Thermal stability, phase transformation characteristics, and thermal properties of T91 steel and welding consumables

A comprehensive dynamic calorimetry and dilatometry characterization of thermal stability, thermal properties, and α-ferrite + carbides → γ-austenite phase transformation kinetics of T91 grade ferritic-martensitic (FM) steel, together with four of its SMAW consumables has been performed. In particular, the sequence of phase changes taking place right up to melting has been investigated. The on-heating α-ferrite → γ-austenite transformation temperatures, namely Ac1, Ac3, the dissolution temperatures of M23C6 and MX-type carbonitrides, and the solidus and liquidus temperatures are accurately measured along with associated enthalpies. The role of (Ni + Mn) content in influencing the equilibrium solidification mode has been clearly delineated. It is found that both Ac1 and Ac3 temperatures vary in a nonlinear fashion with respect to the heating rate. Further, the Ac1 temperature exhibits a pronounced decrease with increasing (Ni + Mn) content. The kinetics of γ-austenite formation upon continuous heating has been modeled using a simple isochronal version of the Kolmogorov-Johnson-Mehl-Avrami (KJMA) formalism. A value of about 280 to 300 kJ mol−1 has been obtained for the effective activation energy (Qeff) of the overall transformation. It is found that Qeff is quite sensitive to the heating rate, and the observed transformation kinetics indicate that the presence of undissolved M23C6 and MX carbide particles plays an important role in the kinetics of reaustenitization reaction in high chromium steels. Finally, the specific heat Cp and the bulk thermal expansion (Δl/lo) for both base metal for all four welding consumables have been determined.