Influence of molten pool mode on microstructure and mechanical properties of heterogeneously tempered 300M steel by selective laser melting

Abstract

Formability, microstructure, nanohardness, tensile properties and impact toughnesses of heterogeneously tempered 300M steel by selective laser melting under the conduction and keyhole mode were investigated. The results indicate that the optimal energy density ranges with a relative density of more than 99.9 % for as-printed 300M steel in both modes are between 89 and 250 J/mm3. The molten pools used for the SLM depositing process under a conduction mode exhibit shallow semi-elliptical shapes but deep “U” shapes under a keyhole mode. The microstructure of as-printed parts presents the co-existence of tempered troostite and tempered sorbite, which results in a spatial interlaced distribution of fine and coarse grains. The inhomogeneous microstructure can be regulated and controlled by varying molten pool sizes and modes. The molten pool remains high hardness and strength while the heat-affected zone possesses good ductility, which generates a “soft-wraps-hard” shell-core structure. The shell-core structure is good for improving the performance of SLMed 300M steel parts but can lead to the anisotropy of mechanical properties. The vertically SLMed samples have higher tensile strengths but poorer break elongations than those of the horizontally SLMed samples under both modes. The tensile properties and impact toughnesses of SLMed 300M steel parts in a conduction mode are superior to those under a keyhole mode.