奥氏体不锈钢应变强化工艺及性能研究

针对奥氏体不锈钢延性好但屈服强度低的问题,提出采用应变强化工艺来提高材料屈服强度。分析应变强化工艺中两个关键工艺参数——应变速度和应变量对材料力学行为的影响,指出应变速度不宜过慢,否则会出现锯齿形屈服行为,对材料性能造成不利影响。经应变强化后的奥氏体不锈钢在显著提高强度的同时,仍能保持较好的韧性。通过金相组织分析、马氏体体积分数测定等结果表明,将应变量控制在10%以下,强化后奥氏体组织仅发生少量的α′马氏体相变,对材料的力学性能影响不大,且材料的微观组织也没有明显变化。研究结果表明,采用应变强化技术在大幅提高奥氏体不锈钢屈服强度的同时,对材料的其他力学性能均不造成大的影响,从而为压力容器的安全运行提供有力保证,可实现压力容器的轻型化设计,经济和社会效益显著,应用前景广阔。     

奥氏体不锈钢压力容器的应变强化技术

奥氏体不锈钢材料本身具有良好的韧性,但它的屈服强度比较低,而应变强化技术能够显著提升奥氏体不锈钢材料的屈服强度,节约材料。奥氏体不锈钢压力容器的应变强化具有两种不同的模式:常温应变强化模式和低温应变强化模式。本文通过对应变强化基本原理的介绍,对奥氏体不锈钢压力容器的应变强化技术进行分析探讨。     

基于应变强化技术的奥氏体不锈钢压力容器轻型化设计探讨

奥氏体不锈钢材料韧性好但屈服强度低,通过应变强化技术可显著提高奥氏体不锈钢的屈服强度,从而提高奥氏体不锈钢压力容器的承载能力,减薄容器壁厚,达到节约材料的目的。介绍了奥氏体不锈钢应变强化的基本原理和基本过程,从强度、抗腐蚀能力、应力腐蚀开裂和氢脆等方面综述了奥氏体不锈钢应变强化后性能变化的研究进展,并提出进一步研究的建议,以实现压力容器轻型化这一安全与经济并重的绿色制造理念。     

国产奥氏体不锈钢06Cr19Ni10（S30408）拉伸试验研究

对国产奥氏体不锈钢06Cr19Ni10进行室温单向拉伸试验,研究了预应变量和变形速度等对材料强度、韧性等力学性能的影响。试验结果表明,不同预应变量及不同变形速度对材料的力学性能均有不同程度的影响。预应变处理能提高材料的屈服强度和抗拉强度,但同时会降低其塑性;提高拉伸速度能提高材料的屈服强度、降低抗拉强度、提高屈强比且能明显降低试样的断裂伸长率。     

ASME应变强化本构模型及压力容器安全裕度分析

弹塑性有限元分析需要材料的真实应力—应变曲线,但利用ASME中的应变强化本构模型,按标准保证值和实测值分别建立的ASME真实应力—应变曲线存在较大的差异。运用ANSYS有限元软件模拟同一个1.4301奥氏体不锈钢压力容器模型在这两种材料参数下筒体应力、应变以及爆破压力的差异,并将模拟结果与试验结果对比。同时利用有限元模拟和爆破试验的爆破压力结果,分析奥氏体不锈钢应变强化压力容器在不同预应变下的安全裕度和实际安全裕度。结果表明：按保证值材料参数设计的压力容器,容器的实际塑性应变要比理论值小很多,用实测值材料参数设计大变形压力容器时应严格控制实际的应变值;应变强化压力容器的理论设计应变可达10%,但实际应变应在5%左右,容器才具有足够的安全裕度。     

应变强化对022Cr17Ni12Mo2奥氏体不锈钢显微组织和力学行为的影响

采用电液伺服万能试验机、X射线衍射仪、金相显微镜和透射电子显微镜,研究了应变强化对022Cr17Ni12Mo2奥氏体不锈钢微观组织和力学行为的影响。结果表明：前期预应变强化过程中,材料没有发生相变,形变孪晶数量的增加使材料的屈服强度和硬度得到大幅提高,但塑性有所降低,发生形变孪晶诱发强度效应。随着预应变量的增大,应变强化能力减弱,瞬变应变有所降低,位错的滑移模式发生转变,从单系滑移和平面滑移向多系滑移和交滑移转变。     

奥氏体不锈钢压力容器的应变强化承载能力研究

在压力容器发展过程中,一个重要的课题为如何使轻型化设计中并重安全与经济。随着美国及欧盟制定的压力容器标准中纳入弹塑性分析设计方法,奥氏体不锈钢压力容器在开展轻型化设计时,逐渐应用于弹塑性分析设计为基础的应变强化技术,发展前景十分广阔。利用应变强化技术设计奥氏体不锈钢压力容器后,屈服强度可显著提升,壁厚及压力容器的重量降低,有效的实现节能减排。本文重点分析了基于应变强化技术设计的奥氏体不锈钢压力容器的承载能力。     

