In situ synchrotron analysis of phase transformation at high temperatures in ODS ferritic steel

Abstract

Texture and microstructure evolution in a ferritic/martensitic 9wt%Cr oxide dispersion strengthened (ODS) steels during heat treatment after cold tube forming are studied via in situ synchrotron X-ray diffraction analysis. The initial textures of cold-formed ODS steel tubes present a well-known α-fiber ( //RD). During high-temperature heat treatment, this initial sharp fiber texture, inherited from cold forming, is transformed by the phase transition to austenite into a weak fiber. During cooling, a texture memory effect is observed with a strong dependency of the fiber intensity on the cooling rate. After quenching into martensite, the fiber becomes weaker than in the initial ferrite. By contrast, after slow cooling into ferrite, the fiber develops and becomes stronger than initially. For many materials, texture memory effect can be explained by a variant selection phenomenon. Accordingly, experimental results are compared to the double Kurdjumov–Sachs (DKS) model. During heating, a limited texture heritage is exhibited with essentially a Kurdjumov–Sachs (K–S) relation between the two phases. During cooling the DKS relation correctly predicts the texture fiber components. However, the strong enhancement of texture during slow cooling remains not completely explained. A Williamson–Hall analysis of the size and strain evolutions during the heating cycle showed a remarkable stability of the crystallites size which is the same in ferrite, austenite and martensite.