| Abstract Scope |
Grade 91 steel is known for its superior creep resistance and is considered a candidate material for next-generation high-temperature applications. Owing to its alloying elements, Grade 91 is air-quenchable, and high-strength martensite can be readily formed from austenite through the γ → α′ transformation. However, during the Laser-Directed Energy Deposition (L-DED) process, the rapid solidification and high cooling rates result in a softer fusion zone characterized by a mixed microstructure of δ-ferrite and martensite. The faster the cooling rate, the more δ-ferrite is retained, suggesting that a continuous cooling transformation (CCT) diagram is also applicable to the δ → γ transformation. The primary mechanism behind δ-ferrite retention is that the rapid cooling during L-DED suppresses the δ → γ transformation once the temperature falls below the austenite formation threshold.
This study integrates experimental characterization, thermal cycle modeling, and phase transformation kinetics to investigate the mechanism of δ-ferrite retention in a series of L-DED Grade 91 blocks fabricated with varying preheat temperatures: room temperature, 100 °C, 185 °C, and 350 °C. A finite element-based thermal model was developed to simulate the thermal history of each layer and estimate cooling rates. Diffusion-based kinetic calculations were then employed to quantify the retained δ-ferrite content under different cooling conditions. The δ → γ transformation was modeled in a one-dimensional domain, incorporating temperature-dependent diffusion coefficients extracted from a thermodynamic database and expressed as time-dependent functions to reflect varying cooling rates. Using this kinetic model, cooling rates ranging from 10 °C/s to 10⁶ °C/s were applied to estimate the retained δ-ferrite fractions and construct an extended CCT diagram for the δ → γ transformation. |