Compacted Iron Research Articles

Effect of copper and graphite addition on sinterability of iron

In ferrous powder metallurgy, copper and graphite are used as common alloying elements. Copper melts at low temperature compared to iron and forms liquid which promote interparticle bond formation. However, it also results in compact swelling. To negate this, graphite is used as an additive. This study examines the influence of copper and graphite addition on the densification, dimensional changes, and mechanical properties of iron compacts sintered at 1120°C. These properties have been correlated with the microstructure.

Contact formation in iron powder compacts during electric sintering

A high-speed x-ray diffraction real-time unit (XRD-RTU) is used for electric sintering of iron powder. Variation in the resistivity, temperature, and γ-line intensity is analyzed. The samples produced by electric sintering in different conditions are tested by bending. It is shown that physical and mechanical contacts form simultaneously after heating above the phase transition point.

Effect of interconnected porosity on carbon diffusion depth in vacuum carburising process of iron compacts

The research work was aimed at determining the influence of macro- and micropores on carbon diffusion depth in the process of vacuum carburising of iron compacts with density within 5·6 to 6·4 g cm–3. Vacuum carburising of the compacts made of iron powder ASC100·29 was performed at 1050°C in a laboratory vacuum furnace. The carburising effects were compared and evaluated in terms of thickness, structure and surface carbon content of the case. Simplified carbon concentration gradients in the carburised layers were determined for the examined densities. Empirical relationships were formulated for carbon content in the case at a fixed distance from the surface as a function of the compact density for the accepted process parameters. It was found that interconnected macropores are decisive for the carburising depth.

An optimizing design method for a compact iron shielded superconducting magnet for usein MRI**This work is supported partly by the Natural Science Foundation of China (no. 60871001).

A method is developed for designing a special iron shielded superconducting magnet forMRI in this paper. The shield is designed as an integral part of the cryostat and highpermeability and high saturated magnetization iron material is adopted. This scheme willresult in a compact iron shielded magnet. In the presented design, the finite element (FE)method is adopted to calculate the magnetic field produced by superconducting coils andnonlinear iron material. The FE method is incorporated into the simulated annealingmethod which is employed for corresponding optimization. Therefore, geometricalconfigurations of both coils and iron shield can be optimized together. This method candeal with discrete design variables which are defined to describe the cable arrangements ofcoil cross sections. A detailed algorithm of the present design is described and anexample for designing a 1.5 T clinical iron shielded magnet for MRI is shown.

Finite element modelling of crack propagation in metal powder compaction using Mohr–Coulomb and Elliptical Cap yield criteria

In the modelling of the compaction process of particulate materials into a coherent green body, an appropriate yield criterion for the deformation process has to be selected. In this paper, two commonly used yield criteria for powder compaction, namely Mohr–Coulomb and Elliptical Cap are utilized in the finite element modelling of iron powder compaction process, which incorporates a fracture criterion of granular materials in compression. The simulated crack growth patterns obtained by using these two yield criteria were compared in terms of the influence of shear stress and relative density distributions. This work has shown that the application of the Elliptical Cap yield model gives a more realistic simulated Mode II crack propagation during the densification stage of the metal powder compaction process in comparison to the Mohr–Coulomb yield model.

Changing structure to aid engineering steel applications

The application of vacuum carburising to iron compacts can combine carburising and sintering operations in one process. Work in Wroclaw, Poland, indicates that the effect of high temperature and silicon on increasing the carbon diffusion coefficient accelerates the diffusion of carbon deep into the material, through-carburising the compacts in a relatively short time…

Effect of phosphorus on vacuum carburising depth of iron compacts

The work was aimed at determining the effect of phosphorus on vacuum carburising of iron compacts with density over 7.2 gcm−3. An attempt was made to determine the effectiveness of phosphorus on carbon diffusion rate into the material of compacts with no additional effect of interconnected porosity. Vacuum carburising of compacts, made with a blend of powders ASC100.29 and PASC60, was carried out at 1050 °C in a laboratory vacuum furnace. The effect of phosphorus content within 0.1% to 0.6% on the vacuum carburising depth was analysed. It was found that the phosphorus addition up to 0.4% increased the carburising depth by ca. 20% in comparison with the compacts of pure iron.

Optimising grey iron powder compacts

The grey iron microstructure Fe–2C–2Si powder based compact is tailored by different kinds of in situ and post sintering processing. This has been achieved by combining thermodynamic and kinetics modelling of microstructure development with sintering and controlled heat treatment experiments of tensile test specimens die compacted at 600 MPa. Applying optimised sintering conditions led to a grey iron like microstructure with 95% relative sintered density. Sinter hardening the compacts led to 500 MPa in yield strength and 600 MPa in ultimate tensile strength in combination with ductile fracture. Quenched and tempered condition showed the same strength values, but combined with brittle fracture due to martensitic structure. Pore rounding and partial pore filling by graphite were obtained by austenising isothermal hold during the cooling of the sintering cycle.

