Austenite memory during reverse transformation of steels at different heating rates

Abstract

The austenite (γ) structure reversely transformed from martensite in Fe–0.55C mass% C alloy was investigated at high temperatures using in-situ an electron backscatter diffraction (EBSD) method. When the heating rate was varied, a rapidly heating produced fine γ grains, in contrast, a slow heating induced coarse γ grains having the same crystal orientations as those of the prior γ grains; known as “γ memory”. The most likely mechanism for the γ memory could be variant restrictions owing to cementite (θ). To investigate the effect of θ on the γ memory, the sample was tempered at 700 °C for 24 h and θ was precipitated. The tempered sample exhibited the γ memory irrespective of the heating rate. To clarify the origin of the γ memory, we focused on the reversely transformed γ formed adjacent to the θ and directly observed the orientation relationships (ORs) using the in-situ EBSD. A crystallographic analysis on the between neighboring ferrites (α), θ, and γ revealed the existence of Kurdjumov–Sachs OR between γ and two α (α1 and α2), Isaichev OR between α and θ, and Pitsch OR between θ and γ. In the all possible combinations, the reversely transformed γ satisfying the K–S OR for two α variants was limited to four variants. The θ variant satisfying the Isaichev OR for α1 and α2 was limited to one variant, which was close to one of the four variants by Pitsch OR, which suggests the γ memory is due to these variant restrictions among α-θ-γ.