Formability, microstructure and mechanical properties of 300M steel single tracks fabricated by high power selective laser melting

•Molten pool behaviors of HP-SLMed 300 M steel are initially researched.•Formation mechanism of defects such as humping during HP-SLM is revealed.•Effects of laser power and layer thickness on HP-SLMed 300 M steel are studied. High power selective laser melting (HP-SLM) has been used to manufacture the 300M steel single tracks. The influences of laser power and layer thickness on the formability, microstructure and mechanical properties of single tracks were researched. The results show that the optimized parameter window for HP-SLM of 300M steel is determined as laser power 500–1800 W, scanning velocity 400–600 mm/s, layer thickness 80–160 μm. Laser power has a great influence but layer thickness has little effect on the shape and mode of the molten pool (MP). The microstructure of SLMed 300M steel presents an epitaxial growth of columnar crystals and mainly consists of lath α' martensites with no obvious element micro-segregation. Laser power has little effect on the morphology but a great influence on the size and misorientation of the microstructure. However, layer thickness has little effect on the morphology and size but significant influence on the misorientation of the microstructure. The average microhardness increases firstly (~453 to ~551 HV) and then decreases (~551 to ~533 HV) when laser power varies from 500 to 2000 W. However, the average microhardness nearly remains constant (~530 HV) when layer thickness varies from 80 to 200 μm.