基于相变诱导塑性Satoh试验的有限元模拟

通过将相变诱导塑性应变增量引入到总应变增量的分解式中,建立了温度-组织-应力耦合计算的有限元模型.在此基础上,以9Cr1Mo钢作为研究对象,对其Satoh试验进行了有限元模拟,对温度-组织-应力耦合计算以及传统的热弹塑性计算结果进行了对比分析.结果表明,无论在应力的演变上,还是在残余应力水平上,两种计算得到的结果均存在较大差异.由于忽略相变的影响,热弹塑性计算得到的结果无法反映在相变过程中所发生的应力"松弛"现象,与实际结果偏差较大.准确地对焊接应力应变进行预测,需要通过温度-组织-应力耦合考虑相变的影响.     

焊接热循环和变形历史相关的A7N01-T6本构关系

建立了与焊接热循环温度和热变形历史相关的铝合金本构关系,利用MSC.MARC二次开发接口和Fortran语言,以塑性变形有限元计算增量理论为基础,开发了适用于焊接过程的材料本构关系用户子程序.采用弹塑性(混合硬化)和蠕变性质(应变软化)描述低温应变硬化特征和高温动态回复及再结晶引起的应变软化特征,不同温度的本构关系形式一致而参数不同.结果表明,焊件的残余应力和应变结果与理论结果吻合良好.与采用理想弹塑性本构关系相比,采用新开发的本构关系,高温应变软化和低温应变硬化导致等效残余应力基本不变,纵向残余压缩塑性应变较大,相应的焊接残余变形也较大.     

Ti-6Al-4V基体上等离子喷涂羟基磷灰石涂层的热-力耦合和残余应力

基于热弹塑性理论建立了等离子喷涂羟基磷灰石涂层的热-力耦合模型。一方面,通过引入变形对温度的影响项对经典的Fourier瞬态热传导方程进行了修正,另一方面,在考虑了温度软化效应、应变硬化效应、和应变率强化效应后,采用适用于高温、高应变率条件下的Johnson-Cook模型作为本构方程。基于上述耦合模型,采用有限元方法对涂层内的残余应力场进行了模拟,同时还模拟了基体预热温度以及涂层厚度对残余应力的影响。为了验证数值模拟的可靠性,还采用“材料去除”的实验技术测试了涂层在界面处的残余应力。结果表明：在界面边缘有明显的应力集中;适当提高基体的预热温度有利于减小残余应力;涂层的残余应力随厚度的增加而增大。     

焊接残余应力的温度-组织-应力耦合分析

在应变加和分解式的基础上,通过引入相变应变增量描述模型,采用均匀化方法对多相组织力学行为进行描述,构建可以对温度-组织-应力耦合关系进行表征的模型,并将该模型应用于对9Cr1Mo钢管道环焊缝接头焊接残余应力的分析中.结果表明,在数值模拟中,使用三维有限元模型进行计算,计算结果很好的体现了在焊接过程中马氏体相变随热源移动逐渐进行的过程.在对焊接残余应力的表征上,无论是在残余应力沿管道轴向分布的特征,还是在具体的残余应力水平上,三维有限元计算的结果均与实际测量结果取得了较好的吻合.     

应力应变、疲劳与脆断、断裂力学

基于ADINA软件的焊接结构应力场与温度场研究;焊接残余应力对焊接接头蠕变性能的影响;钛合金焊接热弹塑性应力应变过程全图;低相变点焊条焊接接头残余应力场的数值模拟;双孔微剪切测定铝合金焊接接头的局部本构特性     

Q&P钢热处理过程有限元法数值模拟模型研究

以国内典型淬火-分配(QP)高强钢——QP980钢为例,进行热处理全过程物理模拟研究,提出一种耦合温度及时间影响的类蠕变应变方程用以描述材料在QP热处理分配过程的体积变化,建立考虑淬火温度影响的QP热处理两次淬火过程相变动力学方程、相变应变及相变塑性方程,获得了QP钢各相的热膨胀系数。根据温度场、组织场、应力场三场耦合原理,基于物理模拟得到的弹塑性增量本构模型,对商业有限元软件ABAQUS用户子程序进行二次开发,建立了针对QP热处理全过程的三场耦合数值仿真模型;通过Gleeble热-力模拟试验机上的QP热处理实验对模型进行了实验验证,实验结果与数值模拟结果吻合良好。     

