316LN钢的高温本构模型与热加工图

目的 研究316LN钢的高温变形行为,确定最佳加工区间并优化工艺参数。方法 利用Gleeble热模拟实验机在变形温度为1 000～1 150℃、应变速率为0.001～10 s^-1条件下对316LN钢进行热压缩实验。根据实验数据分别绘制不同变形温度和不同应变速率下的流变应力曲线。在传统Arrhenius双曲正弦关系的基础上,考虑应变量的影响,通过五次多项式拟合建立316LN钢的改进型本构模型,基于动态材料模型及Prasad塑性失稳判据计算得到材料的能量耗散图和流变失稳图,将二者叠加得到316LN钢的热加工图。结果 流变应力曲线呈现典型的动态再结晶特征,且随着应变速率的增大和变形温度的升高,316LN钢的压缩应力逐渐减小,耦合应变量的本构模型预测值与实验值的相关系数达0.9888,吻合度较高。通过建立热加工图并对比金相组织发现,316LN钢在“安全区”能量耗散效率较大的区域更容易发生动态再结晶行为。结论 高变形温度、低应变速率条件更有利于软化机制的发生,改进型本构模型精度较高,可对316LN钢热变形过程中的流变应力进行准确预测。通过构建热加工图确定了316LN钢的最佳...     

延性材料全阶段单轴本构关系获取方法

材料拉伸直至破坏的单轴本构关系对于材料与结构的大变形分析和裂纹问题中的变形行为研究有重要意义。该文提出一种将实验与数值分析相结合获取材料拉伸直至颈缩破断全程本构关系的新方法——TF法(tensile test-finite element method)。通过构造初始微小缺陷实现拉伸试样的颈缩有限元模拟;应用自制对中夹具,结合DIC(digital image correlation)方法和应变传感器来测量材料拉伸全程的标距位移、试样颈缩根部直径和颈缩区轮廓线;应用逐步逼近原理并借助开发的ANSYS APDL命令流程序实现材料全程单轴真实本构关系求解的迭代计算。应用TF方法得到的全程材料本构关系对SS316L和T225NG合金拉伸试样的颈缩行为进行有限元模拟分析,结果表明:颈缩区两端的载荷-位移曲线、最小截面直径-位移曲线和颈缩区轮廓线的数值分析结果与实验结果吻合良好;对SS316L小曲率半径漏斗试样的载荷-位移曲线的模拟结果也与实验结果吻合良好。还给出SS316L和T225NG合金两种材料的Ramberg-Osgood全程单轴本构模型参数与等效破断应力应变,讨论两种材料单轴拉伸试样破断时颈缩根部横截面的应力分布。     

2024-T42铝合金低中应变率力学性能及本构关系

为研究2024-T42铝合金的低中应变率力学性能和本构关系,采用电子万能实验机和高速液压伺服材料实验机,进行常温下2024-T42铝合金准静态、低中应变率拉伸实验,得到材料在不同应变率下的应力-应变曲线。考虑颈缩对真实应力-真实应变的影响,采用仿真反演方法对颈缩后的真实应力-真实应变曲线进行修正,并基于Johnson-Cook本构模型进行拟合。结果表明:2024-T42铝合金在低中应变率范围的率敏感性较弱;但具有较强的应变硬化效应;基于仿真分析的反演修正方法能较好重构材料颈缩点后的真实应力-真实应变曲线;并通过铝管压溃实验和仿真分析,验证了反演修正方法的合理性和所获本构模型参数的准确性。     

基于改进Johnson-Cook模型的Mg-Gd-Y-Zn-Zr 合金高温流变行为研究

以Mg-Gd-Y-Zn-Zr合金为研究对象,分别在变形温度范围为250～400℃、应变速率范围为0.001～1 s-1的变形条件下,利用Gleeble-1500热模拟试验机,进行恒温等应变速率的热拉伸实验,研究该合金的高温流变行为.综合考虑温度、应变速率和应变在高温变形过程中的影响,建立了Mg-Gd-Y-Zn-Zr合金改进的Johnson-Cook本构模型.实验结果表明:Mg-Gd-Y-Zn-Zr合金的流变应力与变形温度、应变速率和应变呈非线性关系,应变速率的升高和变形温度的降低均会导致合金的流变应力明显升高.改进Johnson-Cook本构模型的预测数据与实验数据的平均相对误差(Δ)为4.5％,相关度(R)为0.994,所建立的本构模型能够准确地描述Mg-Gd-Y-Zn-Zr合金的高温流变行为.     

