Influence of Post-Processing on S-Phase Formation During Plasma Nitriding of Additively Manufactured Inconel 939

Area of Science:

• Materials Science
• Surface Engineering
• Additive Manufacturing

Background:

• Active screen plasma nitriding (ASPN) is an understudied surface enhancement technique for additively manufactured nickel-based superalloys.
• Inconel 939 (IN 939) is a critical superalloy for high-temperature applications, but its surface properties require optimization.

Purpose of the Study:

• To systematically investigate the effect of different post-processing routes on ASPN of additively manufactured IN 939.
• To evaluate how heat treatment atmosphere and mechanical finishing influence nitriding behavior and layer properties.

Main Methods:

• ASPN was applied to additively manufactured IN 939 under identical nitriding parameters.
• Four distinct post-processing routes were evaluated, combining air vs. argon cooling, vibratory finishing, and lapping.
• X-ray diffraction (XRD) and hardness testing were used for material characterization.

Main Results:

• The air-cooled route formed an oxide layer inhibiting nitrogen diffusion, resulting in thin, discontinuous nitrided layers.
• The inert atmosphere (argon) route prevented oxide formation, enabling a continuous 6.4 µm S-phase layer.
• The S-phase layer achieved exceptional hardness (~1200 HV), a 3-4x enhancement over the base material.
• XRD confirmed S-phase formation with refined lattice parameters and secondary nitride precipitates.

Conclusions:

• Post-processing sequence, especially heat treatment atmosphere and mechanical finishing, critically controls S-phase formation and properties.
• Controlled surface modification via post-processing is essential for achieving desired nitrided layer properties in additively manufactured superalloys.
• This study establishes a pathway for optimizing ASPN in additively manufactured nickel-based superalloys, distinct from conventional systems.