Experimental Evaluation of Mechanical and Tribological Behaviours of Gas Nitride treated AISI 316LN Austenitic Stainless Steel

Enhancing the Corrosion Resistance of Austenitic Steel Using Active Screen Plasma Nitriding and Nitrocarburising

AISI 316L steel was subjected to active screen plasma nitriding and nitrocarburising. The processes were carried out at 440 °C for 6 h. The nitriding process employed an atmosphere of nitrogen and hydrogen, while nitrocarburising was carried out in nitrogen, hydrogen and methane. The processes yielded structures consisting of nitrogen and nitro-carbon expanded austenite, respectively. Microhardness was measured via the Vickers method, surface roughness using an optical profilometer, microstructure by means of light microscopy, while a scanning electron microscope (SEM) served to determine surface topography. Phase composition, lattice parameter and lattice deformation tests were carried out using the X-ray diffraction (XRD) method. Corrosion resistance measurements were performed in a 0.5 M NaCl solution using the potentiodynamic method. The produced layers showed very high resistance to pitting corrosion, while the pitting potential reached 1.5 V, a value that has not yet been recorded in a chloride environment. After the passive layer was broken down, there was a clear deceleration of pitting in the nitrocarburised layer. It was found that in the case of nitro-carbon expanded austenite, pits are formed much slower compared to the nitrogen austenite layer.

Effect of Nitrided and Nitrocarburised Austenite on Pitting and Crevice Corrosion Resistance of 316LVM Steel Implants

Harmful lesions occur in the body around multielement stabilisers made of AISI 316 LVM (Low Vacuum Melted) steel, caused by products of pitting, fretting or crevice corrosion. Preventing the effect is possible by modifying the surface of the steel implants. Therefore, the goal of the paper is the comparison of the mechanical and physiochemical properties of plates for treating deformations of the anterior chest wall made of AISI 316 LVM steel, subjected to diffusion and sterilisation processes and exposed to Ringer's solution. The surface of the implants was subjected to electrochemical polishing, chemical passivation and, in order to modify their properties, nitrocarburised and nitrided diffusion layers were created on selected stabilisers under glow discharge conditions with the use of an active screen at a temperature of 420 °C, over 60 min. The conducted studies involved the examination of the microstructure of the formed layers, surface roughness testing, analysis of contact angles and surface free energy, examination of resistance to pitting and crevice corrosion and examination of nanohardness. On the basis of the results of the conducted studies, it was established that the most advantageous set of properties after sterilisation and exposure to Ringer's solution was displayed by implants with a formed diffusion nitrocarburised layer.

Hardening of HVOF-Sprayed Austenitic Stainless-Steel Coatings by Gas Nitriding

Austenitic stainless steel exhibits an excellent corrosion behavior. The relatively poor wear resistance can be improved by surface hardening, whereby thermochemical processes offer an economic option. The successful diffusion enrichment of bulk material requires a decomposition of the passive layer. A gas nitriding of high velocity oxygen fuel spraying (HVOF)-sprayed AISI 316L coatings without an additional activation step was studied with a variation of the process temperature depending on the heat-treatment state of the coating. A successful nitrogen enrichment was found in as-sprayed condition, whereas passivation prevents diffusion after solution heat treatment. The phase composition and microstructure formation were examined. The crystal structure and lattice parameters were determined using X-ray diffraction analysis. The identified phases were assigned to the different microstructural elements using the color etchant Beraha II. In as-sprayed condition, the phase formation in the coating is related to the process temperature. The formation of the S-phase with interstitial solvation of nitrogen is achieved by a process temperature of 420 °C. Precipitation occurs during the heat treatment at 520 °C. In both cases, a significant increase in wear resistance was found. The correlation of the thermochemical process parameters and the microstructural properties contributes to a better understanding of the requirements for the process combination of thermal spraying and diffusion.