一种高强度微合金钢的高温流变和软化行为

利用Gleeble 2000热模拟试验机对一种屈服强度为800MPa的高强度微合金钢的高温变形行为进行了研究,对不同变形条件下的流变应力－应变关系进行了分析,并利用Roberts提出的思想,用扩展了的蠕变方程建立了实验钢的流变应力模型,对不同变形条件下的流变应力进行了预测利用双道次变形试验,得出了试验钢在高温变形保持时间的软化率曲线,并对曲线进行了分析。     

用蠕变方程建立高强微合金钢的流变应力预测模型

利用Gleeble 2000热模拟试验机对一种屈服强度为700MPa级的微合金钢的热变形行为进行了试验研究。分析了不同变形条件下的流变应力与应变的关系，并根据Roberts提出的用蠕变方程预测不同应变对应的流变应力的方法，用扩展的蠕变方程建立了试验钢的流变应力模型。用建立的模型预测了不同变形条件下的流变应力，预测结果与试验结果基本吻合。     

0.07C-0.85Mn-0.16S-0.05Bi钢的热变形行为

采用MMS-200热力模拟试验机,在变形温度950 ～1200℃以及变形速率0.01～10 s-1条件下对0.07C-0.85Mn-0.16S-0.05Bi钢进行一系列热压缩实验.结果 表明,实验钢的流变应力曲线呈现明显的动态再结晶特征,并且流变应力随变形温度的提升或者应变速率的下降而降低.根据不同变形条件下的峰值应力,由Arrhenius模型构建了峰值应力下的本构方程,计算实验钢热变形激活能Q并基于动态材料模型绘制真应变为0.1、0.3、0.5、0.7的热加工图.研究分析了实验钢在不同应变下的失稳区域和合理热加工区域,随着应变的增大,失稳区均出现在高速率变形区,且由低温高速率区向高温高速率区转变.最佳热加工参数为变形温度1020～1200℃、变形速率0.01～0.3 s-1.     

0.07C-0.85Mn-0.16S-0.05Bi钢的热变形行为

采用MMS-200热力模拟试验机,在变形温度950 ～1200℃以及变形速率0.01～10 s-1条件下对0.07C-0.85Mn-0.16S-0.05Bi钢进行一系列热压缩实验.结果 表明,实验钢的流变应力曲线呈现明显的动态再结晶特征,并且流变应力随变形温度的提升或者应变速率的下降而降低.根据不同变形条件下的峰值应力,由Arrhenius模型构建了峰值应力下的本构方程,计算实验钢热变形激活能Q并基于动态材料模型绘制真应变为0.1、0.3、0.5、0.7的热加工图.研究分析了实验钢在不同应变下的失稳区域和合理热加工区域,随着应变的增大,失稳区均出现在高速率变形区,且由低温高速率区向高温高速率区转变.最佳热加工参数为变形温度1020～1200℃、变形速率0.01～0.3 s-1.     

T91钢的高温塑性变形及动态再结晶行为

采用Gleeble-1500热模拟试验机进行了T91钢的压缩试验,研究了变形温度为1100~1250℃、应变速率为0.01~1 s-1时该钢的变形行为,分析了流变应力与应变速率和变形温度之间的关系,计算了高温变形时应力指数和变形激活能,并采用Zener-Hollomon参数法构建该钢高温塑性变形的本构关系,绘制了动态再结晶图和热加工图.结果表明:在试验变形条件范围内,其真应力-真应变曲线呈双峰特征;钢中发生了明显的动态再结晶,且再结晶类型属于连续动态再结晶.T91钢的热变形激活能为484 kJ.mol-1,利用加工图确定了热变形的流变失稳区,结合力学性能,可以优先选择的变形温度为1200~1 250℃,应变速率不高于0.1 s-1.      

0.07C-0.85Mn-0.16S-0.05Bi钢的热变形行为

采用MMS-200热力模拟试验机,在变形温度950 ～1200℃以及变形速率0.01～10 s-1条件下对0.07C-0.85Mn-0.16S-0.05Bi钢进行一系列热压缩实验.结果 表明,实验钢的流变应力曲线呈现明显的动态再结晶特征,并且流变应力随变形温度的提升或者应变速率的下降而降低.根据不同变形条件下的峰值应力,由Arrhenius模型构建了峰值应力下的本构方程,计算实验钢热变形激活能Q并基于动态材料模型绘制真应变为0.1、0.3、0.5、0.7的热加工图.研究分析了实验钢在不同应变下的失稳区域和合理热加工区域,随着应变的增大,失稳区均出现在高速率变形区,且由低温高速率区向高温高速率区转变.最佳热加工参数为变形温度1020～1200℃、变形速率0.01～0.3 s-1.     

