Microstructure control during deposition and post-treatment to optimize mechanical properties of wire-arc additively manufactured 17-4 PH stainless steel

The solidification and microstructural evolution during deposition, as well as the structural evolution during post heat treatment, determine the mechanical properties of wire-arc additively manufactured maraging stainless steels. In the present work, we tune the austenite reversion and nanoscale precipitation during post heat treatment and achieve an excellent combination of strength and ductility (ultimate tensile strength ~1340 MPa and uniform elongation ~10.5 %). The structural evolution is studied through computational thermodynamics, electron microscopy, in situ small-angle neutron scattering, and synchrotron X-ray diffraction. The as-built microstructure is composed of mainly martensite and retained austenite (~30 vol%) together with a minor fraction of δ-ferrite, M23C6, Nb(C, N), spherical and ellipsoidal Cu precipitates and some inclusions. The presence of these phases cannot be fully predicted by the Scheil-Gulliver model due to the complicated thermal history and non-homogenous elemental distribution. The reverted austenite formed during the post heat treatments has high stability and fine grain size (~1 µm), which contributes to the excellent ductility, while the nanoscale precipitation hardening contributes to the achieved high strength.