3D‐FE Modelling and Simulation of Multi‐way Loading Process for Multi‐ported Valves

Multi‐ported valves are widely used in the marine, sanitary, petrochemical and power industry. Multi‐way loading forming technology provides an efficient approach for integral forming of high strength multi‐ported valves, such as tee pipe coupling, high‐pressure cross valves, large‐scale complex valves, and so on. Since the multi‐way loading process is a very complicated plastic forming process due to the complexity of loading path, finite element numerical simulation is adopted to investigate the multi‐way loading process in order to predict and control the multi‐ported valve forming process. A reasonable model of the process is developed under DFEORM‐3D environment based on the coupled thermo‐mechanical finite element method. Then the reliability of the model is validated with respect to geometry development and forming defects. Numerical simulations of multi‐way loading forming for a tee valve and a cross valve have been carried out via using the developed model. Further, the forming processes of tee valve and cross valve have been compared. Moreover, the modelling method is also suitable for multi‐way loading processes of other complex components.     

Theoretical and Experimental Damage Prediction in Cold and Semi‐Hot Bulk Forming of Ductile Steels

Bulk forging is among the most important manufacturing methods in metal forming, due to its wide applicability from some ounces to several tons of steel in a high diversity of shapes and forming conditions. Economical constraints demand for further optimisation and cost‐effective production. This requires the application of suitable finite elements simulation software, in order to support the already digitalised construction processes. Ductile damage is one of the most severe problems to arise during the production sequences, not only in cold but also in semi‐hot forging operations. Mathematical approaches exist for the modelling and simulation of ductile fracture in steel. In this paper some widespread used damage models are introduced and discussed. Their damage prediction quality has been verified by experiments, the tensile test and the collar specimen upsetting with several different steels under cold and semi‐hot forging conditions. The methods for the experimental fracture detection are introduced as well. In cold forging the passive ultrasonic testing with integrated statistical filtering algorithms is used. As this method is not applicable to semi‐hot forging experiments, optical fracture detection by means of a high‐speed camera is used instead. A very interesting material behaviour of the steels tested has been identified in the semi‐hot upsetting of collar specimen. For every steel a distinct temperature crossover interval exists, in which the forging process abruptly changes from damaged to undamaged state. This interval amounts to some degrees Celsius only for each of the seven materials investigated. Among the damage models proposed, the Model of Effective Stresses by Lemaitre is chosen for the application to a cold and a semi‐hot forging operation. These industrial processes of an axle end (cold) and a journal bearing (semi‐hot) are susceptible to damage for reasons to be discussed in this paper. It will be shown that the internal fracture of the axle end (chevrons) and the surface fissures of the journal bearing can be predicted with high accuracy. Moreover, the application of the damage model in the finite element software MSC.SuperForm 2004 offers a promising approach for process optimisation. Several possibilities could be tested for their suitability of reducing the calculated damage: geometry variation of the forming tools, process annealing, different materials. The use of damage models in finite element simulation can be regarded as a further step towards an optimal process design.     

Validation of Multi‐scale Model Describing Microstructure Evolution in Steels

Properties of deformed steels depend on various microstructure parameters such as distribution of grain size and precipitates. Strain, strain rate and temperature inhomogeneities make quantitative prediction of microstructure difficult but the Finite Element method is able to model these inhomogeneities. Different scales of phenomena occurring in deformed materials are another difficulty in modelling. Microstructure evolution can be described by more realistic methods (e.g. Cellular Automata CA, Monte Carlo), which, on the other hand, are unable to simulate larger samples. Therefore, development of the methods capable of spanning multiple scales became a current challenge. CAFE modelling, which couples FE and CA methods, is the objective of the paper. The model consists of two layers. The micro‐scale layer, simulated by CA, represents microstructure evolution including nucleation and growth of the grains. Evolution of a dislocation density is described for every grain separately by solving differential equation. The FE thermal‐mechanical model is used as a macro‐scale part. Multistage plane strain compression tests for niobium steel are considered. Distributions of initial and final grain size are measured during the tests. The results from the CAFE model are compared to the measurements and to the predictions by a conventional model. The comparisons confirm the capability of the CAFE method to predict flow stress, recrystallized fraction and grain size distribution. Conventional approach gives a good agreement with experiments for an average grain size only.     

