Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy

Phase transformations during nitriding of a ternary carbon iron-based alloy Fe–3%Cr–0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr7C3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon.The authors acknowledge Aubert & Duval (Eramet Group) and the Safran Group for their support, as well as CP2M laboratory, Paul Cezanne University, Marseille, for TEM investigations

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed

Surface Characteristics and Performance of Stainless Steel AISI 316L of Low Temperature Hybrid Thermochemical Treatment

Austenitic stainless steels are the most popular materials in the stainless steel due to their excellent corrosion resistance and good forming characteristics. Nevertheless, this type of materials has low hardness as well as poor wear resistance due to the inherent austenitic structure. Earlier attempts to increase the surface hardness and wear resistance of austenitic stainless steels by surface treatments have led to the deterioration in corrosion resistance arising from the depletion in chromium in the hardened layer. The development of low temperature surface treatment carried out at temperatures lower than 500°C has been successfully conducted by innovative techniques including plasma nitriding, ion beam nitriding and ion implantation

THE EFFECTS OF NITROGEN ION IMPLANTATION ON FATIGUE PROPERTIES OF PLAIN CARBON STEELS

Plain carbon steels are widely used for simple structural applications and machinery parts because they are cheap and readily available, but they perform poorly under fatigue when compared with alloy steels. Plasma ion immersion implantation (PIII) is a surface modification technique which has shown to improve the hardness, corrosion, fatigue, and wear properties of steel. Most of the research works featuring this technique used nitrogen as the implantation species however, the materials used were mostly of alloy steels. In the present study, AISI 1018 and AISI 1045 plain carbon steels were bombarded with nitrogen ions at a bias voltage of 5 kV and a working pressure of 15 mTorr for 1 h, 2 h, 3 h and 5 h. The effect of treatment conditions on implantation depth, hardness and phase evolution were analyzed using surface roughness measurements, microhardness and nanohardness tests, X-ray photoelectron spectroscopy and X-ray diffraction. The fatigue properties of nitrogen implanted steels were evaluated using a rotating bending fatigue test at 50 Hz, while fractured surfaces of failed samples were examined using scanning electron microscopy. The surface roughness of AISI 1018 and AISI 1045 specimens remained practically unchanged after PIII treatment. It was also found that AISI 1045 steel had higher concentration of implanted nitrogen than AISI 1018 at the near surface. However, the implanted nitrogen atoms were distributed over a smaller depth from the surface in AISI 1045 compared with AISI 1018 steel. Both grades of steel showed an increase in microhardness after nitrogen implantation for all treatment times compare to their untreated counterparts. However, after 2 h of PIII treatment the hardness values measured remained practically the same. Two distinct regions characterized the fatigue behavior of both grades of steel after nitrogen ion implantation, namely: the high cycle fracture domain (HCFD) and low cycle fracture domain (LCFD). The fatigue life of both steels was increased in the HCFD, while it decreased in the LCFD. This effect was more pronounced in AISI 1045 steel than AISI 1018 steel

Amélioration de la résistance à la corrosion de réfractaires exposés à des alliages d'aluminium liquide par des revêtements de nitrure d'aluminium synthétisés par suspension plasma