900 MPa级析出强化钢高温变形行为

 针对900 MPa级析出强化型热轧高强钢,利用Gleeble-3800热模拟试验机研究其在变形温度为950～1 150 ℃、变形速率为0.1～10 s-1条件下的压缩变形行为。根据应力-应变曲线图获得峰值应力,并用双曲正弦方程描述热压缩变形过程中的试验钢峰值应力与Zener-Hollomon参数的关系。回归分析得到方程中变形激活能及其他材料变形参数,并对试验在高温条件下的流变应力本构方程并对其进行了验证。结果表明,采用该本构方程计算出的流变应力值与试验所得应力值非常接近,为估算成形时所需的最大载荷及设备选取提供参考。     

SAE9310钢热变形行为的研究

利用Gleeble-3500热力模拟试验机,在温度为1123~1423 K,应变速率为0.1~10 s-1,真应变为0.8的条件下,对一种传动部件用高强度渗碳钢(SAE9310钢)进行了高温轴向压缩试验,测得了SAE9310钢的高温流变曲线,并观察其变形后的显微组织。试验结果表明,SAE9310钢的流变应力和峰值应变随着变形温度的升高和应变速率的降低而减小;SAE9310钢在真应变为0.8的条件下,随着变形速率的提高,其发生完全动态再结晶的温度也逐渐升高,当热变形温度高于1323 K时,应变速率在0.1~10 s-1范围内,试验钢均会发生动态完全再结晶;测得9310钢的热变形激活能Q值为416.78 kJ/mol,并确立了其热变形方程。     

二次硬化超高强度钢的热变形行为

利用Gleeble-3500热力模拟试验机,在温度为1123～1423K,应变速率为0.5～10s-1的条件下,对航空用高强韧性的二次硬化超高强度钢(AF1410钢)进行了高温轴向压缩试验,测得了AF1410钢的高温流变曲线,并观察了变形后的显微组织。试验结果表明,AF1410钢的流变应力和峰值应变随着变形温度的升高和应变速率的降低而减小;AF1410钢在真应变为0.8,应变速率为0.5～10s-1的条件下,随着变形速率的提高,其发生完全动态再结晶的温度也逐渐升高。当变形速率为10s-1时,其变形温度高于1373K,才会发生完全动态再结晶;AF1410钢的热变形激活能Q值为430.39kJ/mol,并确立了其热变形方程。     

氮含量对0Cr16Ni5Mo马氏体不锈钢高温热变形行为影响

采用Gleeble3800试验机对三种不同N含量的0Cr16Ni5Mo马氏体不锈钢进行等温热压缩实验.通过对真应变-应力曲线及压缩后变形组织的观察,发现相同热变形条件下,N含量的增加提高了试验钢的流变应力,抑制了再结晶晶粒长大.采用Zener-Hollomon参数,以流变应力方程为基础,构建三种本构模型.通过观察拟合应力值与实验值的离散性,确定双曲正弦模型更适用于本试验钢的本构方程计算,优化此计算模型,获得了三种试验钢的本构方程.      

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10^-5 s^-1 and 1 s^-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.     

Hot workability of three grades of tool steel

Three tool steels, a cold-work air-hardening grade, a hot-work die grade, and a high-speed type, were deformed by torsion in the range of 900 to 1100 °C at rates of 0.1 to 5 s^•1. In a series of continuous deformation tests the flow stress and ductility were determined. The exponent of the flow stress was proportional to the strain rate and to the temperature in a reciprocal Arrhenius relationship. In general the flow stress for a given deformation condition, the activation energy, and the strain for the start of dynamic recrystallization increased for the steels in the order listed above; however, the ductility of the hot-work grade is superior to the other two grades. Multistage tests were carried out on each steel to determine its softening behavior during intervals between passes. Each test was carried out under isothermal conditions with constant strain rate, pass strain, and interval duration. Softening occurred by both recovery and recrystallization with the amount increasing with temperature, strain rate, pass strain, and accumulated strain. The first two steels were similar in behavior having extensive softening at 1000 °C, whereas the high-speed steel experienced considerably less softening.     

Influence of nonsteady state behavior on superplastic deformation of a 25.7 Cr-6.6 Ni stainless steel

Superplastic behavior of a microduplex stainless steel (IN744) was studied by deforming specimens with initially elongated grains. As a result of deformation, grain growth and breaking up of elongated grains were noticed to occur concurrently. These microstructural instabilities result in strain hardening or strain softening, respectively. This then leads to nonuniqueness in the flow behavior. Some examples of nonuniqueness in stress-strain rate relation include the presence of: strength anisotropy, increase in strain rate sensitivity, constant stress level for varying grain size, and two distinct stress-strain rate curves over comparable test condition for the same initial microstructure. These have been explained by the concomitant microstructural changes and the influence of such changes on instantaneous flow stress. Emphasis is made on the need to eliminate microstructural instability in order to characterize the deformation phenomena under steady state.     

