Kinetic modeling and characterization of silicon-free pack boriding on Inconel 738 superalloy using the Taylor expansion model

Abstract

Abstract Inconel 738 superalloy was treated using a silicon-free pack boriding process with a powder mixture of boron carbide (B 4 C) and sodium tetrafluoroborate (NaBF4) at 850 °C, 900 °C, and 950 °C for 2 h–8 h. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses confirmed the formation of dense, continuous, and uniform boride layers with thicknesses ranging from 16 µm to 61 µm. X-ray diffraction (XRD) results identified a multi-phase structure consisting of Ni 2 B, Ni 4 B 3 , CrB, and Cr 5 B 3 , where elevated temperatures promoted chromium-rich phase formation. Microhardness profiles exhibited a diffusion-controlled gradient, with surface hardness increasing from approximately 2,000 Vickers hardness (HV) to 2,600 HV. Kinetic studies showed that layer growth follows a parabolic law. Using the Taylor expansion model, the activation energy for boron diffusion was calculated as 168.10 kJ mol −1 . This study demonstrates that silicon-free pack boriding effectively enhances the surface hardness and structural integrity of Inconel 738 for high temperature applications.