| |
PROCESSES: STUDY AND CHARACTERISATION OF THE BEHAVIOUR OF METALLIC MATERIALS AT HIGH TEMPERATURES
A. Mechanical behaviour at high temperatures
One of the strong points of CTM´s Area of Forming Processes is in fact, the description of the characterisation of the behaviour of metallic materials at high temperatures. It is well known that during hot deformation the material has to undergo some antagonistic phenomena such as hardening by deformation or softening by restoration and/or dynamic recrystallisation. In spite of the outstanding developments of the last decades, the correct description and quantification of these phenomena is still subject of discussion in the scientific community. From CTM, we have put special emphasis on the forecast of flow stress curves at high temperatures according to the initial microstructure, deformation temperatures and speed using for this purpose equations with a physical foundation. This approach is complemented with the capacity of forecasting even the microstructure that results from hot plastic deformation. The approach used is shown in the following figure:
In the first place, the experimental curves are determined by means of a battery of compression tests at different temperatures, deformation speeds and initial microstructures. From these, through the adjustment by the least-square method we obtain the constitutive equations that explain the hardening by deformation and softening by dynamic restoration (Estrin, Mecking and Bergstrom´s equation), maximum tensions and of stable-state (Jonas´s equation modified by Prado and Cabrera), dynamic recrystallisation kinetics (Avrami equation) and the size of the recrystallised grain (Derby equation). All of this gives rise to a predictive model that reproduces with a high level of precision the experimental behaviour. This approach has been successfully used with carbon and microalloyed steels, single-phase and duplex stainless steels, electric steels, advanced steels with a high elastic limit (DP and TRIP), coppers, aluminiums, superalloys, titaniums and low fusion point alloys.
A.1. Carbon steels and microalloys
Special emphasis has been put on the study of the constitutive equations of these steels, mainly with the idea of being able to determine the effect of the chemical composition on them and also the effect of micro alloys elements, either as solute or as precipitate. These studies have also been useful for preparing the constitutive equations and the working methodology that are finally used in all kinds of materials. Likewise, the results have made it possible to count with a wide perspective of the physical metallurgy of microalloyed steels, specially of the ones with medium carbon content. The results obtained from the effect of carbon at high temperatures have been of special interest. Recently, studies are focusing on the effect of boron with the objective of observing how microalloy quantities of this element can interfere with hot plastic deformation. These steels are of special interest for the hot stamping processes., In this work have collaborated, besides CTM, UPC, the McGill University (Canada), CENIM (Madrid), the Universidad Michoacana de San Nicolás de Hidalgo (Mexico) and companies such as SEAT, Industrias Puigjaner, Global Steel Wire, etc.
A.2. Stainless steels
One-phase stainless steels constitute an ideal material for the study of the hot deformation phenomena because they can easily retain the deformed microstructure at room temperature. It is for this reason that these kind of steels have been used to get a better understanding of the processes of dynamic recrystallisation, specially if they are joined to the modern microstructural observation techniques such as EBSD. These analyses have made it possible to explain the important role of twins as nucleation elements of the dynamic recrystallisation as well as the influence of the size of the grain on the transition between cyclic dynamic recrystallisation and single pick recrystallisation. They have also made possible to determine the partial effect of some alloy elements. Another interesting aspect that has been analysed is the hot deformation of duplex stainless steels paying special attention to the accommodation of the deformation between the two phases (austenite and ferrite) coexisting in the steel. The studies in this field are carried out in collaboration with UPC, the Ecole des Mines de Saint-Etienne (France) and the Universidade Federal de Sao Carlos (Brasil).
A.3. Electric steels (Fe-Si)
Steels with a high Si content are mainly used as material in transformers-nuclei and electric motors because of their high features, since they increase resistivity reducing the magnetricity and the total losses. The industrial production of this kind of steels via the conventional way of casting and lamination is limited to compositions inferior to 3.5 weight percentage of Si, due to the brittle fracture phenomena that prevent its lamination at cold temperatures. Nevertheless, the optimum compositions concerning the electrical features are at 6.5 Si % being produced through alternative methods such as CVD (Chemical Vapour Deposition). The microstructural and textural characterisation of such steels is a very important aspect since it defines their final application by classifying them in Orientated and Not orientated. The first ones are mainly used as sheet for transformers-nuclei since these require an anisotropic material in the conduction direction of the magnetic flow with an ideal crystal-graphic texture for the grains in the GOSS {110}<100> component. On the other hand, the second ones are used in rotative machines such as electrical motors since the latter require an isotropic material with an ideal texture on the Cubic fiber <001>//DL. Much of CTM´s work on this kind of steels is orientated to their microstructural characterisation and formability at high temperatures. The work is carried out in collaboration with UPC, the Universidad Autónoma de Nuevo León (Mexico), the University of Gante (Belgium) and the University of Freiberg (Germany).
