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I.

INTRODUCTION

MORPHOLOGICAL classification of the microstructural constituents in steels is based on the appearance of ferrite or cementite or of ferrite-cementite aggregates. The addition of strong carbide-forming elements, for example, V, Nb, and Ti, is known to alter the normalized or isothermally transformed microstructure of steels; even when added in small amounts, they can cause fine dispersions of alloy carbide (known as interphase precipitation) to form within the ferrite phase of ferritic or pearlitic steels.[1–5] These altered microstructures have been studied in great detail, and the knowledge of them has reached a sufficient degree of maturity to allow their use in industy. Consequently, they too can now be considered as conventional microstructures. Notwithstanding that chromium is a weaker carbide former, it can also cause significant alteration to the ferrite/pearlite microstructures of steels. Like the strong carbide formers listed earlier, chromium can also produce microstructures containing interphase precipitation.[6,7] However, chromium is further associated with some unusual transformation products.[8–20] These novel microstructures in chromium-containing Fe-C alloys have drawn less attention, despite the fact that they have been known to occur since the early electron metallography of Schrader,[8] or even from evidence in the earlier optical metallography of Jolivet, who described them as arborescent,[21] and Lyman and Troiano.[22] These unusual austenite decomposition products occur in the lower-temperature range of the pearlitic region, or between the pearlitic and bainitic region if the latter is present. A.A. KAYA, formerly Research Student with the Department of Materials, University of Oxford, Oxford, United Kingdom, is Research Fellow, School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia. D.V. EDMONDS, formerly Lecturer, Department of Materials, University of Oxford, Oxford, OXI 3PH, is Professor of Metallurgy, Department of Materials, University of Leeds, Leeds LS2 9JT United Kingdom. Manuscript submitted December 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A

The curved transformation front of pearlitic nodules becomes progressively more irregular as the isothermal transformation temperature is lowered, while, at the lowest temperature, the structure is distinctly acicular, akin to bainite under the light microscope. Composition measurements carried out on thin foils and replicas has shown the presence of preferential partitioning of chromium to cementite at the ‘‘spiky’’ interface, and that the extent of partitioning was less at the spiky interface than in the relatively smooth regions from which the spikes were projecting. Chance and Ridley[14] and Ridley et al.,[16] attempted to relate this spiky structure to interphase precipitation. These unusual structures have been observed over a wide range of carbon contents, from hypoeutectoid to hypereutectoid steels, and after quite modest Cr additions of as little as 1 to 2 wt pct.