The microstructures of complex phase 780C steel and TRIP 690T steel joints with thickness of 1.5 mm welded using fibre laser process were evaluated. The microstructures and microhardness of butt joints were characterized in the fusion zones and heat affected zones. The microstructures in the fusion zones were predominantly built of martensite and lower bainite. In both heat affected zones near 780C steel and near 690T steel grain coarsening of dominantly martensitic microstructure with small portion of lower bainite and retained austenite was observed. The microhardness increased in the fusion zone and both heat affected zones of joints. The highest microhardness of 485 HV0.1 was measured in the heat affected zone near 780C steel region of joint. The average tensile strength of all joints was 750 MPa and it achieved the strength of the 690T steel, because the failures during the tests occurred in this steel base material.Keywords: complex phase steel, TRIP steel, microstructure, microhardness, fibre laser welding
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