Design of T22/800H transition joints through thermodynamic and kinetic modeling

When T22 ferritic steel and 800H austenitic alloy were welded, premature failures often appear on the heat-affected zone side of ferritic steel, due to carbon migration. The carbon chemical potential gradient is the driving force of carbon diffusion. Usually, adding a transition layer between dissimilar alloys is an effective way to reduce the chemical potential gradient. Therefore, in this study, thermodynamic and kinetic modeling tools were used for designing transition layers between T22 and 800H to mitigate carbon migration. Firstly, simulation results show that a linear transition zone can decrease the migration of carbon to reduce the decomposition of carbide on the T22 side. In addition, two layers transition was designed to further depress carbon migration. Compared with the wider linear transition zone, it can gain a similar favorable effect. Moreover, the width of the transition zone can be significantly minimized. The designed constant composition is propitious to drop the experiment complexity.