装甲防护厚板淬火过程形状畸变的预测

利用装甲钢的实测物性参数及淬火过程数值模拟平台NSSQ,建立了某型号装甲防护厚板淬火过程的计算模型.该模型考虑了装甲防护厚板的实际淬火工艺,采用1/2对称模型进行简化.材料本构考虑了热变形、相变变形、经典弹塑性变形、相变塑性等效应.厚板水淬过程采用了实测的换热系数.各特征点淬火后变形量的模拟值与实测值基本吻合.对淬火过程变形温度、等效应力、等效应变及变形演化的分析,结果表明,装甲加热阶段的变形主要为均匀胀大变形,冷却阶段由于冷却速度的不均匀性,凹槽的存在对淬火变形的不均匀程度影响显著.     

多晶NiTi形状记忆合金相变的细观力学本构模型

假设NiTi单晶在相变过程中具有层状的微观结构及理想的界面连续条件,推导出各相微观量与宏观量之间的关系,及相变驱动力的表达式,建立了单晶相变的控制方程,从而得到单晶的本构模型.以此为基础,利用Tayloy假设,建立了NiTi多晶的本构模型.通过控制应变进行加载,数值模拟了恒温条件下具有{111}织构的NiTi合金的力学响应,得到的应力-应变曲线与实验结果吻合较好.利用模拟结果讨论了拉伸与压缩的不对称性、软化和温度对NiTi合金变形的影响.     

冷轧辊淬冷过程数值模拟的研究

采用三维非线性有限元分析方法模拟了小型冷轧辊淬火冷却过程,直观地显示出任一时刻轧辊上的瞬成温度场,组织分布和应力场,中以给了轧辊上任意位置的冷却曲线。在所采和的数学模型中,用增量叠代法处理边界条件非线性,物理参数非线性,相为潜热非线性等复杂问题,在相变量计算中引入应力状态的影响,在应力场分析中采用热弹塑性模型,考虑了相变应变,相变塑性应变,热应变,材料力学性能的温度效应和相变影响等因素,计算机模拟     

考虑TRIP效应的0.1C-5Mn中锰钢本构关系研究

通过XRD试验获得了中锰钢板材在单向拉伸状态下奥氏体转变与应变之间的规律,中锰钢中的残余奥氏体体积分数随应变量的增加呈现出非线性下降趋势.由马氏体相变动力学模型得出了中锰钢中的残余奥氏体与等效塑性应变之间的函数关系.根据中锰钢的微观结构特点和细观力学方法,建立了包含TRIP效应的中锰钢本构模型,在该本构模型中考虑了马氏...     

相变对22CrMo钢淬火应力影响的数值模拟

利用有限元法和热弹塑性理论,结合马氏体相变动力学方程,对22CrMo钢圆柱体试样在淬火过程中的温度场、组织场和应力场进行数值模拟分析,重点模拟分析了试样表面和心部在淬火过程中应力的变化及马氏体相变对淬火应力的影响。模拟发现,相变对淬火应力影响比较明显。在一定条件下,在油中淬火,组织应力对淬火应力分布的影响要远大于热应力的影响,最终试样的淬火残余应力将以表层为拉应力而心部为压应力的组织应力形式存在。通过对残余应力的测量结果表明,计算结果与实测值相吻合。     

