Introduction: Grade 91 steel components are used in the nuclear industry for its excellent strength, creep resistance and radiation tolerance. Directed energy deposition (DED) is a potentially attractive method to manufacture Grade 91 steel components for the nuclear power industry. Structurally sound, defect-free and reliable parts of Grade 91 steel made by DED require selection of appropriate process conditions to achieve superior microstructure and properties. DED exposes components to repeated thermal cycles when previously formed martensitic structure experiences in-situ tempering. Careful selection of processing conditions such as laser power, scanning speed, powder mass flow rate and preheat temperature is important for tuning the thermal cycles responsible for the tempering. The main objective of this work is to combine the thermal cycles calculated using a mechanistic model with appropriate phase transformation kinetics to quantitatively compute the extent of tempering of Grade 91 steel components for different processing conditions.
Technical approach: Thermal cycles at different processing conditions were calculated using a three-dimensional, transient heat transfer and fluid flow model. The model solves the equations of conservations of mass, momentum, and energy in a 3-D discretized solution domain that includes deposit, substrate, and shielding gas. Thermophysical properties of Grade 91 steel required for the heat transfer and fluid flow model were computed by thermodynamic calculations using a commercial software JMatPro. Rates of phase transformations during tempering were calculated using the Johnson-Mehl-Avrami (JMA) equation that gives fraction converted as a function of time. The thermal cycles were subdivided into multiple small isothermal segments, each representing a very short time increment to evaluate the effects of multiple thermal cycles on tempering. The constants of the JMA equation were determined from isothermal tempering experiments of wrought Grade 91 steel samples.
Results and discussions: Temperature variations with time for different processing conditions were compared with the corresponding independent experimentally determined values. Besides, the fusion zone shape and size obtained from the modeling were compared with several independent experimental data. The agreement provides confidence that the heat transfer and fluid flow model can provide a reliable basis for estimating the necessary thermal cycle for tempering. The peak temperatures, cooling rates and solidification parameters for various processing conditions were evaluated. It is found that for a set of typical deposition parameters, the cooling rates were sufficiently rapid to form martensite during the cooling of austenite. However, in a previously deposited layer and nearby areas, significant tempering of previously formed martensite takes place like temper bead welding. Tempering kinetics necessary to determine the extent of tempering was determined from isothermal tempering experiments at various temperatures. These data were fitted to a form of Johnson-Mehl-Avrami (JMA) equation. The JMA equation was used to compute the extent of tempering under different thermal cycles at various locations of the three-dimensional specimen. The extent of tempering at various places was sensitive to process parameters. This finding is important because it provides hope that by tuning process parameters, microstructures similar to temper bead welding could be obtained to achieve superior strength, creep resistance and radiation tolerance.
Summary and conclusions: Significant tempering of previously formed martensite occurred during the deposition of components by multi-layer directed energy deposition of Grade 91 steel like temper bead welding. The extent of tempering was sensitive to DED process parameters. This finding provides hope that desirable microstructures and properties can be obtained in Grade 91 steel components by adjusting the DED process parameters.
Isothermal tempering kinetics of grade 91 steel was determined experimentally. The data were fitted to the JMA kinetic equation. A heat transfer and fluid flow model for multi-layer deposition of Grade 91 steel components was used to quantitatively evaluate thermal cycles at various locations in the entire three-dimensional specimens for various sets of DED process parameters. The extent of tempering of martensite during multi-layer directed energy deposition of Grade 91 steel was computed for various DED processing variables by combining the computed thermal cycles with the JMA equation. This approach is shown to be useful in selecting appropriate laser power, scanning speed and powder mass flow rate to control the extent of tempering in the additively manufactured Grade 91 steel components.