超细晶纯铝微塑性变形本构关系的建立

为了研究超细晶纯铝热变形行为,利用等通道挤压工艺制备了超细晶纯铝,并采用INSTRON 5965材料电子万能试验机对超细晶纯铝进行了高温微压缩实验,分析了超细晶纯铝在150~250℃、应变速率3.3!10-4~1!10-1s-1条件下流动应力的变化规律.研究表明,流动应力随试样尺寸的减小呈下降趋势,且变形温度和应变速率对流动应力有显著影响,流动应力随变形温度的升高和应变速率的降低而降低.基于双曲正弦形式修正的Arrhenius关系的本构模型,建立了超细晶纯铝高温微压缩变形本构关系.实验验证表明,所建立的本构关系能够较好地反映超细晶纯铝高温微塑性变形行为特征.     

核电装备用奥氏体不锈钢的高温本构模型及动态再结晶

采用热模拟压缩实验研究核电装备用316L奥氏体不锈钢在变形温度为900～1 100℃、应变速率为0.01～5 s~(-1)时的高温变形行为。根据压缩实验数据绘制流变应力曲线;基于Arrhenius关系并考虑应变量因素,建立耦合应变量因素的改进型本构方程;结合光学显微镜(OM)观察材料变形过程中微观组织的特征;根据加工硬化率-流动应力曲线确定316L不锈钢的动态再结晶临界应变并基于Avrami方程建立其动态再结晶体积分数模型。结果表明:在316L不锈钢热变形过程中,较低的温度和较快的应变速率对应的流变应力也较大;耦合应变量因素的本构模型预测316L不锈钢的流变应力,预测值与实验值的相关系数为0.986 88,平均相对误差仅4.6%,该模型能较好地预测316L不锈钢在热变形过程中的变形抗力。316L不锈钢易在高温、低速的加工条件下发生动态再结晶行为,其动态再结晶体积分数与应变呈S形变化。该模型所得的预测值与实验数据之间的相关性较好,能很好地预测316L不锈钢在热加工过程中发生动态再结晶的体积分数。     

材料模型对1Cr18Ni9Ti管材拉伸有限元仿真的影响

为研究材料模型对有限元模拟1Cr18Ni9Ti管拉伸的影响,将管材单向拉伸试验获取的真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元模拟。经对比分析认为,采用真实应力应变模型的分析结果与实验结果吻合良好,并能正确显示出颈缩发生时刻和颈缩形状;采用指数硬化模型的有限元模拟结果接近真实应力应变模型,颈缩区应力应变分布略显分散;采用线性硬化模型的有限元模拟结果未能显示实际管拉伸后期的局部颈缩形状。     

基于修正的Arrhennius方程钛合金高温本构方程研究

为了更准确地描述钛合金的高温变形行为,对Arrhennius方程进行修正得到钛合金高温本构方程.通过对一种新型钛合金在热模拟试验机上进行恒应变速率等温压缩实验,研究其在700~1 000℃、应变速率0.01~10 s-1条件下的热变形行为,分析了材料的真实应力-真实应变曲线.采用最小二乘拟合的数据回归处理,得到该钛合金在α+β双相区和β单相区的热变形激活能,并通过引入温度变量,获得了Arrhennius方程参数A随温度变化的函数关系,建立了该材料的高温流变应力本构方程.实验结果表明,随着变形增加,流变应力开始急剧增加,随后出现软化并趋于稳态,同时峰值应力对于温度和应变速率具有很强的敏感性.通过在Arrhenius方程中引入温度变量,有利于提高本构方程的准确性.     

基于GA优化的7055铝合金Johnson–Cook流变本构建模及其FEA应用

目的 研究7055铝合金高温流变行为,建立高精度流变本构模型和有限元分析（Finite Element Analysis,FEA）仿真模型。方法 基于Gholamzadeh温度修正模型和Evans摩擦修正模型,计算修正7055铝合金热压缩流动应力,以排除试验过程中变形温升和摩擦对流动应力的影响;针对修正后的流动应力构建Johnson–Cook本构模型,依托MATLAB编程采用遍历法优化模型参考条件,并引入遗传算法（Genetic Algorithm,GA）对模型参数进行优化,通过流动应力子程序二次开发实现优化后的模型在商用软件DEFORM中的应用,以预测变形工件的应力应变分布与成形载荷。结果 经温度和摩擦修正的流动应力与试验值相近;采用遍历法优选参考条件后的Johnson–Cook本构模型流动应力预测值与试验值之间的平均相对误差绝对值（Average Absolute Relative Error,AARE）为4.57%,经GA优化后降至3.50%,实现了精度的提升。基于该模型二次开发的DEFORM模拟平台能准确预测成形载荷,预测值与试验值之间的AARE为2.42%。结论构建了具有较高...     

