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. Knowledge based approach to designing power plant steels calls for accurate information on high temperature phase & microstructural stability. In the present study, Dynamic Calorimetry, Electron Microscopy, together with Thermodynamic Simulations, have been carried out to rationalize the observed high temperature phase phase transformation behaviour of W and Ta added Reduced Activation Ferritic Martensitic (RAFM) Steel.