Duplex and superduplex stainless steels: microstructure and properties evolution by surface modification processes

The paper presents an overview of diffusion surface treatments, especially nitriding processes, applied to duplex and superduplex stainless steels in the last five years. Research has been done mainly to investigate different nitriding processes in order to optimize parameters for the most appropriate procedure. The scope has been to improve mechanical and wear resistance without prejudice to the corrosion properties of the duplex and superduplex stainless steels. Our investigation also aimed to understand the effect of the nitriding layer on the precipitation of secondary phases after any heating step.Peer ReviewedPostprint (published version

Surface-functionalised materials for microplastic removal

Microplastic (MP) pollution is a matter of great concern attracting increasing attention due to its adverse effects on the environment. Different technologies and methodologies have been developed to remove these pollutants. Herein, we focus on a promising environmental solution that involves surface modification to change the wettability properties of MPs or solid materials by conferring superhydrophobicity and superoleophilicity to increase the selectivity for MP separation. Both processes can be used to selectively separate MPs because of the changes in the wettable properties of the MP or by changing the oil used in the case of superhydrophobic surfaces. We show two distinct methods based on changing the wettability properties of surfaces that could lead to innovative and environmental applications. We also discuss some of the challenges that need to be overcome

Non-fluorinated, sustainable, and durable superhydrophobic microarrayed surface for water-harvesting

Water scarcity is a worldwide issue that significantly affects the environment, population, and economy of the arid zones. In this study, we report a straightforward method for water-harvesting based on modifications of the surface wettability. Using magnesium chloride, lauric acid, and electrodeposition process, a superhydrophobic surface (155°) is obtained. Morphological characterization techniques allow determination of the characteristic flower-like microstructures combined with close packed nanoarrays that lead to the hierarchical structure. Furthermore, the coating presents vertically aligned microarrays in a non-linear cone morphology formed by dynamic templating of hydrogen bubbles. From a chemical point of view, magnesium laurate is responsible for the surface tension decrease. To determine the durability of the obtained surface ultra-violet (UV) light test and abrasive paper test, tests are carried out revealing high durability against these severe conditions. The water-harvesting ability of the superhydrophobic surface is studied at 45° and 90° tilted samples. The capacity of the water to be harvested efficiently is found to be at 90° tilt under fog conditions. The use of green reactants associated with this hierarchical structure broadens a new scope for sustainable freshwater collection and it becomes an excellent example of a green solution

Identification of the mechanism that confers superhydrophobicity on 316L stainless steel.

This study develops a rapid method to confer superhydrophobicity on 316L stainless steel surfaces with an amphiphilic reagent such as dodecanoic acid. The highest contact angle (approaching 173°) was obtained after forming hierarchical structures with a non-aqueous electrolyte by an electrolytic process. Our goal was to induce superhydrophobicity directly on 316L stainless steel substrates and to establish which molecules cause the effect. The superhydrophobic behaviour is analysed by contact angle measurements, scanning electron microscopy (SEM), IR spectroscopy and atomic force microscopy (AFM). The growth mechanism is analysed using FE-SEM, TOF-SIMS and XPS in order to determine the molecules involved in the reaction and the growth. The TOF-SIMS analysis revealed that the Ni2 + ions react with lauric acid to create an ester on the stainless steel surface

Superhydrophobic and nanostructured CuFeCo powder alloy for the capture of microplastics

A superhydrophobic CuFeCo powder alloy, obtained by combining high-energy ball milling (HEBM) and liquid phase deposition (LPD), was used to remove high-density polyethylene fibres from water. After 48 h of HEBM, CuFeCo solid solution powder with ferromagnetic properties was obtained. High-resolution transmission electron microscopy showed a crystallite size of 20 nm, confirming its nanostructure. The metallic CuFeCo powder surface was functionalised with dodecanoic acid to confer superhydrophobicity (water contact angle = 162 ± 1°) and superoleophilicity (oil contact angle ~ 0°). Taking advantage of its superwettable properties, superhydrophobic CuFeCo particles were used to capture microplastics (270 μm < size < 1240 μm), which is an innovative application of superhydrophobic materials. This study demonstrates an innovative way of using superhydrophobic materials in environmental applications such as the removal of solid pollutants like microplastics