Abstract : The objective of this thesis is to identify and synthesize a corrosion-resistant refractory composition when in direct contact with the chemically aggressive molten Al-5wt%Mg alloy. The reviewed literature suggests solutions for resolving the corrosion issue: (i) the addition of non-wetting agents (NWAs) to the refractory, and (ii) deposition of a protective coating on the refractory surface. The first part of this project aims at finding the NWAs that would protect the white fused mullite (WFM) refractory from corrosion, and the experimental results showed that a mixture of CaF2 and BaSO4 is the most impactful NWA protecting the WFM refractories from corrosion. For solution (ii) i.e., the coating, the existing literature suggests that: a) its melting point should exceed the furnace service point, b) its thermal expansion coefficient should be close to that of the substrate, c) it should be chemically resistant to molten Al alloy. The corrosion-resistant property of the selected coating candidate in contact with molten Al alloy was assessed using FactSage thermochemical software. The results showed that aluminum nitride (AlN) meets the required criteria and its in-situ synthesis and deposition using radio-frequency (RF) induction suspension plasma spray (SPS) technology on various substrates (Ti, Mo, and carbon steel) was performed and studied in the second part of the thesis. The corrosion-resistant performance of hexagonal and cubic phases of AlN in contact with molten Al was studied using the ab initio molecular dynamic (BOMD) simulation module of BIOVIA Materials Studio software. Such analysis was needed for the quantification of corrosion resistance of hexagonal versus cubic phases of AlN which is not possible to get using FactSage software. The results show that both phases have interfacial energy within the physisorption regime confirming that neither cubic nor hexagonal AlN phases are reactive in contact with molten Al. Different parametric studies have been done to understand the most important parameters which affect the cubic AlN coatings synthesis. The main results are: 1) The best particle size distribution ratio by mass is 75% of the (– 5 +1 μm) size range mixed with 25% of (– 35 + 17 μm). Rietveld quantitative analysis (RQA) using X-ray diffraction (XRD) data indicated that the coatings consisted of up to 82% AlN (80% cubic + 2% hexagonal crystal structure). 2) Melamine is advantageous over urea by producing more active nitrogen species in the plasma. 3) Hexadecane suspension carrier outperforms ethylene glycol, light mineral oil, and ethanol in the formation of AlN. 4) The use of promotional additives such as boron, BN, Mo, Y2O3, AlN, or Al4C3 can stimulate the formation of AlN. The optimum amount of promotional AlN was 0.22-wt% of the total suspension mass, producing up to 72% AlN in the coating. 5) Among Ti, Mo, and carbon steel substrates, the formation of AlN was significantly higher with the latter than with the two others. 6) Between the range of 6 to 10 cm, the standoff distance of 8 cm provides a residence time long enough for the reaction of Al and nitrogen active species, maximizing the formation of AlN. Finally, the plasma synthesized AlN coating was tested for corrosion resistance by direct contact with molten Al-5wt%Mg alloy at 1123 K and found to be stable after the test.L'objectif de cette thèse est d'identifier et de synthétiser un matériau réfractaire résistant à la corrosion lorsqu'il est en contact direct avec l'alliage Al-Mg fondu qui est très agressif chimiquement. La revue de littérature suggère des solutions pour résoudre le problème de la corrosion : (i) l'ajout d'agents non mouillants (NWA) au réfractaire, et (ii) le dépôt d'un revêtement protecteur sur la surface réfractaire. La première partie de ce projet vise à trouver les NWA qui protégeraient le réfractaire de mullite fondue blanche (WFM) de la corrosion, et les résultats expérimentaux ont montré qu'un mélange de CaF2 et de BaSO4 est le NWA le plus efficace pour protéger ces réfractaires de la corrosion. Pour la solution (ii) c'est-à-dire le revêtement, la littérature existante suggère que : a) son point de fusion doit dépasser la température de service du four, b) son coefficient de dilatation thermique doit être proche de celui du substrat, et c) il doit être chimiquement résistant à l’alliages d'Al fondu. La propriété de résistance à la corrosion du revêtement candidat sélectionné en contact avec l'alliage d'Al fondu a été évaluée à l'aide du logiciel thermochimique FactSage. Les résultats ont montré que le nitrure d’aluminium (AlN) répond aux critères requis. Sa synthèse et déposition in situ à l'aide de la technologie de projection par plasma de suspension par induction radiofréquence (RF) sur divers substrats (Ti, Mo et acier au carbone) a été réalisée et étudiée dans la deuxième partie de la thèse. Les performances de résistance à la corrosion des phases hexagonales et cubiques d'AlN en contact avec l'Al fondu ont été étudiées à l'aide du module de simulation de dynamique moléculaire ab initio (BOMD) du logiciel BIOVIA Materials Studio. Une telle analyse était nécessaire pour la quantification de la résistance à la corrosion des phases hexagonales par rapport aux phases cubiques d'AlN, ce qui n'est pas possible avec le logiciel FactSage. Les résultats montrent que les deux phases ont une énergie interfaciale dans le régime de physisorption confirmant que ni les phases AlN cubique ni hexagonale ne sont réactives au contact de l'Al fondu. Différentes études paramétriques ont été réalisées pour comprendre les paramètres les plus importants qui affectent la synthèse des revêtements d'AlN cubique. Les principaux résultats sont : 1) Le meilleur ratio de distribution granulométrique en masse est de 75 % de la gamme granulométrique (– 5 +1 µm) mélangé à 25 % de (– 35 + 17 µm). L'analyse quantitative de Rietveld (RQA) utilisant des données de diffraction des rayons X (XRD) a indiqué que les meilleurs revêtements étaient constitués de 82 % d'AlN (80 % cubique + 2 % de structure cristalline hexagonale). 2) La mélamine est avantageuse par rapport à l'urée en produisant plus d'espèces azotées actives dans le plasma. 3) Le liquide de suspension d'hexadécane surpasse l'éthylène glycol, l'huile minérale légère et l'éthanol dans la formation d'AIN. 4) L'utilisation d'additifs promotionnels tels que le bore, le BN, le Mo, le Y2O3, l'AlN ou l'Al4C3 peut stimuler la formation d'AlN. La quantité optimale d'AlN promotionnel est de 0,22 % en poids de la masse totale de la suspension, produisant jusqu'à 72 % d'AlN dans le revêtement. 5) Parmi les substrats en Ti, Mo et acier au carbone, la formation d'AlN est significativement plus élevée avec ce dernier que pour les deux autres. 6) Entre 6 et 10 cm, la distance de déposition de 8 cm fournit un temps de séjour suffisant pour la réaction d'Al et des espèces actives d'azote, maximisant ainsi la formation d'AlN. Enfin, le revêtement AlN synthétisé par plasma a été testé pour sa résistance à la corrosion par contact direct avec un alliage fondu Al-5wt%Mg à 1123 K et s'est avéré stable après le test

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

Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy

Phase transformations during nitriding of a ternary carbon iron-based alloy Fe–3%Cr–0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr7C3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon.The authors acknowledge Aubert & Duval (Eramet Group) and the Safran Group for their support, as well as CP2M laboratory, Paul Cezanne University, Marseille, for TEM investigations