Enhancing material performance by controlling retained austenite in laser powder bed fused 17-4PH steel

Retained austenite is a fundamental factor in steels, capable of transforming to martensite under load, thus enhancing strength and ductility through TRIP effects (TRansformation Induced Plasticity). This study investigates the influence of nitrogen concentration (0.0081 to 1.58 wt.%) and normalized energy density (3.13 to 21.95) on retained austenite content in laser powder bed fused 17-4PH steel. Our findings demonstrate that higher nitrogen levels and lower energy densities significantly increase retained austenite fractions, achieving a range in measured austenite fraction of 1.40 % to 98.60 %. Conventional methods of measuring retained austenite using mechanical polishing can distort results due to unintended deformation-induced martensite. Therefore, we present a special magnetization measurement setup that characterizes the phase composition by differentiating the magnetization of body-centered (e.g., martensite, ferrite) and non-magnetic, face-centered crystal structures (e.g., austenite), validated by X-ray diffraction, thereby accurately characterizing phase compositions and overcoming limitations of traditional methods. The presence of metastable austenite correlates with substantial enhancements in mechanical properties, evidenced by an ultimate tensile strength (UTS) increase from 898 to 1131 MPa and improvements in elongation at break from 14.8% to 24.6%. This novel systematic exploration of the interplay between nitrogen concentration and energy density marks a significant advancement in controlling material properties of laser powder bed fused 17-4PH steel, highlighting the potential for tailored applications, such as in high-energy absorption contexts.