E. Ramous

Microstructure and Properties of TiB/Ti-6Al-4V Coatings Produced With Laser Treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB2/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB2 and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure.

Investigation of secondary phase effect on 2205 DSS fracture toughness

Abstract The fracture toughness properties of duplex stainless steels (DSS) are strongly reduced by intermetallic phases, precipitating at 600–1000°C. In this investigation, the embrittlement effect induced by a low content of these phases has been studied in a 2205 DSS, by means of instrumented Charpy V impact testing. Different contents of intermetallic phases have been produced by isothermal treatments at 800–950°C. The microstructure has been analysed by scanning electron microscope/backscattered electron images. The impact test results, the examination of the fracture surfaces and of the cross-sections close to the notch root allows us to elucidate the role of secondary phases contents in reducing impact fracture toughness. When the quantity of secondary phases is <0·5%, these precipitates are rare and insignificant, reducing the absorbed energy by facilitating a ductile fracture. When the content is >1%, the fracture is completely brittle. The plastic deformation at the notch root and in the resisting section provokes a secondary phase breakage with a high density microcrack formation, inducing an easy nucleation and propagation of the final fracture crack. When the content of secondary phases is between 0·5 and 1%, fracture is mixed. Both the sigma σ and chi χ phases have been found to be responsible for the embrittling of the 2205 DSS, and fracture toughness mainly depends on their density and dimension.

Development of solidification microstructures in atomized Fe-Ni alloy droplets

Abstract Inert-gas atomization, using nitrogen as the atomizing medium, has been used to process a series of Fe-Ni alloys. The weight distributions of the powders were bi-modal with relative maxima in the 38 and 60–90 μm ranges. Droplet cooling rates were calculated and the final solidification microstructures were investigated by optical microscopy, X-ray diffraction and transmission electron microscopy (TEM). The smallest droplets were predominantly dendritic with the largest droplets being a mixture of dendritic and grain-refined. For additions of 2 at.% Ni, a number of droplets were found to show solid-state transformations, having a characteristic martensitic microstructure. The primary phase to nucleate was the equilibrium b.c.c. δ-Fe phase and the transformation path was determined to be δ-Fe->-γ-Fe,Ni→α-Fe. TEM examinations of the transformed microstructure showed an interlocked structure with a number of different orientations. In these particles, some duplex b.c.c.-c.c.p. structures were also present. For higher Ni contents, the metastable b.c.c. δ-Fe phase nucleated in preference to the equilibrium γ-Fe,Ni phase.

Precipitation of secondary phases in super duplex stainless steel ZERON100 isothermally aged

Abstract In the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850–1000°C for 180–2400 s, are presented. During the isothermal heat treatments, at 850°C, the χ phase is the only phase to precipitate at α/γ boundaries and triple points, while at 900 and 950°C, χ phase is the first phase to precipitate after 180 s, followed by σ phase 300 s later. At all the temperatures the total amount of secondary phases is ∼3% after <600 s aging.

A simplified model for gas atomization

A simplified model describing the cooling behaviour and the solidification of undercooled droplets in gas atomization has been developed. The resulting computer code can predict the cooling behaviour both in the liquid and in the solid state for any powder size. The droplet velocity in the collecting chamber as well as its temperature, solid fraction and cooling rate can be predicted as a function of the characteristics of the processed alloy, the superheating of the melt, the atomizing gas, its velocity at the nozzles and the droplet diameter. The model overcomes the problem of the undetermined value of the undercooling and evaluates the effects of the simplified approach.

Effects of Isothermal Ageing and Continuous Cooling after Solubilization in a Duplex Stainless Steel

The kinetics of precipitation of secondary phases in a duplex stainless steel (SAF 2205) after isothermal and continuous cooling treatment were investigated. The evolution of the phases chemical composition in relation with time and cooling rate is presented.

Microstructure of a nitrided steel previously decarburized

In this study the effects of a surface-controlled decarburization on the structure of a nitrided steel are analyzed. Samples of a quenched and tempered 42CrMo4 steel were decarburized by heating in air at different depths and submitted to gaseous nitriding. After decarburization and nitriding, the microstructure of surface layers was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The nitrogen and carbon profiles in the diffusion layers were determined by SEM equipped with a wavelength dispersive spectrometer (EPMA-WDS) and by glow discharge optical spectrometry (GDOS). The effect of nitriding was determined by microhardness measurements. Our results indicate that a previous decarburization only slightly affects the surface hardness, but reduces the conventional nitriding depth. The decarburization also favors the nitrogen take-up and produces increased nitrogen concentrations in the compound layer and in the narrow zone beneath it.

The application of micro structure-selection maps to droplet solidification

Abstract The techniques of electromagnetic levitation, drop-tube processing and atomization are used to study containerless solidification of the Cu-Ni and Fe-Ni systems. Microstructure-selection maps are developed for the case of droplet solidification where droplet diameter and undercooling are the process parameters. The microstructure varies systematically with undercooling and composition, such that grain-refined microstructures are produced at both low and high undercoolings and dendritic microstructures are produced at intermediate undercoolings. The results are analyzed within a recently developed model for grain refinement which is based on the fragmentation of primary dendrites. The role of cooling rate in determining whether the microstructure transforms in the solid state or is retained as a primary structure is explored. Keywords: Cu-Ni; Dendrites; Droplet solidification; Fe-Ni; Grain refinement; Microstructure-selection maps

New Lean Duplex Stainless Steels for Structural Applications

The continuous improvement of the applications of duplex stainless steels (DSS) is justified mainly by their excellent corrosion resistance in very aggressive environments and by their mechanical strength, often twice than that of conventional austenitic grades. Therefore more extensive applications have been suggested, like constructions, structural components, automotive applications, etc. However for such not advanced but quantitatively important applications the base cost of the alloy is one of the more constraining features. A way to obtain leaner alloys is to reduce the Ni content and to compensate with manganese and nitrogen additions. In the present paper the structural and mechanical properties of two low Ni duplex grades are analyzed in order to investigate the structural stability of the austenite against its possible transformation to martensite and the secondary phase’s precipitation. The detailed characterization has been performed with SEM-EDS on as received and heat treated (600-850 °C) alloys. A limited precipitation of chromium carbides and nitrides at the grain boundaries has been detected in both grades. The martensite formation has been revealed only in the 2101 grade. With regard to the impact toughness the behaviour of 2101 grade is quite similar to that of the common DSS, while in the 2304 the drastic drop of toughness is avoided. The corrosion behaviour of both grades in aggressive chloride environments is quite similar to that of austenitic AISI 304 grade.

Phase transformation in duplex stainless steels after isothermal treatments, continuous cooling and cold working

This paper concerns the phase transformation induced by heat treatment and cold rolling in four duplex stainless steel. In 2205 and 2507 , during the isothermal heat treatments, chi-phase precipitates as small particles at the ferrite/austenite boundaries, followed by sigma precipitation. At the lowest temperature the formation kinetic of chi-phase is favoured, with the increasing of time and temperature a progressive transformation of chi to sigma occurs and the kinetic of sigma is favoured. During continuous cooling, the chi -phase appears at low cooling rates. In low Ni grades the grain boundaries precipitation of chromium nitrides were detected , but no sigma and chi. In 2101 the austenite transforms to martensite both after cold rolling and quenching