The International Institute of Welding (IIW) microstructure classi cation scheme for ferrous weld metals has been investigated as a basis for the quanti cation 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 brie y reviewed. The classi cation 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 classi cation 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 classi cation scheme has been de ned in the form of ow 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 Widmanstä tten 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.MST/5675

Inclusion assisted microstructure control has been a key technology to improve the toughness of C-Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.

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