Hot Deformation Behavior of an Ultra-High-Strength Fe–Ni–Co-Based Maraging Steel

Hot processing behavior of an ultra-high-strength Fe–Ni–Co-based maraging steel was studied in temperature range of 900–1200 °C and strain rate range of 0.001–10 s~(-1). Deformation processing parameters and optimum hot working window were characterized via flow stress analysis, constitutive equation construction, hot processing map calculation and microstructure evolution, respectively. Critical strain value for dynamic recrystallization was determined through theoretical mathematical differential method: the inflection point of θ–σ and -αθ/ασ-σ curves. It was found that the flow stress increased with the decrease in deformation temperature and increase in the strain rate. The power dissipation maps in the strain range of 0.1–0.6 were entirely similar with the tendency of contour lines which implied that strain had no strong effect on the dissipation maps. Nevertheless, the instability maps showed obvious strain sensitivity with increasing strain, which was ascribed to the flow localization and instability. The optimized hot processing window of the experimental steel was obtained as 1100–1200 °C/0.001–1 s~(-1) and 1000–1100 °C/0.001–0.1 s~(-1), with the efficiency range of 20–40%. Owing to high Mo content in the experimental steel, high dynamic activation energy, Q = 439.311 kJ mol~(-1), was achieved, indicating that dynamic recrystallization was difficult to occur in the hot deformation process, which was proved via microstructure analysis under different hot deformation conditions.     

Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850–1050 ℃ and strain rates of 0.05–10 s~(-1).The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization(DRX).The flow behavior was characterized by significant flow softening at deformation temperatures of 950–1050 ℃ and lower strain rates of 0.05–5 s~(-1), which was attributed to heating during deformation, DRX and flow instability.A step-by-step calculating procedure for constitutive equations is proposed.The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel.Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium Mn steel should be carried out at 1050 ℃, and the strain rate of 0.05–10 s~(-1) resulted in significantly recrystallized microstructures in the in steel.The flow localization is mainly flow instability mechanism for experimental steel.     

2507超级双相不锈钢的流变应力本构关系及热加工图

采用Gleeble-3800热模拟试验机研究了热变形温度为950~1200 ℃、应变速率为 0.01~10 s^-1条件下2507超级双相不锈钢的热压缩变形行为,并借助光学显微镜观察了不同变形过程中的微观组织演化。基于试验数据分析,建立2507超级双相不锈钢的流变应力本构关系及热加工图。结果表明:流变应力随着变形温度的升高和应变速率的降低而逐渐降低,在高应变速率下,流变曲线出现“类屈服平台”。试验钢的热变形激活能为414.57 kJ·mol^-1,应力指数为4.18,峰值应力本构方程为ε·=3.69×10¹⁵[sinh(0.0101σ)]^4.18exp-414.57RT,根据微观组织分析及热加工图确定出试验钢的最佳热加工区域为热压缩温度1060~1120 ℃,应变速率0.01~0.1 s^-1。     

Hot Deformation Behavior of a New Nuclear Use Reduced Activation Ferritic/Martensitic Steel

The hot deformation behavior and workability of a new reduced activation ferritic/martensitic steel named SIMP steel for accelerator-driven system were studied.The flow curve and its microstructure were studied at 900-1200℃ and strain rate range of 0.001-10 s~(-1).The results showed that the deformation behavior of the SIMP steel during hot compression could be manifested by the Zener-Hollomon parameter in an exponent-type equation.Based on the obtained constitutive equation,the calculated flow stresses were in agreement with the experimentally measured ones,and the average activity energies QDRV and QHW for the initiation of dynamic recrystallization and the peak strain were calculated to be 476.1 kJ/mol and 462.7 kJ/mol,respectively.Furthermore,based on the processing maps and microstructure evolution,the optimum processing condition for the SIMP steel was determined to be 1050-1200 ℃/0.001-0.1 s~(-1).     

