Re-engineering of a dual phase steel by laser surface softening: Process development deploying a thermo-kinetic model and its mechanical response

The work presents a novel technological approach based on laser surface softening (LSS) to locally re-engineer the microstructure of dual phase steel (DP780) and thereby modify its mechanical response. The approach integrates pulsed-wave laser irradiation with an overlapping spot matrix process design for treatment, enabling precise control of softened layer with pre-determined depth and uniformity. A diffusion-controlled time-temperature equivalence (t eq ) framework devised representing effective accumulated subcritical thermal exposure governing martensite tempering. By maintaining the laser induced thermal exposure below the critical austenization temperature, LSS promotes controlled tempering and recovery. As a result, a softened surface layer comprising tempered martensite and recovered ferrite is formed, while bulk microstructure is preserved. The process was modelled using an analytical transient heat conduction model. Validation was performed through in-situ pyrometry and microstructural observations, which confirmed the reproducibility of the thermal cycles and the resultant transformations. SEM and micro-focused XRD analyses revealed partial decomposition of martensite accompanied by ferrite recovery in the treated region. In line with the martensite decomposition and ferrite recovery, micro and nano indentation measurements showed a significant reduction in surface hardness. Mechanical testing demonstrated that one-sided LSS (300 μm depth, 50% spot overlap) lead to a reduction in yield strength and a concurrent improvement in elongation compared to base material. Dual-sided LSS further increased ductility relative to base material, accompanied by a 36% reduction in yield strength. These findings demonstrate LSS as a controllable and reproducible surface technology for localized softening of advanced high strength steels (AHSS), offering significant potential for tailored formability in automotive and structural applications.