Texture and microstucture obtained by EBSD of an electric high silicon steel deformed this time at cold temperatures
A.4. Advanced high strength steels (DP, TRIP, TWIP)
The requirements set by the car industry in what regards strength, energy absorption at impact and formability are leading towards the development of new qualities of steels. These steels have the characteristic of presenting excellent combinations of strength and ductility that are the result of the activation of deformation mechanisms such as martensitic transformation, twinning or slip bands. A design of the alloys based on thermo-dynamic calculations together with a suitable design of the manufacturing processes is the key for achieving mechanical properties which will make possible, in the near future, to reduce the weight of vehicles increasing the safety of their passengers. These steels are manufactured at high temperatures and this involves carrying out a study of their formability and prediction of their behaviour at such temperatures. The area of Forming Processes of CTM has carried out studies of DP (Dual-Phase), TRIP (Transformed induced plasticity) and TWIP (Twinning induced plasticity) steels. These works are carried out in collaboration with UPC, CEIT (San Sebastián), ITMA (Asturias) AIMEN (Galicia), McGill University (Canada) and the Universidad Michoacana de San Nicolás de Hidalgo (Mexico) and companies such as SEAT, Industrias Puigjaner, GESTAMP, etc.
A.5. Coppers
Another material that can be used as a model for the analysis of the plastic deformation processes at high temperatures is pure copper (at least of commercial purity) which is used in applications for sanitary piping or electrical cabling. This material has been studied with the aim of observing the influence of residual elements on the mechanical behaviour at high temperatures and also with the objective of developing a predictive model of the cyclic dynamic recrystallisation. The work has been carried out in collaboration with UPC and the company TERTUB (LaFargaTub at present).
A.6. Nickel based superalloys
Ni-based superalloys have found a wide range of applications in the aerodynamic industry because of their excellent mechanical properties and resistance to oxidation at high temperatures. These properties are achieved by controlling the precipitation of different phases such as δ, γ’ or γ’’. We must take into account that in spite of the fact that the precipitation of certain species has a beneficial effect on the mechanical properties, these phenomena might interfere with the manufacturing processes. If the precipitation takes place during such processes, the forces may increase considerably making it difficult to form these materials. Therefore, it is necesarry to characterise the behaviour at high temperatures of these superalloys as well as their precipitation kinetics so as to being able to define suitable processing routes which will make it possible to control the microstructure and properties of the final product. The work is being carried out in collaboration with UPC, technological centres such as FATRONIK and companies such as FRISA AEROSPACE and INDUSTRIAS PUIGJANER.
A.7. Solder type alloys (Sn-Pb y Sn-Ag)
More and more, printed circuit boards have to comply with higher requirements apart from environmental restrictions. The failures of electronic devices are usually due to a loss of connectivity because of a failure of the soldering alloy caused by variations in temperature (by the current flow or by variations of the environment). In such conditions, soldering alloys are working at high temperatures and at the same time under creep-fatigue mechanisms. So as to go deeper into the study of this behaviour, CTM has studied the behaviour at high temperatures of the typical 63Sn-37Pb soldering alloy and also that of one of its alternatives, 96,5Sn-3,5Ag; since Pb´s must be gradually withdrawn. The study was focused on the determination of the mechanical behaviour at high temperatures, detection of the appearance of the phenomenon of superplasticity and influence of the initial microstructure. The work was completed with the mechanical fatigue analysis at different temperatures and work frequency. The work is carried out in collaboration with UPC and Lear Corporation.
B. Hot ductility
There are processes such as continuos casting that are carried out in conditions of temperature and deformation speeds which can favour the appearance of brittle fracture mechanisms and the consequent superficial cracking of the product. These brittle fracture mechanisms depend on the alloy and metallurgical phenomena that might occur. For example, in microalloyed steels brittle fracture can be related with the precipitation in the limits of the grain, the transformation of austenite into ferrite or, for low deformation speeds and high temperatures, with diffusive phenomena such as creep. So as to carry out a suitable design of the manufacturing operations that minimice the risk of appearance of cracks it is necesarry to know the nature of these mechanisms for each alloy. As a rule, the study involves carrying out traction tests at high temperatures with the corresponding fractographic and metallographic evaluation of the samples tested for fracture. The results developed by the group with regards carbon steels have been of special interest and have been extended to other materials such as nickel-based superalloys. The works have been carried out in collaboration with UPC, CENIM and the University McGill (Canada).
C. Prediction of solidification microstructures in steels
This task comes up naturally as a continuation of the previous one. In fact, it is proveen that an important factor that promotes the hot brittle fracture is strongly related with the possibility of chemical segregation during solidification which is closely linked to the dendritic microstructure of solidification and in particular to the primary and secondary dendritic spacings. CTM works in collaboration with UPC, IPN (Mexico) and K&E Technologies (Mexico) and companies such as Global Steel Wire for implementing a macro-micro methodology for the prediction of the solidification microstructures depending on the chemical composition of the steel and the operational parameters of continuos casting (heat evacuation, casting speed, etc…).
<< Return to the index of activities
|