Hot Deformation Behavior and Dynamic Recrystallization Characteristics in a Low-Alloy High-Strength Ni–Cr–Mo–V Steel

This study presented a quantitative investigation of deformation behavior and dynamic recrystallization of low-alloy highstrength Ni–Cr–Mo–V steels during hot deformation.A series of isothermal compression experiments were performed at temperatures ranging from 800 to 1200°C and strain rates from 0.01 to 10 s~(-1)with a height reduction of 60%.A complete Arrhenius constitutive model and processing maps were developed.The results showed that the constitutive model had the ability to predict the flow stress with an average absolute relative error of\5.7%.The processing maps constructed at strains of 0.2,0.4,and 0.8 showed that flow instability was prone to occur at higher strain.Dynamic recrystallization tended to take place at higher temperatures(900–1200°C)and lower strain rates(0.01–1 s~(-1)).The critical strain for the onset of dynamic recrystallization was determined,and a kinetics model was developed.The predicted results for recrystallization volume fraction and flow stress were compared with the experimental data,which indicated that the model was accurate and reliable.     

基于摩擦-温度双修正的Maraging250钢热变形行为及热加工图

采用Gleeble-3800热模拟试验机,通过热压缩试验研究了变形温度900~1200 ℃、应变速率0.001~10.0 s^-1时,Maraging250钢的热变形行为,综合考虑摩擦效应和变形热效应,对流变应力曲线进行摩擦修正和温度修正,建立双修正条件下的Maraging250钢本构方程和热加工图,并针对真应变为1.2的热加工图分析了试验钢在不同变形条件下的微观组织变化。结果表明,在相同试验条件下,变形温度降低或应变速率升高,摩擦效应对试验钢流变应力影响越显著;变形热仅在低温、高应变速率条件下对流变应力有显著影响。由变形热引起的最大温升约80 ℃、流变应力最大变化约20 MPa。利用双修正的流变应力曲线计算出试验钢的热变形激活能为393.552 02 kJ/mol,并建立了Z参数方程和本构方程,绘制了真应变ε=0.4、0.8和1.2的热加工图。结合微观组织分析,Maraging250钢在1000～1125 ℃、0.001～1.0 s^-1范围内能获得均匀细小的动态再结晶组织,具有较佳的热加工性能。     

X12CrMoWVNbN钢的热变形行为及热加工图

利用Gleeble3180热模拟试验机,在变形温度为950~1100 ℃,应变速率为0.001~1 s^-1,真应变为0.7的条件下,对X12CrMoWVNbN钢进行了高温单向热压缩试验。通过不同条件下的高温流变曲线分析了变形温度和应变速率对试验钢热变形力学行为的影响。以Arrhenius方程为本构模型,建立了能够预测该钢流动应力的本构方程。基于动态材料模型和试验参数、结果,绘制了该钢不同应变量下的热加工图并结合图进行了组织分析。结果表明,流变峰值应力和稳态应力随温度降低或应变速率升高而升高;功率耗散系数随应变速率降低和变形温度的升高而增大;最优热加工区域功率耗散系数η的值都在0.4以上,且这些区域的变形组织晶粒均匀细小;0.3、0.4、0.5和0.6应变下的最优热加工区域都处于变形温度1050~1100 ℃、应变速率0.001~0.003 s^-1的范围。     

022Cr钢的热变形行为及热加工图

用Gleeble 3180热模拟试验机对022Cr钢的热变形行为进行研究,揭示了变形抗力与变形程度、变形温度和应变速率的关系。在950~1200 ℃温度范围和应变速率为0.001~5 s^-1下进行热压缩,并利用动态材料模型(DMM)建立了022Cr钢热变形的工艺图。结果表明,随着变形温度的升高和应变速率的降低,022Cr钢的流动应力降低。根据流动应力曲线数据计算其变形激活能为381.615 kJ/mol。当应变不小于0.5时,022Cr钢热加工的最佳变形条件有两个区域,第一个区域在温度范围1100~1200 ℃,应变速率范围0.001~0.01 s^-1内,第二个区域在温度范围1130~1180 ℃,应变速率范围1~5 s^-1内,其功耗效率都能达到0.4以上。     

13Cr超级马氏体不锈钢热压缩变形行为与组织演变

通过Gleeble-3500热模拟试验机对13Cr超级马氏体不锈钢进行单道次压缩变形试验,系统研究变形温度在950~1150 ℃、应变速率为0.001~10 s^-1条件下的热变形行为。利用双曲正弦模型建立了13Cr超级马氏体不锈钢的流变应力本构方程,求得试验钢的热变形激活能为412 kJ/mol,并基于动态材料模型(DMM)理论绘制了材料的热加工图,得出材料的最佳热变形工艺参数窗口为:变形温度1032~1072 ℃,应变速率0.039~0.087 s^-1。组织演变结果表明,试验钢在高变形温度和低应变速率的条件下,容易发生动态再结晶。当应变速率一定时(0.01 s^-1),变形温度从950 ℃升到1050 ℃,动态再结晶的体积分数从18.7%升高到60.1%,组织的再结晶程度提高,晶粒均匀细小;当变形温度一定时(1050 ℃),随着应变速率的降低,动态再结晶的晶粒长大粗化。     

Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel

The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel(2205-Cu DSS) were investigated at temperatures of 950-1150℃and strain rates of 0.01-10 s~(-1).The effects of Cu addition and different deformation parameters on deformation behavior were,respectively,characterized by analyzing flow curves,constitutive equations and microstructures.The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases.When deformed at low strain rate,DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX.At high strain rate,flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite.The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain,strain rate and deformation temperature.The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150℃and strain rate range of 0.1-1 s~(-1)with a peak power dissipation efficiency of 41%.Flow localization was the main way to lead to flow instability.Meanwhile,the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000℃.The interaction between dislocations and Cu-rich precipitates at high strain rate,as well as the limited DRV in ferrite and DRX in austenite,contributed to the complex microstructure and flow behavior.     

Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr–24Ni–6Mo Super-Austenitic Stainless Steel During Hot Deformation

Hot compression tests were carried out with specimens of 20 Cr–24 Ni–6 Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s~(-1) in the temperature range from 950 to 1150 °C, and flow behavior was analyzed. Microstructure analysis indicated that dynamic recrystallization(DRX) behavior was more sensitive to the temperature than strain rate, and full DRX was obtained when the specimen deformed at 1150 °C. When the temperature reduced to 1050 °C, full DRX was completed at the highest strain rate 10 s~(-1) rather than at the lowest strain rate 0.01 s~(-1) because the adiabatic heating was pronounced at higher strain rate. In addition, flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation, especially at higher strain rates and lower temperatures. Therefore, flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation. The results showed that adiabatic heating became greater with the increase of strain level, strain rate and the decrease of temperature, while the frictional effect cannot be neglected at high strain level. Moreover, based on the revised flow curves, strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data. The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves, even though some difference existed at 950 °C and0.01 s~(-1). The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior, eventually resulted in dramatic increase of deformation resistance.     

Hot Working Characteristic of Superaustenitic Stainless Steel 254SMO

Hot deformation characteristic of superaustenitic stainless steel 254SMO has been studied by isothermal compression testing in the temperature range of 950–1,200 °C and strain rate range of 0.01–10 s-1.The activation energy of 496 kJ/mol was calculated by a hyperbolic-sine type equation over the entire range of strain rates and temperatures.In order to obtain optimum hot working conditions,processing maps consisting of power dissipation map and instability map were constructed at different strains.The power dissipation map exhibits two domains with relatively high efficiencies of power dissipation.The first domain occurs in the temperature range of 990–1,070 °C and the strain rate range of0.01–0.1 s-1.Microstructure observation in this domain indicates the partial dynamic recrystallization(DRX) accompanied with precipitation of tetragonal sigma phase.The second domain occurs in the temperature range of 1,140–1,200 °C and the strain rate range of 0.01–1 s-1with a peak efficiency of power dissipation of 39%,and in this domain,the microstructure observation reveals the full DRX.The instability map shows that flow instability occurs at the temperatures below 1,140 °C and the strain rates above 0.1 s-1.     

Hot Deformation Behavior and Hardness of a CoCrFeMnNi High-Entropy Alloy with High Content of Carbon

A CoCrFeMnNi high-entropy alloy with a high content of carbon was synthesized, and its hot deformation behavior was studied at the temperatures 800–1000 ℃ at the strain rates ranging from 0.001 to 0.1 s~(-1).As-prepared alloy is a face-centered cubic-structured solid solution, with a large amount of carbides residing at grain boundaries.True stress–strain curves were employed to develop the constitutive equation of apparent activation energy.The apparent activation energy( Q) was found to be 423 kJ mol~(-1), indicating a dynamic flow softening behavior.The size of dynamic recrystallized(DRXed) grains increases with increasing the temperature or decreasing the strain rate.A processing map was sketched on the basis of the flow stress.The temperature range of 900–1000 ℃ and 10~(-3)–10~(-2.6) s~(-1) strain rate were found to be the optimum hot-forging parameter.With increasing temperature or decreasing strain rate, the volume fraction of fine carbides(≤ 1 μm) increases.A lot of coarse carbides can be found in the matrix after deformation at 800 ℃, which leads to a high hardness value of 345 HV.The carbides after deformation at 1000 ℃ are mainly nano-sized M_7C_3 and M_(23)C_6, which can promote the nucleation of DRX.     

