High‐Temperature Phase Stability of 9Cr–W–Ta–V–C Based Reduced Activation Ferritic–Martensitic (RAFM) Steels: Effect of W and Ta Additions

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.