Structure-property relationships in bainitic steels
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INTRODUCTION
D U R I N G the study of phase transformations in metals, the challenge of understanding the mechanisms of bainite formation has provided a continuing stimulus to several generations of physical metallurgists, t~-4~In the context of current developments in coupling the well-documented structural aspects of diffusionless martensitic transformations with the bainite reaction which involves compositional change, the transformation to bainite has recently been suggested to represent the current frontier of the science of phase transformations, ts] However, the significance of bainite as a transformation product lies in its effect on mechanical properties and the extent to which the characteristic features of the transformation can be exploited in the design and heat treatment of commercial alloys, notably those of steels. Bainitic microstructures can arise in commercial steels by appropriate design of the alloy composition, by suitable control of processing, or as a result of fabrication conditions, particularly during welding operations. For many of the steel types currently exploited commercially, with some exceptions to be discussed later, reference to a continuous-cooling-transformation (CCT) diagram will show that their microstructures will not consist entirely of bainite, and the mechanical properties may vary significantly as a function of the process route. Being conscious of current efforts being made to forge a better understanding of the bainite transformation, we shall try to show where possible how these recent developments are helping in the interpretation of structureD.V. EDMONDS, Lecturer, is with the Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom. R.C. COCHRANE, Manager, Welding Research, is with British Steel Sheffield Laboratories, Swinden House, Moorgate, Rotherham, United Kingdom. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
property relationships and in the design of improved steels. Before embarking on this task, however, it is essential to discuss the problem of the recognition of bainitic microstructures. The first part of this review will address these aspects and link them to early studies of the structureproperty relationships in commercial steels. Such steels, however, can be further improved by careful optimization of both alloy chemistry and processing, and these aspects will be addressed in the second part of this review. A recent development has been the use of accelerated cooling or direct quenching to achieve levels of mechanical properties characteristic of expensive heat-treated alloy steels from comparatively alloy-free compositions. A major advantage of steels processed in this way is that the weldability c
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