Morphology and crystallography of austenite formed during intercritical annealing

Abstract

The formation of austenite during intercritical annealing of ferrite–spheroidized cementite and ferrite–pearlite microstructures in low carbon steels has been investigated. It has been established that austenite which nucleates at a ferrite grain boundary does so with a Kurdjumov–Sachs crystallographic orientation relationship with one or both ferrite grains. It has been shown also that austenite/ferrite interfaces displaying low energy planar facets need not have a rational crystallographic relationship across them. The size and distribution of cementite particles in the parent material, particularly in relation to ferrite grain boundaries, has a marked effect on the distribution and morphology of austenite; the morphologies are similar to those displayed by proeutectoid ferrite. Austenite growth has been shown to be initially associated with carbon diffusion through austenite and subsequently with carbon diffusion through ferrite. Primary and secondary WidmansHitten sideplates occur only when diffusion ahead of the interface is rate controlling.