Mathematical Modelling to Predict Nitrided Layer Thickness of Low Temperature Gas and Plasma Nitriding of AISI 316 L Stainless Steel (Austenitic)

Austenitic stainless steel is one of the world most produced alloy for stainless steel production mainly due to its high corrosion resistance properties. However, austenitic stainless steel low surface hardness have always been an important issue to address. Therefore, many studies have been conducted in order to increase the surface hardness of the austenitic stainless steel without significantly affect the corrosion resistance characteristic of the stainless steel. The author’s study compose only on the austenitic stainless steel type AISI 316L which is among the most produced stainless steel in the whole world

Plasma surface engineering and characterisation of biomedical stainless steels

Low temperature plasma surface alloying with nitrogen (nitriding), carbon (carburising) and both (carbonitriding) has been successfully employed in hardening medical grade ASTM F138, ASTM F1586 and ASTM F2581 as well as engineering grade AISI 316 by the formation of a modified layer better known as S-phase or expanded austenite. In this study, systematic plasma treatments and characterisation were performed on medical grade stainless steel in order to establish the optimised treatment conditions, especially temperature, which can maximise the hardened case depth without any detriment in corrosion resistance. The surface of a biomaterial must not adversely affect its biological environment and return the material surface must not be adversely affected by the surrounding host tissue and fluids. Experimental results have shown that this duality of concern can be addressed by creating S-phase. It has been shown that low-temperature nitriding (430°C), carburising (500°C) and carbonitriding (430°C) improved the localised corrosion, corrosion-wear and fretting-wear resistance of these medical grade stainless. Also biocompatibility studies have proved that these hardened surfaces were biocompatible under the realms of the tests conducted in this study therefore the use of hardened medical grade austenitic stainless steel might be suitable in implant applications.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Low temperature plasma surface modification of stainless steels for ice sliding applications

Stainless steels used in ice sliding applications, such as in the blades of ice skates, ski edges and runners of bobsleigh’s have notoriously poor tribological properties. During the study, microstructure and tribological behavior of AISI 420 and AISI 316 were studied and through the application of low temperature plasma surface modification, it has been possible to improve the base properties exhibited by the materials. Both grades of steel were subjected to a range of plasma surface modifications, using nitriding, carburizing and carbo-nitriding techniques over a set temperature range. Improvements in the surface hardness were documented in all cases. Characterization of the depth of the modified layer was carried out using SEM, GDOES and cross-sectional micro-hardness to obtain optimum conditions for treatment of replica steel blades for use in simulation wear testing. A synthetic ice substitute was created, for tribological testing, and an improvement in performance was observed following the surface modification procedure

Mathematical Modelling to Predict Nitrided Layer Thickness of Low Temperature Gas and Plasma Nitriding of AISI 316 L Stainless Steel (Austenitic)

Austenitic stainless steel is one of the world most produced alloy for stainless steel production mainly due to its high corrosion resistance properties. However, austenitic stainless steel low surface hardness have always been an important issue to address. Therefore, many studies have been conducted in order to increase the surface hardness of the austenitic stainless steel without significantly affect the corrosion resistance characteristic of the stainless steel. The author’s study compose only on the austenitic stainless steel type AISI 316L which is among the most produced stainless steel in the whole world

Study for the optimization of interfacial properties between metallic substrates and polymeric coatings by plasma-based surface modification methods to improve performance of vascular stents