5083铝合金热压缩流变应力曲线修正与本构方程

在Gleeble-3800热模拟机上采用等温压缩实验研究5083铝合金在变形温度为523~723K、应变速率为0.01~10s-1、真应变为0~0.7条件下的高温流变应力行为。基于热传导对合金变形热效应的影响,对流变应力曲线进行了变形热修正。结果表明:热传导对变形过程中产生的温升影响不可忽略,其影响随着真应变的增加而更加显著;修正后的流变应力对峰值应力影响不大,但稳态流变应力软化趋势得到一定程度的减弱。建立了Zener-Hollomon参数的本构方程,可对5083铝合金在不同变形条件下的流变应力进行预测,温升修正后的流变应力值与本构方程的预测值吻合较好,平均相对误差仅为5.21%。     

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.     

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

Inter-relation between true stress at the onset of necking and true uniform strain in steels – A manifestation of onset to plastic instability

A linear relation is observed between the true stress at the onset of necking and true uniform strain in type 316L(N) austenitic steel and P91 ferritic steel over wide range of test conditions. The observed linear relation between the true stress at the onset of necking and true uniform strain has been rationalised in terms of the onset to plastic instability. The strain regime, over which the alloy deforms by stable plastic flow, is directly proportional to the tensile strength.     

BIAXIAL CYCLIC DEFORMATION BEHAVIOUR OF STEELS

Abstract— The cyclic stress-strain curves for 1% Cr-Mo-V steel and AISI 316 stainless steel were determined under biaxial loading conditions at various temperatures and strain rates. It is shown that these curves may be correlated in terms of the maximum shear stress and strain amplitudes. It is argued that, even though metals obey the von Mises yield criterion for monotonie loading, the micromechanisms of slip which produce the stabilized cyclic stress-strain behaviour are governed by the Tresca criterion.     

Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder

Stainless steel (316) foams of varying porosities have been made through powder metallurgy route using NH₄HCO₃ as a space holder. Green compacts of stainless steel powder with NH₄HCO₃ were sintered at two different temperatures: 1100 °C and 1200 °C. At higher sintering temperatures, neighboring stainless steel powders fused together to form polycrystalline grain structure with iron–chromium intermetallic phases segregated along the grain boundaries. Whereas, the fusion of neighboring stainless steel powders was limited around the particle–particle contact only when the green compacts were sintered at 1100 °C, which resulted in a larger amount of microporosities in the cell wall. These foams exhibited strain hardening behavior in the plateau region under compressive loading. The yield stress and the flow stress (at lower strain levels) of foams, sintered at 1100 °C were higher. But, the reverse is true for the flow stress at higher strain levels. The exponents and the coefficients of the power law relationships varied with sintering temperature and strain levels.     

Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Super304H钢拉伸流变特性的研究

采用电液式万能试验机测试了奥氏体不锈钢Super304H在20～600℃、不同应变率下的流变应力和应变的关系。实验结果表明,应变速率和变形温度的变化强烈地影响Super304H钢的流变应力,在高温下出现明显的动态软化。根据得到的流变应力曲线,拟合出了JC模型和其修正的JC模型中的相关参数。经与实验对比验证,修正的JC本构模型能够很好地描述Super304H钢的动态力学性能,为生产工艺的制定和钢管质量的控制提供了重要的材料参数。     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

High temperature tensile properties of 316LN stainless steel investigated using automated ball indentation technique

Type 316LN stainless steel (SS) is the principal structural material for the components of sodium cooled fast reactors operating under elevated temperature conditions. In order to assess the degradation in strength of service exposed components using a small specimen testing technique such as automated ball indentation (ABI), it is necessary to carry out prior detailed ABI studies on the virgin material. In this investigation, the tensile behaviour of as-received 316LN SS were investigated at several temperatures in the range 298–973 K using ABI technique. The load-depth of indentation data measured from ABI tests was analyzed using semi-empirical relationships to obtain the tensile properties. The yield stress and the flow curves were determined by correlating ABI results with corresponding uniaxial tensile test results. Trend curve for tensile strength with temperature, as estimated from ABI tests, exhibited a plateau region in the temperature around 823 K, similar to uniaxial tensile tests. The variations of strength coefficient, strain hardening exponent, yield ratio, hardness and uniform ductility with temperature were evaluated from ABI tests. The ABI technique was found to estimate the influence of temperature on tensile properties sensitively.