Finite Element Modelling of TRIP Steels

A constitutive model that describes the mechanical behaviour of steels exhibiting “Transformation Induced Plasticity” (TRIP) during martensitic transformation is presented. Multiphase TRIP steels are considered as composite materials with a ferritic matrix containing bainite and retained austenite, which gradually transforms into martensite. The effective properties and overall behaviour of TRIP steels are determined by using homogenization techniques for non‐linear composites. The developed constitutive model considers the different hardening behaviour of the individual phases and estimates the apportionment of plastic strain and stress between the individual phases of the composite. A methodology for the numerical integration of the resulting elastoplastic constitutive equations in the context of the finite element method is developed and the constitutive model is implemented in a general‐purpose finite element program. The prediction of the model in uniaxial tension agrees well with the experimental data. The problem of necking of a bar in uniaxial tension is studied in detail.     

Physical Metallurgy of Multi‐Phase Steel for Improved Passenger Car Crash‐Worthiness

The dynamic testing of high strength automotive steel grades is of great practical importance if their crash‐worthiness is to be evaluated. During forming operations, steels are processed in a controlled dynamic manner. In collisions, the deformation is different in the sense that the deformation is not controlled, i.e. both strain and strain rate are not pre‐determined. No clear standard testing procedures are currently available to test high strength steels dynamically, in order to evaluate their performance during car crashes. High tensile strength TRIP‐aided steels have been developed by the steel industry because of their promising high strain rate performance. The present contribution focuses on the effect of the strain rate and temperature on the mechanical behaviour of the low alloy high strength TRIP steel. The tests were carried out on the separated phases in order to determine their specific high strain rate deformation response. The temperature‐dependence of the transformation rate of the retained austenite is presented. It is argued that the adiabatic conditions present during high strain rate deformations have a beneficial effect on the behaviour of TRIP steel.     

Simulation of Solidification Microstructure and Columnar to Equiaxed Transition in Free‐cutting Steel 9SMn28 based on a CAFE Method

The solidification microstructure in 9SMn28 free‐cutting steel is simulated by the finite element – cellular automaton (CAFE) method based on the calculation of convection in a casting. The simulation results are consistent with experimental findings; the simulated crystallisation process conforms to the actual situation. The solidification of 9SMn28 alloy is a volume solidification mode under slow cooling condition. The columnar‐to‐equiaxed transition (CET) is also studied in the CAFE model. The mechanism of the CET in the CAFE model is thermal interaction. The CET is not abrupt but occurs gradually, the long columnar grains are first blocked by elongated grains. The grains become more equiaxed as the thermal gradient is decreased with the development of solidification.     

Multi‐Layer Compression Tests under Hot Forming Conditions

Exact simulation of forming processes using the finite element method requires as precise specification as possible of the prevailing material data and physical boundary conditions. The article presents a method of determining the flow‐stress for sheet metals under hot‐forming conditions. The value is determined in a cylindrical multi‐layer compression test which up to now has been underestimated even for cold testing conditions. This article presents some special adaptations which are necessary for testing under high temperature conditions. The method is able to supply reliable data for FEM simulation, as is demonstrated in some examples.     

Computer Aided Design of New Forging Technology for Crank Shafts

Application of numerical simulations to improve forging technology for crank shafts is the objective of this work. Plastometric tests were performed for steels used for manufacturing of crank shafts and a rheological model for these steels was determined. Inverse analysis was applied for the identification of the model parameters. This model was implemented in the finite element software and simulations of various variants of forging were performed. Results of simulations were used to select the best variant, which gives the lowest loses of the material and proper shape of the final product.     

Semi‐Solid Rheoforging of Steel

To produce steel components with complex shapes excessive machining is necessary frequently since high pressure die casting of steel is not industrially applied. Forming steel in the semi‐solid state can in principle produce new components and geometries which cannot be realised by conventional closed die forging. Semi‐solid forging of steel combines the possibility of producing geometries not conventionally forgeable in one forming operation and of adding further functions during the same operation. In previous investigations on thixoforming of steels, the semi‐solid steel was generated by reheating precursor material billets. An alternative approach for generating semi‐solid steel from the liquid state with subsequent forging operation is presented in this paper for the first time. The steel grades X210CrW12 cold work tool steel and 100Cr6 bearing steel are molten and driven into a globular semi‐solid state using a cooling slope and a cup. By cooling the steel into the semi‐solid range instead of heating it, the required process temperatures are lower than in the process route via heating. Therefore, the load on the dies in a semi‐solid forging operation is decreased. Suggestions for the respective layout of the process are made for both steel grades. Future potentials and challenges to be solved are discussed, showing advantages especially in the field of high melting point alloys such as steels. This technique enables to produce pre‐shaped semi‐solid billets to optimise the materials flow and the homogeneity of the mechanical properties.     