Au cours de 15 dernières années, les maladies coronariennes et les accidents vasculaires cérébraux demeurent les causes principales de décès dans le monde. Selon l'Organisation Mondiale de la Santé, en 2015, ces deux maladies ont causé 15 millions des décès sur les 56,4 millions dans le monde. Des traitements chirurgicaux ont été élaborés et améliorés pour soigner ces maladies en maintenant les vaisseaux sanguins ouverts. Parmi les traitements chirurgicaux, l'angioplastie avec utilisation d’un stent est le traitement le plus populaire et le moins invasif. Les stents, qui sont des tubes métalliques en treillis, vont soutenir mécaniquement les vaisseaux sanguins après l’implantation et les maintenir ouverts pour améliorer le flux sanguin. Ceux-ci sont principalement composés d’acier inoxydable AISI316L (SS316L), d'alliage de cobalt-chrome et d'alliage de titane. Depuis plus d'un demi-siècle, lorsqu'un stent a été implanté pour la première fois, ils ont été considérablement améliorés. Cependant, la libération d'ions métalliques, potentiellement toxiques, et la détérioration des propriétés mécaniques à cause de la corrosion ainsi que la diminution de l'adhérence des revêtements, dans le cas de stents avec les revêtements en polymère, constituent encore des préoccupations majeures lors de l’utilisation des stents. Dans le cas des stents en SS316L, afin d’éviter la libération d'ions métalliques, au laboratoire de biomatériaux et de bioingénierie de l'Université Laval (LBB), lors de précédentes recherches, un revêtement fluorocarboné (CFx) a été étudié pour isoler complètement le stent de l'environnement biologique. Ce revêtement permet également le greffage ultérieur de molécules bioactives pour améliorer son intégration dans le corps. Cependant, l'interface de SS316L / CFx devait être améliorée pour augmenter l’adhésion du revêtement CFx sur le SS316L. Dans mon projet de doctorat, l’oxydation au plasma a été utilisé pour élaborer une nouvelle interface entre le substrat SS316L et le revêtement. Les propriétés de cette nouvelle interface, qui est composée d’une couche d'oxyde, ont été modifiées en faisant varier les paramètres du procédé plasma afin de préserver les propriétés de cette couche d’oxyde lorsqu’elle subit une déformation plastique de 25%, c’est-à-dire le pourcentage de déformation maximale que subira le stent lors de son implantation. Cette interface a permis de diminuer la libération des ions du substrat SS316L en réduisant son taux de corrosion plus que trois fois et d’améliorer l’adhérence adéquate du revêtement CFx sur le substrat, après déformation et après immersion dans une solution aqueuse saline. La nouvelle couche d'oxyde sur SS316L est une couche d'oxyde amorphe avec une épaisseur d'environ 6 nm qui se distincte bien de la microstructure polycristalline du substrat. L'amélioration des propriétés de l'interface a été attribuée à cette couche d'oxyde amorphe nano-épaisse, qui est résistante aux déformations plastiques. Cette couche d'oxyde peut être appliquée sur des stents métalliques nus composés de métaux passivables. En outre, elle crée une interface favorable pour les revêtements en polymère, qui sont utilisés pour les stents à relargage de principes actifs ainsi que pour améliorer l'intégration des stents dans le corps humain.Over the past 15 years, ischemic heart disease and stroke have remained the leading causes of death, worldwide. According to the World Health Organization, 15 million of the 56.4 million global deaths, in 2015, were caused only by ischemic heart disease or stroke. For the treatment of these diseases, surgical treatments have been introduced and improved to hold the blood vessels open. Among the surgical treatments, angioplasty with stenting is the most popular and the least invasive treatments. Stents, which are wire mesh tubes, prepare a mechanical support for blood vessels and hold them open to restore the blood flow. They are mostly made up of AISI316L stainless steel (SS316L), cobalt-chromium, and titanium alloys. More than half a century ago, when a stent first used, it has considerably evolved. However, release of potentially-toxic metallic ions and deterioration of mechanical properties due to corrosion, and decrease of polymeric coatings adhesion, in case of coated stents, still constitute major concerns in SS316L stents. In the case of SS316L stents, to circumvent the release of metallic ions, in the laboratory for biomaterials and bioengineering of Université Laval (LBB), a fluorocarbon (CFx) coating was previously investigated to isolate the stent completely from the biological environment. The coating also enables subsequent grafting of bioactive molecules to improve its integration in the body. The results were promising; however, the interface of SS316L/CFx needed to be modified to improve the adhesion of the CFx coating. In this Ph.D. research project, a new interface between the SS316L substrate and the CFx coating was created by plasma oxidation. The properties of this new interface, which was an oxide layer, was modified by varying the plasma-process parameters in order to preserve its properties after a 25% plastic deformation. This deformation is the maximum plastic deformation that imposes on a stent during its implantation. The new interface decreased the release of ions by decreasing the corrosion rate of the SS316L substrate by a factor of three. It was also found that the new interface produced an adequate adhesion of the CFx coating to the substrate after deformation as well as after immersion in an aqueous saline solution. The new oxide layer on SS316L was an amorphous oxide layer with an approximately 6 nm thickness, which was clearly distinguished from the polycrystalline microstructure of the substrate. The enhancement of the interface properties was ascribed to this nano-thick amorphous oxide layer, which was found to be more resistant to plastic deformation. This new oxide layer can be produced on bare-metal stents made of passivating metals. Moreover, it can create a favorable interface for coated stents, which have been used in drug-eluting stents, and also to improve stents integration in the human body

