The Fundamental Thermodynamic Investigation on the Grade 91 Alloy




Abstract: Grade 91 (Gr.91) steel has been applied at various fossil-fuel industries because of its high creep strength. Creep strength in Gr.91 is derived from the phase formation of its precipitates, such as M23C6 and MX phases. However, during creep M23C6 precipitates coarsen over time due to the applied heat and stress. This creates even more dislocations within the grain matrix eventually causing what is known as a Type IV failure, which are located within the Heat-Affected-Zone (HAZ). Investigations were be carried out by the Computational Thermodynamics, specifically the CALPHAD or calculation of phase diagrams approach to predict the phases within the HAZ of the Gr.91 such as the Coarse-Grain HAZ, Fine-Grain HAZ, and Inter-Critical HAZ. Other parameters such as the AC1 and AC3 temperatures and the relationship between the stability of the precipitates and small additions of Titanium (Ti) were also simulated using Thermo-Calc. Calculations that will be presented will be the baseline study for creep of Gr.91 alloy.

Authors: Andrew Smith, Mohammad Asadikiya, and Yu Zhong

Keywords: Grade 91, Ferritic Steels, Microstructure, Thermodynamics, Calphad, Alloying Elements

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9 pages


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