Q&P钢残留奥氏体含量的热动力学计算

基于CALPHAD方法建立了Q&P钢的配分扩散模型,并建立了一套特定成分在特定QP工艺下的组织转变计算任务流,通过计算QP钢一次淬火过程的马氏体/残留奥氏体含量和配分过程中残留奥氏体的碳富集量,并结合Thermo-Calc软件内置的基于吉布斯自由能的马氏体相变本构模型,预测稳定保留至室温的残留奥氏体含量。利用该模型计算文献钢种(Fe-0.2C-1.28Mn-0.37Si-0.0018B, wt%)的室温残留奥氏体含量,结果显示计算马氏体转变温度比试验数据高60 ℃,计算室温残留奥氏体含量为4.41%,与试验数据基本吻合,从而验证了该计算模型的半定量性。利用该模型进一步计算分析了碳、锰元素含量和热处理制度对AQT980和AQT1180钢一次残留奥氏体含量的影响规律,计算结果显示碳、锰元素含量的增加可使钢中相变点(A₃、Ms、Mf)温度下降;在固定淬火温度下,钢中的碳含量和锰含量增加可使一次残留奥氏体含量大幅增加;当碳、锰元素含量一定时,一次淬火温度的上升会使一次残奥含量大幅增加。     

The mathematical modeling of phase transformation of steel during quenching

In the heat treatment of steel, uneven cooling in variably introduces residual stresses in the workpiece. These residual stresses can combine with the thermomechanical stresses encountered in operation to cause premature fatigue failure of the material. A prediction of the residual and thermoelastoplastic stresses developed during heat treatment would be beneficial for component design. In this article a numerical model is developed to predict the thermoelastoplastic and residual stresses during rapid cooling of a long solid cylinder. The total strains developed during cooling of the cylinder comprise elastic, thermal, and plastic strains and strains due to phase transformation. For plastic deformation an extension of Jiang’s constitutive equations developed by Jahanian is adopted. The properties of the material are assumed to be temperature dependent and characterized by nonlinear strain hardening. For phase transformation two parts are considered: nucleation according to Scheil’s method and phase growth according to Johnson and Mehl’s law. For martensitic transformation, a law established by Koisteinin and Marburger is used. Non-additivity of pearlitic and bainitic nucleation suggested by Manning and Lorig is taken into account by means of a correction factor to Scheil’s summation of the transition from pearlitic to bainitic. The effect of phase transformation and temperature dependence of material properties is investigated. It is shown that by neglecting the temperature dependency and phase transformation in numerical calculations, the results are underestimated. The numerical results are compared with the available experimental data in the literature, and good agreement is observed.     

Finite-element simulation of quenching incorporating improved transformation kinetics in a plain medium-carbon steel

An experimental and numerical investigation was conducted to acquire more precise continuous cooling transformation kinetics for the quenching simulation of a plain medium-carbon steel by improving the conversion model of transformation strains to phase fractions. Previous conversion models have limited applications in plain medium-carbon steels with variable bainite start temperatures (B_s) and martensite start temperatures (M_s), which are cooling rate dependent and cannot be distinguished on dilatometric curves. Therefore, we propose new methods for determining variable B_s and M_s. More accurate diffusive transformation kinetics models were developed based on transformation kinetics data converted from continuous cooling dilatometric curves and considering variable B_s. The martensitic transformation kinetics model was also improved using the quantitative relationship between the variable M_s and the enriched carbon content in residual austenite during diffusive transformations. Finite-element simulation of quenching incorporating the improved transformation kinetics was performed to predict the temperature, microstructure, residual stress and distortion of a plain medium-carbon steel cylinder. The pearlite, bainite, and martensite fractions, axial and hoop stresses near the surface, and the degree of distortion calculated using the improved transformation kinetics exhibited markedly better agreement with the measured results than those calculated using the previous transformation kinetics with B_s and M_s held constant. Furthermore, a comparison of the two simulation results provided a detailed understanding of the effects of transformation kinetics on residual stress and distortion.     

