ON THE FORMATION OF AUSTENITE GRAINS FROM PRIOR MARTENSITE STRUCTURE.

Abstract

Austenite partially transformed at between Ac//1 and Ac//3 from a martensitic structure was studied by electron microscopy. It is found that each acicular austenite grain which is formed from ferrite laths with the same orientation has the same crystallographic orientation, and the austenite has the Kurdjumov-Sachs orientation relationship with ferrite. Both lath boundary and cementite play an important role in the formation of acicular austenite grains with the same orientation. When austenite is formed adjacent to precipitated cementite on the boundary, the austenite must have the 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 above Ac//3 after acicular austenite structure has sufficiently been developed, coarse austenite grain structure is formed from the prior coarse gamma 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 corresponds to recovered ferrite regions which have been formed by the recovery of lath martensite during heating.