Analysis of density and mechanical properties of high velocity compacted iron powder

A new method for producing higher density PM parts, high velocity compaction (HVC), was presented in the paper. Using water atomized pure iron powder without lubricant admixed as the staring material, ring samples were compacted by the technique. Scanning electron microscopy (SEM) and a computer controlled universal testing machine were used to investigate the morphologies and the mechanical properties of samples, respectively. The relationships among the impact velocity, the green density, the sintered density, the bending strength and the tensile strength were discussed. The results show that with increasing impact velocity, the green density and the bending strength increase gradually, so the sintered density does. In addition, the tensile strength of sintered material is improved continuously with the sintered density enhancing. In the study, the sintered density of 7.545 g/cm 3 and the tensile strength of 190 MPa are achieved at the optimal impact velocity of 9.8 m/s.

Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use of High Heating Rates

An investigation of the effect of heating rates on the densification behavior of carbonyl iron powder compacts, particularly on the exaggerated grain growth during the α-γ phase transformation, was carried out in this study. Compacts heated at 1200 °C/min and then sintered for 90 minutes at 1200 °C attained 7.14 g/cm3, while those heated at 10 °C/min reached only 6.61 g/cm3. Dilatometer curves using heating rates of 2 °C/min, 5 °C/min, 10 °C/min, 30 °C/min, and 90 °C/min demonstrate that 90 °C/min yields the highest sintered density. The microstructure analysis shows that high heating rates inhibit exaggerated grain growth during the phase transformation by keeping the interparticle neck size small and pinning the grain boundaries. This explanation is supported by the calculation that shows that the energy barrier preventing the grain boundary from breaking away from the neck is reduced hyperbolically as the neck size and the amount of shrinkage increase. The high heating rate, however, shows little beneficial effect for materials that have no allotropic phase transformation or have less drastic grain growth during heating, such as nickel and copper. Thus, bypassing the low temperatures to suppress the surface diffusion mechanism, which does not contribute to densification, is ruled out as the main reason for the enhanced densification of carbonyl iron powders.

Effect of boron on vacuum carburising depth of iron powder metallurgy compacts

The work was aimed at determining the effect of boron on vacuum carburising of iron compacts with density over 7·2 g cm–3. An attempt was made to determine the effectiveness of boron on carbon diffusion rate into the material of compacts with no additional effect of interconnected porosity. Vacuum carburising of compacts made of iron powder with an addition of boron was carried out at 1050°C in a laboratory vacuum furnace.The effect of boron content within 0·005 to 0·02% on the vacuum carburising depth was analysed. It was found that the boron addition up to 0·01% increased the carburising depth by ∼0% in comparison with the compacts of pure iron.

Compaction and strain hardening of metal powders and their mixtures when pressed

The compaction of carbonyl nickel powders, nickel-based mixtures, and iron, aluminum, and magnesium powders in cylindrical molds is experimentally studied, the billet height being continuously recorded in a testing machine. The theory of plasticity for powder bodies is used to analyze the strain hardening of the matrix forming a powder body in pressing. It is established that shear yield stress and strain hardening of the matrix determine the shape of the compaction curve and the final density of the powder body.

Surface treatment and nickel plating of iron powder metallurgy parts for corrosion protection

The aim of this study is the evaluation of corrosion behavior of nickel coated iron compacts produced by powder metallurgy (PM). To improve the quality of PM products three different secondary operations: copper infiltration, resin impregnation and steam-treatment were applied for pore closuring of iron PM specimens before being electroplated by nickel. After microstructural studies, salt fog exposure and electrochemical tests were carried out to study the corrosion behavior of the nickel coated specimens. The results showed that, nickel coated steam treated specimens exhibit better performance on corrosion testing in comparison with others. Although copper infiltrated specimens showed less porosity in PM products, observed limited corrosion behavior can be due to its galvanic effect.

Mechanical properties and porosity relationship of porous iron compacts

Experimental design technique was used in this investigation to illustrate the relationship between the porosity degree as well as its morphology on the mechanical properties and the wear resistance of iron metal compacts. Two different porous iron compacts of porosities 21 and 46% were chosen for the present investigation as metal compacts of moderate porosity (10–70%). The results indicated that the 2N factorial design technique could be used in evaluating the wear resistance of the iron compacts dependent on the percentage of porosity. It has been indicated that the increase in porosity is largely affected the wear behaviour of such porous metals. However, the stress–strain relationship of these metals is largely dependent on the degree of porosity. Since the compacts poses lower porosity (21%) provides distinct stages of elastic plastic behaviour, the compacts having higher porosity (46%) exhibit identified ultimate strength point.

