NEW BAINITIC STEELS BY DESIGN Displacive Phase Transformations

Bainitic steels are now at the forefront of new and exciting developments in steel technology. This paper discusses how phase transformation theory is being exploited in the systematic design of every aspect of the new alloys, emphasising the immense influence that Professor Wayman has had on the development of the subject [1]. Technology of Bainitic Steels Gone are the days when industry could simply make a whole matrix of alloys, test them, and then hope for the best. Such methods are less acceptable in the modern competitive environment. This realisation has caused a great increase in research leading to the development of models which help reduce the time and expense of alloy development, and maintain the enormous success of steel as a structural material. It is therefore fortunate that bainitic steels have become popular at a time when it is possible to systematically design them using a panoply of quantitative techniques based on phase transformation theory. The purpose of this paper is to show how such theory has led already to the development of important new alloys. The rise in popularity of bainitic steels is for a number of reasons. Modern processing techniques (such as accelerated cooling mills) have enabled steels which would traditionally be considered as having poor hardenability, to be produced with homogeneously bainitic microstructures. Consequently, coarse carbides are avoided and the ultrafine, submicrometer grain size of the bainitic ferrite can be exploited to give a strong and tough steel with weldability. The inevitable solidification–induced chemical segregation present in commercial steels leads to a “banding” when the microstructure is a mixture of . allotriomorphic ferrite and pearlite. This is not the case when the transformation occurs at a larger supersaturation to bainite, giving major advantages in corrosive environments. We shall begin this review by discussing aspects of the mechanism which are necessary in order to calculate the structure of bainite, and then illustrate how the mechanism can be exploited in design. Table 1 lists some of the properties which are of interest in industrial design, and which need to be related directly to phase transformation theory. Only a few of these relations are discussed here; more can be found elsewhere [2].