Modeling the ductile-brittle transition behavior in thermomechanically controlled rolled steels

The Charpy impact transition temperature (ITT) is well modeled for hot-rolled or normalized steels having uniform grain size using empirical equations. However, the ITT of nonhomogeneous steel microstructures, such as duplex (mixed fine and coarse) grain sizes, and the scatter in experimental Charpy energy values, observed in the transition region, are not accurately modeled. This article describes research on the microstructure-fracture property relationship and the prediction of the ITT using a cellular automata finite element (CAFE) model in thermomechanically controlled rolled (TMCR) Nb-microalloyed steels. The ferrite grain size distributions for two TMCR steel plates were analyzed and used for the prediction of the local fracture stress (σ _F ) values based upon the Griffith model. It was found that the coarse grain size distribution could be used to predict the range of σ _F values observed. The CAFE model was used to predict the ITT using the predicted σ _F distribution for a TMCR steel. Results showed that the CAFE model realistically predicted the Charpy ITT; in particular, it was able to reproduce the scatter in values in the transition region. Within the model, the percentage of brittle failure and the upper shelf ductile energy were predicted well. However, the lower shelf brittle energy was overestimated due to computational limitations in the commercial FE software used with the current CAFE model.     

Evaluation of the Static Stress‐Strain Behaviour of Phosphorus Alloyed and Titanium Micro‐alloyed TRIP Steels

A considerable research effort has been done in the field of cold rolled TRIP steels submitted to a two‐step annealing cycle. After annealing, these steels contain retained austenite, which offers them superior mechanical properties required for specific applications in automotive industry. In the present work, a physically based microstructural model has been applied to describe the static stress‐strain behaviour of phosphorus alloyed TRIP steel. The impact of the TiC precipitation on the static stress‐strain behaviour for a Ti micro‐alloyed TRIP steel was simulated. The model calculations were compared with experimental stress‐strain curves. An excellent agreement between simulation and experimental data was demonstrated.     

Stability and Constitutive Modelling in Multiphase TRIP Steels

Multiphase TRIP steels are a relatively new class of steels exhibiting excellent combinations of strength and cold formability, a fact that renders them particularly attractive for automotive applications. The present work reports models regarding the prediction of the stability of retained austenite, the optimisation of the heat‐treatment stages necessary for austenite stabilization in the microstructure, as well as the mechanical behaviour of these steels under deformation. Austenite stability against mechanically‐induced transformation to martensite depends on chemical composition, austenite particle size, strength of the matrix and stress state. The stability of retained austenite is characterized by the M^σ_S temperature, which can be expressed as a function of the aforementioned parameters by an appropriate model presented in this work. Besides stability, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure. This amount is determined by the combinations of temperature and temporal duration of the heat‐treatment stages undergone by the steel. Maximum amounts of retained austenite require optimisation of the heat‐treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat‐treatment of multiphase TRIP steels, and which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of multiphase TRIP steels under deformation, taking into account the different plastic behaviour of the individual phases, as well as the evolution of the microstructure itself during plastic deformation. This constitutive micromechanical model is subsequently used for the calculation of forming limit diagrams (FLD) for these complex steels, an issue of great practical importance for the optimisation of stretch‐forming and deep‐drawing operations.     

Validation of a Model of Plastic Deformation of C‐Mn Steels in the Two‐Phase Temperature Region

The paper describes the validation of a thermal‐mechanical‐microstructural model of deformation of carbon‐manganese steels in the two‐phase temperature region. The model has been developed on the basis of dilatometric and plastometric tests performed for a wide range of temperatures and strain rates. Dilatometric tests were used to identify the phase transformation model and plastometric tests were used to identify the flow stress model. Inverse analysis was applied to find the parameters of the models which were further implemented into a finite element code. Numerical simulations of the deformation of steels in the two‐phase temperature range were performed. Multi‐stage plane strain compression tests were performed to validate the model. The samples were quenched after subsequent stages of the tests and metallographic analysis was performed. Predicted loads, grain size and volume fractions of the microstructural components were compared with measurements and the coefficients in the models were updated.     

