M. Pietrzyk

Identification of rheological parameters on the basis of various types of plastometric tests

Abstract Axisymmetric compression and plane-strain compression tests have been performed on specimens of carbon–manganese steel containing 0.17% of carbon. Load–displacement relationships were monitored for each test and the inverse analysis method used to determine the material flow stress for constant strain rate.

Testing of the inverse software for identification of rheological models of materials subjected to plastic deformation

The general objective of the present work was to perform numerical tests for the inverse analysis of various plastometric tests. Uniaxial compression, plane strain compression and ring compression were investigated for different materials. The experimental results, in the form of load vs. displacement measurements carried out in two laboratories for various sample dimensions, were used as input for inverse calculations. As a result, a large number of data was obtained and the comparison of flow stress values determined in various tests and in various laboratories was possible. The capabilities of the inverse analysis as well as the influence of the method of testing on the material properties were examined. It is shown, in general, that when the inverse analysis is applied to the interpretation of the plastometric tests, the properties of the material are insensitive to the method of testing and to the sample dimensions.

Modelling of the influence of thermomechanical processing of Nb-microalloyed steel on the resulting mechanical properties

Abstract Two-stage compression tests were performed at various temperatures and reductions. The influence of finish deformation temperature in the range of 650–820°C and strain on microstructure and final mechanical properties of Nb steel is investigated. Starting from detailed simulation of microstructure development, i.e. recrystallization, precipitation and grain growth kinetics, the material behaviour was modelled. An empirical model of yield strength, developed in previous work, is employed to predict mechanical properties of microalloying steel during hot deformation above and below the γ–α transformation temperature. The comparison of simulated and measured results shows that the real thermomechanical behaviour as well as the microstructure development of Nb-steel has been correctly implemented. The results provided by both finite element analyses and microstructure modelling were compared with earlier similar investigations of C–Mn steel.

Modelling phase transformations in steel

Abstract: This chapter discusses aspects of the physical and numerical modelling of phase transformations in steels. The basic features of these phase transformations are described. Dilatometric tests, which are performed to identify the parameters of phase transformation models, are explained. Four models of phase transformations of various complexity and various predictive capability are described. The chapter includes a case study of the simulation and optimization of two industrial processes for dual-phase steel strips: laminar cooling after hot rolling, and cooling after continuous annealing.

Use of the computer simulation to predict mechanical properties of C-Mn steel, after thermomechanical processing

Abstract The objective of the paper is to demonstrate the ability of a computer simulation to analyse the development of microstructure and finally to the predict mechanical properties of C-Mn steels. The microstructure of ferrite is the main parameter which controls the mechanical properties of steels after hot deformation. This microstructure depends on the grain size and morphology of austenite just before the transformation. However, when the last deformation takes place below the γ-α transformation temperature, the mechanisms connected with substructure and dislocation forest becomes a significant part of the strengthening process. The microstructural model makes it possible to separate the relative contributions of solid solution, ferrite microstructure, substructure and dislocation strengthening. The change in microstructure at lower temperatures requires an improvement in the model which will allow to account for the substructure and dislocation density. In the present work hot deformation conditions were simulated using Finite Element Method. The investigation was focused on the case when the last deformation takes place in the two phase or ferrite region. The two stage constant strain rate compression tests were conducted, in continuous cooling condition, at the temperature range 1050 – 650 °C. The material after deformation was investigated to obtain the microstructure and verify the model of substructure and dislocation strengthening mechanisms. The experimental results were used to validate and improve the empirical equations that were employed to the general model. The computer simulation suggested in the work can be used to predict mechanical properties, including all the events that occur under industrial processing conditions which cannot be reproduced in the laboratory.

Microstructure evolution in metal forming processes

Part 1 General principles: Understanding and controlling microstructural evolution in metal forming: an overview Techniques for modelling microstructure in metal forming processes Modelling techniques for optimising metal forming processes Recrystallisation and grain growth in hot working of steels Severe plastic deformation for grain refinement and enhancement of properties. Part 2 Microstructure evolution in the processing of steel: Modelling phase transformations in steel Determining unified constitutive equations for modelling hot forming of steel Modelling phase transformations in hot stamping and cold die quenching of steels Modelling microstructure evolution and work hardening in conventional and ultrafine-grained microalloyed steels. Part 3 Microstructure evolution in the processing of other metals: Microstructure control in creep-age forming of aluminium panels Microstructure control in processing nickel, titanium and other special alloys.

A study of thermal-mechanical modeling of hot flat rolling

Mathematical modeling of the thermal and mechanical events during hot flat rolling is considered. The difficulties in arriving at accurate and consistent results are identified as those involving the boundary conditions of the process. These refer to the roll/strip heat transfer coefficient and the shape and magnitude of the roll gap contact surface. The effects of the variations of these parameters on the resulting calculations are studied.

Parallel finite element calculation of plastic deformations on Exemplar SPP1000 and on networked workstations

Abstract In this paper we present an approach to parallelization of the program for computation of axisymmetrical forging process. The parallel algorithm we have applied is based on non-overlapping domain decomposition method. A mesh of elements is divided into layers assigned to different processes. The parallel program was written in C using PVM and it was implemented on Convex Exemplar SPP1000 and on networked workstations IBM RS/6000-320. We have investigated dependence of performance of the elaborated parallel program on number of processs and on number of nodes in the mesh.

Multistage compression of microalloyed steels - FE simulation and measurements of grain size

Abstract The objective of the project is to investigate the process of the multistage compression in application to the testing of the microalloyed steels. Theoretical part of the work is based on the thermal-mechanical finite-element model of the compression process. This model is connected with the conventional closed-form equations describing processes of recrystallization, precipitation and grain growth in the microalloyed steels. The experiment was performed on the hydraulic testing machine and it included both single- and multistage compression. The samples were air cooled or quenched after the deformation. The microstructure on the cross section of each sample was investigated and a comparison between the measured and calculated grain size was done.

The effect of deformation zone geometry on internal defects arising in plane strain rolling

Abstract One of the main aims of the paper is to develop proper criteria in order to avoid the occurence of internal defects such as split ends and central burst which can arise during rolling of metallic products. Using an upper bound method with assumption of a rigid body uni-triangular velocity field for deformation zone the model of these defects is proposed. The power solutions obtained for analysed cases allowed to classify the rolling parameters into safe and defect ranges. The suggested upper bound model has been compared with the rigid-plastic finite-element approach and the stress distribution across the workpiece at the exit plane has been calculated using this approach. The finite element calculations and results obtained from experimental rolling of aluminum alloy confirm the criteria obtained by the upper bound method. Both, perfectly plastic and strain-hardening materials are studied. It has been found that split end iand internal burst formation is more likely to ocur in thick plates and sheets when small reduction are applied.