The factors controlling the transition from a microstructure consisting of austenite grain boundary nucleated sheaves of bainite, to one containing intragranularly nucleated plates of acicular ferrite are explored. The work confirms that in weld metals containing inclusions, the transition from bainite to acicular ferrite can be stimulated by the prior formation of a small amount of allotriomorphic ferrite along the austenite grain surfaces. For a successful transition, the allotriomorphic ferrite has to be inert, i.e. unable to develop into Widmanstätten ferrite or bainite sheaves. Detailed experiments are reported to verify that the allotriomorphic ferrite can be rendered inert by the build up of carbon in the austenite ahead of the allotriomorphic ferrite/austenite boundary.