.     

Study on plastic deformation behavior of hot splitting spinning of TA15 titanium alloy

The plastic deformation behavior of hot splitting spinning of TA15 titanium alloy is a complex metal forming problem with multi-factor coupling interactive effects. In this paper, on condition of considering various thermal effects, a three-dimensional (3D) elastic–plastic coupled thermo-mechanical finite element (FE) model of hot splitting spinning of TA15 titanium alloy is established using the dynamic, temp-disp, explicit module of FE software ABAQUS. Based on the analysis of flow behaviors of TA15 titanium alloy, the mechanism and influence of materials plastic deformation behavior during the forming process are studied. The results show that, the flow stress of TA15 titanium alloy generally decreases with the increase of deformation temperature; at the same strain rate, the higher temperature is, the lower flow stress is. The temperature distributions along the circumferential direction of disk blank are even and the temperature of plastic deformation area is about 984 °C. The heat from plastic deformation and friction at local plastic deformation area is less than the dissipated heat, so the temperature just falls into approximately 945 °C. Radial spinning force as the driving force of plastic deformation increases gradually and reaches about 35 kN at the end. The maximum value of equivalent stress is presented in fillet part between disk blank and two mandrels. The distributions of equivalent plastic strain appear the large strain gradients and the obvious characteristics of inhomogeneous deformation. When friction factor on interfaces between disk blank and dies ranges from 0.4 to 0.6, the forming quality and precision are highest.     

Phase transformation and microstructure evolution of pearlite heat-resistant steel during heating

ABSTRACT

The phase transformation and microstructure evolution of pearlite heat-resistant steel during heating were observed with an ultra-high temperature confocal scanning laser microscope. The α-ferrite completely disappeared earlier than Fe₃C during the formation of γ-austenite, which is inconsistent with the fact that the Fe₃C should disappear completely earlier under equilibrium conditions. After the Fe₃C?+?α→γ transformation, static recrystallisation of γ-austenite occurred, accompanied by the dissolution of cementite. During the γ→δ transformation, the δ-cell first precipitated at the triple point of the γ-austenite grain boundaries, and then the δ-cell platelet with one tip appeared in the γ-austenite grain. The law of δ/γ inter-phase boundaries was analysed based on inter-phase boundary types and element diffusion.     

Effect of molybdenum on continuous cooling bainite transformation of low-carbon microalloyed steel

Through simulation of thermomechanical processing/on-line accelerated cooling processing and observation of microstructure, the effect of molybdenum on continuous cooling bainite transformation of ultra-low carbon microalloyed steel was studied. The continuous cooling transformation curves of the trial steels with or without molybdenum addition were also determined. The result showed that the separate temperature of bainite was obviously reduced and the size of microstructure became smaller as 0.40 wt% Mo was added to the steel. At the same time, the martensitic structure, which formed at some cooling conditions, became finer and dispersed more uniformly. The deformed austenite would transform to finer bainite even when the cooling rate was not too high.     

Recrystallization of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation. Part I: Dynamic recrystallization

This paper presents an investigation that characterizes the evolution of the dynamically recrystallized structure of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation, as a starting point for studies of the static recrystallization (SRX) and the metadynamic recrystallization (MDRX) behaviors, by hot compression tests which are performed at the temperatures from 1243 K to 1543 K and strain rates from 0.001 s⁻¹ to 0.1 s⁻¹ on Gleeble-3500 thermo-mechanical simulator, and the corresponding flow curves are obtained. A third-order polynomial is then fitted to the work hardening region of each curve. The critical stress for initiation of dynamic recrystallization (DRX) can be calculated by setting the second derivative of the third order polynomial. By regression analysis, the activation energy in whole range of deformation temperature is determined to be Q = 368.45 kJ/mol. The complete DRX grain size (D_drx) of the test steel is a function of Zener-Hollomon parameter (Z) and is independent of the true strain. The relationship of D_drx and Z is found to be described in a form of power law function with an exponent of −0.24.     

变形温度对贝氏体钢组织性能的影响

为合理制定Si-Mn-MO系无碳化物贝氏体钢的生产工艺,利用GLEEBLE-3800热模拟试验机,在真空条件下开展了变形温度对贝氏体钢组织性能影响的热模拟试验.利用光学显微镜、透射电镜等设备,采用力学性能测试、微观组织观察等技术分析手段,对热模拟试样进行了组织观察和硬度检测分析,绘制了Si-Mn-MO系无碳化物贝氏体钢不同变形温度的动态CCT曲线,得出了变形温度对其组织和硬度的影响规律.结果表明,变形温度越低,无碳化物贝氏体钢的相变温度越低,组织越细小,先析铁素体越易析出,越有利于提高贝氏体钢的强硬性和韧塑性.