核压力容器SA508Gr4钢焊接热影响区微区转变及残余应力分布

采用新型熔池热源分层加载数学模型对SA508Gr4钢双面单道次焊接的温度场和应力场进行计算及试验验证,分析了焊接热影响区各微区转变过程及残余应力分布情况。结果表明,温度场模拟值与试验值基本吻合,性能不稳定的完全淬火区宽度为1560μm,占热影响区宽度的77.6%;首道焊缝两侧20 mm处出现600 MPa高横向拉伸残余应力区,第二道次高横向残余应力区出现在完全淬火区附近,而对于不完全淬火区组织形成压缩塑性变形趋势,X形坡口第二道焊接热作用可以抵消小部分首道焊的横向收缩。而两者纵向残余应力均以拉应力为主。     

基于材料非均匀性铝厚板预拉伸残余应力的预测方法

引入离散化思想将铝厚板划分为若干层,依据淬火残余应力定义各层的等效塑性应变;通过各层材料应力-应变曲线的线性近似后,根据等效塑性应变推导出误差不超过2%的屈服极限分布模型。此外,根据弹性模量随塑性变形进展而不断变化的实验数据,拟合出弹性模量的变化曲线。以淬火7075铝厚板的预拉伸为例,考虑淬火所引起的屈服极限与弹性模量的非均匀性,分别进行预拉伸率为1.8%、2.2%和2.5%的残余应力有限元仿真,无论是分布曲线还是应力的大小,残余应力仿真值与实验测量值均比较吻合,分布曲线拐点处残余应力的平均误差仅为13.5%。     

对流换热系数对熨平板焊接变形和应力的影响

采用有限元模拟不同对流换热系数下熨平板的焊后变形和应力分布,并采用塞尺和X射线法进行试验验证。结果表明:整体变形趋势为U形板呈"鼓泡"状,左连接板伸出端向x正向收缩变形,两侧板自由边向内收缩变形。最大变形位置出现在连接板伸出端最上端,对流换热系数为采用曲线加载时,变形误差为34.3%,系数为0.02时,误差为17.1%;不同对流散热系数对残余应力峰值影响很小,应力分布与对流换热系数为0.02时趋势一致,误差为32.6%,采用曲线加载,不考虑低应力区结果,误差为21.1%,均满足工程应用要求,证明了模拟结果的准确性。两种加载方式均满足焊接变形预测要求,在焊缝较多、应力分布复杂的情况下,应力预测建议采用曲线加载方式,反之则采用系数为0.02加载。     

基于H13钢水淬在线监测的热处理数值模拟优化

为准确预测厚度较厚的H13钢热处理过程中的温度场和淬火残余应力,对规格为φ300 mm×350mm的H13钢工件进行淬火实时温度监测,分别设置了位于芯部、亚表面(离外表面10 mm)、1/3R(离圆心50 mm)和2/3R(离圆心100 mm)处的监测点。考虑钢/水对流换热系数受沸腾条件和热辐射的影响,对对流换热系数h进行了优化。结果表明各个温度监测点的试验结果与计算结果相符,证明经过优化后的对流换热系数可应用于大截面尺寸工件温度场的预测。模拟工件淬火后的应力场并结合显微组织观察发现,淬火在750 s前后出现芯部、表面最大的拉压峰值应力差,证明大尺寸工件淬火存在淬火危险期。     

A357铝合金大型复杂薄壁构件的淬火温度场、残余应力及变形的有限元分析(英文)

基于ABAQUS软件,采用有限元模拟计算的方法对A357铝合金大型复杂薄壁构件的淬火过程进行研究。通过采用传统的反传热方法,对不同淬火介质在不同温度下的换热系数进行精确求解。精确的换热系数确保对A357铝合金大型复杂薄壁构件淬火过程中温度场预测的准确性。采用3种淬火介质(水、机油,5%-UCON淬火试剂A)。通过综合考虑淬火介质及温度因素,对薄壁构件的残余应力及变形的分布和大小进行有限元预测,得到构件淬火结束后的最大残余应力及变形。     

