Mechanical and Chemical Properties of Low Temperature Plasma Surface Alloyed 316 Austenitic Stainless Steel

Abstract

Low temperature plasma nitriding and carburising are well known as methods for improving tribological properties without deterioration of corrosion properties of austenitic stainless steels. ‘S phase’ is a key alloyed layer, achieved from these two plasma thermochemical processes, referred as nitrogen and carbon S phase, respectively. The present work has been focused on full characterisation of the mechanical and chemical properties of carbon and nitrogen S phase on austenitic stainless steel AISI 316 produced during low temperature plasma processes with different process parameters. A series of theoretical, experimental, and analytical studies have been conducted in order to lay the necessary foundations for fully realising application potential of these processes in various stress and environmental conditions.

An Investigation on Low-Temperature Thermochemical Treatments of Austenitic Stainless Steel in Fluidized Bed Furnace

“…In all the resultant layers, no nitride and carbide precipitates were detectable by XRD. This is in close agreement with previous results (Ref 5,9,17) on individual plasma nitriding and carburizing which conclude that the formation of nitride and carbide precipitates requires a higher temperature, and the temperature of 450°C does not favor the formation of these precipitates. Thermodynamically, chromium nitrides and carbides could form under this process condition, however, at this relatively low processing temperature; the rate of precipitation is sluggish such that for a short treatment period the produced nitrided and carburized layers are precipitation-free.…”

“…The purpose of using low temperatures is to suppress the formation of chromium nitrides and carbides in the alloyed layers, such that chromium is retained in solid solution for corrosion protection (Ref 5,8). Hardening of the nitrided layer and the carburized layer is due to the incorporation of nitrogen and carbon, respectively, in the austenite lattice, forming a structure termed expanded austenite, which is supersaturated with nitrogen and carbon, respectively (Ref 7,9). More recently, a hybrid process has also been developed, which combines the nitriding and carburizing actions in a single process cycle by introducing nitrogen and carbon simultaneously into the austenite lattice to form a hardened zone comprising a nitrogen expanded austenite layer on top of a carbon expanded austenite layer (Ref [10][11][12].…”

Guest editorial: In memory of Tom Bell

“…44 The pioneering work by Tom and his colleague, Zhang, in 1985, 46 demonstrated for the first time that it was possible to effectively improve the surface hardness and wear resistance without loss of corrosion resistance of austenitic stainless steel by low temperature plasma nitriding. 172,182 properties of S-phase were fully evaluated and compared. 171 Recent work by Tom's team has demonstrated that S-phase can also be generated in hcp/fcc dual phase structured Co-Cr alloys.…”

Residual stress measurement of low temperature plasma surface alloyed layer using X-ray diffraction techniques

“…1,2 The surface alloyed layers produced at low temperatures appear as dense, white, precipitate free structures and they are supersaturated with interstitial solid solution of nitrogen and/or carbon. 3 They consist of a single phase having a unique X-ray diffraction (XRD) pattern where diffraction peak positions deviate markedly from the expected theoretical positions of common Bravais lattices. 4 The XRD pattern is similar to that of a face centred cubic (fcc) austenitic structure, but the peak positions are at lower diffraction angles and the reflections contain both broadening and asymmetry.…”

“…The levels of plain and fretting fatigue resistance are quite similar for both surface alloyed layers. 3 Surface modification processes generally increase the case depth or hardness and may encourage a residual stress distribution, producing the longest component life. 7 Internal stress is an extrinsic property and must be calculated via a directly measurable property such as strain, hence the normal method of residual stress determination is to calculate stress from strain, assuming linear elasticity, 7 using XRD to measure d spacing of suitable lattice planes in the structure under investigation.…”