Dynamics of austenite to ferrite phase transformation in ferrous weld metals

The dynamics of the austenite to ferrite phase transformation in ferrous weld metals have been investigated in order to define the nucleation and growth processes leading to the intragranular microstructure commonly known as acicular ferrite. Weld metals of increasing hardenability have been thermally cycled, partially transformed, and the nucleation and growth reactions frozen at temperatures in the reconstructive and displacive regimes by quenching. Information has also been obtained about the rates of transformation at different temperatures in continuously cooled fully transformed specimens. The present work has shown that acicular ferrite is not a single transformation product, but forms as a result of a succession of competing nucleation and growth processes at austenite grain boundary and intragranular nucleation sites during undercooling. The sequence of transformations is consistent with the theoretical activation energy barriers against nucleation of the different sites. The present work demonstrates that large intragranular inclusions (greater than similar to 3 mu m) nucleate acicular ferrite at reconstructive transformation temperatures but the amount of acicular ferrite is limited by low inclusion numbers and slow rates of reaction. Small intragranular inclusions ( similar to 0.5 mu m) nucleate large proportions of acicular ferrite at displacive reaction temperatures because of their substantial numbers and the rapid rates of reaction. Boron retards reconstructive ferrite transformation at austenite grain boundary and large intragranular inclusion sites so that the temperature at which they become operative is significantly lowered. A greater proportion of acicular ferrite is then formed displacively. Boron has little effect on the onset of displacive transformation, which occurs at similar to 630 degrees C in all the weld metals investigated. The maximum rate of transformation occurs at temperatures in the range 597-610 degrees C, depending on the alloying. These temperatures are 40-70 K above the calculated and experimentally observed bainite start temperatures, indicating that the greater proportion of acicular ferrite formed displacively is Widmanstatten ferrite rather than bainite. (C) 1997 The Institute of Materials.