RESEARCH PAPER Thermal stability, phase transformation characteristics, and thermal properties of T91 steel and welding consumables S. Raju 1 & Jeyaganesh B 2 & H. Tripathy 1 & S. Murugesan 1 & Saroja Saibaba 1 & S.K. Albert 1 & A.K. Bhaduri 1 Received: 12 January 2016 /Accepted: 6 May 2016 # International Institute of Welding 2016 Abstract A comprehensive dynamic calorimetry and dilatom- etry 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 trans- formation temperatures, namely Ac 1 , Ac 3 , the dissolution tem- peratures of M 23 C 6 and MX-type carbonitrides, and the solidus and liquidus temperatures are accurately measured along with associated enthalpies. The role of (Ni + Mn) content in influenc- ing the equilibrium solidification mode has been clearly delin- eated. It is found that both Ac 1 and Ac 3 temperatures vary in a nonlinear fashion with respect to the heating rate. Further, the Ac 1 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) for- malism. A value of about 280 to 300 kJ mol -1 has been obtained for the effective activation energy (Q eff ) of the overall transfor- mation. It is found that Q eff is quite sensitive to the heating rate, and the observed transformation kinetics indicate that the pres- ence of undissolved M 23 C 6 and MX carbide particles plays an important role in the kinetics of reaustenitization reaction in high chromium steels. Finally, the specific heat C p and the bulk ther- mal expansion (Δl/l o ) for both base metal for all four welding consumables have been determined. Keywords (IIW Thesaurus) High alloy Cr Mo steels . Transformation . Solidification . Cooling rate . Thermal properties 1 Introduction The design of materials, together with appropriate processing and fabrication into final engineering components is basically a complex decision-making process in the light of all available and critically assessed knowledge base on materials [ 1]. In the case of materials and process design for strategic applications, such as nuclear and advanced fossil power plants, it is often imperative to vet the choice of material composition and the associated fabrication process, against the stringent plant-safety auditing protocols as well. This latter requirement demands a high(er) degree of maturity and reliability on the part of design database(s) [1]. Welding constitutes a major joining process in the fabrica- tion of power plant components [2]. The advent of newer grades of ferritic-martensitic (F-M) steels for nuclear applica- tions [3–8] has catalyzed an extensive basic research program on welding metallurgy; especially, on those aspects that are related to tailoring the composition of welding consumables with a view to minimize, or avoid if possible, the formation of delta ferrite during the course of primary weld solidification [2, 9]. Thus, for an example, a fully primary austenitic mode of solidification may be ensured by carefully adjusting the com- bined concentration of γ-austenite stabilizing elements, such as Ni + Mn + Co + Cu + N + C etc., in welding consumables. In some high-temperature creep-resistant and radiation-induced Recommended for publication by Commission IX - Behaviour of Metals Subjected to Welding * A.K. Bhaduri arunkbhaduri@gmail.com 1 Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2 Formerly at Indira Gandhi Centre for Atomic Research, presently at Nuclear Advanced Manufacturing Research Centre, University of Sheffield, Rotherham S60 5WG, UK Weld World DOI 10.1007/s40194-016-0353-5