轴孔超声冲击强化及有限元模拟研究

目的提高渗碳淬火轴使用寿命。方法针对渗碳淬火轴关键部位孔口,采用超声冲击处理工艺对其进行表面强化处理,利用电子显微镜、硬度计结合有限元模拟对处理后的微观组织、硬度、残余应力等进行分析。结果在孔口处理区,材料产生了明显的加工硬化,形成了梯度硬化层,硬化层厚度达到80μm,表层硬度约60HRC。对超声冲击孔孔口部分的表面残余应力值进行测量,最小残余应力值为-374 MPa,最大残余应力值为-530 MPa,采用上述超声冲击处理后,样品的残余应力平均值在-450 MPa。利用有限元模拟了孔口附近沿轴向深度的残余应力分布,其残余压应力层深约1.4 mm,最大残余应力-891 MPa,疲劳危险点处的残余应力平均值约-760 MPa。轴孔边缘第三主应力基本上沿轴线方向。结论通过超声冲击处理工艺对渗碳淬火轴孔口进行处理后,在孔角处形成硬化层,同时产生残余压应力,上述处理后可有效降低工作应力造成的疲劳载荷幅。     

胀形对2219铝合金环形件淬火残余应力的影响

利用ABAQUS有限元软件对2219铝合金环形件淬火过程及后续胀形过程中径向截面上残余应力的分布进行数值模拟,研究了胀形量对淬火残余应力消减率的影响。结果表明:淬火后环件的径向应力在上下端面呈压应力,在-106 MPa左右,在芯部应力值较小;周向应力和轴向应力均呈外部压应力、芯部拉应力的应力分布状态。胀形对环件的径向应力、周向应力和轴向应力均有显著的消减效果。胀形量在0.25%~4.00%范围内增加时,环形件的残余应力减小,周向应力和轴向应力的消减率都增大并趋于定值,且周向应力消减率的增大速度比轴向应力消减率的增大速度快。当胀形量<3%时,周向应力的消减率大于轴向应力的消减率,当胀形量>3%时,周向应力的消减率略小于轴向应力的消减率;当胀形量>2%时,周向应力和轴向应力的平均消减率>80%,环件的淬火残余应力得到很好的消减。     

中厚板辊式淬火换热系数的确定方法

为了确定中厚板辊式淬火过程中的最佳换热系数,进行了12MnNiVR钢板淬火过程中的温度场和应力场计算.依据温度场分析提出了中厚板在淬火机高压区内淬火过程中换热系数的确定方法,并用该方法确定了不同厚度12MnNiVR钢板在高压区淬火的换热系数.应力场分析表明,在低压区采用低的换热系数,可显著降低钢板淬火过程中产生的热应力及残余应力.20mm厚12MnNiVR钢板低压区淬火,平均换热系数确定为1 kW/(m2·K),与采用8 kW/(m2·K)相比,钢板表面残余压应力与中心残余拉应力的差值可降低181.2 MPa.     

组织不均匀性对TA15钛合金电子束焊焊接接头热影响区应变集中的影响

通过对TA15钛合金电子束焊焊接接头热影响区进行纳米压痕试验和小试样拉伸试验,结合基于组织的有限元模拟,研究了热影响区中α相和α'相不均匀性对热影响区应变集中的影响. 结果表明,α'相的屈服强度和加工硬化系数均高于α相,α'相是热影响区中较“硬”的组织,α相是热影响区中较“软”的组织,两种组织具有不同的力学性能,因此热影响区组织具有不均匀性. 组织不均匀性导致其细观应力应变分布不均匀,α'相承受较高的应力而α相有较大的应变,从而引起局部塑性变形的不协调,最终形成沿α相的塑性应变集中带.     

