Abstract:

The microstructural and mechanical properties of a newly designed tool steel (L-40), specifically designed to be employed in the laser powder bed fusion (LPBF) technique, were examined. Melt pool boundaries with submicron dendritic structures and about 14% retained austenite phase were evident after printing. The grain orientation after high cooling rate solidification is mostly < 110 >  _α∥ building direction (BD). Then, the heat treatment converted the microstructure into a conventional martensitic phase, reduced the retained austenite to about 1.5%, and increased < 111 >  _α∥BD texture. The heat-treated sample exhibits higher tensile strength (1720 ± 14 MPa) compared to the as-printed sample (1540 ± 26 MPa) along the building direction, mainly due to hardening caused by a lower volume fraction of retained austenite phase and precipitation of carbides. As a consequence of the strength-to-ductility trade-off, the heat-treated sample showed lower elongation (10% ± 2%) than that of the as-printed sample (18% ± 2%). It was observed that transformation-induced plasticity (TRIP) occurs in both the as-printed and heat-treated samples during tensile testing, which dynamically transforms the retained austenite into martensite, leading to improved ductility. The minimum driving force to initiate the displacive phase transformation is about 6000 J/mol, which was achieved during tensile testing. The strength and ductility of LPBF-produced L-40 were compared with the other LPBF-produced tool steels in literature; the data indicate that heat-treated L-40 has an excellent combination of strength and ductility complemented with high printability.

Key words: Additive manufacturing, Laser powder bed fusion, L-40 tool steel, Phase transformation