应力与相变相互作用对马氏体淬火残余应力的影响

本文进行了有关钢在马氏体淬火过程中应力与相变相互作用的实验研究，在此基础上提出了一种预测钢的马氏体淬火残余应力的较完整的数学模型。根据实测的材料参数用有限元方法对２６Ｃｒ２Ｎｉ４ＭｏＶ钢圆筒形试件的淬火过程进行了计算分析，着重考察了相变塑性和应力诱导相变对残余应力形成的影响。结果表明，由本模型给出的残余应力预测值与实测值吻合较好，而不考虑相变塑性的模型实际上是不适用的，忽略应力诱导相变则倾向于在较大程度上低估淬火后的残余应力     

Finite Element Analysis of Residual Stress and Distortion in an Eccentric Ring Induced by Quenching

THERE is a pressing need in the metal manufacturingindustries to minimize the amount of rework due tosuch unanticipated effects of quenching as distortionand cracking.Computational modeling offers apowerful tool to predict the component response toquenching.However,physics associated with thequenching process and materials response is verycomplex.There is not yet a complete set of modelsthat can fully incorporate all the relating factorstogether and readily duplicate the experimentalresults.…     

Thermoelastoplastic and Residual Stress Analysis during Induction Hardening of Steel

A theoretical model was developed to predict the thermoelastoplastic and residual stresses developed in a round steel bar during induction hardening. For numerical analysis, a quasi- static, uncoupled thermoelastoplastic solution based on the hyperbolic sine law of Tien and Richmond was formulated. The properties of the material were assumed to be temperature dependent. The phase transformation was considered in the numerical calculation, and the results were compared with the case where phase transformation is avoided. The cylinder was heated rapidly; once the temperature of the outer surface exceeded the transformation temperature, the cylinder was rapidly cooled. Accordingly, in the numerical calculation, only the area at the vicinity of the outer surface was assumed to transform to martensite. The results showed that the compressive residual stresses at the vicinity of the outer surface were considerably higher than the tensile stresses at the center.     

The application of rational approximation in the calculation of a temperature field with a non-linear surface heat-transfer coefficient during quenching for 42CrMo steel cylinder

The calculation of a temperature field has a great influence upon the analysis of thermal stresses and stains during quenching. In this paper, a 42CrMo steel cylinder was used an example for investigation. From the TTT diagram of the 42CrMo steel, the CCT diagram was simulated by mathematical transformation, and the volume fraction of phase constituents was calculated. The thermal physical properties were treated as functions of temperature and the volume fraction of phase constituents. The rational approximation was applied to the finite element method. The temperature field with phase transformation and non-linear surface heat-transfer coefficients was calculated using this technique, which can effectively avoid oscillationin the numerical solution for a small time step. The experimental results of the temperature field calculation coincide with the numerical solutions.     

Transient thermal stress analysis on rapid post-weld heat treatments applied to flash butt welded rails

Abstract

A finite element (FE) model has been established to estimate the transient thermal stresses developed during rapid post-weld heat treatments (RPWHT) and post-weld quenching (PWQ) of rail flash butt welds. Rapid post-weld heat treatment parameters, such as heat flux intensity, location and heating duration, were varied. Altering the PWQ initiation times was also studied. The sequentially coupled thermomechanical FE model incorporated rate independent plasticity with a constitutive linear kinematic hardening rule. This described the inelastic behaviour of the rail material caused by post-weld thermal cyclic loadings. Representative temperature dependent thermal and mechanical properties including the pearlitic transformation behaviour of rail steel were also used in the FE model. Significant reduction of local tensile residual stresses in the web region of the weld was obtained by RPWHT applied to the entire foot region. RPWHT applied to both the web and the foot regions of the weld resulted in further reductions in vertical tensile residual stresses but a lesser effect on the longitudinal tensile residual stresses. A series of PWQ conditions, which were initiated after completion of the austenite–pearlite phase transformation, had significantly affected residual stresses. Numerical predictions revealed that PWQ initiated after transformation induced higher tensile residual stresses in the web region of the weld with respect to normal cooled welds, without affecting the microstructure.     

基于ANSYS软件的T8钢淬火过程三维温度场的模拟

用有限元软件ANSYS模拟计算了T8钢圆柱体水淬过程中三维温度场的变化.计算时综合考虑了非线性的材料热物性参数、界面换热系数及相变潜热的影响.通过对计算硬度与实测淬火试样硬度分布的比较,表明模拟结果与实际情况较为符合.