T. Gómez-Acebo

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 � C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)3O4. The intermediate layer was porous magnetite (Fe3O4) followed by a compact thinner layer of hematite (Fe2O3). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time. � 2003 Elsevier Ltd. All rights reserved.

Thermodynamic optimization of the Ni-Zn system

Optimization of thermodynamic and phase diagram data has been performed and consistent sets of coefficients for the calculation of the phase equilibria in the system Ni-Zn have been obtained using the program BINGSS. The δ phase has been modeled as a stoichiometric compound (NiZn8). The binary liquid and the solid Ni-based solutions have been treated as disordered substitutional phases. The intermediate β, β 1, and γ compounds have been modeled as phases with substitutional defects and vacancies on two sublattices. The calculated phase diagram and thermodynamic quantities are in excellent agreement with the experimental data.

Thermodynamic assessment of the Ag-Zn system

Abstract A thermodynamic assessment of the Ag-Zn system has been done using a computerized CALPHAD (calculation of phase diagrams) technique. The liquid, α, β, η and e phases are described by a regular solution model, the γ phase by a four-sublattice model, and the ζ phase by a two-sublattice model, both based on considerations of their crystal structure and compatibility with the same phase in other systems. A set of parameters describing the Gibbs energy of the different phases is given and calculated phase diagrams are presented.

The application of spreadsheets to the analysis and optimization of systems and processes in the teaching of hydraulic and thermal engineering

This article shows the capability of current spreadsheets to define, analyze and optimize models of systems and processes. Specifically, the Microsoft spreadsheet Excel is used, with its built‐in solver, to analyze and to optimize systems and processes of medium complexity, whose mathematical models are expressed by means of nonlinear systems of equations. Two hydraulic and thermal engineering‐based application examples are presented, respectively: the analysis and optimization of vapor power cycles, and the analysis and design of piping networks. The mathematical models of these examples have been implemented in Excel and have been solved with the solver. For the power cycles, the thermodynamic properties of water have been calculated by means of the add‐in TPX (Thermodynamic Properties for Excel). Performance and optimum designs are presented in cases studies, according to the optimization criteria of maximum efficiency for the power cycle and minimum cost for the piping networks.

Effect of nitrogen on supersolidus sintering of modified M35M high speed steel

Abstract Water atomised high vanadium M35M high speed steel powders with different carbon contents were cold compacted at 700 MPa and sintered under an N2 rich atmosphere. Full density specimens were obtained at temperatures between 1140 and 1105°C for carbon contents between 1·8 and 2·5 wt-% respectively. The sintering behaviour of these powder mixtures was correlated with dilatometric curves and phase diagram calculations made by Thermo-Calc software. It is demonstrated that densification takes place above the solidus temperature, within the austenite + carbide + liquid phase field, and that nitrogen widens this region, which is responsible for increased sinterability with regards to vacuum sintered high speed steel.

Microstructural development during liquid phase sintering of Fe and Fe-Mo alloys containing elemental boron additions

Abstract The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm−3 were obtained for these alloys.

Nitridation of SiC for the production of SiC-Si3N4 Nanocomposites

Abstract High pressure sintering of SiC green compacts, both with and without additions of Al 2 O 3 and SiO 2 , has been carried out in a HIP press under a nitrogen atmosphere. The amount of conversion of SiC to Si 3 N 4 has been quantified after nitridation by XRD, and the results were interpreted in terms of phase stability diagrams taking into account the chemical composition of the specimens, the carbon activity and the nitrogen and the oxygen partial pressure. As a result of the process the final microstructure of the sintered materials was observed to consist of a combination of SiC and Si 3 N 4 grains in the nanosize range and whose relative volume fractions depend on sintering conditions and the initial chemical composition. A minor proportion of free carbon, in the form of both amorphous carbon and nanosized graphite grains, was also part of the microstructure. This method is therefore regarded as a means for the production of SiC-Si 3 N 4 nanocomposites with varying amounts of SiC, α-Si 3 N 4 and β-Si 3 N 4

Processing Routes for Obtaining Novel High Performance Mn-Containing PM Steels

A Mn-containing master alloy (MA) has been specially designed, through thermodynamic and metallurgical criteria, for obtaining high performance low alloy PM steels by SPSS or DPDS. This MA exhibits improved characteristics with respect to ferromanganese and other Mn carriers for alloying PM steels preventing oxidation, keeping a high compressibility of the powder mixture and providing opportunities for low temperature processing. The improved sinterability through the formation of a transient liquid phase leads to dimensional stability and high reproducibility of mechanical properties after sintering at 1120°C. The microstructural development of the PM steels was studied during the sintering cycles. The final microstructure of these PM steels, after defined sintering cycles, was characterised by LOM while the mechanical properties of the consolidated materials were determined by tensile testing.

Sintering of modified M35MHV HSS under diferent nitrogen pressures

Abstract The sintering behaviour of high carbon–high vanadium water atomised M35MHV HSS (1·8 wt-%C, 4·2 wt-%V) is analysed as a function of the nitrogen pressure in the sintering atmosphere. Uniaxially pressed compacts were sintered to full density (≥ 98%TD) under different N2 atmospheres in a range of pressures from vacuum to 8 bar. It is observed that the optimum sintering temperature (OST) depends on the absorbed nitrogen and is as low as 1050°C when the nitrogen content in the steel is 1·2 wt-%. The absorbed nitrogen affects not only the OST but also the matrix and carbides composition and the phases present after sintering. Compared with other powders processed under the same conditions, it is shown that the amount of absorbed nitrogen depends not only on the nitrogen partial pressure in the sintering atmosphere but also on the amount of vanadium and carbon and even on the heating rate. Hardness, fracture toughness, and fracture strength values are reported after heat treatment.

Development of PM Stainless Steels with Improved Properties through Liquid Phase Sintering

The use of boron for successfully obtaining high density PM stainless steels with improved mechanical properties and corrosion resistance is presented. Boron is added as part of master alloys which have been specifically designed to provide the formation of wetting liquid phases with excellent characteristics for producing controlled densification and alloying of 316L and 304L austenitic stainless steels. The as-sintered density and properties of these alloys is determined by the amount of master alloy, the chemical composition of the stainless steel powder, the sintering temperature and time. The microstructural development and alloy homogenisation are determined by the chemical composition of the Fe-based powder and the chemical reactions taking place between the basic powder and the master alloy particles during high temperature sintering. The use of this master alloy is shown to lead to stainless steels with outstanding combinations of strength and ductility. The influence of alloying and the sintering conditions on the final microstructure, density, corrosion resistance and tensile properties is also discussed.