Modelling and Closed‐loop Control of Complex Tube Forming Based on an Optimized Application of Martensite Formation in Austenitic Stainless Steels

Metastable austenitic steels undergo deformation‐induced martensitic transformation which can lead to a distinct increase of fatigue strength at an optimal volume fraction of martensite. This effect was used in the present study to define the local strength behaviour of a structural component part for the very high cycle fatigue (VHCF) regime. The investigation was on a discontinuous two‐stage forming process that consists of U‐O‐forming and rotary draw bending and results in a cross tube of a trailer coupling as exemplary dynamically loaded component. The volume fraction of martensite can be adjusted by means of plastomechanical simulation of the forming process and its parameters as part of the online process control. The formation of martensite shows a strong dependence on forming parameters (e.g. temperature and strain‐rate) and batch variations. These disturbance variables can only be taken into account by a closed‐loop control. Non‐isothermal material models were analysed according to their simulation accuracy of the martensite evolution. For the online control various hierarchical mathematical models were studied with regard to time effort and model error.     

冷变形工艺对SUS301L不锈钢组织性能的影响

对亚稳定奥氏体不锈钢SUS301L进行了不同变形量的冷轧,并分别用X射线衍射仪和MP30铁素体测量仪测出了应变诱导马氏体及其含量.采用金相显微镜观察了试样的组织演变.研究结果表明:形变马氏体在剪切带的交叉点形核,新晶核的不断形成促使了形变马氏体的长大；形变马氏体随着冷轧压下量的增加而增加；另外,抗拉强度和硬度的增加幅度相当；平均抗拉强度与平均显微硬度的比值在2.82～3.17之间.     

Behavior of an Austenitic–Martensitic VNS9-Sh Sheet TRIP Steel during Static and Cyclic Deformation

The properties of austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) sheet TRIP steel during static and cyclic loading are studied. The specific features of the mechanical behavior of the steel during static tension that are related to shearing, twinning, and martensite formation processes are detected. The static stress–strain curve of the steel has a developed microyield stage, a long yield plateau, and a serrated stage of strain hardening (Portevin–Le Chatelier effect). The shear mechanisms at the initial stages of cyclic deformation and fatigue crack propagation mechanisms are investigated.     

Effect of Primary Factor on Cavitation Resistance of Some Austenitic Metals

Thecavitationerosionisasolidsurfacedamageattributedtomechanicaleffectsofmicro jet′sshock ingloadingresultedfrom gaseousand orvaporouscavityimplosion .Cavitationdamageisaseriousprobleminhydraulicmachines ,resultinginlossesofefficiencyandreductionofpower .Soapplicationofmorecavitationerosionresistantmaterialsisveryim portantto preventorreducecavitationdamage .Manymetalsandalloyshavebeenresearchedundercavitationerosionconditions[1-12 ] .Thecrystalstruc ture ,martensitictransformation ,workharden…     

Effects of Strain Rate and Plastic Work on Martensitic Transformation Kinetics of Austenitic Stainless Steel 304

The martensitic transformation behavior and mechanical properties of austenitic stainless steel 304 were studied by both experiments and numerical simulation.Room temperature tensile tests were carried out at various strain rates to investigate the effect on volume fraction of martensite,temperature increase and flow stress.The results show that with increasing strain rate,the local temperature increases,which suppresses the transformation of martensite.To take into account the dependence on strain level,strain rate sensitivity and thermal effects,a kinetic model of martensitic transformation was proposed and constitutive modeling on stress-strain response was conducted.The validity of the proposed model has been proved by comparisons between simulation results and experimental ones.     

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Influence of the tempering temperature on the mechanical properties and the phase composition of thin sheet TRIP steel

The strength and the plasticity properties of sheet high-strength austenitic–martensitic VNS9-Sh TRIP steel (23Kh15N5AM3-Sh) are studied as functions of the tempering temperature in the range 125–600°C. A nonmonotonic decease in the strength and the plasticity properties of the steel has been detected when the tempering temperature increases, and they increase in the range 300–450°C. The influence of aging processes, the precipitation of carbide, and the phase transformations in tempering on the mechanical properties of austenitic–martensitic corrosion-resistant steel is discussed.     

