Ferritic/martensitic 9Cr-1Mo-V-Nb steel also designated as ASTM A335 used in construction of several components of power plants operating in temperature range of 600–650 °C. In present investigation, dissimilar weld joints of P91 and P92 steel were prepared using the autogenous tungsten inert gas welding (A-TIG) and multi-pass gas tungsten arc (GTA) welding process. A comparative was performed on evolution of δ-ferrite patches in weld fusion zone and heat affected zone (HAZ) of welded joints. The evolution of δ-ferrite patches was carried out in as-welded and post-weld heat treatment (PWHT) condition. PWHT was carried out at 760 °C for 2 h for both A-TIG and GTA welding process. For A-TIG weld joint higher yield strength, ultimate tensile strength (UTS) and peak hardness were measured due to higher heat input in single pass and higher carbon content in weld fusion zone. Higher heat input, fast cooling rate and higher weight percentage of ferrite stabilizer like vanadium (V) and niobium (Nb) in A-TIG weld joint were resulted in formation ferrite patches in weld fusion zone and heat affected zone. Higher weight percentage of V and Nb in A-TIG weld zone was resulted poor impact toughness of A-TIG weld fusion zone than GTA welds in as-welded condition. After the PWHT at 760 °C–2 h, Charpy toughness of A-TIG weld fusion zone (48 J) just meet the minimum required value as required by the EN: 1557:1997 (47 J). © 2017