Easily Sinterable Low-Alloy Steel Powders for P/M Diamond Tools

The work presents the design and fabrication procedures used to manufacture inexpensive iron-base powders employed as a matrix in diamond-impregnated tool components. Three newly developed low alloy steel powders, containing from 94.4 to 99.4 wt.% Fe, have been formulated with the assistance of ThermoCalc software and produced by means of a proprietary process patented by AGH-UST. It has been shown that the powders are readily pressureless sintered to a closed porosity condition (>95% theoretical density) at a temperature range between 840 and 950 °C. All as-consolidated materials achieve the desired tool matrix hardness of more than 200 HV. One of the experimental powders has been designed to partly melt within the sintering window. This is particularly important in fabrication of wire saw beads by the conventional press and sinter route because sintering of a diamond-impregnated ring and its further brazing to a tubular steel holder can be combined into one operation.

STUDY OF CHANGES IN THE COMPOSITION OF OXIDE NON-METALLIC INCLUSIONS AT THE STAGES OF THE METALLURGICAL PROCESS

Using optical and electron microscopy methods, a study was made of the contamination of C45E carbon steel with non-metallic inclusions in samples taken at all stages of the metallurgical process, and the chemical compositions of non-metallic inclusions were determined. Smelting was carried out according to the following technology: production of an intermediate in an arc steel-smelting furnace → melt processing at out-of-furnace processing →vacuum treatment of steel → continuous casting. Using the methods of recalculation of the EDS-spectrum data, using the Thermocalc software, the phase compositions of non-metallic inclusions of metal samples determined at each stage of the metallurgical process are determined. It was found that complex oxide inclusions with a slag composition are increase in size with each subsequent step and are completely removed from the metal by the final stages of the treatment. At the same time, inclusions of calcium aluminates, on the contrary, decrease in size and, with inclusions of corundum, which remain almost unchanged throughout the treatment, are the main type of oxide that enters the finished metal.

Computational thermodynamics: towards an improved Gibbs minimization tool for geodynamic modelling

<p>In the last decade, the development of numerical geodynamic tools helped the geoscience community to explore thermo-mechanical processes at play during plate tectonics. Yet, the high computational cost of thermodynamic calculations hampers our ability to quantify multi-phase systems in which the interplay between plate-tectonics and phase transformations leads to magmatism.  Here we use the 'igneous set' of HPx-eos (thermodynamic models for minerals and geological fluids that are based on the Holland & Powell dataset and defined in the THERMOCALC software) to calculate stable phase equilibria in the system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3–Cr2O3 (KNCFMASHTOCr). The calculation is performed by Gibbs free energy minimization at prescribed pressure, temperature and bulk-rock composition and is achieved in two steps. First, we employ a levelling method (iterative change of base) to reduce the number of potential stable phases. Then, the composition and proportions of stable phases at equilibrium are determined using several constrained optimization methods. We explore the computational efficiency of linear programming (e.g., Simplex), Gradient-based (e.g., SLSPQ) and Hessian-based (e.g., Newton-Raphson) methods. The accuracy and performance of tested methods are compared, and applications to geodynamic modelling are discussed.</p>

Precipitation Hardenable High Entropy Alloy for Tooling Applications

ABSTRACTWe present a high entropy alloy (HEA) from the system Al-Co-Cr-Fe-Ni with small additions of W, Mo, Si and C which was designed to allow for precipitation hardening by annealing in the temperature range from 600 to 900 °C. The alloy development was supported by thermodynamic computations using ThermoCalc software and the specimens were produced by arc melting. The microstructure of one selected sample in as-cast and annealed conditions was analysed using SEM/EDS, SEM/EBSD and TEM. The as-cast microstructure consists of spinodally decomposed BCC dendrites enveloped by FCC+Cr23C6 eutectic. Upon annealing at 700 °C for 24 h nanoscale precipitates form within the spinodal BCC as well as from FCC. Precipitation is exquisitely uniform leading to an increase in microhardness from 415 HV0.5 in the as-cast state to 560 HV0.5 after annealing. We investigated coarsening of this microstructure using varying holding time for a constant temperature of 700 °C. The microstructure evolution during coarsening and the corresponding mechanical properties obtained from instrumented indentation experiments are presented in this work.

Microstructure Optimization in δ-TRIP Steels with Al and Nb

δ-TRIP steel is a recent concept and has been developed over the last ten years aiming to combine good mechanical strength and ductility. This class of steels is multiphase and contains δ and α ferrites, as well as austenite, bainite and/or martensite. The TRIP (Transformation Induced Plasticity) effect is influenced by those phases proportion, which depends on alloying contents. This paper investigates a chemical composition that allows adequate proportion among the phases, optimizing the microstructures by means of computational methods. These microstructures are designed to contain between 10 to 50% austenite, 10 to 70% α-ferrite and 20 to 80% δ-ferrite at the eutectoid temperature. The ThermoCalc Software [1] was used to predict the fractions of the microconstituents, producing graphs describing areas of interest of microconstituents as function of alloying elements variations that leads to the desired microstructure. Results indicate that the designed volume of the phases can be found for certain proportions among the alloying elements, higher concentrations of Al and Nb combined with C allow or not the occurrence of carbides and other phases in smaller quantities.

