A mechanistic study of retained δ-ferrite in additively-manufactured Grade 91 steel

A combined experimental, characterization, and simulation study has been conducted to develop a quantitative description for microstructural evolution during Laser-Directed Energy Deposition (L-DED) of Grade 91 steel. Initial L-DED deposits of Grade 91 made without preheat exhibiting much greater volume fractions of δ -ferrite and much less martensite than anticipated, with non-uniform hardness values and lower average hardness levels. Based on our hypothesis for this result, preheating was applied to subsequent deposits, but not for the conventional reasons related to control of austenite decomposition . Rather, the results were consistent with the concept that greater times in the higher temperature range for the δ to γ transformation permitted the transformation to progress further toward completion during L-DED of Grade 91 owing to slower cooling rates resulting from increased preheating. Increasing preheat temperatures up to 350 °C resulted in deposits with progressively lower volume fractions of retained δ -ferrite with greater fractions of martensite and higher average hardness values. The final phase fractions and resulting hardness levels are dependent on the δ to γ transformation kinetics that are dictated by the cooling rates for transformation, which in turn are governed by the preheat temperatures. The results of our work have broader implications toward rationalizing the microstructures and properties for AM with rapid cooling rates of many hardenable alloy steels that solidify as δ-ferrite. Finally, the model results were used to develop a novel revision to the continuous cooling transformation diagram for Grade 91 that captures the kinetics of the δ to γ phase transformation.