D. V. Edmonds

Bio: D. V. Edmonds is an academic researcher from University of Cambridge. The author has contributed to research in topics: Austenite & Ferrite (iron). The author has an hindex of 5, co-authored 5 publications receiving 958 citations.

The bainite transformation in a silicon steel

Abstract: An experimental silicon steel has been used in a detailed kinetic and structural study of the bainite transformation in an attempt to resolve some of the controversies concerning the reaction mechanism. Distinct reaction ‘C’ curves and transformation mechanisms were observed for the upper and lower bainite reactions. The observed set of three minima in transformation kinetics were found to be incompatible with the solute drag explanation of the kinetic Bs temperature. Transmission electron microscopy indicated the growth of both upper and lower bainite by the propagation of displacive subunits, with adjacent nucleation in the latter case. Definite evidence for carbon supersaturation was obtained for the lower bainitic ferrite. The results are best explained in terms of a shear mechanism for the ferritic component of bainite rather than a ledge mechanism (as is observed in Widmanstatten ferrite growth). Carbide precipitation events were also characterized and the evidence suggested that precipitation resulted from the aging of a supersaturated matrix in lower bainite. The evidence also suggests that carbide precipitation events are of secondary importance to the essence of bainite formation. It was further proven that the concept of a metastable equilibrium1 controlling the transition from upper to lower bainite was not applicable to the present steel and indeed, if any metastable equilibrium does exist in any other steel, it does not constitute a general phenomenon and hence is not essential to the bainite transformation mechanism.

Bainite in silicon steels: new composition–property approach Part 1

Abstract: Recent work on the mechanism of the bainite transformation has shown that the extent of transformation to bainitic ferrite, and the carbon content of the remaining austenite, can be estimated thermodynamically. This paper is concerned with the application of this recent work to the development of a quantitative relationship between the composition and some important mechanical properties of silicon containing steels, which can be transformed isothermally to an aggregate of bainitic ferrite and carbon enriched retained austenite only. It is demonstrated that the method has predictive capabilities, and forms the basis of further work (Part 2 of this study) on the theoretical design, optimization, and testing of two promising steel compositions.

Interfacial embrittlement in liquid-phase sintered tungsten heavy alloys

Abstract: The variations in toughness of the liquid-phase sintered heavy alloys W-Ni-Fe and W-Ni-Cu have been examined. Toughness was found to be controlled primarily by the strength of the tungsten particle-matrix interfaces, which is sensitive to the rate of cooling from the sintering temperature. Furnace-cooling led to the embrittlement of these interfaces; in the case of W-Ni-Fe transmission electron microscopy identified interfacial precipitation of a W(NiFe) intermetallic compound, and in W-Ni-Cu Auger electron spectroscopy indicated interfacial segregation of the trace elements phosphorus and sulfur. This embrittlement was effectively reduced by solution treating beneath the sintering temperature and quenching

Unidirectional transformation of Fe-0.8C-Co alloys: Part I. Process per structure relationships and the significance of pearlite interlamellar spacing measurements

Abstract: Fe-0.8C-Co alloys have been unidirectionally transformed by translation through a temperature gradient which establishes a single interface between the austenite and pearlite phases. A comparison between the temperature gradient calculated to maintain a single planar interface by preventing nucleation ahead of the interface and that actually required in the experiment led to the conclusion that the temperature gradient must retard the nucleation of pearlite. The relationship between interface morphology and the resultant pearlitic microstructure was examined, and comparisons made with more familiar pearlite structures produced by conventional isothermal heat treatments. The degree of alignment of the pearlite lamellae was analyzed metallographically and found to be predominantly within 12 deg of the growth direction. A detailed analysis of interlamellar spacing measurements as a function of growth velocity established that pearlite can possess a wide range of interlamellar spacings at any constant growth velocity and that this range seems to be dependent on growth velocity.

Unidirectional transformation of Fe-0.8C-Co alloys: Part II. Kinetics of the eutectoid reaction

Abstract: Unidirectional transformation techniques have been applied to a study of the kinetics of eutectoid growth in Fe-0.8C-Co alloys. The technique readily yielded kinetic data which it is shown could be used to indicate the rate controlling process for pearlitic growth. Accurate measurements of interlamellar spacing (λ) could be made at controlled growth rates (V) and analyzed in terms of the expressionVλn, where the exponentn can indicate the rate controlling process. The results obtained by unidirectional transformation were compared with those achieved by conventional isothermal transformation, both to aid in the initial interpretation of the more unfamiliarV:λ data and also to show that the two different experimental routes lead to equivalent kinetic data. Analysis of the results obtained for Fe-0.8C-Co alloys suggested control by interfacial diffusion of carbon at high growth rates (n=3) changing towards volume diffusion of carbon at lower growth rates (n=2), but also revealed an unexpected region at very slow growth rates (n=4). This anomalous region could be explained in terms of the partitioning of cobalt as the growth rate decreased. It was also shown that cobalt additions decreased the pearlite interlamellar spacing at constant undercooling or growth rate.

Bainite in Steels

Abstract: The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

Structure–properties relationship in TRIP steels containing carbide-free bainite

Abstract: The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIP-aided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.

Quenching and partitioning martensite-a novel steel heat treatment

Abstract: A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.