Superhydrophobic PDMS coated 304 stainless-steel mesh for the removal of HDPE microplastics

The use of microplastics is a global issue that affects the environment, the economy and human health. Here we describe a superhydrophobic 304 stainless steel obtained by combining chemical etching and PDMS modification. Among other techniques, field emission scanning electron microscopy (FE-SEM) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) were used to identify the hierarchical structure as well as the chemical composition of the surface. The stainless-steel mesh was superhydrophobic (159°) and superoleophilic (0°). The coating presented high stability against abrasion of SiC abrasive paper as well as in the presence of different pH values in acidic or alkaline conditions. In addition, taking advantage of the coating's wetting properties, we show that the superhydrophobic surface can also be used to remove high-density polyethylene microplastics from water. A surface mechanism promoting the removal or microplastics is also proposed, considering the surface properties of the solid pollutants as well as the wetting properties of the superhydrophobic coating

Evolution of Shrinkage with Carbon Equivalent and Inoculation in Ductile Cast Irons

Studying how shrinkage porosity changes size when varying the composition of ductile irons is still of interest for manufacturing sound cast parts and defining optimised processing conditions. Usual changes in carbon and silicon contents strongly affect shrinkage, so that a detailed analysis of the effect of alloy composition and of inoculation level on porosity was carried out in the present study. Two test castings have been used to evaluate the extent of porosity formed using different compositions and inoculation levels. It has been observed that increasing carbon content from hypoeutectic to near-eutectic compositions reduces the amount of defects while a further increase of shrinkage porosity is detected for hypereutectic compositions. Thus, a minimum in shrinkage tendency exists for slightly hypereutectic compositions. Although inoculation decreases shrinkage, the relevance of this parameter varies as a function of carbon equivalent

Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel

In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated

The effect of B and Si additions on the structural and magnetic behavior of Fe-Co-Ni alloy prepared by high-energy mechanical milling

Nanocrystalline Fe50Co25Ni15X10 (X = Bamorphous, Bcrystalline, and Si) powdered alloys were prepared by mechanical alloying process. Morphological, microstructural, and structural characterizations of the powders milled several times were investigated by scanning electron microscopy and X-ray diffraction. The final crystallographic state strongly depends on the chemical composition and the grinding time; it can be single-phase or two-phase. The crystallite size reduction down the nanometer scale is accompanied by the introduction of high level of lattice strains. The dissolution of Co, Ni, B (amorphous and crystalline), and Si into the α-Fe lattice leads to the formation of highly disordered Fe-based solid solutions. Coercivity (Hc) and the saturation magnetization (Ms) of alloyed powders were measured at room temperature by a vibration sample magnetization. The magnetic measurements show a contrasting Ms and (Hc) in all alloy compositions. Conclusively, soft magnetic properties of nanocrystalline alloys are related to various factors such as metalloid addition, formed phases, and chemical compositions

Microstructural and Magnetic Behavior of Nanocrystalline Fe-12Ni-16B-2Si Alloy Synthesis and Characterization

The nanocrystalline Fe70Ni12B16Si2 (at.%) alloy was prepared by mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. Phase evolution, microstructure, thermal behavior and magnetic properties were investigated. It was found that a body-centered cubic structured solid solution started to form after 25 h milling and a faced-centered cubic structure solid solution started to form after 50 h of milling; its amount increased gradually with increasing milling time. The BCC and the FCC phases coexisted after 150 h of milling, with a refined microstructure of 13 nm and a 10 nm crystallite size. The as-milled powder was annealed at 450 °C and 650 °C and then investigated by vibrating sample magnetometry (VSM). It was shown that the semi-hard magnetic properties are affected by the phase transformation on annealing. The saturation magnetization decreases after annealing at 450 °C, whereas annealing at 650 °C improves the magnetic properties of 150 h milled powders through the reduction of coercivity from 109 Oe to 70 Oe and the increase in saturation magnetization