Influence of post-process heat treatment on microstructure and mechanical properties of laser-directed energy deposition repaired Q690D high-strength steel

Laser-directed energy deposition (LDED) has emerged as a key technology in high-value manufacturing for repairing large-scale engineering components, such as damaged structures, offering sustainability benefits through material restoration. However, the process often results in non-equilibrium microstructures and high residual stresses, compromising mechanical properties and stress corrosion resistance in repaired parts. This study is among the first to explore post-process heat treatment effects on wire-based LDED-repaired Q690D high-strength steel, addressing a critical gap in the repair of marine engineering components. It explores the effects of post-process heat treatment on the mechanical properties and fracture behavior of Q690D high-strength steel repaired via LDED with flux-cored wire. Repaired specimens were subjected to three heat treatment protocols: (A) 420 °C for 2 hours with air cooling, (B) 500 °C for 2 hours with air cooling, and (C) 580 °C for 2 hours with air cooling. Microstructural analysis revealed that increasing heat treatment temperatures promoted the formation of tempered martensite in the deposition zone, enhancing microstructural stability. The as-repaired specimens exhibited tensile strength comparable to the substrate (yield strength: 785 MPa, ultimate tensile strength: 830 MPa), but with reduced elongation (25% versus 31% for the substrate). Heat treatment led to a progressive decline in tensile strength with rising temperature, while improving elongation. Flexural strength of as-repaired specimens surpassed the substrate by 365 MPa (∼2232 MPa versus ∼1868 MPa), though it decreased post-treatment. Conversely, ductility and impact toughness at −20 °C improved with higher temperatures, with all treated specimens exceeding ISO 16834:2021 standards (≥27 J absorbed energy). Overall, heat treatment at 500 °C optimized the balance of strength, ductility, and toughness by promoting microstructural homogenization.