Collaboration between CeSMEC and WPI enables the thermodynamic evaluation and modeling of Grade 91 Alloy through the CALPHAD approach



Advances in Engineering recently highlights the work of the collaboration between CeSMEC and Worcester Polytechnic Institute (WPI) for the thermodynamic evaluation and modeling of Grade 91 Alloy through the CALPHAD approach. The results have been published in the recent issues of Engineering and Computational Materials Science.

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.
A team of scientists at WPI led by Prof. Yu Zhong (a former FIU faulty at CeSMEC) and the CeSMEC researcher, Prof. Jiuhua Chen, examined the role of various alloying elements of Gr.91 through computational thermodynamics and optimized the compositions of the specific elements according to the stability of secondary phases and critical temperatures in the system. By tuning the constituents of the system, they proposed a possible solution to increasing both long-term and short-term creep resistance for Gr.91.

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  1. Andrew Smith, Mohammad Asadikiya, Mei Yang, Jiuhua Chen, Yu Zhong. An investigation of creep resistance in Grade 91 steel through computational thermodynamics. Engineering, 6 (2020) 644-652.
  2. Andrew Smith, Mohammad Asadikiya, Jiuhua Chen, Yu Zhong. The compositional optimization and secondary phases evaluation regarding the creep resistance in Grade 91 steel through the CALPHAD approach. Computational Materials Science, issue 177 (2020) 109591.