Influence of Microstructural Evolution on the Hot Deformation Behavior of an Fe–Mn–Al Duplex Lightweight Steel

The hot deformation behavior of Fe–26 Mn–6.2 Al–0.05 C steel was studied by experimental hot compression tests in the temperature range of 800–1050 °C and strain rate range of 0.01–30 s21 on a Gleeble-3500 thermal simulation machine. The microstructural evolution during the corresponding thermal process was observed in situ by confocal laser scanning microscopy. Electron backscattered diffraction and transmission electron microscopy analyses were carried out to observe the microstructural morphology before and after the hot deformation. Furthermore, interrupted compression tests were conducted to correlate the microstructural characteristics and softening mechanisms at different deformation stages.The results showed that hot compression tests of this steel were all carried out on a duplex matrix composed of austenite and d-ferrite. As the deformation temperature increased from 800 to 1050 °C, the volume fraction of austenite decreased from 70.9% to 44.0%, while that of d-ferrite increased from 29.1% to 56.0%. Due to the different stress exponents(n) and apparent activation energies(Q), the generated strain was mostly accommodated by d-ferrite at the commencement of deformation, and then both dynamic recovery and dynamic recrystallization occurred earlier in d-ferrite than in austenite.This interaction of strain partitioning and unsynchronized softening behavior caused an abnormal hot deformation behavior profile in the Fe–Mn–Al duplex steel, such as yield-like behavior, peculiar work-hardening behavior, and dynamic softening behavior, which are influenced by not only temperature and strain rate but also by microstructural evolution.     

Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

Hot deformation behavior of the Cu–Cr–Zr alloy was investigated using hot compressive tests in the temperature range of 650–850 °C and strain rate range of 0.001–10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800–850 °C and under0.001–0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu–Cr–Zr alloy are in the temperature range of 800–850 °C and the strain rate range of 0.001–0.1 s-1.     

17Cr2Ni2MoVNb和20Cr2Ni4A齿轮钢的动态再结晶行为及热加工图

利用Gleeble-3800热模拟试验机得到17Cr2Ni2MoVNb和20Cr2Ni4A齿轮钢在1000~1150 ℃、0.01~10 s^-1的流变应力曲线,构建了两种钢的动态再结晶Avrami动力学模型和热加工图。结果表明,两种钢在高变形温度、低应变速率下易发生动态再结晶。17Cr2Ni2MoVNb钢中较高的Nb和Mo含量对动态再结晶的抑制作用大于20Cr2Ni4A钢中的高Ni含量的影响,导致在相同的热变形条件下17Cr2Ni2MoVNb钢的动态再结晶体积分数小于20Cr2Ni4A钢。17Cr2Ni2MoVNb钢的最佳热加工工艺参数为:温度为1050~1150 ℃、应变速率为0.1~0.6 s^-1;20Cr2Ni4A钢的最佳加工参数为:温度为1100~1150 ℃、应变速率为3.3~5.5 s^-1。     

316LN高温热变形行为与热加工图研究

通过Gleeble热模拟实验机在1000~1200℃,应变速率为0.01~10 s~(-1)条件下的近等温热模拟压缩实验,建立了316LN双曲正弦的流动应力预测模型及其热加工图。该流动应力预测模型考虑了实验过程中塑性变形和摩擦引起的温升,对流动应力进行了修正,考虑应变对流动应力预测模型参数的影响,获得了统一流动应力预测模型,模型预测值与实验值的相关系数为0.992,平均相对误差为4.43%;热加工图基于Prasad动态材料模型分别获得了不同应变速率、温度条件下的能量耗散率和失稳系数;分析了应变量、温度和应变速率对于能量耗散率和失稳系数的影响。结果表明:实验条件下最大能量耗散率值为0.38,且高应变速率下失稳,并通过显微组织分析对热加工图进行了验证。     

