On the Formation of Austenite Grains from Prior Martensitic Structure

Electron microscopic studies have been made of austenite partially transformed from prior martensitic structure in the temperature range between Ac1and Ac3.It has been found that each acicular austenite grain which has been formed from ferrite laths with same orientation has same crystallographic orientation and the austenite has the KURDJUMOV-SACHS orientation relationship with the ferrite. The both lath boundary and cementite play an important role in the formation of acicular austenite grains, with same orientation. When austenite is formed adjacent to precipitated cementite on the boundary, the austenite must have PITSCH orientation relationship with cementite and the KURDJUMOV-SACHS orientation relationship with ferrite. Consequently, the variant of formation of austenite is limited to only one, so that acicular austenite grains are identically oriented with each other. When the specimen is heated over Ac3after acicular austenite structure has sufficiently been developed, coarse austenite grain structure is formed from the prior coarse γ grain structure. It is observed that a coarse austenite grain does not consist of a single crystal grain but consists of several divided regions, each of which is corresponding to recovered ferrite regions which have been formed by the recovery of lath martensite during heating.