Weldability Evaluation of Computationally Designed Filler Wires for Wire-Arc Additive Manufacturing of Functionally Graded Materials in Harsh Service Environments

Olefin furnaces contain gravity cast U-bend fittings from Fe-Cr-Ni alloys that can experience premature failure due to a combination of harsh service conditions. The fittings undergo steep temperature variations during startup and shutdown, outer diameter (OD) oxidation from furnace flue gasses, and inner diameter (ID) carburization from process fluids. As a result, thermal fatigue cracking often occurs along large solidification grain boundaries from connected networks of carbides and secondary phases. To address these degradation concerns, Wire-Arc Addition Manufacturing (WAAM) is being used to produce a functionally graded fitting that provides increased oxidation, carburization, and thermal fatigue resistance. Three weld wire chemistries have been designed based on thermodynamic and kinetic modeling techniques to address the appropriate corrosion resistance and mechanical properties needed in the OD, core, and ID regions of the U-band fitting cross-section. A solid Fe-35Cr-45Ni-0.7Nb weld wire is being used for the core section, and metal-cored wires based around this composition with additions of Al or Si are being used for the ID and OD sections respectively. This study involves weldability evaluation focused on understanding the microstructures and potential challenges for AM processing associated with graded WAAM structures using these weld wires. Scheil calculations performed in Thermo-Calc CALPHAD software were used to predict solidification microstructures. To validate the results, optical and SEM characterization is being conducted on deposited weld beads of each wire to identify phase and precipitate formation. Cast Pin Tear Testing (CPTT) is also being performed to evaluate solidification cracking susceptibility of the weld wires. These unique data are informing WAAM process parameters needed to produce a functionally graded material (FGM) that improves the lifetime of Fe-Ni-Cr U-bend fittings in Olefin Furnaces.