The Thermodynamic Evaluation and Modeling of Grade 91 Alloy Through the CALPHAD Approach

Grade 91 steel (modified 9Cr-1Mo steel) is considered a prospective material for many highly sensitive applications such as construction of nuclear power plants and boilers. This steel has exceptional properties such as high creep resistance which is mainly sought after in numerous delicate endeavors such as the one aforementioned. However, research has shown that a detrimental flaw that manifests in form of type IV cracks based on numerous experimental creep failure tests is present in the grade 91 steel. It has been previously reported that the cracks prorogate along the heat-affected-zone (HAZ) which is formed during the welding process and can ultimately cause premature creep failure for the alloy. At present, the main mechanism behind the cause of type IV cracks is yet to be determined although a few promising hypotheses have been advanced. In fact, one such hypothesis recently explored a computational thermodynamic model (using CALPHAD; i.e. CALculation of PHAse Diagram approach) between the effects of C concentration in Gr.91 and the stability of the M₂₃C₆, MX, and Z-Phase which are important in determining the creep resistance of the Gr.91 steel.

Overall, it is evident that there still lack of comprehensive computational thermodynamic simulation on the type IV cracks. Consequently, it remains vital to establish a baseline understanding of Gr.91 to help further the efforts of increasing creep resistance for future high Cr alloys. In this view, scientists from the Mechanical Engineering Department at Worcester Polytechnic Institute in the United States: Mr. Andrew Smith, Dr. Mohammad Asadikiya and Professor Yu Zhong together with Dr. Jiuhua Chen at Florida International University proposed to expand on their previous research by accurately simulating the effect of concentration variations of its main element chromium (Cr), as well as different concentrations of vanadium (V), niobium (Nb), and nitrogen (N) in correlation with the stability of M₂₃C₆, MX, and Z-Phase under short-term and long-term conditions. Their work is currently published in the research journal, Computational Materials Science.

In their approach, the critical secondary phases along with critical temperatures in regard to the creep resistance of Grade 91 steel alloy were evaluated in order to optimize the composition of the alloy to improve the creep resistance. The critical temperatures: Ac1 (the threshold temperature in which austinite begins to form), and Ac3 (the threshold temperature at which ferrite is fully transformed into austenite) were used. Furthermore, optimization was presented through various Cr, V, Nb, and N concentrations in Gr.91 in relation to the critical temperatures and the mole fraction of M₂₃C₆, MX, and Z-Phase as critical secondary phases.

On basis of the equilibrium cooling and Scheil simulations, the authors found out the MX phase is thermodynamically unstable and might disappear at operation temperatures, which successfully explains the experimental observations of the formation of M₂₃C₆ as well as Z phase, and the disappearance of the MX phase under creep test conditions. In addition, elements’ effects on the phase stabilities as well as creep resistance were also investigated. The authors reported that increasing V and Nb increased MX stability, whereas decreasing Cr and N lead to a decrease in Z-Phase and M₂₃C₆ stability, which was seen to lead to an increase in creep resistance of the material. Further, simulation results showed that elimination of N with a dramatic increase in Nb concentrations resulted in the stability increase of only M₂₃C₆ and MX2 phases at temperature regions between 600 °C and 1370 °C and a complete removal of Z-Phase precipitation.

In summary, the study examined the role of various alloying elements of Gr.91 through computational thermodynamics. In this approach, the compositions of the specific elements were optimized according to the stability of secondary phases in the system and also Ac temperatures. In a statement to Advances in Engineering, Professor Yu Zhong, the lead author highlighted that following elimination of N, which destabilizes Z-Phase precipitation, and an increase in Nb for NbC carbide stability, a possible solution to increasing both long-term and short-term creep resistance for Gr.91 could be achieved, though further optimization and creep experiments need to be conducted for final conclusions.