Precipitation in microalloyed steel has been studied by applying thermodynamic calculations based on a description of the Gibbs energies of the individual phases over the full multicomponent composition range. To validate and improve the thermodynamic description, new experimental investigations of the phase separation in the cubic carbides/nitrides/carbonitrides in alloys containing Nb, V, Mo, and Cr, have been performed. Model alloys were designed to obtain equilibrium carbides/carbonitrides that are sufficiently large for measurements of compositions, making it possible to study the partitioning of the elements into different precipitates, showing distinctly different composition sets. The reliability of the calculations, when applied to multicomponent alloys, was tested by comparing with published experimental studies of precipitation in microalloyed steel. It is shown that thermodynamic calculations accurately describe the observed precipitation sequences. Further, they can reproduce several important features of precipitation processes in microalloyed steel such as the partitioning of Mo between matrix and precipitates and the variation of precipitate compositions depending on precipitation temperature.
Oxide reduction in Cr alloyed PM steel has been simulated experimentally by measuring the oxygen loss from powders when exposed to a heating cycle, by photo acoustic spectroscopy (PAS) and by thermogravimetric analysis (TGA). Pressed samples were heated in dry hydrogen gas using a defined, slow, heating rate up to 1300oC. The oxides in the powder are reduced by the hydrogen gas. From the PAS curves distinct reduction temperatures are observed, that can be correlated to the type of oxide by comparing with thermodynamic calculations of the oxide stabilities. The experimental results were analyzed by diffusion simulations that involve a description of the diffusion controlled oxide reduction. It is shown how the calculated rate of oxygen diffusion in the samples, compared with the experimental results, has been applied as a tool for increased understanding of the reduction during sintering.
Compaction of additively manufactured Ti6Al4V components by Hot Isostatic Pressing (HIP) is often applied to eliminate porosity, producing fully dense material. In the present work shelled samples produced by Electron Beam Melting with the Arcam process (EBM) were compacted by HIP to produce fully dense samples. Cylindrical samples were studied. The walls of the cylinders were built with EBM, and the powder from the process was left uncompacted inside the cylinders. Samples with different wall thicknesses were produced. The samples were thereafter subjected to a HIP compaction. The critical wall thicknesses needed for compaction were evaluated, and the microstructures characterized. The results show that fully dense samples, with very fine microstructures, are possible.
Electron Beam Melting (EBM) was used to build Ti-6Al-4V cylindrical shell samples with different wall thickness filled with powder. Built shell samples were HIPed and the difference in microstructure between the EBM-built walls and densified powder inside the shell components was studied as well as the cohesion between these two regions. Components characterization utilizing LOM and SEM+EBSD indicates that columnar grain growth was consistent before and after HIP in the EBM-built part of the components (walls), whereas the densified material in the center of the component had a fine isotropic microstructure, characteristic for HIPed material. The combination of EBM and HIP is shown to be an attractive way of manufacturing complex-shape full density components for high performance applications, involving shortening of built time in the EBM-processing and lead time in capsule fabrication for HIP.
Spherical gas atomised 100Cr6 steel powder, processed with the MMS-Scanpac® process to 95% density (agglomeration, followed by conventional pressing, low temperature sintering and re-strike using high velocity adiabatic compaction) has been fully compacted using capsule-free hot isostatic pressing. The material is characterised at different steps of the process and the results are discussed in this paper. Sintering steel powder with high content of carbon requires carbon control at sintering. By continuously measuring the atmosphere at sintering the ingoing gases are adjusted so that carbon control is achieved. Computational work has been made in order to determine how the sintering atmosphere should be adjusted based on the oxygen release and moisture content in the furnace at sintering. KEYWORDS: Capsule free HIP, high velocity compaction, 100Cr6, carbon control.
The addition of alloying elements in low alloyed PM steels in the form of a master alloy gives the advantage of introducing oxidation sensitive but less expensive elements and also allows manipulation in composition adjustment to achieve desired properties. In this work, interrupted sintering trials of the Fe-2MA-0.5C (%) (MA=Cu based master alloy) are performed. The behaviour of the liquid forming master alloy, for instance in terms of liquid phase formation, alloying element redistribution and effect on the dimensional changes, is investigated. The results show that master alloy particles melt over a range of temperature, which is also supported by the thermodynamic calculations. The low swelling in the master alloy system, compared to a reference system of Fe-2Cu-0.5C, is attributed to the progressive melting of the master alloy. The mean diffusion distance of Cu in Fe at the interparticle boundaries is 5.8 μm after 34 min of isothermal holding.