Type 410 martensitic stainless steel is used in some downstream hydro-processing installations, due to its good resistance to sulfide corrosion and chloride stress corrosion cracking. Industry experience with Type 410 steel welds, using generic welding consumables, has shown difficulties in meeting the weld metal and HAZ hardness and toughness requirements. Recent research has pointed out the wide composition specifications of Type 410 base metal and welding consumables as the leading cause for significant hardness and toughness variations, related to exceeding the A1 temperature during PWHT and formation of fresh martensite, and to retention of significant amounts of delta ferrite. Predictive equations for the A1 temperature and the content of retained delta ferrite were used to identify optimal composition for Type 410 welding consumables with delta ferrite content below 20% and A1 temperature close to the upper end of the ASME specified PWHT range. Experimental metal core filler wire was manufactured and tested to validate the A1 temperature and delta ferrite content. A test weld in Type 410 steel was produced with the new filler wire and subjected to PWHT, metallurgical characterization, and mechanical testing. The weld metal and HAZ properties met the corresponding NACE and ASME hardness and toughness requirements.