Microstructural change during isothermal aging in high manganese austenitic steels

Microstructural change during isothermal aging has been investigated 13%Mn-0.9%C and 22%Mn-0.6%C steels by means of optical and electron microscopy and X-ray diffractometry. High manganese austenitic steels undergo three kinds of reactions during isothermal aging; (1) grain boundary precipitation of carbide, (2) precipitation of platelet carbide within austenite (γ) grains and (3) formation of lamellar structure through eutectoid transformation (γ→ferrite (α)+carbide). In 13%Mn-0.9%C steel, all of the reactions occur and the carbide concerning the reactions is M₃C in any case. On the other hand, in 22%Mn-0.6%C steel, only two of them occur; grain boundary precipitation of M₂₃C₆ carbide (not M₃C) and the eutectoid transformation (γYLDα+M₃C). Besides, both of the two reactions in 22%Mn-0.6%C steel were effectively suppressed due to the chemical composition; high Mn and low C content. Eutectoid transformation proceeds by being supplied carbon from untransformed γ, so that this results in the shortage of carbon in untransformed γ during isothermal aging and the γ phase undergoes γ→ε (hcp) martensitic transformation on the following cooling.