Classification and quantification of microstructures in steels

Abstract

The International Institute of Welding (IIW) microstructure classification scheme for ferrous weld metals has been investigated as a basis for the quantification of complex microstructures in steels. The aim has been to cover the full range of microstructures observed in plain carbon and low alloy steel products, as well as ferritic weld metals and parent plate heat affected zones. The mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials have been briefly reviewed. The classification and terminology used for intragranular as well as austenite grain boundary microstructural constituents have been considered, and also the way in which transformation products are orientated in space. Problems encountered in relating microstructural constituents to principal structures have been discussed in detail and solutions proposed. The microstructure classification and terminology used in the IIW scheme have been built upon and new terminology incorporated into a table providing descriptions of the principal structures and sub-category components. A new classification scheme has been defined in the form of flow charts with guidelines for identifying the principal structures. Evaluation exercises have been carried out with the new scheme. These have shown that a reasonable degree of consistency may be obtained between operators in identifying primary ferrite, pearlite, martensite and the transformation products constituting ferrite sideplate and acicular ferrite structures, notably Widmanstätten ferrite and bainite. A means is thus provided of obtaining database information for developing microstructure–property relationships, or generating data for calibrating physical models, which have the principal structures as their output.