A Material Model for TRIP‐Steels and its Application to a CrMnNi Cast Alloy

A material model is presented that accounts for strain rate dependent inelastic deformation and strain‐induced phase transformation in TRIP‐steels. Modifications for the kinetics equations of the strain‐induced phase transformation, introduced by Stringfellow, are proposed to overcome a drawback of Stringfellow's model. A parameter identification strategy that relies on Gauss‐Markov estimates is used to determine the model parameters from experimental data of a recently developed cast TRIP‐steel. Good agreement is observed between experimental results of the compression test and the corresponding finite element simulation employing the proposed model. This forms the basis for future applications of the material model in the design of composites and structures.     

Work Hardening Behavior of Ultrafine Grained Duplex Medium‐Mn Steels Processed by ART‐Annealing

The ultrafine grained duplex steels were fabricated by austenite reverted transformation annealing of the medium manganese steels after quenching or cold rolling. The microstructures were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The mechanical properties were determined by uniaxial tensile test. It was demonstrated that both the quenched and cold rolled structures were transformed into ultrafine grained duplex structures with large fractioned austenite by ART‐annealing. Long time annealing is essential to obtain the large fractioned austenite in quenched steel, but only short time annealing is needed to get large fractioned austenite in the cold rolled sheet. The mechanical examinations indicated that ART‐annealing results in the superhigh tensile elongation (>40%) and superhigh strength (1000 MPa) in quenched steels after long time annealing but in cold rolled steels after short time annealing. Based on the analysis on the work hardening behaviors of these ART‐annealed steels, the abnormal work hardening rate was presented and analyzed. The substantially enhanced ductility was attributed to the Lüders band propagation of the ferrite matrix and/or the TRIP effects of the large fractioned austenite. At last the dynamic phase natures of both fraction and stress was proposed to interpret the abnormal hardening behaviors and the “S” shaped stress–strain curves.     

Cold Work Hardening of High‐Strength Austenitic Steels

Mechanisms of cold work hardening in three austenitic steels containing (mass%) 12Mn and 1.2C (Hadfield steel denoted as C1.2); 21Cr, 23Mn, 2Ni and 0.9N (Böhler steel P‐560 denoted as N0.9); 18Cr, 18Mn, 0.345C, 0.615N (CARNIT steel denoted as CN0.96) were studied using mechanical tension tests and TEM studies of substructure formed in the course of plastic deformation. Hadfield steel C1.2 reveals the smallest yield and ultimate stresses and elongation but the highest cold work hardening. Similar yield and ultimate stresses were obtained for steels N0.9 and CN0.96 with a higher elongation and cold work hardening for the latter. The analysis of TEM results leads to the following conclusions: Cold work hardening of the carbon steel C1.2 is mainly due to intensive twinning with rather thick twins. Localized planar slip is a feature of the substructure in the nitrogen steel N0.9 and carbon+nitrogen steel CN0.96 at strains up to 10 %, whereas twinning is involved in deformation at strains in the range of 10 to 50%. The strain‐induced ∊ martensite is rarely observed in both of these steels at strains above 30 %. The substructure and cold work hardening are discussed in terms of stacking fault energy, short‐range atomic order and binding between interstitial atoms and dislocations.     

Rheological Model for Simulation of Hot rolling of New Generation Steel Strips for Automotive Applications

The main objective of this paper is the development of a rheological model for automotive steels for the conditions of hot strip rolling and implementation of this model in a finite element program is. Three types of steels were investigated, IF, dual phase and TRIP steel. Plastometric tests were performed on a Gleeble 3800 simulator for the temperature range 850‐1200°C and strain rates 3‐150 s^?1. Inverse analysis was applied to eliminate the influence of disturbances occurring in the plastometric tests and to determine the real flow stress of the material. The coefficients in the flow stress equation were evaluated and this equation was implemented in the FEM code as the constitutive law. The model was validated by comparison of measured and calculated loads in the compression tests and by strip rolling experiments conducted in the laboratory mill. Validation confirmed a good predictive capability of the rheological model.