Consideration of Phase Transformations in Numerical Simulation of Press Hardening

The importance of high‐strength steel concepts for car bodies has increased in the last years due to the need of reduction in weight and enhanced crash safety. It is possible to produce components with a much higher strength, e.g. a tensile strength of about R_m = 1500 MPa, compared to cold forming processes when using press hardening of boron alloyed heat‐treatable steels. Moreover, parts with complex shapes can be realized. Numerical simulation by means of Finite‐Element Method has become an indispensable tool for process design and construction. But for a more realistic prediction of the resulting component properties, for instance residual stresses and distortion, it is essential to consider the complex effects of phase transformation within the simulation. Because it is not a standard task currently, a material model was implemented in the commercial FE‐Code LS‐Dyna. The diffusion‐controlled phase transformation is modelled with Johnson‐Mehl‐Avrami equation for isothermal transformation. The formation of martensite is described by Koistinen‐Marburger equation for diffusionless transformation. The latent heat caused by austenite decomposition is also considered by implementation of a thermal model via a user subroutine. The needed isotherm time‐temperature‐transformation diagram is approximated by a diagram of related steel. These approaches are applied to a simple model process. In this process a round sheet metal is formed and subsequently quenched by cooled tools, therefore a thermal‐mechanical sequential coupled simulation of a model process is implemented. The transport from furnace to the press and the closing of the tools are simulated in order to get a realistic temperature distribution in the sheet metal at the beginning of the forming process. The tools are modelled as deformable bodies and heat‐transfer is taken into account. The simulation results show that nearly the whole austenite is transformed into martensite after the cooling phase.     

NiAl应力诱发马氏体相变的分子动力学模拟

利用ＮｉＡｌ合金的嵌入原子势，通过分子动力学模拟方法研究了标准化学计量比ＮｉＡｌ合金中应力诱发马氏体相变的微观机理。系统径向分布函数的变化表明，模拟中发生了Ｂ２结构奥氏体逐渐转变为四方Ｌ１０结构马氏体的相变，分析了马氏体形核和长大过程中系统微观结构的变化规律。通过分析形核前后系统应变的变化过程，探讨了应力诱发马氏体形核的微观机理。     

Acoustic emission analysis of the stages of deformation of TRIP steel

The kinetics of damage accumulation and deformation martensite formation in cold-rolled austenitic–martensitic VNS9-Sh TRIP steel during static tension at room temperature is studied at various stages of plastic deformation and fracture by acoustic emission and X-ray diffraction. The threshold stresses that correspond to the beginning of dislocation motion and intense dislocation generation predominantly in surface layers are determined. Deformation martensite is shown to form after a yield plateau and its formation is most intensely at the stage of strain hardening.     

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology D. MARI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

Modelling of Incremental Bulk and Sheet Metal Forming

In incremental forming operations, forming is accomplished by a number of single, local forming steps with simple, generic tools, and in some cases even by laser or plasma beams. This makes incremental forming processes very flexible. However, complex tool kinematics with a large number of degrees of freedom can cause severe problems for process planning and optimization. In addition, new questions regarding the material behaviour under cyclic plastic deformation arise. This article gives an overview of current problems in the finite element modelling of incremental forming processes, focusing on two examples, hammer forging and incremental CNC sheet metal forming. Based on this, insight is given into current research at the Metal Forming Institute (IBF) regarding adapted simulation methods for incremental forming processes.     

