The Discovery of Lower Acicular Ferrite

There is general agreement that a weld microstructure primarily containing acicular ferrite will exhibit high strength and excellent toughness due to its small grain size, high dislocation density, and the way in which the plates are dispersed in the microstructure (Pargeter, 1983; Sneider and Ken, 1984). In contrast, the presence of allotriomorphic ferrite, ferrite plates with aligned carbides, or grain-boundary nucleated bainite is considered detrimental to the toughness of the weld, because these constituents allow cracks to propagate without much deflection. The problem is, in fact, more complicated, since it appears that a microstructure consisting totally of acicular ferrite does not exhibit optimum toughness, and that, ideally, a finite amount of allotriomorphic ferrite should also be present in the microstructure (Sneider and Kerr, 1984). Whatever the optimum microstructure, it is clear that a better understanding of the phases involved would permit more detailed investigation on the relationship with mechanical properties. In this context, acicular ferrite is the least understood of all the main phases that occur in steel welds. This work follows on from the work of Yang and Bhadeshia (1987), and Strangwood and Bhadeshia (1987), aimed at identifying and classifying acicular ferrite.