• In-situ DIC and EBSD reveal distinct microvoid initiation mechanisms in quenched vs. tempered SA508 Gr.3 steel, linking microstructure to ductile fracture.
• Tempering reduces UTS but significantly enhances ductility, with local strain reaching 191% at necking, due to carbide morphology changes.
• Microvoids initiate via PAGB decohesion at M-A islands in quenched steel, but via carbide fracture and PAGB decohesion in tempered steel.
• Both conditions exhibit ductile dimple fracture, confirming microvoid coalescence as the dominant failure mode, critical for nuclear pressure vessel reliability.