Case Study: Weld Microstructures

The microstructure obtained as the weld cools from the liquid phase to ambient temperature is called the as–deposited or primary microstructure. Its major components include allotriomorphic ferrite, Widmanstätten ferrite, and acicular ferrite (Fig. 1). There may also be some martensite, retained austenite or degenerate pearlite. These latter phases occur in very small fractions, and are known by the collective term microphases. Bainite, consisting of sheaves of parallel platelets, is not generally found in well–designed welding alloys. Instead, acicular ferrite is induced to nucleate heterogeneously on nonmetallic inclusions.