Some transformations occur in conditions where the atoms cannot diffuse during the course of the experiment. In these circumstances, the change in crystal structure is achieved by a mechanical deformation. This displacements associated with this deformation are called transformation plasticity, which is important both in helping understand the mechanism of phase transformation and in industrial treatments where the displacements lock stress into the material. Such residual stresses can be very detrimental to the performance of engineering structures.
This investigation dealt with a unique technique for demonstrating the transformation plasticity associated with a high-temperature displacive transformation called bainite. Having established that the strains caused by the stress-affected growth of bainite are anisotropic, a detailed model was developed which could explain the peculiar experimental data.
Further high-resolution atomic-force microscopy confirmed the basic mechanism assumed for the formation of bainite. The work was subsequently extended to other transformations in steel.
The work has led to the first quantitative modelcapable of explaining the development of strain during the course of certain phase transformations that occur in common steels. The importance of this to industrial practice has been assessed, and the detailed assessment has been published in a in a book used by people who model the development of stresses in engineering structures. The fundamental information gained is steadily being incorporated into microstructure models which are used in the design of steels.
The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom.
Mechanical alloying is a process in which mixtures of fine powders consisting of elemental metals or master alloys are changed into solid solutions, apparently without any melting. Very unusual alloys can be produced by a process in which the powders are forced to collide with each other and with much larger, hardened steel balls whilst contained in a ball mill. The collisions are energetic, involve large contact pressures, and lead eventually to the formation of an intimate solid solution. Refractory oxides (commonly yttrium oxide) can also be finely dispersed into the mechanically alloyed powder in order to obtain dispersion strengthening. The mechanically alloyed powder is finally extruded to form full density bulk samples in rod, sheet or other useful shapes.
After consolidation by extrusion, the alloys are usually very hard and possess an extremely small grain size, typically a small fraction of a micrometre. The material has to be softened before further fabrication. The grain boundary area locked into the material gives it an unusually large stored energy, which under suitable conditions triggers recrystallisation into very coarse grains which are highly anisotropic. The recrystallisation front generally moves along the extrusion direction, leading to "directional recrystallisation" and a columnar grain structure reminiscent of directionally solidified samples.
There are two major aspects of the recrystallisation of mechanically alloyed yttria dispersion strengthened steels and nickel-base superalloys which are not understood. Recrystallisation occurs at exceptionally high homologous temperatures, of the order of >0.9 of the melting temperature TM . This contrasts with the well established recrystallisation temperatures of around 0.6 TM in ordinary variants of similar metallic alloys. The discrepancy could not be explained. Secondly, the reason why these alloys recrystallise to a columnar grain structure is not understood. Both of these difficulties influence the exploitation of these materials; it has not been possible to effectively control the microstructure and hence avoid the highly anisotropic mechanical properties associated with the columnar grain structures.
The mechanism of directional recrystallisation has been identified in such detail, that it has for the first time ever been possible to control the grain shape by heat-treatment alone. Thus, we can change the highly anisotropic grains into fine equiaxed grains. An exciting result is that we can now quantitatively explain the extraordinarily high recrystallisation temperatures in these materials. The starting grain structure is so fine that the grain boundary junctions are strong pinning points relative to the size of the grain boundary bulge required to initiate recrystallisation. This is an entirely new concept which explains many otherwise confusing observations, even in other nanostructures. The theoretical work done in the project has resulted in a computer program which enables the estimation of the recrystallisation temperatures as a function of the dispersion characteristics, heat treatment and other parameters. New alloys have been designed and manufactured as promised, and the knowledge has been widely disseminated.
The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom.
The work summarised here is a part of a larger EPSRC/LINK project for the design of construction steels. The overall project dealt with the computer-aided design of steel sections for civil engineering, but this particular project focussed on microstructure and mechanical property modelling.
Transformations in steels rarely occur in isolation. We have modified Avrami overall transformation kinetics theory to deal with the simultaneous occurrence of two or more transformations. The method is demonstrated to faithfully reproduce published data on the volume fractions of allotriomorphic, Widmanstatten ferrite and pearlite as a function of chemical composition, austenite grain size and heat treatment (isothermal or continuous cooling transformation).
In addition, we have modelled the strength of such steels using elasticity and plasticity theory combined with neural network modelling. Some of the work has been published, both to disseminate the research and to enhance the teaching of design and modelling.
The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom.
The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom. The results of the project have been assessed by EPSRC referees as follows:
See also British Steel's Slimdek documentation.
The development of new welding alloys for power plant steels has in the past been achieved by trial and experience. The purpose of this work was to enable a significant proportion of the development procedure to be done by computation. A variety of methods have been used towards this end, ranging from physical models to other methods which rely heavily on patterns in experimental data.
A thermodynamic method based on the theory of martensite nucleation has been applied to the calculation of martensite-start temperatures of highly alloyed steels. This information is necessary for welding procedures. The formation of martensite is suppressed as the strength of austenite is increased by alloying. Thus, the critical driving force necessary to trigger martensite is larger for stronger austenite.
Some confusing mechanical property data on 9Cr1Mo type steel welds have been thoroughly explained by considering the relative stabilities of delta-ferrite and austenite as a function of chemical composition. Excessive concentrations of austenite stabilising elements such as nickel cause the formation of austenite during post weld heat treatment. The austenite then transforms to untempered martensite on cooling, thereby causing a drastic increase in strength and reduction in toughness. By contrast, large concentrations of tungsten make it impossible to fully austenitise the alloy, rendering it exceptionally soft.
A weld typically might contain more than twenty important solute additions and impurities. Its properties also depend on the welding conditions and post weld heat treatment. It is a formidable task, therefore, to attempt to predict the yield and ultimate tensile strengths, elongation and Charpy toughness, all of which are elementary design parameters. A massive dataset was compiled using detailed information from the published literature, and subjected to neural network analysis. This is a highly flexible and powerful empirical method, but it is demonstrated that with care the network can be trained to recognise metallurgically sound relationships. The resulting models have been validated in a variety of ways with emphasis on data previously unseen by the models. Having done this, the models have been used successfully to design a new welding alloy.
The work has led to the first comprehensive model for the design of welding electrodes as a function of the weld metal chemical composition and heat-treatment. This is the only model capable of estimating the yield and tensile strength, the ductility and Charpy toughness as well as microstructural features.
This has resulted in the invention of a new welding alloy for power plant applications. Furthermore, the development resulted directly from computer modelling of the metallurgy, with only one chemical composition ever being made, with the total elimination of the usual trial and error alloy development procedures!
The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom.