Austenite in Transformation-Induced Plasticity Steel Subjected to Multiple Isothermal Heat Treatments

The thermodynamic limit to the progress of the bainite reaction in steels containing a cementite inhibitor often leaves large quantities of thermally or mechanically unstable austenite. Such austenite is not effective in delaying the onset of plastic instabilities during the course of deformation. In such circumstances, it is useful to conduct isothermal transformation at a high temperature where the rate of reaction is relatively rapid, followed by a lower temperature step that permits more bainite to be generated. This in turn increases the stability of the refined austenite, which then transforms gently over a large range of strain during a tensile test. A significant corollary is that the two-step heat treatments are unnecessary in low-carbon steels, where the bainite reaction is able to proceed to a greater extent before reaching the thermodynamic limit. Furthermore, the two-step process can be counterproductive in low carbon steel, because the austenite content is reduced to a level below which it does not enhance the mechanical properties. Other circumstances in which multiple heat treatments are necessary are also discussed.The authors are grateful to POSCO for support through Steel Innovation Programme, and to the World Class University Programme of the National Research Foundation of Korea, Ministry of Education, Science and Technology, project number R32-2008-000-10147.This is the accepted manuscript version. The final published version is available from Springer at http://link.springer.com/article/10.1007%2Fs11661-014-2405-z

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

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

Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels

A Semi-Empirical Model for the evolution of Retained Austenite via Bainitic Transformation in Multiphase TRIP Steels

Multiphase TRIP steels exhibit unique combinations of strength and cold formability, characteristics especially desirable in automotive applications. This behaviour is largely determined by the amount and stability of the retained austenite dispersion in the microstructure, produced by a two-stage heat-treatment, consisting of intercritical annealing followed by an isothermal bainitic treatment. The amount and stability of retained austenite is mainly determined by the proper selection of the temperature and temporal duration of the bainitic treatment. In the present work an approach is presented which allows for the calculation of the amount of retained austenite in the microstructure, as a function of bainitic treatment conditions. The approach is based on the physical characteristics of the bainitic transformation and on the stabilizing effects of the formation of bainitic ferrite in austenite. Each bainitic ferrite platelet is considered to chemically stabilize a part of the surrounding austenite due to carbon rejection. The spatial and temporal extent of this stabilization is determined by solving the corresponding carbon-diffusion problem, and thus the amount of retained austenite contributed by any individual platelet is determined. Subsequently, the evolution of the population of the platelets in the entire microstructure is determined and, thus, the volume fraction of retained austenite as a function of transformation time is calculated. Application and comparison of calculations with experimental results, obtained from four different multiphase TRIP steel compositions, exhibited very good qualitative and quantitative agreement

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

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