Materials Science Forum Vols. 638-642

Abstract: Dual-phase steels, developed in the 1970’s, found until now wide applications related to their optimum combination of strength and ductility, in particular as flat semi-products used for further shaping by deep drawing and cold forming operations. Actually the need appears to manufacture long semi-products of dual-phase steels for further processing by cold forging, in order to obtain high-strength final products, like e.g. fasteners, without additional heat-treatment. An additional aim has been to explore the possibility of achieving from the same chemical compositions dedicated properties for particular applications. In this work a thermo-mechanical route was developed of modifying an originally bainitic-martensitic low-alloy steel to achieve a dual-phase microstructure. For this purpose physical simulation was used on Gleeble 3800 thermal-mechanical simulator, programmed to affect austenite in its dynamic recrystallisation range followed by separation of microstructures in the two-phase austenite-ferrite range. Observation of relaxation was used to monitor advancement of transformation / recrystallisation in subsequent stages of the processing. In the simulated hot-warm rolling process it appeared possible to convert the original bainitic microstructure, having prior austenite grain size ~15μm and the martensite-bainite laths of usual length throughout these grains, to the dual-phase microstructure containing well-recrystallised ferrite of an average grain size 1*1.5μm and fine second-phase islands of less than 1.0μm diameter. SEM and TEM analysis have been applied to describe details of the resulting microstructures.

Abstract: High chromium cast iron material 1.4405 GX4CrNiMo 16-5-1 is successfully used in chemical process industry components loaded with hot, highly corrosive and abrasive at high pressures. Overheating test made a discontinuous yielding knee to appear caused probably by dissolution of carbon containing smaller phases but the overall strength and ductility are not affected. Microstructures are studied by optical and SEM microscopy and hardness measurements. This alloy has three main phases appearing as a mixture of rounded and lamellar phases. The work hardening behaviour is studied by analytical composite phase models and 2D FEM models with individual bilinear hardening phase models. The results agreed reasonably. FEM models showed the weak points of the alloy. Fit to test data was better with rounded phase than lamellar phase model. Combination of these tools is useful for innovative special alloy developments.

Abstract: In design of process equipment and hot vessel beam joints the real effect of relaxation is considered. Standards make an allowance that some relaxation of stresses at joint could be advantageous in relieving the stress peaks due to reaction moments. Magnitude of the moments in itself is not the problem but the stress distribution caused by it. The goal in this study is to estimate the possibility of taking advantage of relaxation in design using as test case a rectangular beam. Analytical theory of creep is applied with constant bending moment load. Stress distribution is first linear elastic and then flattens to nonlinear form resembling the elastic plastic bending. FEM gave analogous results constant moment and time dependent strain. One advantage is that fatigue life is increased at these joints after moderate amount of creep relaxation.

Abstract: The Portevin Le Chatelier (PLC) effect appears in many metallic materials at different temperatures. Some numerical simulations of this effect for different alloys (aluminium, steel, nickel based superalloy) are presented in this article. The mechanical model remain the same for all studied materials but the behavior parameters and identification methods differ. The scale at which this effect is investigated also varies from the microstructure to aeronautic components.

Abstract: The system casting-mould is considered. The thermal processes proceeding in a casting sub-domain are described using the one domain approach. The model of solidification process is supplemented by the energy equation concerning the mould sub-domain, the continuity conditions given on the contact surface between casting and mould, boundary conditions on the outer surface of the system and the initial ones. To solve the problem the generalized variant of finite difference method (GFDM) is used. Temporary and local values of temperature can be found at the optional set of collocation points from the domain considered. This essential advantage of GFDM allows to locate and thicken nodes at the regions for which the temperature gradients and cooling (heating) rates are considerable. In the final part of the paper, the example of numerical simulation is shown.