Breakdown Behavior of Eutectic Carbide in High speed Steel during Hot Compression

 The evolution of eutectic carbide in as-cast M2 high-speed steel on different deforming conditions was investigated. Initially, specimens were deformed by hot compression in the range of 1223 to 1398K at strain rates of 0.01 to 1.0s-1. In a series of continuous deformation tests the flow stress curves were determined for the downstream process simulation. Subsequently, metallographic examination was carried out before and after deformed specimens for carbides analysis. As-cast microstructure is characterized as a spherical matrix of austenite and a continuous network of rodlike or irregular eutectic carbide, whereas deformed microstructure has broken carbide network and smaller granular products on all experimental conditions. It suggests that eutectic carbides fracture to particles during hot deformation by thermomechanical disintegration, while diffusion-controlled phase transformation was not remarkable. Combination with numerical simulation, the relationship between breakdown ratio of carbide network and deforming parameters were concluded. Strain was the most important one to shatter eutectic carbides and disperse products. Furthermore, critical strain values were obtained, beyond which carbide network disappeared, and fractured carbides kept a stable profile and deformed with matrix coordinately. A higher temperature or lower strain rate resulted in a lower critical strain.     

变形奥氏体静态软化行为的研究

利用MMS-100热力模拟试验机研究了X60管线钢和JB700低碳贝氏体钢在不同变形温度、变形程度和变形后等温保持时间对奥氏体静态软化行为的影响。随变形温度、道次停留时间及变形量的增加，道次间隔时间内的静态软化量也增加，奥氏体再结晶越容易发生。     

基于热加工图的中碳钢加工性能分析

??Taking LZ50 steel as deformation material?? the stress- strain curves during hot compression was analyzed. The mathematical model of peak stress and strain?? critical stress and strain?? steady stress and strain as well as the stress and strain at which material exhibited maximum flow softening were established by liner regression method. The hot processing map of LZ50 steel under different strains was plotted to predict microstructure evolution behavior during forging process in order to guide production and processing. The results indicate that work hardening rate increases when temperature decreases or strain rate increases. The hot processing map of Murty criterion is optimum to predict the microstructure evolution of LZ50 steel during hot forming by comparing three different instability criterion??s hot processing maps??Prasad criterion?? Murty criterion and Poletti criterion??. The zone of high temperature and high strain rate hasn??t obvious microstructure defects?? so it??s ??false instability range??. The most optimum range for LZ50 steel deformation is zone of medium temperature and medium strain rate?? such as 1020?? and 0. 5s-1?? where the structure is homogeneous and the grain keeps equiaxed after deformation.     

含Nb-Ti微合金汽车用钢高温变形组织及力学性能的研究

研究了含Nb-Ti微合金汽车用钢高温变形时的软化行为和晶粒析出，对析出物的尺寸和分布进行了电镜分析。通过单道次和双道次压下模拟实验，分析了试验钢在不同温度和不同变形条件下的流动应力及显微组织。这些研究为制定最佳控轧工艺，得到细化奥氏体晶粒，获得良好的组织、性能打下了理论基础。     

Effect of Strain Rate on the Subsequent Softening and Precipitation Kinetics in a Nb‐Microalloyed Steel

The influence of strain rate on the subsequent softening kinetics has been investigated for a Nb‐containing steel using stress relaxation. It was found that strain induced precipitation during static recrystallisation was enhanced with an increase in strain rate, resulting in an obvious plateau in the stress relaxation curve and a marked retarded softening. At the higher strain rate, since dynamic precipitation is suppressed during deformation, more solute Nb is left for the subsequent strain induced precipitation after deformation. The stress relaxation data were supported by grain size measurements and TEM observations. A significant influence of strain rate on the recrystallised grain size was not observed for longer annealing times after deformation, indicating that strain induced precipitation lost its retarding effect due to coarsening of precipitates. Clearly, care should be taken when extrapolating softening kinetics measured after deformation at low strain rates to softening kinetics at industrial rolling conditions.     

7085铝合金的高温压缩流变应力及软化行为

对均匀化炉冷态7085铝合金进行高温压缩实验,研究该合金在变形温度为350~450℃、变形速率为0.001~0.1 s 1和应变量为0~0.6条件下的流变应力及软化行为。结果表明:流变应力在变形初期随着应变的增加而迅速增大,出现峰值后逐渐软化进入稳态流变;随着变形温度的升高和应变速率的降低,峰值流变应力降低。采用包含Zener-Hollomon参数的Arrhenius双曲正弦关系描述合金的流变行为。分析和建立了应变量与本构方程参数(激活能、应力指数和结构因子)的关系,研究发现本构方程参数随应变量的增加而减少。合金的流变行为差异与动态回复再结晶和第二相粒子相关。