Surface hardening of ductile austenitic lightweight steel through powder bed fusion 3D printing

To save energy and reduce CO2 emissions, the lightweight design of structural components has recently become a global issue. Fe–Mn–Al–C based alloys with a low mass density have received considerable attention as structural materials enabling such lightweight designs. However, typical strength-ductility trade-off dilemma appears in Fe–Mn–Al–C lightweight steels. Dispersion of nano-sized Fe3AlC-type κ-carbides achieves excellent tensile properties of high strength (∼1 GPa) and large elongation (∼50 %). However, further increase in strength (∼1.2 GPa) caused by κ-carbide coarsening reduces elongation significantly (<10 %), limiting the potential applications of lightweight steels in structural parts that require ultrahigh strength and high ductility, such as wear-resistant components. Here, we resolve this drawback of lightweight steels by reinforcing the surface layer through 3D printing. The composition of base steel plate is Fe–30Mn–8Al-0.7C (wt%), and a lightweight steel powder with a relatively higher Al and C contents (Fe–30Mn-9.5Al-1.0C (wt%)) was then deposited on the surface of base steel plate through laser powder bed fusion (L-PBF). After L-PBF, an aging treatment led to more precipitation of κ-carbides in the surface layer, producing a functionally graded hard surface layer. A developed surface-hardened ductile lightweight steel thus has the potential to replace conventional wear-resistant steels, as it has excellent tensile ductility (51 %), high surface hardness (410 HV), high wear resistance, and 12 % lower mass density.