Formation Mechanism of Microchannels and Lining Layers in Sintered Iron Powder Compacts with Copper Sacrificial Cores

The formation mechanism of microchannels with Fe-Cu alloy lining layers in iron bodies produced by a powder-metallurgical microchanneling process has been investigated. Copper wire was used as a sacrificial core that gives the shape of the microchannel and supplies the alloying element for the lining layer. An iron powder compact containing the sacrificial core was heated and sintered at temperatures between the melting points of copper and iron. Quenching experiments showed that the microchannel was produced just after melting of copper. In a quenched specimen with a newly-formed microchannel, fine copper-rich regions were observed between the iron powder particles in the lining layer. These results established that infiltration of molten copper into the iron powder is the dominant mechanism for the Fe-Cu microchanneling process. It was also found that the liquid copper infiltrated via preferential flow pathways between the iron powder particles.

Combustion synthesis of TiC-based cemented carbide alloy and effect of preheating treatment on porosity

In the present study, it is shown that TiC–20vol%Fe composites can be successfully fabricated by combustion synthesis from elemental powder compacts of titanium, carbon and iron. Particular concern is directed to reduction of the porosity in the product by preheating procedure of the powder compact. There exists 36% of porosity in the fabricated composites without the preheating treatment. However, the preheating treatment leads to the reduction of the volume fraction of porosity to about 10% and, furthermore, a two-step preheating treatment reduces the volume fraction of porosity down to about 5%. Vickers hardness of the TiC–Fe composites fabricated in this study was measured to be around 1300 HV regardless of the preheating condition.

Shape-controlled synthesis and cathodoluminescence properties of elongated α-Fe2O3 nanostructures

α -Fe 2 O 3 (hematite) nanostructures with various morphologies have been grown by thermal oxidation of compacted iron powder at temperatures between 700 and 900 °C. Different thermal treatments have been found to induce the growth of single-crystalline nanowires, nanobelts, nanoplates and featherlike structures, free and caped nanopillars, and pyramidal microcrystals or cactuslike microstructures. The experimental conditions leading to the different morphologies have been systematically investigated, as well as the possible growth mechanisms. The obtained nanostructures have been characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy, x-ray diffraction, and cathodoluminescence (CL) spectroscopy in the SEM. The formation of the nanostructures induces changes in the intensity and spectral distribution of the CL emission, as compared with the bulk material. Ligand to metal charge transfer transitions as well as Fe3+ ligand field transitions are thought to be involved in the observed luminescence. The evolution of the panchromatic CL intensity in the visible range as a function of temperature shows some anomalies that may be induced by magnetic ordering effects.

Investigation of magnetic properties, residual stress and densification in compacted iron powder specimens coated with polyepoxy

Soft magnetic powders are the main component of soft magnetic composites that are covered by an insulation layer. In this work, iron powder with high purity is covered by an inorganic material (iron phosphate) and then by a polyepoxy (DDS + DGEBA). The effect of amount of resin, different curing treatments and compaction pressure on the magnetic properties, residual stress, densification and microstructure are investigated. Results show that the sample with 3 wt% polyepoxy has an acceptable real part of permeability and minimum imaginary part of permeability in comparison with other samples. Also, different curing treatments have different effects on the physical and magnetic properties of the samples. For example, partial curing treatment can be applied for reducing the imaginary part of permeability; and annealing treatment can reduce this part of permeability and increase the real part of permeability.

Elaboration of metallic compacts with high porosity for mechanical supports of SOFC

The development of third generation Solid Oxide Fuel Cells (SOFC) with metallic mechanical supports presents several advantages over that of ceramic stacks by offering a lower cost and longer lifetime of the stacks. As a consequence, it is necessary to prepare metallic porous compacts that remain stable at the operating temperature of the SOFC (700–800 °C) under reductive atmosphere. This paper presents an innovative process to elaborate iron, nickel and cobalt porous compacts. The process is based on the thermal decomposition of metal oxalate precursors with controlled morphology into metallic powders with coralline shape. Uniaxial compaction of such powders (without binder addition to the powders) under low uniaxial pressures (rising from 20 to 100 MPa) gave rise to green compacts with high porosity and good mechanical properties. After annealing at 800 °C under H 2 atmosphere, the compacts still present interconnected porosity high enough to allow sufficient gas flow to feed a SOFC single cell in hydrogen: the porosity rises from 25 to 50% for iron compacts, from 20 to 50% for cobalt compacts, and is higher than 40% for nickel compacts. Results from physicochemical characterization (XRD, SEM, gas permeation, Hg porosimetry) corroborated the process for SOFC application.

Characteristic of interconnected porosity in ferrous PM compacts

The purpose of this work was to quantitatively describe the interconnected porosity in iron compacts, both in macro- and microscale. Size and volume fraction of micro-, meso- and macropores were examined in the compacts with density within 5˙6–6˙4 g cm−3, made in laboratory conditions of two iron powders: NC100˙24 and ASC100˙29 manufactured by Höganäs Company. The interconnected porosity was determined using the method based on measuring the sorption isotherms of CO2 and benzene at T=25°C in static conditions in a high vacuum gravimetric appliance equipped with McBain–Bakr weighers.Volume distributions of individual size classes of micro- and mesopores in the compacts made of both iron powders with fixed density were compared.Relationships between density and the interconnected micro- and macroporosity of the examined compacts were determined.