S phase layers produced on AISI 316 stainless steel by low temperature plasma nitriding and plasma carburising were examined in order to investigate their mechanical and chemical properties. Cylindrical disc specimens were used for testing hardness, pin on disc wear and electrochemical corrosion. Toroidal specimens were used for the fatigue and fretting fatigue tests. Both plasma nitriding and carburising processes created S phase layers on the surface of AISI 316 and greatly improved the steel's tribological properties. SEM showed that more precipitation of nitrogen occurred in the plasma nitrided specimens and GDS revealed that the nitrogen in the nitriding specimens did not penetrate as far into the substrate as the carbon in the carburising specimens. This is due to the nitrogen bonding easily with chromium in the surface layer which obstructs the diffusion of nitrogen into the substrate. The hardness tests showed that the nitriding layer was harder than the carburised layer but was more brittle and did not extend far into the substrate whereas the hardness of the carburised layer reduced gradually to a depth of 10 μm. The nitrided layer did better in the abrasion tests whereas the carburising layer performed better in the fatigue tests. The importance of these discoveries is discussed.
A low temperature plasma carburising process has been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. Previous studies have investigated the chemical, mechanical and structural characteristics of this carburised layer produced on AISI 316 steel at temperatures between 400° C and 600° C. The present paper focuses on the thermal stability of this carbon S phase layer. The investigation included isothermal annealing of the S phase layer as well as microstructure and property characterisation of the specimens. The results show that the S phase is metastable. When thermally annealed at certain temperatures for long enough, the carbon S phase decomposes into chromium carbides. Correspondingly, the hardness and corrosion resistance also varied. A preliminary isothermal transformation diagram has been constructed, which provides a basic guideline for the application of low temperature plasma carburised 316 austenitic stainless steel.
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.
Low temperature plasma surface alloying with nitrogen (nitriding) and carbon (carburising) has been conducted to enhance the wear and corrosion resistance of AISI 316 austenitic stainless steel. In addition to detailed general characterisation of the microstructure and carbon and nitrogen depth profiles, a series of wear corrosion tests were carried out, employing a recently developed electrochemical scratching technique, to study the simultaneous effects of corrosion and wear. Results show that both plasma nitriding and carburising can improve the corrosion wear resistance of AISI 316 austenitic stainless steel but that the nitrided layer out performs the carburised layer.
X-ray diffraction (XRD) techniques including stress measurement were applied to untreated, low temperature plasma nitrided and low temperature plasma carburised AISI 304 austenitic stainless steels treated at 425uC for 12 h in H 2 /N 2 and H 2 /CH 4 gases respectively. Relationships between surface microhardness and XRD peak broadening were established. The results also showed that both surface treated layers were under a compressive residual stress. The compressive residual stresses of the low temperature plasma nitrided and the low temperature plasma carburised layers were 2?19 and 1?58 GPa.
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