Petrography and phase equilibria modeling of mid-pressure aluminous gneisses derived from hydrothermally altered protoliths, Grenville Province, Canada

A large portion of the hinterland of the central Grenville Province is characterized by mid-pressure granulite-facies metamorphic rocks. In the Manicouagan – Lac du Milieu region of Quebec, aluminous gneisses derived from hydrothermally altered felsic protoliths provide a record of anatectic processes and of the pressure–temperature (P–T) evolution during orogenesis. Samples collected in areas separated by several tens of kilometres consist of garnet + biotite + quartz + K-feldspar ± plagioclase ± sillimanite, with retrograde cordierite in some, and most display microstructural evidence of partial melting. However, they have a wide range of bulk compositions and textures, and may be grouped into two types: sillimanite-rich rocks, with a high alumina index and sillimanite-poor rocks with low alumina index and in which sillimanite is not part of the peak assemblage. Phase equilibria modeling with THERMOCALC software constrained the P–T field of the peak mineral assemblage(s) at 800–900 °C and 6–11 kbar (1 kbar = 100 MPa), with melt solidification in the range of 800–865 °C and 6–8 kbar. The presence of sillimanite inclusions in garnet, and the scarcity of retrograde cordierite are consistent with moderate dP/dT gradient “hairpin” P–T paths. The data suggest that this part of the mid-P hinterland in the central Grenville experienced a rather uniform high-temperature metamorphic evolution, with large temperature variations relative to pressure. This is consistent with the thermal evolution of middle crust beneath an orogenic plateau.

Microstructural and Mechanical Characterization of Solidified Austenitic Stainless Steels

Among the family of stainless steels, cast austenitic stainless steels (CASSs) are preferably used due to their high mechanical properties and corrosion resistance. These steels owe their properties to their microstructural features consisting of an austenitic matrix and skeletal or lathy type δ-ferrite depending on the cooling rate. In this study, the solidification behavior of CASSs (304L and 316L grades) was studied using ThermoCalc software in order to determine the solidification sequence and final microstructure during cooling. Theoretical findings were supported by the microstructural examinations. For the mechanical characterization, not only hardness measurements but also tribological studies were carried out under dry sliding conditions and worn surfaces were examined by microscopy and 3D profilometric analysis. Results were discussed according to the type and amount of microstructural features.

Precipitation of Secondary Phases in a Supermartensitic 13Cr6Ni2.5MoTi Steel during Heat Treatment

The balance between strength and toughness of supermartensitic steels can be controlled by heat treatment parameters. Quality heat treatment usually consists of quenching and single or double tempering above the Ac1 temperature. Such a treatment results in stabilization of reverted austenite in the tempered martensite. Tempering can also be accompanied by intensive precipitation processes, especially in high alloyed grades. A detailed TEM characterisation of precipitation processes in a 13Cr6Ni2.5MoTi supermartensitic steel proved that single tempering at temperatures of 600°C and 690°C was accompanied by precipitation of three minor phases: MX (TiX), M23C6 and Laves phase (Fe2Mo type). Precipitation processes were more intensive at 690°C. Volume fractions of MX and M23C6 phases were low. Laves phase precipitation was intensive and particles of this minor phase grew fast. However, thermodynamic calculations using the Thermocalc software suggest that Laves phase is not an equilibrium phase in the steel under consideration.

Thermodynamics of Bi2O3-SiO2 system

Thermodynamic properties of the liquid Bi2O3-SiO2 solutions were determined from the results of the electrochemical measurements by use of the solid oxide galvanic cells with YSZ (Yttria-Stabilized-Zirconia) electrolyte. Activities of Bi2O3 in the solutions were determined for 0.2, 0.3, 0.4, and 0.5 SiO2 mole fractions in the temperature range 1073-1293 K from measured electromotive force (e.m.f) of the solid electrolyte galvanic cell: Bi, Bi2O3-SiO2 | YSZ | air (pO2 = 0.213 bar) Additionally, heat capacity data obtained for two solid phases 6Bi2O3?SiO2 and 2Bi2O3?3SiO2 were included into optimization of thermodynamic properties of the system. Optimization procedure was supported by differential thermal analysis (DTA) data obtained in this work as well as those accepted from the literature. Using the data obtained in this work, and the information about phase equilibria found in the literature, binary system Bi2O3-SiO2 was assessed with the ThermoCalc software.