Ultrasonic　welding　(UW)　of　duplex　stainless　steel　(DSS)　substrates　with　high　deformation　resistance　presents　challenges　for　fabricating　high-strength　joints.　This　research　addresses　this　challenge　by　employing　heat　treatment　to　modify　the　grain　size　and　mechanical　properties　of　DSS　substrates　to　match　the　UW　process　requirements.　The　differences　in　weld　formation　between　heat-treated　and　as-rolled　DSS　substrates　were　comprehensively　analyzed　via　mechanical　tests,　microstructural　characterization,　and　molecular　dynamic　simulation.　Lap　shear　test　results　found　that　welded　joints　using　heat-treated　substrates　exhibited　up　to　a　71　%　increase　in　strength　compared　to　those　using　as-rolled　substrates,　which　was　attributed　to　the　expansion　of　high-hardness　welded　regions　and　a　higher　linear　welding　density.　A　phase　transformation　of　ferrite-to-austenite　induced　by　strain-driven　atomic　misalignment　was　found　by　electron　backscatter　diffraction　analysis　and　molecular　dynamic　simulation.　Moreover,　initial　grain-coarsening　microstructures　in　the　heat-treated　substrates　facilitated　dislocation　motion　and　dynamic　recrystallization　during　UW,　resulting　in　thicker　grain-refined　austenite　layers.　An　energy-based　model　for　interfacial　deformation　was　further　established　to　elucidate　the　mechanisms　of　accelerated　interfacial　bonding　and　joint　enhancement　induced　by　grain-coarsening.　This　provides　a　new　insight　into　optimizing　the　weld　formation　for　UW　of　hard　metals.　©　2025　Elsevier　B.V.