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

## TL;DR

This study shows how different post-processing methods affect the formation of a nitrided layer in additively manufactured Inconel 939 during plasma nitriding.

## Contribution

The first systematic investigation of ASPN on additively manufactured Inconel 939, highlighting the impact of post-processing on S-phase formation.

## Key findings

- Air-cooled thermal treatment promotes oxide layer formation, inhibiting nitrogen diffusion and resulting in thin nitrided layers.
- Inert atmosphere processing enables continuous S-phase layer development with enhanced surface hardness (~1200 HV).
- Post-processing sequence significantly influences S-phase formation and mechanical properties in additively manufactured superalloys.

## Abstract

Active screen plasma nitriding (ASPN) of additively manufactured nickel-based superalloys represents an understudied surface enhancement pathway. This study presents the first systematic investigation of ASPN applied to additively manufactured Inconel 939 (IN 939), evaluating four distinct post-processing routes combining heat treatment atmospheres (argon versus air cooling), vibratory finishing, and lapping under identical nitriding parameters (450 °C, 8 h, 25% N2 + 75% H2, 3 hPa). Contrasting nitriding behaviours emerged as a function of the post-processing route: the air-cooled thermal treatment (HT-air-vibr-lap) promotes formation of a thick Al/Cr-rich oxide layer (10–15 µm) that substantially inhibits nitrogen diffusion, resulting in thin and discontinuous nitrided layers. Conversely, the inert atmosphere route (HT-Ar-vibr-lap) circumvents oxide formation, enabling continuous S-phase (expanded austenite, γN) layer development of a 6.4 ± 0.3 µm thickness with exceptional surface hardness (~1200 HV, representing 3–4× enhancement relative to base material). X-ray diffraction confirmed S-phase formation with refined lattice parameter (3.609 Å) and secondary nitride phases (CrN-type and NbN/TaN-type precipitates). The post-processing sequence—particularly heat treatment atmosphere and mechanical finishing methodology—emerged as a critical controlling parameter for S-phase formation efficiency and mechanical properties of nitrided layers in additively manufactured nickel-based superalloys. This work addresses a knowledge gap distinct from the existing literature on conventional Inconel systems, establishing that controlled surface modification through post-processing can achieve the required properties.