国产S30408奥氏体不锈钢应变强化低温容器许用应力及应变确定

对奥氏体不锈钢低温压力容器常规设计与应变强化设计进行比较,可知应变强化技术可大幅提高奥氏体不锈钢材料的许用应力,减薄简体壁厚,减轻容器重量。根据预应变拉伸试验确定国产S30408奥氏体不锈钢应变强化压力容器的应变上限值,并建立国产S30408奥氏体不锈钢材料的ASME和双线性这两种应力应变曲线,对两者进行比较后,以ASME应力应变曲线为计算依据,考虑抗拉强度的影响,确定了国产S30408奥氏体不锈钢材料制造应变强化低温容器时的许用应力及其对应的应变。     

奥氏体不锈钢应变强化容器结构的可靠度分析

由于较好的低温性能,奥氏体不锈钢被广泛应用于LNG低温储罐,而奥氏体不锈钢的应变强化技术能提高材料的屈服强度实现容器的轻量化设计。在工程上,奥氏体不锈钢材料性能数据呈现一定的离散性,在压力容器制造和使用过程中,容器的尺寸和使用条件也是随机变量。利用可靠性设计中的一次二阶矩法和ANSYS软件中的Prob Design模块,可以得到了应变强化前后容器关键参数的随机分布,从而得到强化前后结构可靠度的变化,为奥氏体不锈钢应变强化容器的设计和制造提供支持。     

奥氏体不锈钢制压力容器强度裕度研究

分析了国内外压力容器标准中关于奥氏体不锈钢的许用应力,并通过工业规模奥氏体不锈钢压力容器爆破试验得到了容器的实际爆破压力,根据爆破压力与设计压力的比值比较不同标准下的压力容器强度裕度,结果说明GB 150在许用应力方面相对其他国际标准较为保守,建议以残余应变为1%的屈服强度代替残余应变为0.2%的屈服强度.     

A comprehensive study on a propagating buckle in externally pressurized pipelines

Buckle propagation is a unique phenomenon occurring in deep-sea pipelines. In previous works, this phenomenon was investigated using a ring technique in which the pipeline was assumed to be in plane strain condition and the energies absorbed in membrane stretching and longitudinal bending were ignored. This paper presents a three-dimensional analysis of the buckle propagation phenomenon with an emphasis to address more complete factors that were not accounted for in the ring analysis. The analyses are based on the available solutions of the transition zone obtained in our previous works. A comprehensive mechanism for buckle propagation phenomenon is described from the point view of plastic stability theory for shells which enables the incorporation of the effects of transverse and longitudinal bending, membrane stretching and material strain hardening. The nondimensionalized buckle propagation pressure is represented in terms of yield coefficient, strain hardening coefficient and membrane stretching factor. It is found that a buckle once initiated in a pipeline may or may not propagate along the pipeline depending on its radius-to-thickness ratio. By comparing with various experimental results the theoretical predictions from this analysis are shown to provide very accurate estimations of the buckle propagation pressure for different materials with diverse geometric parameters and material properties. This paper points to the need for more complete information regarding the effects of transverse bending, membrane stretching and material strain-hardening on the buckle propagation pressure. Upon the requirement of application variations of the yield coefficient, strain hardening coefficient and membrane stretching factor with respect to the radius-to-thickness ratio are sketched out. This eliminates the need for recourse the curves and allows a fast and convenient resolution of buckle propagation pressure for certain pipeline. Most importantly, the present analysis offers the potential for future design of pipelines being at once more rationally and parametrically complete, and yet compact and simple to apply.     

Identification of sheet metal plastic anisotropy using heterogeneous biaxial tensile tests

The plastic behavior of anisotropic steel and aluminum sheets is identified by combining the results of classical uniaxial tensile tests and heterogeneous biaxial tensile tests on non-standard cruciform specimens specifically designed for obtaining a high sensitivity of strain fields to material anisotropy. The strain fields are measured on the surface of the specimens by means of an image correlation method. The 8-parameter anisotropic yield function proposed by Ferron et al. [1] is adopted for identification. On the one hand, the results of uniaxial tensile tests are analyzed to determine the strain-hardening parameters and yield function parameters related to transverse strain-anisotropy (angular variation of the anisotropy coefficient R) and stress-anisotropy (angular variation of the yield stress σ). On the other hand, strain fields measured in the biaxial tests are used as input data in an optimization procedure that consists of fitting simulated fields with experimental ones in order to determine the material parameters describing the shape of the yield surface in the biaxial stretching range. The identified yield function is validated using experimental data issued from biaxial tests that were not considered during the optimization process.     