30CrNi3MoV钢的热变形行为及热加工图

采用Gleeble-3500热模拟试验机对30CrNi3MoV钢进行单向热压缩试验,研究了其在变形温度950~1150 ℃、应变速率0.01~10 s^-1的热变形行为,构建了应变补偿型流变应力本构方程,并绘制出该钢的热加工图。结果表明,30CrNi3MoV钢真应力-真应变曲线有3种不同特征:高温小应变速率时,表现为典型的动态再结晶过程;低温小应变速率时,曲线为动态回复特征;应变速率较大时,应力随应变的增大而增大,无明显的峰值应力。采用5次多项式拟合构建的应变耦合流变应力本构方程具有高的精确度,采用该方程获得的预测值与试验值的平均相对误差为3.2%,相关性系数R值为0.993。从热加工图中得到试验钢最佳的热加工工艺参数范围是:变形温度为1020~1150 ℃、应变速率为0.03~0.35 s^-1。     

Characterization of Hot Deformation Behavior of a Novel Al–Cu–Li Alloy Using Processing Maps

The isothermally compression deformation behavior of an elevated Cu/Li weight ratio Al–Cu–Li alloy was investigated under various deformation conditions.The isothermal compression tests were carried out in a temperature range from 300 to 500 °C and at a strain rate range from 0.001 to 10 s-1.The results show that the peak stress level decreases with temperature increasing and strain rate decreasing,which is represented by the Zener–Hollomon parameter Z in the hyperbolic sine equation with the hot deformation activation energy of 218.5 k J/mol.At low Z value,the dynamic recrystallized grain is well formed with clean high-angle boundaries.At high Z value,a high dislocation density with poorly developed cellularity and considerable fine dynamic precipitates are observed.Based on the experimental data and dynamic material model,the processing maps at strain of 0.3,0.5 and 0.7 were developed to demonstrate the hot workability of the alloy.The results show that the main softening mechanism at high Z value is precipitate coarsening and dynamic recovery;the dynamic recrystallization of the alloy can be easily observed as ln Z B 29.44,with peak efficiency of power dissipation of around 70%.At strains of 0.3,0.5 and 0.7,the flow instability domains are found at higher strain rates,which mainly locate at the upper part of processing maps.In addition,when the strain rate is 0.001 or 0.02 s-1,there is a particular instability domain at 300–350 °C.     

Hot Deformation Behavior and Workability of(SiC_p + Mg_2B_2O_(5w))/6061 Al Hybrid and SiC_p/6061 Al Composites

The hot deformation behavior of(3 vol%SiC_p + 3 vol%Mg_2B_2O_(5w))/6061 Al(W_3P_3) hybrid composite and6 vol%SiCp/6061 Al(P_6) composite have been characterized in the temperature range of 300-450 ℃ and strain rate range of 0.0001-0.1 s~(-1) using isothermal constant true strain rate tests.The flow behavior and processing maps have been investigated using the corrected data to eliminate the effect of friction.Under the same deformation conditions,the compressive resistance of the singular composite remains superior to that of the hybrid composites.The processing map of W_3P_3 hybrid composite exhibits a single hot working domain at the temperature between 350 and 450 ℃ with strain rate between 0.0001 and 0.003 s~(-1)(domain A).Two hot working domains exist for P_6 composite:(i) 300-400 ℃/0.0001-0.003 s~(-1)(domain Bl);(ii) 380-450 ℃/0.01-0.1 s~(-1)(domain B2).The processing maps also reveal the flow instability of the two composites,which is associated with whiskers breakage,whisker/matrix interfacial debonding,SiCp/matrix interfacial decohesion,adiabatic shear bands or flow localization,and wedge cracking in the corresponding regions.The estimated apparent activation energies are about 224 kJ mol~(-1) in domain A for W3P3 hybrid composite,177 kJ mol~(-1) in domain Bl and 263 kJ mol~(-1) in domain B2 for P_6 composite,respectively.These values are higher than that for self-diffusion in Al(142 kJ mol~(-1)),suggesting that there is a significant contribution from the back stress caused by the presence of particles and/or whiskers in the matrix.The deformation mechanisms corresponding to domain Bl and domain B2 are dislocation climb controlled creep and cross-slip for P_6 composite,respectively.For W_3P_3 hybrid composite,the deformation mechanisms contain dislocation climb controlled creep and grain boundary sliding caused by DRX in domain A.