Vanadium - Transforming Possibilities

Influence of V Precipitates on Acicular Ferrite Transformation, Part 1: The Role of Nitrogen and Part 2: Transformation Kinetics

Abstract:

This paper (part 1 of a two part study) deals with the influence of N in its combination with V, as V(C, N) precipitates, on the decomposition of austenite into acicular ferrite. Likewise, the intragranular nucleation potency of V(C, N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C–Mn–V steels with different contents of N under two different austenitising temperatures, i.e. different austenite grain sizes. The results clearly show that for austenite to decompose into acicular ferrite is necessary, first to have a representative fraction of V(C, N) precipitates within austenite, and second to decorate the austenite grain boundaries with proeutectoid ferrite so bainite can not form. Part 2 of the study concerns with the influence that those precipitates have on the kinetics of acicular ferrite formation during austenite isothermal decomposition. A combination of thermodynamic models as well as some physical metallurgical principles has been used to analyse the influence that V(C, N) precipitates have on the transformation kinetics of acicular ferrite by isothermal decomposition of austenite. Those precipitates were found absolutely necessary for the nucleation of acicular ferrite as reported in Part 1 of this work, now in Part 2, through the proper design of a heat treatment route we have studied the effect that those precipitates have on the kinetics of acicular ferrite transformation.

Keywords:

acicular ferrite, bainite, V(C,N), continuous cooling transformation diagrams, transformation kinetics