F. G. Caballero

Very strong low temperature bainite

Abstract Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.

Design of novel high strength bainitic steels: Part 1

Abstract Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental verification of the design process.

Application of dilatometric analysis to the study of solid–solid phase transformations in steels

Abstract This article outlines the use of dilatometry in solid–solid phase transformation research in steel. It describes how dilatometric data are interpreted, with an emphasis on continuous heating and cooling transformation diagrams. These diagrams show the microstructural constituents that result from given heating and cooling conditions, and are an invaluable tool for the metallurgist in characterizing steels with respect to their response to heat treatments. Several practical examples and applications of dilatometry in steel research are briefly described in this work.

Tempering of hard mixture of bainitic ferrite and austenite

Abstract Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV30, tensile strength ~2.3 GPa and toughness ~30 MPa m1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out in relation to the tempering resistance of steel over the temperature range 350 – 750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from decomposition of the carbon enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery during annealing.

Mechanical Properties of Low-Temperature Bainite

The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (< 45 MPa m0.5). This unusual combination of properties is attributed to the exceptionally fine scale of the carbidefree bainitic microstructure and the associated retained austenite.

Modelling of kinetics and dilatometric behavior of non-isothermal pearlite-to-austenite transformation in an eutectoid steel

European Coal and Steel Community (ECSC-7210. EC/939) and the Spanish Comisio´n Interministerial de Ciencia y Tecnologi´a (CICYTMAT95-1192-CE)

Metallographic techniques for the determination of the austenite grain size in medium-carbon microalloyed steels

Abstract Different techniques have been investigated to seek the best procedure to reveal the prior-austenite grain boundaries in three medium-carbon microalloyed steels. This study has been carried out over a wide range of temperatures (950–1250°C) and it has been found that thermal etching (TE) is the best technique to reveal the prior-austenite grain boundaries in these steels.

Revealing austenite grain boundaries by thermal etching: advantages and disadvantages

Abstract This article reviews the method of thermal etching for revealing the prior-austenite grain boundaries in steels. This method involves preferential transfer of material away from grain boundaries when the steel is exposed to a high temperature in an inert atmosphere. Thus, during austenitization of a prepolished sample, grooves are formed at the intersections of the austenite grain boundaries with the polished surface. These grooves remain intact after cooling and are clearly visible at room temperature outlining the austenite grain boundaries. However, at very high austenitization temperatures, those grooves might interfere with the advance of the austenite grain boundaries on the surface. In that case, this technique could lead to a wrong measurement of the austenite grain size. The aim of this work is to study the advantages and disadvantages of the technique to reveal the austenite grain boundaries in microalloyed steels.

Evaluation of potential of high Si high C steel nanostructured bainite for wear and fatigue applications

Abstract The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (<300°C). The first part gives an overview of the design principles, allowing very high strength and ductility to be achieved while minimising transformation duration. Wear and fatigue properties are then investigated for over 10 variants of such materials, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strengths above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non-negligible parameters remain to be quantified. Rolling–sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional 100Cr6 isothermally transformed to bainite. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumably introduces compressive residual stresses. Fatigue performance was slightly improved over 100Cr6 for one of the two industrially produced materials but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.

On measurement of carbon content in retained austenite in a nanostructured bainitic steel

In this study, the carbon content of retained austenite in a nanostructured bainitic steel was measured by atom probe tomography and compared with data derived from the austenite lattice parameter determined by X-ray diffraction. The results provide new evidence about the heterogeneous distribution of carbon in austenite, a fundamental issue controlling ductility in this type of microstructure.