Exploitation of the TRIP effect for the development of formable, fracture and fatigue resistant steels for automotive applications

The present paper summarizes recent and on-going work on the exploitation of TRansformation-Induced Plasticity (TRIP) in an effort to develop automotive steels which would possess high-strength combined with high formability while these steels could also exhibit high fracture and fatigue resistance. Especially for the automotive industry, the driving force for these developments is the vehicle weight reduction, which could eventually lead to lower fuel consumption combined with reduced greenhouse gas emissions. The discussion starts with the modelling and characterization of the retained austenite stability (the transforming phase) as well as the modelling of the transformation kinetics, i.e. evolution of transformation with plastic strain. Enhancement of formability is discussed next. Constitutive micromechanical modelling has been employed 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. Fracture resistance can be considerably increased by the TRIP effect. A review is made of the "transformation toughening" arising from metastable austenitic dispersions in ultrahigh strength steels and the high fracture toughness achieved in this class of materials. The potential of the TRIP effect in increasing fatigue strength has not yet received considerable attention. However once we understand the fatigue behaviour of these materials, new applications, e.g. long products made of TRIP steel, might emerge for automotive applications. © Springer Science+Business Media B.V. 2009

Coupled thermodynamic/kinetic analysis of diffusional transformations during laser hardening and laser welding

Several important industrial material processes, such as welding and surface treatments with high energy beams, incorporate rapid thermal cycles characterized by high heating/cooling rates and short dwell times. Computational simulation of the evolution of microstructure under these extreme conditions has received rather limited attention. With the advent of modern computational tools regarding alloy thermodynamics and kinetics, it is possible to simulate the progress of diffusional phase transformations and thus to predict microstructural development. In the present work, moving boundary diffusion problems have been simulated for two cases. In the first case the rapid austenitization during laser transformation hardening of a hypoeutectoid steel was examined. The effects of heating rate, maximum temperature, dwell time and initial microstructure fineness were analyzed. In the second case the aging, dissolution and coarsening of strengthening precipitates in the heat affected zone of laser welds in Al-Mg-Si alloys was examined. The simulation provided the variation of the volume fraction and average size of the strengthening phase during the weld thermal cycle. In both cases the calculations were performed by applying the coupled thermodynamics and kinetics approach, incorporated in the DICTRA program. This kind of simulation provides useful information for the design of the above processes. © 2001 Elsevier Science B.V

Modelling of microsegregation and homogenization of 6061 extrudable Al-alloy

A simulation of the evolution of the as-cast microstructure during the homogenization heat treatment of A6061 aluminum alloy is presented. The microstructure is dendritic with phase and solute segregation. The alloy microsegregation, which results after casting, was calculated with the Scheil equation by employing computational alloy thermodynamics. The microstructure evolution during homogenization was simulated by employing computational kinetics for the solution of the multicomponent diffusion equations. The composition profiles of the alloying elements and the volume fraction of the secondary phases were calculated as a function of homogenization time. The simulation results were validated qualitatively by comparison with metallographic examinations. During homogenization, the Mg2Si dissolution and the transformations of the Fe-intermetallics were confirmed. It is concluded that the model reproduces the homogenization kinetics reasonably and it is capable for the prediction of the homogenization heat treatment completion times. (C) 2007 Elsevier B.V. All rights reserved

On the nanocrystalline structure of TiC and TiB2 produced by laser ablation

Thin films of TiC and TiB2 have been deposited on silicon substrate by pulsed laser deposition technique, under substrate temperatures 25 - 600 degrees C. Transmission electron microscopy revealed the nanocrystalline structure of the coatings. The grain size for the TiC film was between 10 and 70 nm and for the TiB2 film was between 10 and 50 nm

Effect of novel paint removal processes on the fatigue behavior of aluminum alloy 2024

Conventional paint removal processes, based on application of chemicals and abrasion, are becoming inadequate for modern aircraft structures. In addition they are associated with severe environmental problems, mainly due to the production of hazardous waste. Several alternative novel techniques are being developed. However, the aspect of material property degradation due to the application of these new paint removal techniques has not been addressed adequately. The application of laser radiation (carbon dioxide and excimer), as well as plasma etching, has recently been associated with significant ductility deterioration and fatigue life extension. Residual stress measurements, roughness measurements and fractographic analysis were employed in order to rationalize the effect of these novel paint removal processes on the fatigue behavior of aluminum alloy 2024. The observed enhancement of fatigue life is attributed to the development of compressive residual stresses during paint removal processing At low fatigue stresses, the magnitude of the residual stress correlates with the relative enhancement in fatigue life for the three processes investigated. The effect of surface roughening towards decreasing fatigue life is surpassed by the effect of residual stresses in extending fatigue life. Finally, the decrease of toughness and associated damage tolerance ability which follows the application of paint removal processes has been confirmed by fractographic measurements. (C) 1998 Elsevier Science S.A

Carburization of high-temperature steels: A simulation-based ranking of carburization resistance

Carburization is a failure mechanism affecting equipment, such as furnace tubes, operating at high temperatures. Carburization simulations were carried out for the heat-resistant steels referred to the API-530 standard by applying a model for carbon diffusion with the concurrent formation of alloy carbides. The calculated carbon and carbide volume fraction profiles were validated experimentally. The carburization layer is composed from M23C6 and M7C3 carbides. The time required for the carburization front to reach the mid-thickness of the tubes was used to characterize carburization resistance. The austenitic grades exhibit a higher carburization resistance than the ferritic grades at all temperatures. In the ferritic grades, alloy composition has a stronger effect at lower service temperatures (600 degrees C) where carburization resistance increases with Cr and Mo content. The acceleration of diffusion at high temperatures (800 degrees C) dominates the composition effects on carbon diffusion, and the carburization front is controlled by the formation of carbides, which in turn depends on the available amount of Cr in the steel. In the austenitic grades, the highest carburization resistance is exhibited by the stabilized grades 321 and 347 due to formation of TiC or NbC carbides respectively. Regarding the non-stabilized grades, carburization resistance is raised by addition of Mo (316 vs 304) and lower carbon (316L vs 316). The results of this study can be used for material selection for carburization resistance and for planning maintenance procedures for the timely replacement of tubes. (C) 2015 Elsevier Ltd. All rights reserved

Laser-beam carburizing of low-alloy steels

The present study investigated the use of a laser-beam, in order to carburize the surface of DIN 15CrNi6 low-alloy, case-hardening steel. The surface of the material was coated with graphite prior to laser irradiation. Two different kinds of coatings were used: (i) a dilute commercial graphite spray; and (ii) a slurry of graphite powder in ethanol. A CO, laser-beam was used as the heat source, in order to activate the introduction of carbon in the steel surface. Carburizing was achieved by two distinct mechanisms: (a) the surface alloying mechanism, which incorporates melting of the substrate and dissolution of the graphite in the liquid phase; and (b) the solid-state diffusion mechanism, which incorporates austenitization of the substrate and carbon diffusion in austenite. A variety of microstructures and microhardness profiles were produced, depending mainly on the type of graphite coating used, as well as on processing parameters. In general, the carburized layer was accompanied by a heat-affected zone, which was also significantly hardened, mainly due to secondary hardening. Cracking and porosity was observed in some specimens carburized by the surface alloying mechanism, as a result of carbon enrichment and high solidification rates. Finally, an effort was made to investigate the possibility for solid-state diffusion of carbon in austenite, under the short heating times imposed by laser treatment, with the use of computational kinetics simulation. (C) 2001 Elsevier Science B.V. All rights reserved

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