Abstract
We study the effect of ion-plasma surface nitriding on the phase composition, microstructure, surface microhardness, and tensile properties of the AISI 321-type stainless steel produced by wire-feed electron-beam additive manufacturing (EBAM). Ion-plasma nitriding at 550 °C for 12 h in N2/H2 gases provides the formation of a 10-μm thick surface layer with solid solution strengthening by nitrogen atoms (Fe-γN и Fe-αN) and dispersion hardening (γ’-Fe4N) with a fivefold increase in surface hardness up to ≈12 GPa. Surface ion-plasma nitriding of additively produced steel does not affect the anisotropy of mechanical properties, but rather increases the yield strength and ultimate tensile strength while maintaining high plasticity in the specimens. In specimens after ion-plasma nitriding, the fracture mechanism changes from initially ductile to a quasi-brittle fracture near the surface and ductile transgranular mode in the central parts of the specimens. The nitrided layer fractured in a transgranular brittle manner with the formation of quasi-cleavage facets and secondary cracks near the surface of the specimens. Brittle fracture of the compositional layer occurs due to the complex solid solution strengthening and particle hardening of austenite.
Highlights
The development of new production technologies, including additive manufacturing (AM), is focused on obtaining new advanced materials, and on adapting the technological cycles to produce industrially important alloys
We study the effect of ion-plasma surface treatment on the phase composition, microstructure, surface microhardness, and tensile properties of the Cr-Ni stainless steel produced by wire-feed electron-beam additive manufacturing
In Cr-Ni stainless steel, obtained by electron-beam wire-feed additive manufacturing, a two-phase γ-austenite/δ-ferrite microstructure is formed with columnar austenite grains elongated in the building direction of the billet
Summary
The development of new production technologies, including additive manufacturing (AM), is focused on obtaining new advanced materials, and on adapting the technological cycles to produce industrially important alloys. A typical two-phase microstructure of the EBAMfabricated Cr-Ni steels, which could contain about 20% of high-temperature δ-ferrite, arises due to the depletion of the melt by nickel and the variation of the crystallization mechanism of the steel [7,8,9]. This disadvantage could be avoided by the post-build heat treatment or excessive concentration of the austenite-stabilizing elements in the raw materials (the most commonly used elements are nickel, manganese, nitrogen, or carbon) [7,9,10,11,12]. In applications, where high corrosion resistance and high surface hardness are mandatory, both could be achieved in one technological cycle via surface modification of the steel products fabricated in the AM process
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