Micro-focused X-ray diffraction imaging analysis of Fe nitrides and carbides in the compound layer of nitrocarburized and gas nitrided steel surfaces

Thermochemical treatments like nitrocarburizing and gas nitriding form hardened surface layers of iron nitrides and carbides, improving wear, fatigue, and corrosion resistance in steel loaded components. This study employs micro-focused X-ray diffraction (µXRD) imaging at synchrotron facilities to characterize the microstructure of nitrocarburized and gas-nitrided steel surfaces in three steel grades (46MnVS3, 34CrNiMo6, 16CrMnNiMo9-5-2). Through line profile analysis with fine-step mesh grid scanning, we spatially resolve phase distributions and elastic/plastic strains in the compound layer. The ε-phase exhibits isotropic residual strain, transitioning from expansion to compression with depth, while the γ ’-phase displays anisotropic strain, expanding perpendicular to the surface and compressing parallel to it. These findings highlight µXRD’s potential for detailed structural analysis, enabling optimization of surface hardening processes. • Nitrocarbuzing and gas nitriding treatments on three steel grades produced a collection of samples with compound layers in a broad composition variation from the majority of ε phase to dominant the ϒ ’ phase • Synchrotron-radiation-based micro-focused X-ray diffraction imaging technique was used to map the phase and strain distribution in the compound layers. • XRD line profile analysis and mapping show that both ε and ϒ ’ phases are subject to depth-dependent strain distribution, with rather isotropic orientations for ε phase and apparent anisotropic orientations for ϒ ’ phase.