The high‐temperature phase stability of reduced activation ferritic–martensitic steels having various W and Ta contents has been investigated using calorimetry, microscopy, and thermodynamic simulations. Calorimetric studies revealed the following transformation sequence as a function of increasing temperature upon slow heating at 1 K min−1: α‐ferrite (ferromagnetic) + M23C6 + MX → α‐ferrite (paramagnetic) + M23C6 + MX → γ‐austenite + M23C6 + MX → γ + MX → γ → δ+ γ → γ + δ + L → δ + L → liquid. This is fully supported by equilibrium thermodynamic simulations. It is found that the complete dissolution of M23C6 carbide required significant overheating above Ac3 that is, α + M23C6 + MX → γ + MX transformation finish temperature. The measured values of lattice parameter, Curie temperature, liquidus, solidus, and carbide dissolution temperatures, as well as melting enthalpy, did not reveal a drastic variation with W content, from 0 to 0.2 wt%. However, the measured dissolution temperature of MX phase exhibited a mild increase with Ta content, for up to about 0.2 wt%. The extent of dissolution of M23C6 especially around Ac3 temperature is found to depend on heating rate. Higher heating rates and higher W content yielded inhomogeneous austenite and this is found to have significant influence on subsequent γ‐austenite → α′‐martensite formation during cooling. The critical cooling rate for γ → α′‐martensite formation is found to be 6 K min−1, for all the RAFM steels. Electron microscopy and thermodynamic simulations support the possibility of simultaneous presence of two types of MX type carbonitrides at service conditions.

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Reduced activation ferritic martensitic (RAFM) 9Cr steels, which are candidate materials for the test blanket module (TBM) of nuclear fusion reactors, are considered to be air hardenable. However, alloy composition and the processing conditions play a significant role during the transformation of austenite to martensite/ferrite on cooling. The presence of retained austenite is known to influence the mechanical properties of the steel. Identifying very low amounts of retained austenite is very challenging though conventional microscopy. This paper aims at identifying a low amount of retained austenite in normalized 9Cr-1.4W-0.06Ta-0.12C RAFM steel using synchrotron X-ray diffraction and Mossbauer spectroscopy and confirmed by advanced automated crystal orientation mapping in transmission electron microscopy. Homogeneity of austenite has been understood to influence the microstructure of the normalized steel, which is discussed in detail.

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