胀形法消减铝合金环形件淬火残余应力研究

针对铝合金淬火残余应力突出的问题,研究了消减铝合金环形件淬火残余应力的方法即胀形法。该方法通过液压胀形机对环形件施加胀形力,使得环形件产生拉伸的效果。采用ABAQUS有限元软件对2219铝合金环形件淬火残余应力及胀形法进行有限元模拟。进行胀形法消减铝合金环形件淬火残余应力工艺试验,采用压痕应变法对环形件的残余应力进行测试。研究结果表明:该环形件淬火残余应力很大,最大值超过300MPa;胀形法可有效消减淬火残余应力,当胀形量为3%时,残余应力幅值降至40MPa以内;试验结果与模拟结果一致。     

40Cr钢喷雾淬火换热系数的计算及数值模拟

为计算ф25 mm×100 mm 40Cr圆柱试件喷雾淬火冷却过程的换热系数,采用四通道采样系统测定了喷雾淬火过程的冷却曲线,并用反传热法中的非线性估算法计算出换热系数。计算结果表明,喷雾淬火过程分3个阶段:膜沸腾阶段、核沸腾阶段和对流换热阶段,并在冷却到120℃时,换热系数达到峰值9800 W·m-2·℃-1。采用此换热系数作边界条件,对40Cr钢的喷雾淬火过程进行了数值模拟,得到淬火过程中不同时刻的温度场、组织场、硬度场和应力场。     

Study on inhomogeneous cooling behavior of extruded profile with unequal and large thicknesses during quenching using thermo-mechanical coupling model

The interfacial heat transfer coefficient between hot profile surface and cooling water was determined by using inverse heat conduction model combined with end quenching experiment. Then, a Deform-3D thermo-mechanical coupling model for simulating the on-line water quenching of extruded profile with unequal and large thicknesses was developed. The temperature field, residual stress field and distortion of profile during quenching were investigated systematically. The results show that heat transfer coefficient increases as water flow rate increases. The peak heat transfer coefficient with higher water flow rates appears at lower interface temperatures. The temperature distribution across the cross-section of profile during quenching is severe nonuniform and the maximum temperature difference is 300 °C at quenching time of 3.49 s. The temperature difference through the thickness of different parts of profile first increases sharply to a maximum value, and then gradually decreases. The temperature gradient increases obviously with the increase of thickness of parts. After quenching, there exist large residual stresses on the inner side of joints of profile and the two ends of part with thickness of 10 mm. The profile presents a twisting-type distortion across the cross-section under non-uniform cooling and the maximum twisting angle during quenching is 2.78°.     

无相变合金淬火热应力演变机理的理论模型--(1)角端和边缘淬火热应力模型与淬火角端效应

淬火过程中率先引发塑性变形的动力是拉应力,此后拉应力和压应力交替引发塑性变形.拉应力可以诱发压应力的出现.在淬火试样表层可以出现压应力,这不同于传统观点认为淬火表层只能是拉应力的结论.淬火热应力与温度高低关系不大,主要由相对温差(即温度梯度)决定,相对温度高的部分会趋向发展成压应力状态,相对温度低的部分趋向发展成拉应力状态,这是淬火和快速加热产生热应力、塑性变形和残余应力的根本原因.建立了角端、边缘两种淬火热应力模型,提出了淬火角端效应,即在角端拉应力率先引发塑性变形并引起温度梯度、热应力、塑性变形等交替衰减的现象.     

Simulation of temperature and stress in 6061 aluminum alloy during online quenching process

The cooling curves of 6061 aluminum alloy were acquired through water quenching experiment. The heat transfer coefficient was accurately calculated based on the cooling curves and the law of cooling. The online quenching process of complex cross-section profile was dynamically simulated by the ABAQUS software. The results suggest that the heat transfer coefficient changes during online quenching process. Different parts of the profile have different cooling velocity, and it was verified by water quenching experiment. The maximum residual stress of the profile was predicted using FEM simulation based on ABAQUS software. The relations between the temperature and stress were presented by analyzing the data of key points.     

