Recent advances in the development of high performance steels presenting improved properties of strength and ductility rely on the TRIP effect, i.e. on the mechanically-induced martensitic transformation of the retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilisation and retention of austenite at room temperature have become of primary importance, leading to specifically designed steel grades and thermal or thermomechanical treatments. Particularly, carbon enrichment of the austenite during intercritical annealing and bainite transformation was found to be very effective in retaining austenite. This metastable austenite then progressively transforms during straining, bringing about a large increase of the work hardening rate. This increase results from the stress and strain partitioning continuously evolving with the appearance of the hard martensite. (c) 2004 Elsevier Ltd. All rights reserved.

Transformation-induced plasticity for high strength formable steels

Multiscale mechanics of TRIP-assisted multiphase steels: II. Micromechanical modelling

Concentrating solar power (CSP) provides the ability to incorporate simple, efficient, and cost-effective thermal energy storage (TES) by virtue of converting sunlight to heat as an intermediate step to generating electricity Thermal energy storage for use in CSP systems can be one of sensible heat storage, latent heat storage using phase change materials (PCMs) or thermochemical storage Commercially deployed CSP TES systems have been achieved in recent years, with two-tank TES using molten salt as a storage medium and steam accumulators being the system configurations deployed to date Sensible energy thermocline systems and PCM systems have been deployed on a pilot-scale level and considerable research effort continues to be funded, by the United States Department of Energy (DOE) and others, in developing TES systems utilizing any one of the three categories of TES This paper discusses technoeconomic challenges associated with the various TES technologies and opportunities for advancing the scientific knowledge relating to the critical questions still remaining for each technology

Technical Challenges and Opportunities for Concentrating Solar Power With Thermal Energy Storage

Slags are an indispensable tool for the pyrometallurgical industry to extract and purify metals at competitive prices. Large volumes are produced annually, leading to important economical and ecological issues regarding their afterlife. To maximise the recycling potential, slag processing has become an integral part of the valorisation chain. However, processing is often directed solely towards the cooled slag. In this article, the authors present an overview of the scientific studies dedicated to the hot stage of slag processing, i.e. from the moment of slag/metal separation to complete cooling at the slag yard. Using in-depth case studies on C 2 S driven slag disintegration and chromium leaching, it is shown that the functional properties of the cooled slag can be significantly enhanced by small or large scale additions to the high temperature slag and/or variations in the cooling path, even without interfering with the metallurgical process. The technology to implement such hot stage processing steps in an industrial environment is currently available. No innovative technological solutions are required. Rather, advances in hot stage slag processing seem to rely primarily on further unravelling the relationships between process, structure and properties. This knowledge is required to identify the critical process parameters for quality control. Moreover, it could even allow to consciously alter slag compositions and cooling paths to tailor the slag to a certain application.

Hot stage processing of metallurgical slags

Modelling of the plastic flow of trip-aided multiphase steel based on an incremental mean-field approach

Labscale freeze layers of an industrial nonferrous slag with Al2O3-CaO-FeOx-MgO-SiO2-ZnO as main components are studied to explore the microstructure and the composition of an industrial freeze lining. The freeze layers were formed by submerging a watercooled probe into a liquid slag bath. The influence of submergence time, of heat input from the furnace, and of the rotational speed of the crucible is studied. The microstructures of the freeze layers are examined using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Thermodynamic software is used to interpret the solidification microstructure. The results show that the freeze layer microstructure consists of different zones, depending on the local thermal history. These zones show different growth morphologies and different microstructure scales, from an amorphous matrix with small crystals to large columnar crystals. Furthermore, two microstructure types are observed, one with melilite columnar crystals and the other with olivine columnar crystals. These microstructure types appear for similar experimental conditions and are even observed within the same freeze layer. An increase in submergence time or in heat input from the slag bath does not seem to favor a particular microstructure type. A high rotational speed of the crucible resulting in a higher convection in the slag bath seems to favor the microstructure type with olivine columnar crystals.

