Effect of cathodic hydrogen charging on the surface of duplex stainless steel

The effect of cathodic hydrogen charging on the mechanical properties of steels has been extensively investigated (1-5). There is a general agreement, that cathodic harging during a tensile test leads to reduction in ductility, and embrittlement (5-7). The effects of cathodic charging on the surface of metals also have been reported in the literature. Electrochemical hydrogen charging of austenitic stainless teels has been shown t

Simulation of Steady Laser Hardening by an Arbitrary Lagrangian Eulerian Method

One of the most practical methods for simulation of steady state thermal processing is the Arbitrary Lagrangian-\ud Eulerian method. Each calculation step is split into two phases. In the first phase, the Lagrangian phase, the element mesh\ud remains attached to the material. The evolution of the state variables is monitored and the state at the end of the phase is\ud calculated. In the second phase, the Eulerian phase, the mesh is, broadly speaking, restored to its original position with\ud respect to a window attached to the moving heat source. The mesh is not restored to its exact original position, but some\ud allowance is made perpendicular to the flow direction in order to capture movement of the free surfaces. In this paper a finite\ud element model for Lagrangian simulation of thermo-mechanical processes with phase transformations is combined with a\ud second order discontinuous Galerkin method for modeling of Eulerian advection

Coupled analysis of material flow and die deflection in direct aluminum extrusion

The design of extrusion dies depends on the experience of the designer. After the die has\ud been manufactured, it is tested during an extrusion trial and machined several times until it works\ud properly. The die is designed by a trial and error method which is an expensive process in terms\ud of time and the amount of scrap. In order to decrease the time and the amount of scrap, research is\ud going on to replace the trial pressing with finite element simulations. The goal of these simulations\ud is to predict the material flow through the die. In these simulations, it is required to calculate the\ud material flow and the tool deformation simultaneously. Solving the system of equations concerning\ud the material flow and the tool deformation becomes more difficult with increasing the complexity\ud of the die. For example the total number of degrees of freedom can reach a value of 500,000 for\ud a flat die. Therefore, actions must be taken to solve the material flow and the tool deformation\ud simultaneously and faster. This paper describes the calculation of a flat die deformation used in the\ud production of a U-shape profile with a coupled method. In this calculation an Arbitrary Lagrangian\ud Eulerian and Updated Lagrangian formulation are applied for the aluminum and the tool finite\ud element models respectively. In addition, for decreasing the total number of degrees of freedom,\ud the stiffness matrix of the tool is condensed to the contact nodes between the aluminum and the tool\ud finite element models. Finally, the numerical results are compared with experiment results in terms\ud of extrusion force and the angular deflection of the tongue

Prediction of the mechanical behaviour of TRIP steel

TRIP steel typically contains four different phases, ferrite, bainite, austenite and martensite. During deformation the metastable retained austenite tends to transform to stable martensite. The accompanying transformation strain has a beneficial effect on the ductility of the steel during forming. By changing the alloy composition, the rolling procedure and the thermal processing of the steel, a wide range of different morphologies and microstructures can be obtained. Interesting parameters are the amount of retained austenite, the carbon content of the austenite, the stability of the austenite as well as its hardness. A constitutive model is developed for TRIP steel which contains four different phases. The transformation of the metastable austenite to martensite is taken into account. The phase transformation depends on the stress in the austenite. Due to the differences in hardness of the phases the austenite stress is not equal to the overall stress. An estimate of the local stress in the austenite is obtained by homogenization of the response of the phases using a self-consistent mean-field homogenization method. Overall stress-strain results as well as stress-strain results for individual phases are compared to measurements found in literature for some TRIP steels. The model is then used to explore the influence of some possible variations in microstructural composition on the mechanical response of the steel

Numerical Simulation of Stresses due to Solid State Transformations : The Simulation of Laser Hardening

The properties of many engineeringmaterialsmay be favourablymodified by application of\ud a suitable heat treatment. Examples are precipitation hardening, tempering and annealing.\ud One of the most important treatments is the transformation hardening of steel. Steel is an\ud alloy of iron and carbon. At room temperature the sollubility of carbon in steel is negligible.\ud The carbon seggregates as cementite (Fe3C). By heating the steel above austenization\ud temperature a crystal structure is obtained in which the carbon does solve. When cooled\ud fast the carbon cannot seggregate. The resulting structure, martensite is very hard and also\ud has good corrosion resistance.\ud Traditionally harding is done by first heating the whole workpiece in an oven and then\ud quenching it in air, oil or water. Other methods such as laser hardening and induction\ud hardening are charaterized by a very localized heat input. The quenching is achieved by\ud thermal conduction to the cold bulk material. A critical factor in these processes is the time\ud required for the carbon to dissolve and homogenize in the austenite.\ud This thesis consists of two parts. In the first part algorithms and methods are developed\ud for simulating phase transformations and the stresses which are generated by inhomogeneous\ud temperature and phase distributions. In particular the integration of the constitutive\ud equations at large time increments is explored. The interactions between temperatures,\ud stresses and phase transformations are cast into constitutive models which are suitable for\ud implementation into a finite element model.\ud The second part is concerned with simulation of steady state laser hardening. Two\ud different methods are elaborated, the Arbtrary Lagrangian Eulerian (ALE) method and a\ud direct steady state method. In the ALE method a transient calculation is prolonged until\ud a steady state is reached. An improvement of the convection algorithm enables to obtain\ud accurate results within acceptable calculation times.\ud In the steady state method the steadiness of the process is directly incorporated into\ud the integration of the constitutive equations. It is a simplified version of a method recently\ud published in the literature. It works well for calculation of temperatures and phase distributions.\ud When applied to the computation of distortions and stresses, the convergence of the\ud method is not yet satisfactory.\ud i

Equivalent drawbead performance in deep drawing simulations

Drawbeads are applied in the deep drawing process to improve the control of the material flow\ud during the forming operation. In simulations of the deep drawing process these drawbeads can be replaced by\ud an equivalent drawbead model. In this paper the usage of an equivalent drawbead model in the finite element\ud code DiekA is described. The input for this equivalent drawbead model is served by experiments or by a 2D\ud plane strain drawbead simulation. Simulations and experiments of the deep drawing of a rectangular product\ud are performed to test the equivalent drawbead model performance. The overall conclusion reads that a real\ud drawbead geometry can succesfully be replaced by the equivalent drawbead mode

Constitutive modeling of metastable austenitic stainless steel

A stress-update algorithm is developed for austenitic metastable steels which undergo phase evolution during deformation. The material initially comprises only the soft and ductile austenite phase which due to the phenomenon of mechanically induced martensitic transformation, transforms completely to the hard and brittle martensite. A mean-field homogenization algorithm is developed that can predict the mechanical response of the composite material during transformation. Furthermore, a physically based transformation model is developed that predicts the amount of transformation during deformation