Stress-induced γ→ α martensitic transformation in two carbon stainless steels. Application to trip steels

Abstract

The influence of the temperature θαof a prestraining of austenite above M_don the subsequent stress-induced γ→ α’ transformation in the(M _s, M_d) range is examined in two carbon stainless steels. It is shown that the yield stress, which is controlled by the transformation, increases with θαat given testing temperature and amount of prestraining. This behavior is related to the influence of θαon the nature and arrangement of the defects present in austenite after the prestraining: planar defects(i.e., stacking faults, twins, e platelets) predominate if θαis close to M_dwhereas undissociated dislocation cells are only to be observed if θif higher. This is consistent with the strong increase of the intrinsic stacking fault energy of the austenite, as inferred from measurements using the node method on a hot stage microscope. In addition, the ability of plane defects to propagate under stress is shown to be lower after a prestraining at higher θα, which is attributed to a segregation of impurity atoms on dislocations. It is concluded that the nucleation stress of the γ→ α’ transformation is the stress necessary to allow planar defects to propagate in the prestrained austenite.