TABLE OF CONTENTS Fusion welding is of greatest importance in the fabrication of engineering structures. There are many ways in which fusion welding can be carried out, but all of them involve the deposition of a small amount of molten steel within a gap between the components to be joined. When the steel solidifies, it welds the components together. The metallurgy of the welded joint can be categorized into two major regions, the fusion zone and the heat-affected zone (HAZ). The fusion zone represents both the deposited metal and the parts of the steel component melted during the process, and is a solidification microstructure. The HAZ, on the other hand, represents those regions in the close proximity of the weld, where the heat input during welding changes the microstructure without melting the steel. This chapter describes the development of microstructure in both zones, beginning with the fused regions. Virtually every aspect of phase transformation in steels is relevant to the subject of welding. There is an opportunity for a whole series of transformations to occur successively as the weld cools from the liquid state.
Iron is ferritic at temperatures just below the melting point. As it cools, the ferrite then transforms to austenite, only to revert back to ferrite on continued cooling. Most steels contain modest concentrations of alloying elements, and hence show similar crystal structure changes as pure iron. Weld deposits, therefore begin solidification with the epitaxial growth of columnar delta-ferrite ( ?