Effect of temperature and carbonaceous environment on the fatigue behaviour of AISI 316L austenitic stainless steel

Austenitic stainless steels are widely used in the chemical, petrochemical and food-processing industries due to their excellent corrosion resistance and good mechanical properties. However, due to their inherent austenitic structure, they have relatively low hardness, as well as poor wear resistance and short fatigue life. Thermochemical surface treatments are used to improve the wear resistance, hardness and fatigue life. Austenitic stainless steels are difficult to carburise due to the tenacious Cr2O3 layer on the surface, although plasma and gas carburising have proven to be very effective. However, this investigation sought to understand the effect of high carburising temperature on the mechanical properties of AISI 316L steel, without the removal of the tenacious Cr2O3 surface layer. Pack carburising with 60% BaCO3, 30% activated carbon and 10% sodium chloride was done on AISI 316L austenitic stainless steel at 450ºC, 550ºC, 650ºC, 700ºC and 750ºC for 24 hours. Tensile, impact, hardness testing and fatigue tests were done and optical microscopy, SEM and XRD were used to characterise the specimens. The ultimate tensile strength (UTS) of the as-received samples was 647±4MPa. The values for samples carburised at 450ºC and 550ºC were 651±5.5MPa and 651±3.6MPa with no significant differences. Samples tested at 650ºC, 700ºC and 750ºC showed decreasing tensile strength (638±3.5-603±2.5MPa). The hardnesses of treated samples at 450ºC and 550ºC were similar to the as-received, ranging between 248HV0.5 and 254HV0.5. Hardnesses of samples carburised at 650C, 700C and 750C decreased from the surface to the core, with limited carbide precipitation within the core, which had similar values to the as-received. The number of cycles to failure of the as-received sample was 59298±2520 and was similar to the carburised samples at 450-650ºC (ranging from 61455±15076-51819±5257). A significant reduction in fatigue strength was observed for samples carburised at 700ºC (25387±595) and 750ºC (7146±318), which was due to the effect of carbon intake of the samples. The elongation for as-received material was 48.5±0.4%, and the reduction in area was 76.6±0.97%. These values decreased with increasing carburising temperatures from 450C to 750C. The decrease in ductility was attributed to uptake of carbon, causing surface hardening and minor carbide precipitation in the core. X-ray diffraction of the carburised samples showed a shift in γ peaks compared to the as-received samples, which was attributed to the carbon intake. Samples carburised at 450C and 650C had austenite grains, twin boundaries and slip lines within the grains. The frequency of the defects increased with increasing carburising temperature. The twins were more predominant at the surface and less so towards the core of the steel. The hardness increase was more effected by the carbon increase. There was limited carbide precipitation and a very thin observable carburised case. The results showed that this type of pack carburising of austenitic stainless steel is not suitable for improving the properties of AISI 316L steel

Corrosion Resistance

The book has covered the state-of-the-art technologies, development, and research progress of corrosion studies in a wide range of research and application fields. The authors have contributed their chapters on corrosion characterization and corrosion resistance. The applications of corrosion resistance materials will also bring great values to reader's work at different fields. In addition to traditional corrosion study, the book also contains chapters dealing with energy, fuel cell, daily life materials, corrosion study in green materials, and in semiconductor industry

Heat Treatment of Steels

Steels represent a quite interesting material family, both from scientific and commercial points of view, following many applications they can be devoted to. Following this, it is therefore essential to deeply understand the relations between properties and microstructure and how to drive them via a specific process. Despite their diffusion as a consolidated material, many research fields are active regarding new applications. In this framework, in particular, the role of heat treatments in obtaining complex microstructures is still quite an open matter, which is also thanks to the design of innovative heat treatments.This Special Issue embraces interdisciplinary work covering physical metallurgy and processes, reporting on experimental and theoretical progress concerning microstructural evolution during the heat treatment of steels