Microstructure and mechanical properties of hand-held laser beam welded S700MC high-strength steel

• First results on the hand-held laser beam welding of HSLA steel are presented. • Radiographic and metallographic analysis reveal low amount of porosity. • Hardness and tensile properties correlate well with the cooling rate. • Fractography and impact testing confirm ductile fracture behavior of the weld. Hand-held laser beam welding (HLBW) has gained attention due to its flexibility, high welding speeds, and excellent joint appearance. Since research on this technique remains limited, this study provides first insights into HLBW of S700MC high-strength low-alloy steel. Radiographic analysis reveals that joints with a low degree of porosity can be achieved, addressing a major challenge of manual welding. Mechanical characterization by hardness, V-notch impact, and tensile testing demonstrates good property retention. The welded joint exhibits a yield strength of 686 MPa and a tensile strength of 778 MPa compared to 775 MPa and 840 MPa of the base material, respectively. Hardness measurements show a reduction from 280 HV0.5 in the base material to ≤ 240 HV0.5 in the fine-grained heat-affected zone, consistent with the observed strength decrease and within the limits of the ER100S-G filler wire. Despite a reduction in fracture elongation from 20 % to ≈ 10 %, the absorbed impact energy reaches 36.5 J, exceeding the base material’s 30 J, indicating sufficient ductility. Microstructural analysis reveals distinct cementite-free upper and granular bainite, acicular and polygonal ferrite as well as various morphologies of martensite-austenite constituents in the fusion zone and depending on the distance to the fusion line. A cooling time t 8/5 ≈ 6 s was determined, enabling quantitative process–microstructure–property correlation. Overall, the study confirms that HLBW enables the production of mechanically sound joints in S700MC, offering promising potential application of this emerging technology for high-strength thermo-mechanical controlled processed constructional steel.