On the Microstructure of a Freeze Lining of an Industrial Nonferrous Slag

Lab scale freeze layers of an industrial nonferrous slag with as main components Al2O3-CaO-FeO
 x
 -MgO-SiO2-ZnO were studied to explore the mass transport in an industrial freeze lining. The freeze layers were formed by submerging a water-cooled probe into a liquid slag bath. In previous research,[1] two distinct types of microstructures were observed in these freeze layers: one with only melilite columnar crystals and one with both melilite and olivine columnar crystals. In this article, the mass transport during the formation of these microstructures is investigated. The element distribution, the phase fraction, and the crystal morphology were examined using electron microscopy and electron probe microanalysis. In addition, thermodynamic software was used to calculate the solidification path of the slag. The impact of the mass transport on the growth of a freeze lining depends on the solidification rate. For high solidification rates, only short-range mass transport is observed and small crystals form. For lower solidification rates, long-range mass transport is observed, resulting in an exchange of components between the freeze layer and liquid slag bath and in the formation of large crystals. Different growth mechanisms are observed for melilite and olivine crystals. The broad large melilite crystals have to exchange components with the liquid slag bath to grow, because the amount of liquid slag between the crystals is limited, while the long thin olivine crystals can exchange components with a large amount of liquid slag in between the crystals. Furthermore, the mass transport of minor elements may be very important, because these elements can pile up at the crystal-liquid slag interface and hamper the crystal growth.

On the Mass Transport and the Crystal Growth in a Freeze Lining of an Industrial Nonferrous Slag

A three-dimensional finite-element microstructural cell model involving an inclusion of retained austenite embedded within a ferrite grain, which is surrounded by a homogeneous matrix representing the behavior of a transformation-induced-plasticity (TRIP)-assisted multiphase steel, was developed in order to address the micromechanics of the martensitic transformation in small isolated austenite grains. The transformation of a single martensite plate is simulated after various amounts of prior plastic deformation under different in-plane loading conditions. The values of the mechanical driving force and of the elastic and plastic accommodation energies associated with the transformation are calculated as a function of the externally applied loading conditions. The mechanical driving force and the total accommodation energy are of the same order of magnitude. The mechanical driving force depends upon the stress state and is the highest for plane-strain conditions. The total accommodation energy is almost independent of the stress state. It is affected by the amount of plastic straining prior to transformation and is very much dependent on the level of the shear component of the transformation strain. The results of this study provide guidelines for the development of realistic stress-state-dependent transformation evolution laws for TRIP-assisted multiphase steels.

https://link.springer.com/content/pdf/10.1007/s11661-006-0156-1.pdf

Three-dimensional computational-cell modeling of the micromechanics of the martensitic transformation in transformation-induced-plasticity-assisted multiphase steels

The combined thermodynamic-micromechanical model of Fischer [1] is applied to a low-alloyed TRIP steel with a volume fraction of 16% of retained austenite. The model is implemented in a finite element code. The mesh consists of 9 by 9 by 9 cubical elements, each representing a single grain with a random crystallographic orientation. The retained austenite grains are randomly dispersed through the entire mesh. The extent of the martensitic transformation in all austenite grains is calculated. A large spread in transformation rate is observed. The most favourably oriented grains reach a full martensitic structure, while the martensite volume fraction of less favourably oriented grains is less than 50%. When the chemical driving force is more negative, the onset of the transformation is delayed and the increase of the martensite volume fraction is slower. The calculated results are compared with experimentally obtained values. Although in general a reasonable agreement is found, Fischer's approach leads to some discrepancies with experimental observations.

Tim Van Rompaey

Papers

On the Microstructure of a Freeze Lining of an Industrial Nonferrous Slag

On the Mass Transport and the Crystal Growth in a Freeze Lining of an Industrial Nonferrous Slag

Three-dimensional computational-cell modeling of the micromechanics of the martensitic transformation in transformation-induced-plasticity-assisted multiphase steels

Micromechanical modelling of TRIP steels

Slag solidification with water-cooled probe technique