In modern thermal power plants, superheated steam is transported through the components made of creep strength enhanced ferritic-martensitic (CSEF) steel and austenitic stainless steel (ASS). The welding of different grades of materials is necessary for power plants to satisfy the design requirement, cost savings, and obtain the desired properties. On the low-temperature side, P92 CSEF steel is employed since it is less expensive, has superior creep strength, and has high corrosion resistance. At a high-temperature side, expensive low alloy steel, such as 304L austenitic steel, is used. The P92 steel and 304L ASS is used in the form of tube or pipe in the thermal power plant. Due to this, its welding becomes essential to connect it with the various other components of the power plant. The gas tungsten arc welding (GTAW) technique with filler metal compatible with the base metal (BM) is frequently used. The nickel-based filler rod ERNiCr-3 was adapted to join CSEF P92 steel and AISI 304L ASS. The effect of groove geometry on tensile strength, microhardness, and impact toughness was investigated for P92/304L SS DWJs. The results obtained from the narrow groove design were compared with the conventional V-groove design. After welding, the post-weld heat treatment (PWHT), called tempering, was performed at 760 °C for 2 h to recover the material microstructural and mechanical properties degraded during the welding. The detailed characterization was carried out at the weld fusion zone center and the interface of the weld metal (WM) and base metal (BM) in the as-weld (AW) state and after PWHT using a field emission scanning electron microscope (FESEM). During the room temperature tensile test, the P92/304L SS DWJs failed from the weld metal. According to the Charpy test results, the ERNiCr-3 weld metal having relatively low toughness compared to both the parent metal.