Effect of austenite fraction and stability on strength-hardening-ductility in additively manufactured 17-4 PH stainless steel containing nitrogen

Abstract

Additively-manufactured (AM) 17-4 precipitation-hardening (PH) martensitic stainless steel (SS) built from nitrogen-atomized powder often retains a large volume fraction of austenite. The retained austenite lowers the yield strength compared to both wrought and AM 17-4 SS built from argon-atomized powder, hindering its use in many applications. However, retained austenite allows for high work hardening and high ductility through deformation-induced martensitic transformation (DIMT). This work systematically investigates the influence of various heat treatments on the volume fraction and stability of the retained austenite from tensile loading in AM 17-4 SS containing a mass fraction of 0.13% nitrogen (AM 17-4N SS). Synchrotron-based high-energy x-ray diffraction (HEXRD) is used to quantify the austenite volume fraction resulting from the heat treatments and the extent of DIMT in tensile specimens loaded to various plastic strains and specimens loaded to fracture. We show that by altering the volume fraction and stability of the retained austenite in AM 17-4N SS using different heat treatments, the DIMT process can be utilized to vary the strength, work hardening, and ductility, thus enabling its use in a wide range of applications. The AM 17-4N SS is also shown to meet or exceed the peak aged requirements for wrought 17-4 SS with yield strength (YS) of >1200 MPa, an ultimate tensile strength (UTS) of >1500 MPa, and a uniform elongation of ≥13% while retaining ≈ 20% volume fraction of austenite.