Microstructure of ultrahigh strength steel 300 M fabricated by laser melting deposition

The microstructure of thin plate-like ultrahigh strength steel 300 M fabricated by the laser melting deposition was analyzed by OM and SEM, and the Rockwell hardness (HRC) profile along the depositing direction was measured. Results showed that the as-deposited plate has a rapidly solidified cellular dendrite structure, and the microstructure of the plate varied notably along depositing direction. A mixed structure of tempered martensite and bainite appeared in the bottom part, a mixture of carbide-free bainite and island-like martensite-austenite duplex structure in the middle-lower part and a mixed martensite-bainite structure in the middle-upper part. The hardness profile varied stepwise with increasing deposition height. The above phenomena were attributed to the particular rapid/unstable thermal cyclic behavior during the laser melting deposition process.