Rapid Finite Element Analysis of Bulk Metal Forming Process Based on Deformation Theory

 The one-step finite element method (FEM), based on plastic deformation theory, has been widely used to simulate sheet metal forming processes, but its application in bulk metal forming simulation has been seldom investigated, because of the complexity involved. In this paper, a bulk metal forming process is analyzed by using a rapid finite element simulation method based on deformation theory. The material is assumed to be rigid-plastic, strain hardening. The constitutive relationship between stress and total strain is adopted, whereas the incompressible condition is enforced by penalty function. The geometrical non-linearity in large plastic deformation is taken into consideration. Furthermore, the force boundary condition is treated by a simplified equivalent approach, considering the contact history. Based on constraint variational principle, the deformation finite element method is proposed. The one-step forward simulation of axisymmetric upsetting process is performed by this method. The results are compared with those obtained by the traditional incremental FEM to verify the feasibility of the proposed method.     

Development of a Robot‐Based Sheet Metal Forming Process

This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.     

Process Modeling for Precision Forming of Discrete Parts – State of the Technology and Critical Issues

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing manufacturing processes for precision parts. The processes discussed include forging, sheet metal forming and hydroforming. Determination of reliable input parameters to simulate a process is a key element in successful application of process simulation for process design in all the mentioned areas. These issues are discussed in detail. Practical examples of application of FE simulation are presented for improvement of the existing metal forming process and/or designing new metal forming process for manufacturing discrete precision parts in forging, sheet metal forming and hydroforming.     

Q&P钢热处理工艺的开发进展

Q&P钢的工艺概念已提出了有些年了,它作为一种热处理工艺,生产的组织具有马氏体和富C的亚稳残余奥氏体.工艺包括:控制淬火形成部分的马氏体,然后热处理以使C从过饱和的马氏体中转移到残余奥氏体中.潜在的应用包括具有好的成型性和抗断裂的高强度钢.应用该工艺可生产薄板和厚板,以及用从长锻材生产热处理元件,甚至于铸铁.自2003年一直在实验室研究,早期的工作结果已有发表.近期的研究主要集中于机理方面的细节.国际范围的独立团队在此研究领域也有很多进展.     

Martensitic Transformation during Simultaneous High Temperature Forming and Cooling Experiments

The main target of hot stamping is to combine accurate forming, low forming forces and high material strength in complex steel components. In the present study, the hot stamping process is simulated by means of simultaneous forming and quenching experiments. This is performed by uniaxial compression tests at high temperatures using a dilatometer. The effects of process parameters like strain, strain rate, initial deformation temperature, austenization time and applied forces on the martensitic transformation of the boron steel 27MnCrB5 are investigated. It is concluded that with increasing strain rate and initial deformation temperature, martensite content, hardness and martensite start temperature (Ms) are increased. On the contrary, when applying larger deformations, the above mentioned properties are decreased. It is also concluded that, regardless of the process parameters, higher applied forces retard the successful martensitic transformation during hot stamping.     

Innovative Flexible Metal Forming Processes based on Hybrid Thermo‐Mechanical Interaction

A new approach towards functional gradation of structural parts is presented. This approach is based on the utilization of locally varying thermo‐mechanically coupled effects applied to different initial workpiece geometries. The possible degree of freedom for the gradation of material properties and geometrical shape for sheet metal forming applications as well as for parts produced by bulk metal forming is characterized by the results of metallographic investigations, by mechanical testing and by an indication of the remaining residual stress state. On the basis of experimental results and process simulations, it could be revealed that the ability to exactly control the dynamic microstructural evolution by thermal and mechanical process parameters combined with predefined material design parameters constitutes a key towards the adjustment of flexible material property profiles even for parts with complex three‐dimensional geometry. Beyond that, the integrative aspect of thermal and mechanical treatment already implies the high level of obtainable efficiency resulting from shortening of process chains. However, it is not only the ability to integrate shape generation and property gradation, but also the automatically included positive effect of tailoring process behaviour by a gradation of formability finally allowing to improve process efficiency e.g. by a reduction of forming steps or reduction of (local) tool load.