Grain Size Effect on the Nitrogen Super-Saturation Process into AISI316 at 623 K

Abstract

Coarse and fine grained AISI316 substrates were prepared to describe the grain size effect on the inner nitriding behavior at 623 K by using the high density plasma nitriding without precipitation of nitrides. In case of coarse grained AISI316, the nitriding process advanced homogeneously in one part of nitrided layer with high nitrogen content, and, heterogeneously in its other part. In the former, γ-α ' two-phase, fine microstructure was uniformly formed by the phase transformation and plastic straining with the nitrogen supersaturation. In the latter, the nitrogen super-saturation localized to selectively modify the coarse grains to form the transformed α '-phase zones with the plastically strained γ-phase ones, even below the nitriding front end of 30 μm. In case of fine-grained AISI316, the nitriding took place homogeneously to form fine, two-phase microstructure down to the nitriding front end of 40 μm. This difference in the inner nitriding behavior came from the synergetic relationship between the nitrogen diffusion and super-saturation processes.

Microstructure/Mechanical Characterization of Plasma Nitrided Fine-Grain Austenitic Stainless Steels in Low Temperature

“…The γ N -α N two-phase nanostructure comes back to a nearly single γ N -phase in Figure 6c. As reported in [14,15,19], the nitrogen solute diffuses through the refined grain and zone boundaries in the nitrogen supersaturation. This microstructure change is caused by the autonomous mode change from heterogeneous nitrogen supersaturation to homogeneous nitrogen supersaturation.…”

“…This non-zero nitrogen content beyond the nitriding front end implies that the nitrogen supersaturation advances heterogeneously even below the nitrided layer. EBSD was a powerful tool to describe the microstructure evolution during plasma nitriding as stated in [9,[12][13][14][15]. Figure 6a depicts the IPF distribution from the surface to the depth (d) of 130 µm.…”

“…The wire was uniformly nitrided to have a layer thickness of 40 μm and the un-nitrided FGSS316 matrix is continuously capped by this nitrided surface layer. Since this nitrided layer thickness is nearly equal to 40 μm in the nitrided FGSS316 plates at 623 K in [12][13][14], the nitrogen supersaturation process advances…”

“…As stressed in [10], this nitrogen supersaturation is the key to combining the benefits of a stable austenitic structure with higher strength, better corrosion, and wear resistance and superior biocompatibility in comparison to the currently used austenitic stainless steel, especially AISI316. After [11][12][13][14][15], precise analysis of the micro-/nanostructure of nitrogen supersaturated AISI316 steels reveals that they have a two-phase nitrided layer with the grain size refined down to 0.1 µm. The nitrided layer thickness reaches 60 µm even by the RF (radio frequency)-DC (direct current) high-density plasma nitriding at 673 K. This in situ refinement of the crystalline structure of AISI316 steels is accompanied by homogeneous nitrogen supersaturation and diffusion deep in their matrix.…”

Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing

“…As had been discussed in [29,30], the initial grain size of AISI316 has influence on the inner nitriding behavior; homogeneity in nitriding process is enhanced with reduction of initial grain size. This finding suggested that fine-grained AISI316 or FGSS316 structural components and parts could be homogeneously nitrided on the surface with a little effect of heterogeneous nitriding on the microstructure below NFE.…”