Strengthening of an Fe-C-Cr-Ni hypoeutectic alloy by rapid solidification under additive manufacturing conditions and further strengthening by post heat treatment

This paper investigates the effects of γ-Fe dendrite arm spacing and inter-dendritic eutectic Cr 7 C 3 particle spacing on the hardness of an as-solidified hypoeutectic alloy (Fe-2.14mass%C-32.38mass%Cr-6.48mass%Ni-0.90mass%Si), as well as the effects of subsequent heat treatment on the hardness. When the cooling rate was varied between 10 4 and 10 7 o C/s, the hardness of the as-solidified alloy increased exponentially with decreasing dendrite arm spacing and eutectic particle spacing. When this alloy was subjected to laser powder bed fusion (LPBF) process, the hardness exceeded 700 HV, more than twice that of Inconel 718 Ni-based superalloy fabricated using the same process. During subsequent heat treatment at 800 °C, extremely fine Cr 23 C 6 particles precipitated in both the γ-Fe dendrite arms and the γ-Fe matrix phase of the eutectic structure, increasing the hardness of the alloy samples. Although the initial hardness of the dendrite arms was lower than that of the eutectic region, the increase in hardness was greater in the dendrite arms, indicating greater precipitation hardening in the dendrite arms than in the eutectic region. The average hardness of a large arear of the hypoeutectic structure, consisting of dendrites and eutectic regions, was always higher in the heat-treated samples with higher cooling rates during solidification. This is due to the higher supersaturation of Cr and C during cooling, which resulted in a higher number density and volume fraction of Cr 23 C 6 particles precipitated in the γ-Fe.