Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K for Hardening and Microstructure Control

Abstract

The high-density plasma nitriding at 673 K and 623 K was employed to make 10% of nitrogen supersaturation on AISI316 base austenitic stainless steels. The processing parameters and nitrogen-hydrogen gas flow ratio were optimized to increase the yield of N2+ ion and NH-radical for efficient nitriding. The nitrided AISI316 specimens were prepared for multidimensional analysis to describe the fundamental features of low-temperature plasma nitriding. First, macroscopic evaluation revealed that nitrogen supersaturation induced the γ-lattice expansion and the higher nitrogen content than 4% of mass in depth. The mesoscopic analysis describes the holding temperature and initial grain-size effects on the microstructure changes. Plastic straining, grain-size refinement, and nitrogen zone-boundary diffusion processes advance with nitrogen supersaturation to drive the inner nitriding behavior. The microscopic analysis explains the microstructure refinement, the two-phase structuring, and the microstructure modification. Through this multi-dimensional analysis, the essential characteristics of the low-temperature plasma nitriding of 316 austenitic stainless steels were precisely understood to extend the engineering treatise on the bulk nitrogen stainless steels for surface modification and treatment of stainless steels by nitriding. This plasma nitriding was applied to strengthen and harden the AISI316 wire surfaces toward its application on surgery wires.

Micro-/Meso-Structure Control of Multi-Hostmetal Alloys by Massive Nitrogen Supersaturation

“…That is, the N-rich clustering nanostructure had a bcc structure with higher nitrogen solute content in local, while the N-poor clustering nanostructure had a fcc structure with lower nitrogen solute content. Remember that the NS-AISI316 layer had two-phase nanostructures as analyzed by EBSD [ 5 , 15 ]. This separation into two-clustering nanostructures with different crystallographic structure, shown in Figure 11 , is just corresponding to the formation of two-phase structuring as observed in EBSD analysis.…”

“…In the case of MNS-AISI316, this solid-phase separation occurs under the high nitrogen solute content condition. As partially discussed in [5,15], higher nitrogen content than 3 mass% is needed to drive the interaction between the host metals of Cr, Fe and Ni and the nitrogen interstitial atom. Higher hardness than 1400 HV in [5] and high machinability of NS-AISI316 by the PCD chip in [12,13] were attributed to this high nitrogen interstitial content in host metals.…”

“…No chemical reactions took place between the carbon in the diamond chips and the iron in the NS-AISI316 matrix when machining the NS-AISI316 works by the PCD (Poly-Crystal Diamond)-chipped cutting tools [12,13]. Furthermore, as theoretically estimated and experimentally described in [5,14,15], this nitrided layer had uniform nitrogen content distribution from its surface to the nitriding front end. The average nitrogen content of this plateau reached 4 mass%.…”

In Situ Lubrication in Forging of Pure Titanium Using Carbon Supersaturated Die Materials

“…That is, NS and CS modify the original microstructure of the dies to a clustered nanostructure with different crystallographic structures. The cluster with rich interstitial solutes corresponds to the γ-phase nanograin, whereas the cluster with poor interstitial solutes corresponds to the α-phase nanograin [ 32 ]. These two different nanograins are neighboring each other.…”

Plasma Nitriding-Assisted 3D Printing for Die Technology in Digital Micro-Manufacturing

“…This inner nitriding process at low holding temperature is not only governed by the nitrogen diffusion process but also controlled by the plastic straining. After [26,27], the mismatched strains between the un-nitrided and nitrided zones are induced by the lattice expansion in the nitrogen supersaturation process. Most of nitrogen solute diffuse through the plastically strained zone boundaries so that the high nitrogen content is preserved in all the nitrided layer.…”