Low Temperature Plasma Nitriding of Austenitic Stainless Steels

Abstract

A low temperature plasma nitriding process has become one of the most promising methods to make solid-solution hardening by the nitrogen super-saturation, being free from toxicity and energy consumption. High-density radio-frequency and direct current (RF/DC) plasma nitriding process was applied to synthesize the nitrided AISI304 microstructure and to describe the essential mechanism of inner nitriding in this low temperature nitriding (LTN) process. In case of the nitrided AISI304 at 673 K for 14.4 ks, the nitrided layer thickness became 66.5 μm with the surface hardness of 1550 HV and the surface nitrogen content of 9 mass%. This inner nitriding process was governed by the synergetic interrelation among the nitrogen super-saturation, the lattice expansion, the phase transformation, the plastic straining, the microstructure refinement, and the acceleration of nitrogen diffusion. When this interrelation is sustained during the nitriding process, the original austenitic microstructure is homogeneously nitrided to have fine-grained microstructure with the average size of 0.1 μm. Once this interrelation does not work anymore, the homogeneous microstructure changed itself to the heterogeneous one. The plastic straining took place in the selected coarse grains so that the parts of them were only refined. This plastic localization accompanied with the localized phase transformation.

Low-Temperature Plasma Nitriding of Mini-/Micro-Tools and Parts by Table-Top System

“…This plasma nitriding at a lower temperature than 700 K was characterized by the nitrogen supersaturation; after [5], this processing was expected to be applied to various surface treatments such as carburizing and nitrocarburizing as the S-phase engineering. In addition, this nitrogen supersaturation process accompanied by two-phase nano-structuring to harden and strengthen the stainless steel parts and members as pointed in [8,9]. As demonstrated in [9][10][11], the corrosion toughness was also improved in these nitrogen supersaturated stainless steels.…”

Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals

“…The fine-grained AISI316 (FGSS316) specimen was prepared for plasma nitriding at 623 K [38][39][40][41] Figure 19(A) depicts the high density plasma nitriding system with use of the hollow cathode device to intensify the density of nitrogen ions as well as the NH-radicals. The plasma-processing conditions are also summarized in Figure 19(B) in correspondence to "B" in Figure 18.…”

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

“…Lower temperature plasma nitriding process of austenitic stainless steels than 700 K is governed by the nitrogen supersaturation with nitrogen interstitial occupation of octahedral vacancy sites of fcc-structured supercells as well as the nitrogen diffusion through refined grain boundaries and slipping lines [13][14][15]. FGSS316 plates and wires are employed to describe the nano-structuring process with grain size refinement by this plasma nitriding.…”

“…This theoretical model is experimentally demonstrated in the following. As stated in [13][14][15], the inner nitriding advances homogeneously from the surface to the nitriding front end; it is rather difficult to experimentally describe each fundamental process separately from other processes in Figure 9. A normal-grained AISI316 plate is employed as a work material and nitrided at 623 K for 14.4 ks to decelerate the nitrogen diffusion rate and to describe the synergetic relation among the nitrogen supersaturation, the phase transformation, the plastic straining, the grain size refinement and the local nitrogen diffusion.…”

“…Without externally applied stresses, the slip-line network is formed from the surface to the depth together with the nitrogen interstitial diffusion. In particular, the low temperature plasma nitriding process [10][11][12][13][14][15] works as a powerful means to demonstrate that austenitic and martensitic stainless steel substrates are hardened and modified to have two-phase structure with the average grain size of 0.1 μm. These previous studies proved that grain size as well as crystallographic structure should be significantly controlled by the materials processing other than the shearing process in metal forming.…”