Abstract: In the analysis of the thermomechanical behaviour of the target material subject to Laser Shock Processing (LSP), most of the simplified models used for the analysis of its residual shocked state rely on rather simple estimations or material response equations that rarely take into account a detailed description of the material subject to a simultaneous dynamic compression and either deformation-induced or plasma-driven thermal heating. The calculational system developed by the authors (SHOCKLAS) includes a coupled analysis of the pressure wave applied to the target material as a result of the plasma buildup following laser interaction and the shock wave propagation into the solid material with specific consideration of the material response to thermal and mechanical alterations induced by the propagating wave itself (i.e. effects as elastic-plastic deformation, changes in elastic constants, etc.). The model is applicable to the typical behaviour shown by the different materials through their dynamic strain-stress relations. In the present paper, the key features and several typical results of the developed SHOCKLAS calculational system are presented. In particular, the application of the model to the realistic simulation (full 3D dependence, non linear material behaviour, thermal and mechanical effects, treatment over extended surfaces) of LSP treatments in the experimental conditions of the irradiation facility used by the authors is presented

Abstract: A model for the nucleation and growth processes of Sn whisker is offered. High density of localized screw dislocations by deformation form the dense spiral steps of atomic scale on Sn surface. The spiral steps would induce the nucleation of Sn whisker. Edge dislocations localized at the same region where the dense screw dislocations exist supply Sn atoms to Sn whisker through pipe diffusion. Both screw and edge dislocations would bend along almost one direction, namely, to relax the external shear stress. The image force also helps to bend the dislocations perpendicular to the whisker side-surface. The bending of dislocations at root of whisker leads the bend of whisker. The pipe diffusion of Sn atoms through edge dislocations from bulk Sn toward whisker is suppressed at the bent part of edge dislocation, resulting in release of Sn atoms inside whisker and leading to the growth of whisker near its root.

Abstract: The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This paper reports on a model Fe-30wt%Ni based alloy, which retains a stable austenitic structure at room temperature, and has therefore been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterised in detail using optical microscopy, XRD, SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30%Ni base alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni-Nb alloy in which the strain-induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed.

Abstract: This experimental work deals with the influence of niobium additions to high purity nickel on dynamic recrystallization behavior during hot working. Various high-purity alloys were prepared (unalloyed Ni and Ni–0.01, 0.1, 1 and 10 wt % Nb) and deformed to high strains by hot torsion tests to characterize the rheological behavior within the range 800 – 1000°C at strain rates of 0.03, 0.1 and 0.3 s–1. Niobium additions strongly increased the flow stress. To quantify such behavior, the strain-hardening parameter h and dynamic-recovery parameter r in the Yoshie-Laasraoui-Jonas constitutive equation were determined from the initial part of the experimental stress-strain curves (i.e., at strains before the stress peak) in which dynamic recrystallization does not alter the mechanical behavior. A table showing the variation of h and r as a function of strain rate, temperature, and niobium content was compiled and used to fit a simple empirical model for predicting h and r from the deformation conditions and alloy composition. In addition, microstructures were determined by optical metallography and SEM/EBSD. Based on this work, it appears that niobium additions noticeably refine the steady-state grain size by considerably decreasing the kinetics of dynamic recrystallization in nickel.

Abstract: Recent trends in the production of high strength steel plate call for increasingly sophisticated thermo-mechanical treatment schedules, including the use of high rate accelerated cooling after finish rolling in order to achieve the desired microstructure and mechanical properties. Achieving the necessary cooling process control accuracy in such cases requires a sound understanding and description of the interactions between external heat transfer processes and changes in internal energy due to phenomena such as solid-state phase transformations. The thermal physical properties of the evolving microstructures of complex phase and martensitic steels vary greatly, and are strongly dependent on temperature and constituent phases. As a result, critical parameters such as thermal diffusivity cannot be accurately estimated without appropriate linkage to both phase transformation kinetics and temperature. In the present study, a numerical simulation has been developed to investigate the unsteady heat transfer and phase transformation behaviour of a moving steel plate during accelerated cooling. The simulation includes semi-empirical microstructure evolution sub-models, fitted to measured CCT data using non-linear regression. These are coupled to thermal-physical properties sub-models and thermal conduction calculations. A comprehensive suite of thermal boundary condition models which account for direct water cooling, forced convection film boiling, air cooling, radiation and heat transfer between plate and transport rollers are also included. The required equations for the plate temperature and microstructure evolution are solved numerically using a cell centred finite volume method, and the model has been validated by comparing simulated cooling stop temperatures with measurements obtained on the plate cooling section of an industrial plate mill. The predicted cooling stop temperatures of steel plates for different thicknesses, velocities and water flow rates are in good agreement with plant operational data.