Optimum design of the heat-transfer coefficient during gas quenching using the response surface method

The heat-transfer coefficient during gas quenching is optimized to obtain minimum distortion with stress and hardness distribution requirements satisfied. During the gas-quenching process, the heat-transfer coefficient has a significant influence on the properties of the final product, such as the hardness, strength, residual stress and distortion. Significant distortions after quenching will increase the cost due to post-manufacturing processes such as grinding and hot straightening. Therefore, minimizing the distortion will benefit in most gas-quenching cases. A step function is used to represent the heat-transfer coefficient in terms of the quenching time. Five coefficients of the step function are used as the design variables to minimize the distortion. Two design constraints are used to satisfy the maximum principal stress and the average surface hardness requirements of the final product. The finite element package DEFORM is used to predict the material responses during the quenching process. Phase transformations, deformation and heat transfer are integrated during the simulation process. The response surface method is used to obtain the analytical models of the objective function and constraints in terms of the design variables. Once the closed-form equations of the objective and constraints are obtained, a design optimization tool is used to search the optimum design point. This paper summarizes the methodology that is used to optimize a plane strain example.     

铝合金模锻件热处理过程的热力耦合分析

采用有限元法对一个铝合金模锻件的沸水淬火和时效处理过程进行了热力耦合分析 ,研究了淬火过程中锻件与沸水之间的换热系数对淬火变形的影响、淬火过程中工件内应力的变化和水温对锻件残余应力的影响。对于铝合金锻件热处理过程的有关材料和工艺参数以及热处理结果进行了实验检测 ,并采用数值方法进行了处理。本文的研究对于控制和改进铝合金锻件热处理工艺具有一定的指导意义。     

Using simulation for heat treatment process design: matching quenching process with steel grade and product geometry

Abstract

The performance of steel parts is heavily dependent on the heat treatment process applied. The alloy content of the steel establishes the steel hardenability. The severity of the quenching establishes the local temperature history throughout the body of the part. In combination, the steel hardenability and the quenching process determine the final microstructure, mechanical properties, residual stress state and the performance of the part. The residual stress state, especially the surface stress state, is a significant factor in affecting fatigue life of the part. The steel hardenability and quenching practice can be adjusted to enhance residual surface compression and improve the fatigue life of a component. Computer simulation of the heat treatment process that includes calculation of the metallurgical phase transformations during the heating and cooling processes offers a method for scientifically designing the heat treatment process and selecting the steel alloy to optimise the performance of a particular product. In this paper, the DANTE heat treatment simulation software will be used to demonstrate this design methodology for a spur gear.     

基于模拟的热处理工艺设计:使淬火工艺与钢种和产品几何形状相匹配

钢铁零件的性能主要决定于所采用的热处理工艺。钢的淬透性决定于合金元素含量,而淬火烈度确定了整个零件中不同部位的温度随时间变化过程。总的来说,零件最终的组织、力学性能、残余应力状态和服役性能均取决于钢的淬透性和淬火工艺。残余应力状态,特别是表面应力状态,是影响零件疲劳寿命的重要因素,可通过控制钢的淬透性和淬火工艺来提高零件的表面压应力,从而提高疲劳寿命。热处理工艺的计算机模拟,包括加热和冷却过程中的相变的计算,提供了一种科学地进行热处理工艺设计、钢种选择以优化某一特定产品性能的方法。本文采用热处理模拟软件DANTE来论证这种设计方法在正齿轮上的应用。     

高温喷丸强化Ti6Al4V合金的热力耦合数值模拟

发展一种连接喷丸强化与高温加载传热的热力耦合有限元方法,模拟高温喷丸强化Ti6Al4V合金的过程。首先建立一圆盘模型模拟待喷材料的高温加载传热过程。然后将圆盘模型的受热部分取出一小块建立对称胞元喷丸模型,并且将高温加载所致的温度场和热应力场通过解析场的方式导入到对称胞元喷丸模型,模拟高温喷丸强化过程。最后通过回弹计算获得稳定的残余应力场和温度场。创建4种模拟工况：常温喷丸、单独导入温度场的喷丸、单独导入热应力场的喷丸和高温喷丸,探究高温喷丸的残余压应力强化机理。结果表明：在常温环境下,对称胞元喷丸模型模拟的Ti6Al4V表层残余应力与试验结果具有很好的一致性。在高温加载作用下,随着热流密度的增加,受喷材料表层残余压应力有所减小,材料亚表层的残余压应力逐渐增大。影响高温喷丸强化的残余压应力的主要因素是高温加载所致的温度场,热应力场对残余压应力强化起次要作用。