电流变液力学特性的实验研究

电流变液的力学性能参数是评价电流变液的性能指标之一 ,也是含电流变液智能结构建模所需的基本参数。对电流变液这种新兴的材料 ,其力学参数测试尚没有成熟统一的方法。本文研制了一系列电流变液力学参数专用测试仪器 ,并对所研制的电流变液的基本力学参数进行了系统的测试。结果表明 ,电流变液的拉伸模量和剪切模在屈服转捩前后有明显差异。本文所研制的电流变液在应力应变曲线上无过冲现象 ,其强度满足一定条件下的工程应用要求     

Ideal stretch forming for minimum weight axisymmetric shell structures

A companion paper “Minimum Weight Design of Axisymmetric Shell Structures” by Richmond and Azarkhin describes the design of axisymmetric thin-walled structures and parts that can support specified loads with minimum material. The result is an optimum shape and thickness distribution in the final part when the strength of the material is assumed to be uniform. Here, we demonstrate the application of ideal forming theory to design sheet stretching processes that can produce the optimum shapes and thickness distributions from flat sheets of uniform thickness. Specific designs are achieved for producing minimum weight shell structures that will support a specified uniform pressure assuming both the Mises and the Tresca yield criteria along with the rigid-perfectly plastic flow condition. In the case of the Tresca yield condition, the optimum structure is a spherical shell segment with uniform thickness, and an associated ideal stretching process is hydraulic bulging. Because the effects of strain hardening have been neglected in the structural optimization theory, it has been possible here to design the minimum weight structure and its forming process sequentially. In subsequent work, we plan to include the effects of strain hardening on the shell strength, which will then require coupled design of the structure and its forming process. Also extension of these methods to three-dimensional geometries will be considered.     

Plastic deformation behavior of a uniaxial metal matrix composite under transverse compression

Plastic deformation behavior of a stainless-steel/Sn−Bi composite was examined using transverse compression tests on rectangular specimens under plane strain loadings. Based on the anisotropic yield criterion proposed by Hill, a theoretical analysis on the relationship between the yield strength of the matrix material and the yield strength of the composite was developed and compared to experimental results. Experiments were carried out to investigate the effects of the forming parameters such as yield strength of the matrix material, fiber packing patterns, fiber volume fraction, and lubrication of the compression platens, on the plastic deformation behavior of the metal matrix composite. Failure modes of the composite included shear band formation and eye formation at the fiber-matrix interface. Low deformability in the transverse directions was found for the metal matrix composite specimen. The theoretical and experimental results on the effects of the forming parameters provide basic information for further research on the transverse compression of metal matrix composite materials.     

Methodology for evaluating effects of material characteristics on machinability—theory and statistics-based modelling applied on Alloy 718

The potential machinability for Alloy 718 (Inconel 718) is examined in terms of five material characteristics considered to play a key role in the machinability: ductility (elongation to fracture), strain hardening (ultimate tensile strength over yield strength), thermal conductivity, yield strength and abrasiveness (amount of carbides). The material characteristics are simulated with the software JMatPro from Sente software. The effects of composition, grain size, hardness (size of the precipitated intermetallic particles for given volume fraction), heat treatment, temperature and strain rate have been modelled and statistically evaluated. Combining thermodynamics-based modelling (JMatPro), design of experiments and statistical analysis (Minitab), and machinability polar diagram, a concept on methodology to assess variations in material specifications and to optimise these specifications with respect to potential machinability has been developed. The mechanical properties, predicted from the meta-modelling are found to be affected by the same parameters: hardness (intermetallic particles characteristics), grain size, amount of aluminium, strain rate and temperature. The abrasiveness should only be affected by the amount of carbon. Simulated material characteristics for two different types of turbine discs were compared with measured tool wear from production environment machining experiments. Variations in material characteristics between the discs were small as well as the critical tool wear, revealing a robust metal cutting process.     

A simplified analysis of the effect of microstructure gradient on the stress relief of aluminum plates and extrusions

Many bulk forms of metal products are quenched after heat treatment in order to preserve material characteristics. Rapid quenching results in large thermal stress gradients that lead to high levels of residual stress. For products such as plates and extrusions, stretching by a few percent can reduce these stresses, but the stresses are not completely relieved. This report presents a simplified analytical model of the stress relief process illustrating the effects of through-thickness property gradients on residual stress after stretching. The basic conclusion is that for plates with uniform yield surface shapes through the thickness, the through-thickness residual stress range after stress relief is equal to the strength range in the direction of stretch. In extrusions, the bow after stress relief is proportional to this strength range. Cases are also considered in which the yield surface shapes range from isotropic to anisotropic through the thickness. Plastic anisotropy can perturb the effect of strength range on residual stress.     

基于模糊控制的永磁同步电机控制方法研究

针对传统盘拉机不能根据各种材料的特性而在拉伸过程中转矩抖动和转速不稳的问题,提出了基于模糊控制的永磁同步电机控制方案,即将代表材料可拉伸程度的屈服强度系数弓l入到控制系统中,而且在控制中将SPWM调制方法改为SVPWM调制方法。通过实验对设计的方案进行了验证,结果表明,加入材料的屈服强度系数后,控制系统的控制性能可根据不同材料和线径来达到需要的拉伸效果,拉出的线材均匀度和各项特性更能满足要求;而且使用SVPWM调制方法也可一定程度地增加盘拉机运行过程的平稳性。