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The mechanical properties of bimetallic composites are significantly influenced by their interfacial morphologies. This study delves into the impact of various heat treatment conditions on the microstructure and mechanical attributes of steel/nickel bimetallic (17-4PH/IN625) components produced through extrusion-based sintering-assisted additive manufacturing (ES-AM). The bimetallic composites were annealed at 1150°C for 1, 4, and 8 h, followed by an aging treatment at 482°C for samples annealed for 8 h. After annealing, microstructural heterogeneities, including variations in grain size and elemental distribution within the transition zone close to the interface, were observed. It was found that in the diffusion transition zone between the two alloy layers, the diffusion of iron (Fe) and nickel (Ni) elements increased with longer holding times, as corroborated by microhardness tests and quantified through theoretical parabolic diffusion law. The transition zone exhibited two distinct areas: an Fe-predominant zone and a Ni-predominant zone, with the latter containing oxides and molybdenum (Mo)- and niobium (Nb)-rich precipitates. No new phases emerged post-heat treatment; however, shifts in peak due to stress relaxation and the emergence of precipitates were identified through X-ray diffraction (XRD) observations. Microhardness within the transition zone increased following heat treatment, peaking at 186 HV1.0 after a 4-h annealing. The optimal heat treatment condition was identified as 1150°C for 4 h, which facilitated the development of uniform microstructures and improved bonding strength. This study demonstrates an enhanced interfacial bonding strength in 17-4PH and IN625 bimetallic parts manufactured through ES-AM, suggesting their wide-ranging potential applications in industry.