Effect of powder oxygen content on hierarchical martensitic structure and strength-ductility balance in Fe-Si-Mn-Cr-W medium carbon tool steel manufactured by laser direct energy deposition

초록

A Fe-Si-Mn-Cr-W medium-carbon tool steel (modified G6 composition) was fabricated via directed energy deposition (DED) using two types of powder feedstocks. Two powder feedstocks were produced by gas atomization using molten metal prepared under vacuum (VP, lower oxygen level) and air atmospheres (AP, higher oxygen level). In the DED Fe-Si-Mn-Cr-W tool steels, VP tool steel developed finer prior austenite grain (PAG)-packet-block-lath structures accompanied by stabilized dislocation cell substructures, whereas AP tool steel induced coarser martensitic hierarchy with reduced substructural refinement and a greater fraction of Mn-rich oxides segregated near PAG boundaries. Tensile results showed that VP tool steel exhibited higher strength (814.4 MPa YS, 2333.3 MPa UTS) but lower elongation (9.2%), whereas that AP tool steel showed lower strength (757.7 MPa YS, 1833.6 MPa UTS) but significantly higher elongation (20.8%). Variant distribution and crystallographic orientation within prior austenite grains (PAGs) further supported this trend, with VP tool steel representing strong packet alignment and limited slip diversity, and AP tool steel showing block clustering and expanded slip pathways. These hierarchical differences produced distinct deformation modes: VP tool steel experienced strain localization and deformation twinning due to restricted slip transfer, consistent with its strength-dominated mechanical response; in contrast, AP tool steel accommodated strain through multidirectional slip transfer and dislocation wall formation within coarse blocks, promoting strain delocalization and enhanced ductility. Overall, the results demonstrate that oxygen-induced evolution of the hierarchical martensitic structure regulates the balance between strength and ductility in DED tool steel by controlling deformation accommodation mechanisms.

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